Ascorbic Acid - American Chemical Society

21 Harvesting, Processing, and C o o k i n g

Influences

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o n V i t a m i n C i n Foods JOHN W. ERDMAN, JR. Department of F o o d Science, University of Illinois, Urbana, IL 61801 BARBARA P. KLEIN Department of Foods and Nutrition, Bevier H a l l , University of Illinois, Urbana, IL 61801

Vitamin C is considered the most labile of the vitamins in our food supply. Reduced ascorbic acid (RAA), which is the predominant form found in foods of plant origin, can be reversibly oxidized to dehydroascorbic acid (DHA). Further irreversible oxidation of RAA or DHA to diketogulonic acid or other products results in loss of biological activity. Oxidation can occur in the presence of metal catalysts, or plant oxidase enzymes, particularly following cell damage, or as a result of heat during food processing. Vitamin C is easily leached from foods during processing and is discarded with washing, soaking, or cooking water. Ascorbic acid losses begin with harvesting and continue through handling, industrial or home preparation, cooking, and storage of plant foods.

V

i t a m i n C is w i d e l y d i s t r i b u t e d i n t h e p l a n t k i n g d o m , p a r t i c u l a r l y i n fruits s u c h as c i t r u s , g u a v a , tomatoes, s t r a w b e r r i e s , a n d b l a c k c u r r a n t s

a n d i n most vegetables, e s p e c i a l l y t h e green leafy vegetables. F o r a l i s t i n g of t h e v i t a m i n C contents of some selected f r e s h f r u i t s a n d vegetables, see T a b l e I . T h e v i t a m i n C a c t i v i t y of L - a s c o r b i c a c i d o r r e d u c e d ascorbic (RAA)

a n d its o x i d i z e d f o r m , d e h y d r o a s c o r b i c

acid

a c i d ( D H A ) is essen

t i a l l y t h e same, w h i l e D - a s c o r b i c a c i d ( i s o a s c o r b i c a c i d o r e r y t h r o a s c o r b i c acid)

h a s little of t h e v i t a m i n ' s b i o l o g i c a l p o t e n c y ( I ) . T h e readiness

w i t h w h i c h R A A is r e v e r s i b l y o x i d i z e d t o D H A is t h e basis p h y s i o l o g i c a l a c t i v i t y , a n d o f its u s e as a n a n t i o x i d a n t i n f o o d 0065-2393/82/0200-0499$09.50/0 © 1982 American Chemical Society

In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

of its

systems.

500

ASCORBIC

T h e a e r o b i c o x i d a t i o n of R A A ( F i g u r e 1)

occurs r a p i d l y w h e n m e t a l

catalysts, p a r t i c u l a r l y c o p p e r o r i r o n , o r e n z y m e s s u c h as a s c o r b i c oxidase,

peroxidase,

ACID

a n d cytochrome

polyphenol

present.

T h e a n a e r o b i c d e s t r u c t i o n of a s c o r b i c a c i d m a y p r o c e e d b y a

v a r i e t y of m e c h a n i s m s t h a t h a v e b e e n p o s t u l a t e d ( 2 , 3 )

oxidase

acid

oxidase,

are

but not verified.

D H A c a n b e r e d u c e d t o R A A b y c h e m i c a l agents, s u c h as h y d r o g e n


sulfide o r e n z y m a t i c a l l y , b y d e h y d r o a s c o r b i c a c i d reductase.

T h e conver

s i o n of D H A to d i k e t o g u l o n i c a c i d ( D K G ) is i r r e v e r s i b l e a n d o c c u r s both

aerobically

and anaerobically, particularly d u r i n g heating.

This

r e a c t i o n results i n loss of b i o l o g i c a l a c t i v i t y . T h e t o t a l o x i d a t i o n of R A A m a y result i n t h e f o r m a t i o n of f u r f u r a l b y d e c a r b o x y l a t i o n a n d d e h y d r a tion.

With

subsequent

polymerization, the formation

of

dark-colored

p i g m e n t s results. T h e s e c o m p o u n d s affect t h e c o l o r a n d flavor of c e r t a i n foods, s u c h as c i t r u s juices, a n d decrease n u t r i t i v e v a l u e . B e c a u s e v i t a m i n C is the most l a b i l e v i t a m i n i n o u r f o o d s u p p l y , i t is i m p o r t a n t to define those c o n d i t i o n s t h a t are p a r t i c u l a r l y d e t r i m e n t a l to m a i n t a i n i n g o p t i m a l a s c o r b i c a c i d content i n foods. I n this c h a p t e r w e r e v i e w the effects of

genetic

v a r i a t i o n , e n v i r o n m e n t a l factors

during

g r o w t h , as w e l l as h a r v e s t i n g , p r e p a r a t i o n , c o o k i n g , a n d storage p r a c t i c e s u p o n t h e r e t e n t i o n of v i t a m i n C i n foods "as c o n s u m e d / '

Emphasis will

b e p l a c e d o n those p r a c t i c e s t h a t a d v e r s e l y affect a s c o r b i c a c i d r e t e n t i o n i n foods. N o effort w i l l be m a d e to c o m p l e t e l y r e v i e w t h e l i t e r a t u r e o n t h e subject.

H o w e v e r , r e v i e w articles w i l l b e c i t e d a l o n g w i t h

r e s e a r c h papers.

selected

Sections are also i n c l u d e d o n assay m e t h o d o l o g y

and

k i n e t i c s of d e s t r u c t i o n of the v i t a m i n .

Table I.

Concentration of Ascorbic A c i d in Selected Fresh Vegetables and Fruits

Fruit or Vegetable

Ascorbic Acid (mg/100 g)

Kale Collard T u r n i p green G r e e n pepper Broccoli Brussel sprout M u s t a r d green Watercress Cauliflower

186 152 139 128 113 109 97 79 78

Fruit or Vegetable Kohlrabi Strawberry Spinach Orange Cabbage Rutabaga A p r i c o t , peach, p l u m , grape, a p p l e , p e a r , banana, carrot, lettuce, celery

Ascorbic Acid (mg/100 g) 66 59 51 50 47 43 10 or less

S o u r c e : C o m p i l e d b y S a l u n k h e et a l . (97) f r o m various n u t r i t i o n handbooks.


21.

501

Vitamin C in Foods

ERDMAN AND KLEIN

L-ASCORBIC ACID ANAEROBIC

AEROBIC T2H+

s

DELACTONIZATION


DEHYDROASCORBIC ACID H 0 2

-H 0 2

C0

2

XYLOSONE

DEOXYPENTOSONE

+ AMINO ACIDS

FURFURAL REDUCTONES

BROWN PIGMENTS

Figure 1.

Degradation of ascorbic acid.

Methodology for Determining Ascorbic Acid T h e d e t e r m i n a t i o n of ascorbic a c i d i n foods is b a s e d , i n p a r t , o n its a b i l i t y to b e o x i d i z e d or to act as a r e d u c i n g agent.

T h e most

common

m e t h o d f o r d e t e r m i n a t i o n of v i t a m i n C i n foods is t h e v i s u a l t i t r a t i o n o f the r e d u c e d f o r m w i t h 2 , 6 - d i c h l o r o i n d o p h e n o l ( D C I P ) ( 4 - 7 ) . V a r i a t i o n s i n this p r o c e d u r e i n c l u d e t h e use of a p o t e n t i o m e t r i c t i t r a t i o n ( 6 ) , o r a p h o t o m e t r i c a d a p t a t i o n ( 8 ) to r e d u c e t h e difficulty of v i s u a l l y d e t e r m i n ing

the endpoint i n a colored

extract.

T h e m a j o r c r i t i c i s m s of t h i s

t e c h n i q u e are t h a t o n l y t h e r e d u c e d v i t a m i n , a n d n o t t h e t o t a l v i t a m i n C content of t h e f o o d , is m e a s u r e d , a n d t h a t t h e r e c a n b e interference f r o m other r e d u c i n g agents, s u c h as s u l f h y d r y l c o m p o u n d s ,

reductones,

and

The D C I P

r e d u c e d metals ( F e , S n , C u ) , often present i n foods.

assay c a n b e m o d i f i e d to m i n i m i z e t h e effects of t h e i n t e r f e r i n g b a s i c substances, b u t the m e a s u r e m e n t is still o n l y of t h e r e d u c e d f o r m . E g b e r g et a l . ( 9 ) a d a p t e d t h e p h o t o m e t r i c D C I P assay to a n a u t o m a t e d p r o c e d u r e f o r c o n t i n u o u s analysis of v i t a m i n C i n f o o d extracts.


502

ASCORBIC

ACID

I n most p l a n t foods, t h e p r e d o m i n a n t f o r m of a s c o r b i c a c i d present is t h e r e d u c e d c o m p o u n d . T h u s , t h e e r r o r i n t r o d u c e d b y t h e u s e of D C I P assay is c o n s i d e r e d n e g l i g i b l e b y some investigators

(10).

However,

others h a v e r e p o r t e d that d u r i n g heat p r o c e s s i n g o r storage, t h e a m o u n t of D H A increases s u b s t a n t i a l l y as p e r c e n t of t o t a l a s c o r b i c a c i d ( T A A ) , a n d these w o r k e r s suggest t h a t D H A s h o u l d n o t b e n e g l e c t e d


B r e c h t et a l . (13)

r e p o r t e d t h a t r i p e tomatoes c o n t a i n e d

(11,12). negligible

q u a n t i t i e s o f D H A , b u t g r e e n m a t u r e tomatoes h a d h i g h e r a m o u n t s . T h e s e factors s h o u l d b e c o n s i d e r e d i n selection of assay m e t h o d . D e t e r m i n a t i o n of T A A c a n b e p e r f o r m e d b y s e v e r a l m e t h o d s . T h e official A s s o c i a t i o n of O f f i c i a l A n a l y t i c a l C h e m i s t s ( A O A C ) m e t h o d is t h e m i c r o f l u o r o m e t r y assay d e v e l o p e d b y D e u t s c h a n d W e e k s

(7) (14).

I n t h i s p r o c e d u r e , R A A is o x i d i z e d t o D H A u s i n g a c t i v a t e d c h a r c o a l ( N o r i t ) . T h e o x i d i z e d a s c o r b i c a c i d is r e a c t e d w i t h o - p h e n y l e n e d i a m i n e (OPDA)

f o r m i n g a c o n d e n s a t i o n p r o d u c t t h a t fluoresces

at 4 3 0 n m ,

f o l l o w i n g e x c i t a t i o n at 350 n m . O n e of t h e disadvantages of t h e O P D A m e t h o d is t h e d e v e l o p m e n t of p r e s e n c e of l i g h t .

fluorescing

compounds from O P D A i n the

Therefore, the procedure must be performed

under

reduced light conditions. Another complication m a y be the interference f r o m D K G f o r m e d i n t h e f o o d p r i o r to t h e assay as a result of p r o c e s s i n g a n d storage

(15).

S e v e r a l modifications of t h e O P D A p r o c e d u r e h a v e b e e n r e p o r t e d for a u t o m a t e d analysis. K i r k a n d T i n g (16) d e s c r i b e d a c o n t i n u o u s

flow

analysis i n w h i c h D C I P w a s s u b s t i t u t e d f o r N o r i t i n t h e o x i d a t i o n step. TAA

a n d D H A c a n b e d e t e r m i n e d d i r e c t l y ; R A A is c a l c u l a t e d as t h e

difference

between

T A A and D H A .

manual a n d automated procedures

Good

agreement

between the

was achieved, w i t h a considerable

decrease i n a n a l y t i c a l t i m e f o r t h e a u t o m a t e d assay. K i r k a n d c o w o r k e r s (17-19) h a v e u s e d t h e c o n t i n u o u s flow analysis f o r studies of t h e k i n e t i c s of ascorbic a c i d d e g r a d a t i o n i n m o d e l systems. R o y et a l . (20) d e v e l o p e d a n a u t o m a t e d p r o c e d u r e f o r

fluorometric

d e t e r m i n a t i o n of T A A , D H A , a n d R A A i n w h i c h N - b r o m o s u c c i n i m i d e w a s u s e d f o r t h e o x i d a t i o n step. oxidizes

R A A before

other

T h i s m i l d o x i d i z i n g agent s e l e c t i v e l y

interfering reducing

N - b r o m o s u c c i n i m i d e does n o t react w i t h reductones and

compounds.

Also,

present i n f r u i t s

vegetables. E g b e r g et a l . (21) u s e d N o r i t as t h e o x i d a n t i n a s e m i a u t o m a t e d

total

vitamin

C

e x t r a c t i o n step.

determination, u s i n g a simultaneous

oxidation a n d

D H A could be determined b y omitting Norit i n the

extraction. T h e p r o c e d u r e w a s successfully u s e d o n a v a r i e t y o f f o o d s ; c o r r e l a t i o n w i t h m a n u a l p r o c e d u r e s w a s excellent. T h e automated procedures, w h i c h enable the the operator to analyze m a n y samples a c c u r a t e l y , p r e c i s e l y , a n d r a p i d l y , are p a r t i c u l a r l y i m p o r t a n t


21.

503

Vitamin C in Foods

ERDMAN AND KLEIN

i n laboratories r e s p o n s i b l e for o b t a i n i n g d a t a for n u t r i t i o n a l l a b e l i n g . D u n m i r e et a l . ( 2 2 )

c o m p a r e d the m e t h o d s of E g b e r g et a l . ( 2 1 )

and

R o y et a l . ( 2 0 ) w i t h the m a n u a l A O A C v i s u a l t i t r a t i o n a n d m i c r o f l u o r o metric techniques.

F o r t y p r o d u c t s , i n c l u d i n g cereals, fruits, vegetables,

b a b y foods, juices, a n d p e t foods, w e r e i n c l u d e d i n the s t u d y . F o r those samples e x h i b i t i n g c o l o r i n t e r f e r e n c e , the t i t r a t i o n m e t h o d w a s n o t u s e d .


T h e results of this s t u d y i n d i c a t e d t h a t for laboratories d o i n g a u t o m a t e d analysis of a w i d e s p e c t r u m of p r o d u c t s , the p r o c e d u r e d e s c r i b e d

by

E g b e r g et a l . ( 2 1 ) is the most w i d e l y a p p l i c a b l e . I n the m e t h o d of R o e a n d K u e t h e r ( 2 3 ) suggested for t o t a l v i t a m i n C d e t e r m i n a t i o n ( 6 ) , ascorbic a c i d is o x i d i z e d a n d r e a c t e d w i t h 2 , 4 - d i nitrophenylhydrazine

(DNPH).

The

osazone f o r m e d

is e x t r a c t e d i n

s u l f u r i c a c i d y i e l d i n g a r e d s o l u t i o n w h o s e i n t e n s i t y is p r o p o r t i o n a l to the a s c o r b i c a c i d c o n c e n t r a t i o n .

R e d u c t o n e s a n d D K G m a y also f o r m

osazones, thus c a u s i n g s p u r i o u s l y h i g h v i t a m i n C values ( 2 4 , 2 5 ) .

The

presence of D K G is b e l i e v e d to interfere to some extent w i t h the O P D A m e t h o d as w e l l ( 2 5 ) , a l t h o u g h E g b e r g et a l . ( 2 1 ) In

addition Roe

(24)

noted

d i d n o t observe this.

t h a t h i g h concentrations

of

sugar

can

interfere w i t h this analysis, b u t the sugar osazones d e c o m p o s e i n t h e s u l f u r i c a c i d i f the p r e p a r a t i o n is a l l o w e d to s t a n d . T h e i n t e r f e r e n c e b y sugar c a n be c o m p e n s a t e d for b y the a d d i t i o n of constant a m o u n t s of fructose a n d glucose to the standards ( I I ) .

Pelletier and Brassard ( I I )

suggested the use of a m o d i f i e d m a n u a l or a u t o m a t e d D N P H b a s e d o n a m e t h o d d e s i g n e d for b i o l o g i c a l m a t e r i a l s ( 2 6 ) .

procedure,

A l t h o u g h the

p r o p o s e d a u t o m a t e d assay is less tedious t h a n Roe's m a n u a l assay a n d as accurate, t h e l o n g r e a c t i o n times n e e d e d s t i l l m a k e the

(24)

DNPH

p r o c e d u r e m o r e t i m e - c o n s u m i n g t h a n the t i t r a t i o n or f l u o r o m e t r i c analysis. A

different m e t h o d f o r d e t e r m i n a t i o n of R A A , D H A , a n d D K G ,

b a s e d o n the w o r k of R o e et a l . ( 2 7 ) , has b e e n u s e d b y s o m e investigators (12,13,28).

T o t a l A A is d e t e r m i n e d b y the D N P H m e t h o d p r e v i o u s l y

d e s c r i b e d , w h e r e the R A A is o x i d i z e d w i t h N o r i t or b r o m i n e to D H A . DHA

a n d D K G r e a c t w i t h the D N P H to f o r m t h e r e d osazones.

To

d e t e r m i n e D H A a n d D K G , the extract is n o t o x i d i z e d , b u t is r e a c t e d directly w i t h the D N P H .

T h e R A A is d e t e r m i n e d b y difference.

Total

D K G is m e a s u r e d b y r e d u c i n g t h e D H A i n t h e extract to R A A , p r i o r to osazone f o r m a t i o n .

I n cases w h e r e i t is suspected t h a t a r e l a t i v e l y

l a r g e p r o p o r t i o n of the R A A has b e e n o x i d i z e d , e i t h e r to D H A or D K G , as i n the case of f r o z e n fruits a n d vegetables assay m a y be of use. fluorometric

(12,29),

the differential

H o w e v e r , f o r r o u t i n e assays, t h e D C I P or m i c r o -

assays, o r a c o m b i n a t i o n of the t w o w i l l y i e l d a d e q u a t e

information. T h e use of h i g h p e r f o r m a n c e l i q u i d c h r o m a t o g r a p h y been proposed

(30-32)

b u t not w i d e l y a p p l i e d .

(HPLC)

T h e interference


has of


504

ASCORBIC

ACID

reductones, nonenzymatic browning compounds, and condensation prod ucts formed during processing and cooking of foods would be eliminated by an analytical technique, such as H P L C , specific for the various forms of ascorbic acid. A t this time, the selection of the method for determining ascorbic acid content requires some knowledge of the forms of vitamin C likely to be present in a given food product, the number of assays to be performed, and the spectrum of foods being assayed. In addition, the presence of interfering substances must be assessed. Genetic and Environmental Factors The ascorbic acid content of fruits and vegetables is markedly affected by variety, and to a lesser degree by maturity and climate. Increased exposure to sunlight and ripening on the plant generally en hances the vitamin C content of the edible portion. Ascorbic acid concentration within a fruit or vegetable often varies largely from part to part. The variability of the naturally occurring nutrients, such as ascorbic acid, in fruits and vegetables has been a genuine concern to fruit and vegetable processors who have elected to participate i n voluntary nutri tional labeling programs (33). The large variation of vitamin C content due to genetic and environmental factors makes it necessary to reduce label claims to avoid over labeling. Genetic Variation. The concentration of many individual nutrients in foods of plant origin is under genetic control. Baker (34) reviewed some examples of genetic manipulation that improved the quantity of /^-carotene i n tomatoes, methionine in beans, and lysine i n corn. Varia tions of ascorbic acid content of different varieties of raw vegetables and fruits is notoriously high (35). Twofold variation i n vitamin C concentra tion in different strains of a vegetable or a fruit is common and a fivefold variation can be found. Differences i n ascorbic acid contents (35- to 300-fold) of different strains of a fruit were reported prior to 1950 (36). These reports have not been substantiated. Stage of Maturity. In general, ascorbic acid concentration, but not necessarily total vitamin C content, decreases w i t h maturity. Imma ture cabbage (37), citrus fruit (38), tomatoes (39,40), and potatoes (41,42) were reported to contain higher concentrations (mg/100 g of tissue) of ascorbic acid than when mature. However, there may be some varietal variation. F o r example, the H-1783 variety of tomatoes had higher vitamin C content when fully ripe than when green (Table II) (43). Although concentration decreased during ripening, the total content of vitamin C per citrus fruit tended to increase because of


21.

ERDMAN AND KLEIN

505

Vitamin C in Foods

Table II. Environmental Factors Affecting the Vitamin C Content of the H-1783 Variety of Tomatoes


Environmental

Total Ascorbic Acid Concentration (mg/100 g)

Factor

F u l l y r i p e , l i g h t foliage N o t f u l l y ripe°, l i g h t foliage F u l l y r i p e , h e a v y foliage F u l l y r i p e , h e a v y foliage, h i g h n i t r o g e n f e r t i l i z a t i o n

23 17 18 10

6

Source: Reference 43. • One week prior to fully ripe. Twice the amount for other treatments: 100 lbs/acre. 6

i n c r e a s e d v o l u m e of juice a n d size of f r u i t ( 3 8 ) . H o w e v e r , t o t a l v i t a m i n C

content of R u s s e t B u r b a n k potatoes

harvest ( ~

110 d )

g r a d u a l l y d e c r e a s e d after e a r l y

(41,44).

B r e c h t et a l . ( 1 3 ) h a r v e s t e d e i g h t c u l t i v a r s of m a t u r e - g r e e n a n d table-ripe

tomatoes

o n t h e same

day a n d found

n o differences

in

T A A concentrations. H o w e v e r , table ripe fruits were considerably higher i n R A A t h a n t h e m a t u r e - g r e e n f r u i t s . N e g l i g i b l e q u a n t i t i e s of D H A w e r e f o u n d i n ripe fruits. W a t a d a et a l . ( 4 5 ) r e p o r t e d t h a t R A A content w a s n o t s i g n i f i c a n t l y different i n r i p e o r m a t u r e - g r e e n tomatoes.

E t h y l e n e - t r e a t e d tomatoes

were higher i n ascorbic acid than untreated fruit b u t the c o u l d n o t be d i r e c t l y a t t r i b u t e d to ethylene.

differences

T h e differences

between

c u l t i v a r s w e r e greater t h a n those b e t w e e n m a t u r i t y stages. B e t a n c o u r t et a l . (46) f o u n d t h a t i n t w o varieties of tomatoes, p l a n t r i p e n e d f r u i t a c c u m u l a t e d m o r e ( 2 2 % ) R A A t h a n d i d f r u i t r i p e n e d off the p l a n t .

U n f o r t u n a t e l y , these researchers d i d n o t m e a s u r e t h e t o t a l

a s c o r b i c a c i d c o n t e n t of tomatoes i n t h e i r s t u d y . P a n t o s a n d M a r k a k i s (47) f o u n d t h a t t w o c u l t i v a r s of tomatoes c o n t a i n e d 2 5 - 3 3 % m o r e t o t a l v i t a m i n C w h e n they were r i p e n e d o n the vine rather t h a n artificially. C o n v e r s e l y , M a t t h e w s et a l . (48) r e p o r t e d that R A A of W a l t e r tomatoes h a r v e s t e d at t h e g r e e n - m a t u r e

stage

a n d r i p e n e d off t h e p l a n t w a s

essentially t h e same as i n those r i p e n e d o n t h e p l a n t . T h u s , there m a y b e v a r i e t a l differences i n this r e g a r d . M o s t c o m m e r c i a l l y g r o w n f r e s h m a r k e t tomatoes are h a r v e s t e d a t the m a t u r e - g r e e n o r p a r t i a l l y r i p e ( " b r e a k e r " ) stages a n d are r i p e n e d off t h e p l a n t . F u r t h e r r e s e a r c h s h o u l d c l a r i f y t h e effect of stage of ripeness at p i c k i n g o n v i t a m i n C content. N e a r l y a l l c a n n e d t o m a t o p r o d u c t s are prepared from

field-ripened

tomatoes so v a r i e t a l v a r i a t i o n , b u t n o t stage

of ripeness, d e t e r m i n e s t h e v i t a m i n C i n p r o c e s s e d

products.


ASCORBIC

506

Climate.

ACID

C l i m a t i c factors, p r i n c i p a l l y t e m p e r a t u r e a n d a m o u n t of

s u n l i g h t , h a v e a s t r o n g i n f l u e n c e o n t h e c o m p o s i t i o n of fruits a n d v e g e tables, e s p e c i a l l y a s c o r b i c a c i d .

T u r n i p greens, tomatoes

(Table I I ) ,

a n d strawberries (36) a l l h a d i n c r e a s e d v i t a m i n C c o n c e n t r a t i o n w h e n g r o w n w i t h greater l i g h t exposure.

Tests w i t h n i n e varieties of apples

s h o w e d that t h e side exposed to t h e s u n w a s h i g h e r i n a s c o r b i c a c i d t h a n


the m o r e

shaded

side

(49).

Sites a n d R e i t z

(50) removed

a l l the

oranges f r o m a s i n g l e V a l e n c i a tree a n d d i v i d e d a n d assayed p o r t i o n s of e a c h orange o n t h e basis of t h e i r r e l a t i o n t o t h e d i r e c t i o n of s u n l i g h t exposure a n d t h e a m o u n t of l i g h t or shade ( o u t s i d e , c a n o p y , i n s i d e ) t h a t was received.

T h e authors f o u n d t h a t ascorbic a c i d c o n c e n t r a t i o n w a s

d i r e c t l y r e l a t e d t o exposure of that p o r t i o n o f t h e f r u i t to s u n l i g h t . S u n l i g h t is n o t necessary f o r t h e synthesis o f a s c o r b i c a c i d i n p l a n t s b u t is n e e d e d to p r o d u c e o p t i m a l v i t a m i n C concentrations.

Photosyn

thesis p r o d u c e s t h e p r e c u r s o r hexoses n e e d e d f o r ascorbic a c i d synthesis (38,51). T h e t e m p e r a t u r e o p t i m u m f o r t h e most r a p i d rate o f g r o w t h o f a species of e d i b l e p l a n t is n o t u s u a l l y o p t i m a l f o r t h e synthesis a n d storage of n u t r i e n t s i n its tissue. I n fact, t h e o p t i m a l t e m p e r a t u r e necessary t o p r o d u c e a n d store o n e n u t r i e n t w i l l often b e different f o r greatest storage of a n o t h e r n u t r i e n t (36). A l t h o u g h m o r e r e s e a r c h needs to b e c o n d u c t e d , i t appears t h a t c i t r u s f r u i t g r o w n i n t r o p i c a l or desert c l i m a t e s a c c u m u l a t e d less ascorbic a c i d t h a n fruits g r o w n i n a m o r e m o d e r a t e c l i m a t e (38). A u g u s t i n et a l . (44) c o u l d find n o statistical effect of l o c a t i o n (i.e., C a l i f o r n i a v s . M a i n e ) , p e r se, u p o n ascorbic a c i d content of potatoes.

B u r g e et a l . (43)

reported

t h a t tomatoes of the same v a r i e t y , g r o w n i n different areas of t h e U n i t e d States, m a y h a v e v i t a m i n C concentrations t h a t d e v i a t e as m u c h as 1 7 % f r o m t h e average v a l u e f o r t h a t v a r i e t y . H o w e v e r , s o m e varieties h a d h i g h e r values g r o w n i n C a l i f o r n i a , w h i l e others w e r e h i g h e r i n other areas. Soil F e r t i l i t y .

I n g e n e r a l , t h e p r i n c i p a l effect of i m p r o v e m e n t of

soils is to increase t h e y i e l d r a t h e r t h a n to e n h a n c e t h e c o n c e n t r a t i o n of n u t r i e n t s i n p l a n t s g r o w n o n these soils (36,52).

Increased

nitrogen

f e r t i l i z a t i o n has b e e n r e p o r t e d to decrease v i t a m i n C c o n c e n t r a t i o n i n potatoes

(42),

grapefruit

( 5 3 ) , a n d several other citrus f r u i t s

(38).

H a r r i s (36) n o t e d that some e a r l y researchers r e p o r t e d t h a t i n c r e a s e d n i t r o g e n f e r t i l i z a t i o n r e s u l t e d i n i n c r e a s e d ascorbic c o l l a r d s , s p i n a c h , a n d Swiss c h a r d .

acid i n cabbage,

M o r e r e c e n t l y , B u r g e et a l . (43)

s h o w e d that a p p l i c a t i o n of 100 l b s of n i t r o g e n / a c r e p r o d u c e d

tomato

p l a n t s w i t h h e a v y f o l i a g e a n d tomatoes w i t h l o w v i t a m i n C ( T a b l e I I ) . Fertilization w i t h 50 l b s / a c r e increased v i t a m i n C i n tomato fruit b y 80%.

S h e k h a r et a l . (42) also f o u n d t h a t h i g h n i t r o g e n f e r t i l i z a t i o n


21.

507

Vitamin C in Foods

ERDMAN AND KLEIN

r e s u l t e d i n l o w e r e d ascorbic a c i d content i n potato tubers. T h e effects of p h o s p h o r u s a n d p o t a s s i u m f e r t i l i z a t i o n o n v i t a m i n C are i n c o n c l u s i v e . Once

m i n i m a l m i n e r a l content

of

soil for

o p t i m a l g r o w t h of

a

p a r t i c u l a r c r o p is a c h i e v e d , no i n c r e a s e d c o n c e n t r a t i o n of v i t a m i n C is f o u n d w i t h m o r e f e r t i l i z a t i o n . I n t h e case of n i t r o g e n , i n c r e a s e d f e r t i l i z a t i o n m a y decrease ascorbic a c i d c o n c e n t r a t i o n .


Variation Among Vegetable or Fruit Parts.

F r u i t juices are w i d e l y

r e c o g n i z e d for t h e i r h i g h concentrations of ascorbic a c i d . O t h e r p o r t i o n s of citrus fruits are often h i g h e r i n c o n c e n t r a t i o n of t h e v i t a m i n .

Nagy

( 3 8 ) , i n his r e v i e w of ascorbic a c i d i n c i t r u s p r o d u c t s , p o i n t e d out t h a t o n l y a b o u t 2 5 % of t h e v i t a m i n C content of citrus f r u i t is f o u n d i n t h e j u i c e ( T a b l e I I I ) . F o r f o u r varieties of orange, the p e e l c o n t a i n e d 5 2 % , w h i l e the p u l p a n d r a g c o n t a i n e d 2 1 % , of the t o t a l v i t a m i n C i n the oranges

(54).

Table III.

Vitamin C Contents of Component Parts of Citrus F r u i t Peel

Fruit

Flavedo

Albedo

Pulp

Rag

Juice

377 239 144

206 148 —

— — 49

68 47 —

68 36 34

Orange, pineapple Grapefruit Lemon

Source: Compiled by Nagy (38). Note: Data presented as milligrams of vitamin C per 1 0 0 g of fresh weight.

T h e a p i c a l p o r t i o n s of potato tubers are h i g h e r i n a s c o r b i c a c i d t h a n the basal portions (42).

T h e l o c u l a r (soft, g e l a t i n - l i k e a n d s e e d - c o n t a i n

i n g i n n e r m a t e r i a l ) tissues of f o u r varieties of tomatoes a v e r a g e d

about

2 5 % h i g h e r i n a s c o r b i c a c i d concentrations t h a n the p e r i c a r p ( o u t e r w a l l tissues) ( 1 3 ) a l t h o u g h w i t h greater a m o u n t s of s u n l i g h t , w a l l tissue m a y be

e q u a l or

ascorbic acid

greater

than placental

(inner

core)

tissue i n

reduced

(55).

A s c o r b i c a c i d c o n c e n t r a t i o n i n peaches a n d apples is h i g h e s t just u n d e r the s k i n

(36).

T h e outer g r e e n leaves of c a b b a g e , w h i c h are

g e n e r a l l y t r i m m e d off, a n d the i n n e r core are h i g h e r i n a s c o r b i c a c i d t h a n the e d i b l e p o r t i o n ( 3 7 ) .

T h i s is consistent w i t h the n o t i o n t h a t

w h e n t r i m m i n g foods of p l a n t o r i g i n , t h e n u t r i e n t losses g e n e r a l l y e x c e e d the w e i g h t losses, because n u t r i e n t c o n c e n t r a t i o n is u s u a l l y h i g h e r i n t h e outer layers of vegetables, seeds, tubers, a n d f r u i t s ( 5 6 , 5 7 ) . Effect of Season.

T h e observation that nutrient compositions

of

e d i b l e portions of p l a n t s p r o d u c e d f r o m t h e same v a r i e t y are different f r o m one season to another p r o b a b l y results f r o m differences i n t e m p e r a -


508

ASCORBIC

ACID

t u r e , l e n g t h of d a y , l i g h t i n t e n s i t y , a n d l i g h t s p e c t r u m , as w e l l as f r o m other m i n o r factors ( 3 6 ) . T h e l i t e r a t u r e shows inconsistent fluctuation i n a s c o r b i c a c i d content of vegetables

a n d fruits p r o d u c e d

over t h e f o u r

seasons.


Effect of Harvesting and Storage of Fresh Fruits and Vegetables T h e procedures utilized d u r i n g harvesting a n d the ensuing h a n d l i n g and

storage

period

p r i o r to c o m m e r c i a l

o r at-home

processing c a n

d r a m a t i c a l l y affect b o t h n u t r i t i o n a l v a l u e a n d sensory q u a l i t y of f r u i t s a n d vegetables.

A s c o r b i c a c i d is p a r t i c u l a r l y sensitive t o b o t h e n z y m a t i c

a n d n o n e n z y m a t i c o x i d a t i o n d u r i n g this p e r i o d . M e c h a n i c a l h a r v e s t i n g of some v e g e t a b l e

Harvesting.

crops has i n c r e a s e d m a r k e d l y i n t h e last d e c a d e . C a l i f o r n i a tomato

c r o p is m a c h i n e h a r v e s t e d

a n d fruit

O v e r 9 5 % of the

(43).

T h e mechanical

shaker harvester u s e d f o r some tree f r u i t s c a n cause severe b r u i s i n g o f the f r u i t .

Injury c a n be reduced b y harvesting i n the cool hours of t h e

n i g h t , q u i c k a p p l i c a t i o n of p r e c o o l i n g , r a p i d a n d c a r e f u l t r a n s p o r t a t i o n , and immediate processing

( 5 8 ) . G e n e r a l l y , fruits a n d vegetables

will

r e a c h t h e f o o d processor w i t h i n hours b u t take m u c h l o n g e r to get f r o m the

field

to t h e r e t a i l f r e s h m a r k e t .

F o r this reason t h e m e c h a n i c a l

h a r v e s t i n g m e t h o d has n o t b e e n satisfactory f o r t h e f r e s h f r u i t m a r k e t for oranges, pears, p l u m s , apples, o r apricots ( 5 8 , 5 9 ) . N e w varieties are b e i n g d e v e l o p e d t o w i t h s t a n d better m e c h a n i c a l h a r v e s t i n g . B u r g e et a l . ( 4 3 ) r e p o r t e d that t h e a s c o r b i c a c i d c o n t e n t o f t o m a t o varieties d e v e l o p e d

f o r this p u r p o s e c o n t a i n as m u c h

ascorbic

a c i d as d o c o n v e n t i o n a l varieties. I n t a c t p l a n t tissue a s c o r b i c cellular compartmentation.

a c i d is p r o t e c t e d

from

oxidation b y

H o w e v e r , w h e n tissues a r e d i s r u p t e d after

b r u i s i n g , w i l t i n g , r o t t i n g , o r d u r i n g a d v a n c e d stages of senescence, o x i d a t i o n of t h e v i t a m i n c a n easily t a k e p l a c e . I n p l a n t s at least f o u r e n z y m e s — ascorbic

acid

oxidase,

polyphenol

oxidase,

cytochrome

oxidase, a n d

peroxidase—are thought to oxidize vitamin C i n damaged or overripe tissue (35,58,60).

I t has also b e e n suggested that p l a n t tissues c o n t a i n

r e d u c t a s e systems t h a t c a n regenerate a s c o r b i c a c i d w h e n i t is o x i d i z e d i n situ. these

W h e n t h e c e l l u l a r i n t e g r i t y is d a m a g e d

reductase

systems

(e.g., after b r u i s i n g ) ,

are inactivated a n d ascorbic

acid

oxidation

continues w i t h o u t c o n t r o l ( 3 5 ) . T h e m a x i m u m a c t i v i t y of a s c o r b i c a c i d oxidase is at 4 0 ° C , b u t i t is almost c o m p l e t e l y i n a c t i v a t e d at 6 5 ° C . T h e r e f o r e , b l a n c h i n g o f vegetables or f r u i t s p r i o r to f u r t h e r p r o c e s s i n g

(freezing or canning)

is i d e a l f o r

p r o t e c t i o n of v i t a m i n C f r o m e n z y m a t i c o x i d a t i o n , a l t h o u g h some r e g e n e r a t i o n of h e a t - i n a c t i v a t e d peroxidases m a y o c c u r after b l a n c h i n g ( 3 5 ) .


21.

ERDMAN AND KLEIN

Storage

509

Vitamin C in Foods

of N e w l y Harvested Crops.

E n z y m a t i c d e s t r u c t i o n of

a s c o r b i c a c i d c a n b e g i n as soon as a c r o p is h a r v e s t e d .

K a l e c a n lose

1.5% o f its v i t a m i n C p e r h o u r a n d a b o u t o n e - t h i r d i n 24 h . Storage i n c o o l c o n d i t i o n s a n d i n c r e a s e d h u m i d i t y , factors t h a t r e d u c e w i l t i n g , r e d u c e losses of ascorbic a c i d d u r i n g storage.

W i l t i n g ( m o i s t u r e loss)

t i c u l a r l y p r e v a l e n t w i t h t h e fresh, green, l e a f y vegetables


surface

areas.

Ezell

and Wilcox

(61)

observed

is p a r

with

large

that ascorbic

acid

d e s t r u c t i o n i n k a l e w a s a c c e l e r a t e d b y r o o m t e m p e r a t u r e storage, p a r t i c u l a r l y i f t h e h u m i d i t y w a s l o w . F r e s h r a w s p i n a c h stored o v e r n i g h t at 4 ° C i n a w a l k - i n c o o l e r lost as m u c h as 4 0 % o f its i n i t i a l ascorbic a c i d . T h e degree o f loss d e p e n d e d o n t h e c o n d i t i o n of t h e leaves at t h e t i m e of storage

( 6 2 ) . C o n t r o l of t e m p e r a t u r e a n d h u m i d i t y is essential f o r

p r e s e r v a t i o n o f v i t a m i n C i n these p r o d u c t s . G r e e n beans, w h i c h h a v e less surface area t h a n t h e leafy vegetables, stored at 10 ° C f o r 24 h , lost o n l y 1 0 % of t h e i r a s c o r b i c a c i d c o n c e n t r a tions, b u t w h e n stored at r o o m t e m p e r a t u r e f o r t h e same t i m e , lost 2 4 % of

t h e i r ascorbic

acid concentration

(35,63).

O n l y m i n o r losses

of

a s c o r b i c a c i d are f o u n d after 1 o r 2 m o n t h s storage o f fresh c i t r u s f r u i t , w h i c h h a v e v e r y l o w surface areas a n d a p r o t e c t i v e p e e l , i f t h e y a r e stored i n c o o l temperatures ( 3 . 3 - 5 . 6 ° C )

(38).

E h e a r t ( 6 4 ) f o u n d that t h e ascorbic a c i d c o n t e n t o f b r o c c o l i h e l d at 3 ° C i n c r e a s e d d u r i n g storage.

I n a follow-up

study, E h e a r t a n d

O d l a n d ( 6 5 ) f o u n d n o significant losses of ascorbic a c i d d u r i n g a 1-week storage p e r i o d , a n d i n fact, a 3 6 % increase w a s n o t e d f o r o n e v a r i e t y . T h i s w a s a t t r i b u t e d to a s c o r b i c a c i d synthesis f r o m

monosaccharides

d u r i n g t h e storage p e r i o d . G r e e n beans, o n t h e other h a n d , lost signifi cant a m o u n t s of ascorbic a c i d ( u p to 8 8 % ) after 1 w e e k at t h e same t e m p e r a t u r e , a l t h o u g h decreases w e r e s m a l l i n t h e first 48 h . R e f r i g e r a t i o n ( 0 - 1 0 ° C ) of f r e s h fruits a n d vegetables is c o m m o n l y u s e d to r e t a r d d e t e r i o r a t i o n i n e a t i n g q u a l i t i e s (flavor, texture, a p p e a r ance, a n d c o l o r ) a n d n u t r i t i v e v a l u e .

R e d u c t i o n of t e m p e r a t u r e slows

r e s p i r a t o r y a c t i v i t y i n p l a n t p r o d u c t s , reduces m o i s t u r e loss, a n d decreases the rate of d e c a y d u e to m i c r o o r g a n i s m s . I n t h e h o m e , fresh p r o d u c e is g e n e r a l l y stored r e f r i g e r a t e d f o r a r e l a t i v e l y short p e r i o d of t i m e .

Once

f r u i t is r i p e n e d , i t s h o u l d b e r e f r i g e r a t e d p r o m p t l y to p r e v e n t u n d e s i r a b l e s o f t e n i n g a n d subsequent

b r u i s i n g , w h i c h increases losses of a s c o r b i c

a c i d d u e to e n z y m a t i c a c t i o n a n d o x i d a t i o n .

Vegetables, particularly

l e a f y ones, s h o u l d b e stored i n p l a s t i c bags o r i n a v e g e t a b l e c r i s p e r , to m i n i m i z e m o i s t u r e loss that accelerates a s c o r b i c a c i d d e g r a d a t i o n (61,66). S o m e vegetables a r e subject to c h i l l i n g i n j u r y at r e f r i g e r a t i o n t e m peratures.

T h i s i n c l u d e s f a i l u r e to r i p e n n o r m a l l y ( m a t u r e - g r e e n t o m a

toes, i m m a t u r e bananas, a n d e g g p l a n t s ) , s u s c e p t i b i l i t y to d e c a y potatoes),

u n d e s i r a b l e increases i n sugars

(sweet

( p o t a t o e s ) , as w e l l as a n


510

ASCORBIC

increase i n lesions ( p i t t i n g , i n t e r n a l a n d external d i s c o l o r a t i o n ) .

ACID

Mem

b r a n e d a m a g e d u r i n g c h i l l i n g increases the p o s s i b i l i t y of e n z y m e release i n the p l a n t tissue, r e s u l t i n g i n f u r t h e r softening a n d o x i d a t i v e reactions. T h e s e effects decrease the e a t i n g q u a l i t y a n d a c c e p t a b i l i t y of the p r o d u c e , r e d u c i n g its c o n s u m p t i o n . not r e c e i v e d


important economically. ascorbic

T h e n u t r i t i v e v a l u e of c h i l l - i n j u r e d foods has

m u c h a t t e n t i o n , b e c a u s e the v i s i b l e d e t e r i o r a t i o n is m o r e I n g e n e r a l , c h i l l i n g has

l i t t l e effect o n

a c i d content of fresh fruits a n d vegetables u n d e r

conditions

of d i s t r i b u t i o n , storage, a n d m a r k e t i n g ( 5 8 ) .

controlled

atmosphere

storage o n ascorbic

the

reasonable

T h e effect of

a c i d content of fruits

and

vegetables stored u n d e r those c o n d i t i o n s has not b e e n w e l l i n v e s t i g a t e d . T e m p e r a t u r e of storage is i m p o r t a n t i n m a i n t a i n i n g the ascorbic a c i d c o n t e n t of potatoes. temperatures

I n g e n e r a l , potatoes are stored at r e l a t i v e l y h i g h

( 4 0 - 5 0 ° F ) , w h i c h results i n better ascorbic a c i d r e t e n t i o n

as w e l l as better q u a l i t y

(67).

A major factor c o n t r i b u t i n g to the v a r i a b i l i t y i n v i t a m i n C c o n t e n t of potatoes is the storage t i m e .

A u g u s t i n et a l . ( 4 4 )

reported a sharp

decrease i n the v i t a m i n d u r i n g the first 4 m o n t h s of storage of potatoes at a b o u t 7 ° C a n d 9 5 % r e l a t i v e h u m i d i t y . O v e r the next 4 months there w a s either a c o m p l e t e l e v e l i n g out or a less p r o n o u n c e d

decrease.

After

8-months storage a l l varieties a p p e a r e d to c o n t a i n a b o u t the same c o n c e n t r a t i o n of v i t a m i n C ( 4 0 - 5 0 m g / 1 0 0 g D W B ) .

Kinetics of Ascorbic Acid Destruction During Processing and Storage A n interest i n q u a n t i t a t i v e a p p r o a c h e s to changes i n f o o d q u a l i t y has b e e n s t i m u l a t e d i n p a r t b y l a b e l i n g (68).

government

r e g u l a t i o n of n u t r i t i o n a l

T h e r e l a t i v e i n s t a b i l i t y of a s c o r b i c a c i d u n d e r u s u a l c o n d i

tions of f o o d storage a n d p r o c e s s i n g is w e l l d o c u m e n t e d (69, 7 0 ) .

How

ever, the p r e d i c t i o n of v i t a m i n C losses is c o m p l i c a t e d b y l a c k of i n f o r m a t i o n a b o u t t h e m e c h a n i s m s of d e g r a d a t i o n a n d the factors t h a t i n f l u ence t h e m .

T h e loss of ascorbic

a c i d is d e p e n d e n t o n t h e p r e s e n c e o r

absence of o x y g e n , t h e rate of o x y g e n transfer, p H , a n d w a t e r c o n t e n t a n d a c t i v i t y of the f o o d (17, 71-74). T o p r e d i c t n u t r i e n t d e t e r i o r a t i o n , k n o w l e d g e of t h e r e a c t i o n r a t e as a f u n c t i o n of t e m p e r a t u r e of storage or p r o c e s s i n g is n e e d e d . T h e k i n e t i c s of ascorbic model

acid destruction have been examined

systems, w i t h p a r t i c u l a r a t t e n t i o n b e i n g

m o s t extensively

g i v e n to

in

intermediate

m o i s t u r e foods (17, 7 1 , 7 8 , 7 9 ) .

M o s t o f t h e d a t a a v a i l a b l e for v i t a m i n C

losses i n a c t u a l f o o d systems

are insufficient to

parameters

calculate the

n e e d e d to p r e d i c t losses d u r i n g heat t r e a t m e n t or


kinetic storage.

21.

ERDMAN AND KLEIN

511

Vitamin C in Foods

L u n d (80) a n d L a b u z a (71,76) h a v e r e e x a m i n e d t h e d a t a i n t h e l i t e r a t u r e to d e r i v e some of t h e values. H o w e v e r , i n m a n y cases, t h e i n f o r m a t i o n p r e s e n t e d i n the r e v i e w e d studies is i n a d e q u a t e to establish t h e c o n d i t i o n s t h a t w e r e u s e d f o r processing o r storage.

T h e interpretation

of d a t a w a s sometimes b a s e d o n erroneous a s s u m p t i o n of r e a c t i o n orders, l e a d i n g to i n a c c u r a t e p r e d i c t i o n s of n u t r i e n t losses


A c c o r d i n g to L e n z a n d L u n d of

food components

are n e e d e d

(76).

( 7 2 ) , kinetic models for destruction to improve

products

by minimizing

q u a l i t y changes f o r n e w p r o d u c t d e v e l o p m e n t a n d t o p r e d i c t shelf l i f e d u r i n g storage. N u m e r o u s reports a n d r e v i e w s of t h e k i n e t i c s of a s c o r b i c a c i d d e s t r u c t i o n c a n b e f o u n d i n t h e l i t e r a t u r e (68-88).

A brief overview

is p r e s e n t e d here to i n d i c a t e t h e n e e d f o r f u r t h e r r e s e a r c h i n this area. N u t r i e n t d e s t r u c t i o n is u s u a l l y d e s c r i b e d

i n terms of t i m e a n d

t e m p e r a t u r e effects u s i n g t h e r e a c t i o n r a t e a n d t h e d e p e n d e n c e o f t h e r e a c t i o n rate o n t e m p e r a t u r e .

T h e parameters most f r e q u e n t l y u s e d b y

p h y s i c a l chemists a n d i n e n g i n e e r i n g a p p l i c a t i o n s are t h e r e a c t i o n r a t e constant (k), at a g i v e n t e m p e r a t u r e ( T ) , a n d t h e A r r h e n i u s a c t i v a t i o n e n e r g y (E ). a

I n the food industry, the time to reduce the concentration

of a c o m p o n e n t to 1 0 % of t h e i n i t i a l v a l u e ( D ) , at a g i v e n t e m p e r a t u r e ( u s u a l l y 1 2 1 ° C ) , a n d t h e c h a n g e i n degrees F a h r e n h e i t r e q u i r e d f o r a t e n - f o l d c h a n g e i n D (z), are u s e d to d e s c r i b e t h e r e a c t i o n rates. T h e Q

10

v a l u e , w h i c h is t h e ratio of t h e r e a c t i o n rate (k) at T ( ° C ) +

10, t o

t h e r e a c t i o n rate at T , is often u s e d i n b i o l o g i c a l d e s c r i p t i o n s of k i n e t i c s . F o r ascorbic a c i d , losses are g e n e r a l l y c o n s i d e r e d to f o l l o w

first-order

k i n e t i c s as d e s c r i b e d b y : -dC dt

= fcC

(1)

w h e r e C = t h e c o n c e n t r a t i o n of t h e n u t r i e n t , t = t i m e , a n d k = t h e r a t e constant ( t i m e ' ) . I f C = C at t i m e zero, i n t e g r a t i o n of E q u a t i o n 1 yields: 1

0

C = C exp 0

(-kt)

(2)

F o r t h e first-order r e a c t i o n , a p l o t of l o g C vs. t w i l l y i e l d a straight l i n e , a n d t h e rate constant (k), c a n b e d e r i v e d f r o m t h e slope.

I t is p o s s i b l e

t o p l o t t h e l o g of t h e ratios of concentrations a t t w o t i m e s , o r t h e l o g o f t h e p e r c e n t n u t r i e n t r e m a i n i n g , vs. t i m e t o o b t a i n t h e rate constant a t a

particular temperature

(71,77).

T h e half-life (time

required for

d e s t r u c t i o n of 5 0 % o f t h e i n i t i a l v i t a m i n p r e s e n t ) at a g i v e n t e m p e r a t u r e c a n also b e u s e d to c a l c u l a t e k since t h e h a l f - l i f e is i n d e p e n d e n t o f t h e initial concentration.


512

ASCORBIC

ACID

I f the d e s t r u c t i o n of a c o m p o n e n t is p r e s u m e d to be z e r o - o r d e r , a p l o t of C vs. t s h o u l d g i v e a straight l i n e .

C e r t a i n losses of v i t a m i n C ,

p a r t i c u l a r l y i n f r o z e n foods, are p r e s u m e d to f o l l o w first-order k i n e t i c s (89).

L a b u z a (76)

o b s e r v e d t h a t z e r o - o r d e r r e a c t i o n rates f o r q u a l i t y

losses m a y be a s s u m e d i n some

fluctuating

t e m p e r a t u r e studies, b u t t h i s

m a y l e a d to a m i s c a l c u l a t i o n of p r e d i c t e d changes.

Therefore, from a


t h e o r e t i c a l s t a n d p o i n t , i t is i m p o r t a n t that the p r o p e r o r d e r b e u s e d f o r p r e d i c t i o n s . I n g e n e r a l , a s c o r b i c a c i d d e s t r u c t i o n is a s s u m e d to b e order, o r p s e u d o

first-order

first-

(17,27) except u n d e r specific c o n d i t i o n s o f

heat a n d m o i s t u r e ( 7 9 ) . T h e t e m p e r a t u r e d e p e n d e n c e of the r e a c t i o n is u s u a l l y d e s c r i b e d b y the A r r h e n i u s e q u a t i o n : k = where k =

fc exp 0

(3)

(-E /RT) a

the first-order rate constant, k =

p r e - e x p o n e n t i a l constant o r

0

f r e q u e n c y factor or A r r h e n i u s constant ( t i m e ) , R — gas constant (1.987 - 1

c a l / K - m o l ) , and T =

absolute t e m p e r a t u r e ( K ) .

T h e activation energy

is e q u i v a l e n t to —2.303R times the slope of a p l o t of l o g k vs. 1 / T .

(E ) a

T h e energy of a c t i v a t i o n for most v i t a m i n s is c o n s i d e r e d to be 2 0 - 3 0 k c a l / m o l ( 6 9 ) , b u t the v a l u e c a n be affected b y a n u m b e r of factors. T h e E s for d e s t r u c t i o n of enzymes or m i c r o o r g a n i s m s u s e d as i n d i c a t o r s a

of a d e q u a c y of t h e r m a l p r o c e s s i n g are g e n e r a l l y m u c h h i g h e r .

Because

a n increase i n t e m p e r a t u r e has a greater effect o n the r e a c t i o n rate w h e n the E

a

is h i g h e r , i f k s are c o m p a r a b l e , m o r e r a p i d d e s t r u c t i o n of e n z y m e s 0

a n d m i c r o o r g a n i s m s t h a n v i t a m i n s m a y result at the e l e v a t e d t e m p e r a t u r e s u s e d for t h e r m a l p r o c e s s i n g . T h e r e f o r e , i t has b e e n a s s u m e d t h a t v i t a m i n s are m o r e stable t h a n other f o o d c o m p o n e n t s .

However, relatively little

d a t a are a v a i l a b l e to s u p p o r t this. S o m e i n f o r m a t i o n r e g a r d i n g the E

f o r a s c o r b i c a c i d i n f o o d systems

a

c a n be f o u n d i n t h e l i t e r a t u r e . K i r k et a l . (17) vitamin C destruction i n a model d e s c r i b e d b y the

first-order

determined that the

d e h y d r a t e d f o o d system c o u l d

function.

Rates of d e s t r u c t i o n w e r e

e n c e d b y a , m o i s t u r e , a n d t e m p e r a t u r e of storage. w

be

influ

A c t i v a t i o n energies

for T A A d e s t r u c t i o n at a s a b o v e 0.24 w e r e a p p r o x i m a t e l y 18 k c a l / m o l , w

s i m i l a r to those r e p o r t e d for R A A b y L e e a n d L a b u z a ( 7 8 ) . w e r e r e p o r t e d at a s w

Lower

Es a

less t h a n 0.24, s u g g e s t i n g a different m e c h a n i s m

for a s c o r b i c a c i d d e s t r u c t i o n , p e r h a p s b y a n a n a e r o b i c p a t h w a y .

Rate

constants w e r e also i n f l u e n c e d b y t h e p a c k a g i n g u s e d for the m o d e l f o o d system, w h i c h m a y be a t t r i b u t e d to t h e a m o u n t of d i s s o l v e d

oxygen

present. L e e et a l . (81)

reported that the E

a

for the a n a e r o b i c d e s t r u c t i o n of

a d d e d a s c o r b i c a c i d i n t o m a t o j u i c e at p H 4.0 d u r i n g storage w a s


3.3

21.

513

Vitamin C in Foods

ERDMAN AND KLEIN

k c a l / m o l , w h i c h is l o w e r t h a n t h e values f o r ascorbic a c i d d e s t r u c t i o n i n b u f f e r e d solutions r e p o r t e d b y B l a u g a n d H a j r a t w a l a ( 8 2 ) . T h e a n a e r o b i c d e s t r u c t i o n of ascorbic a c i d is g e n e r a l l y b e l i e v e d to p r o c e e d at a s l o w e r r a t e t h a n aerobic d e g r a d a t i o n .

I n c a n n e d foods, t h e absence o f o x y g e n

alters the m e c h a n i s m of d e s t r u c t i o n , a n d thus t h e r e a c t i o n rate ( 7 2 , 7 5 ) . L a t h r o p a n d L e u n g (75) reported that the E

a


degradation i n canned kcal/mol.

peas d u r i n g p r o c e s s i n g

for ascorbic

acid

at 1 1 0 - 1 3 2 ° C w a s 4 1

T h i s w a s h i g h e r t h a n t h e E s r e p o r t e d f o r other n u t r i e n t s , a

a l t h o u g h i n a s i m i l a r r a n g e (83,84).

C o n t r a r y to other studies, these

investigators suggested t h a t a s c o r b i c a c i d d e s t r u c t i o n i n c a n n e d peas w a s m u c h m o r e heat sensitive t h a n t h a t i n m o d e l systems.

R a o et a l . (90),

i n a s i m i l a r i n v e s t i g a t i o n w i t h c a n n e d peas, r e p o r t e d

an E

a

of 13.1

k c a l / m o l , significantly lower than that f o u n d b y L a t h r o p a n d L e u n g (75).

Differences i n t h e d e s t r u c t i o n m e c h a n i s m ( a e r o b i c vs. a n a e r o b i c ) ,

o x y g e n c o n c e n t r a t i o n , a n d a c t u a l t e m p e r a t u r e i n t h e center of t h e f o o d m i g h t account f o r some of t h e v a r i a t i o n . N a g y ( 3 8 ) r e p o r t e d t h a t f o r stored c i t r u s juices, t h e loss of v i t a m i n C w a s n o t necessarily a first-order r e a c t i o n . F o r g r a p e f r u i t juice, t h e E

a

w a s 18.2 k c a l / m o l , a n d t h e r e a c t i o n w a s

first-order.

F o r orange

juice,

t w o E s w e r e d e t e r m i n e d : 12.8 k c a l / m o l i n the t e m p e r a t u r e r a n g e 4 - 2 8 ° C , a

a n d 24.5 k c a l / m o l i n t h e r a n g e 2 8 - 5 0 ° C . T h e c h a n g e i n r e a c t i o n k i n e t i c s w a s a t t r i b u t e d to different d e s t r u c t i o n m e c h a n i s m s , a l t h o u g h n o e x p l a n a t i o n w a s offered. T h e l a c k o f d a t a f o r a c t u a l f o o d systems, a n d t h e d i s c r e p a n c i e s c i t e d a b o v e , i n d i c a t e t h e difficulty i n p r e d i c t i n g a s c o r b i c a c i d losses t h a t m i g h t o c c u r d u r i n g p r o c e s s i n g a n d storage of foods. W i t h t h e a d v e n t o f n u t r i t i o n a l l a b e l i n g , t h e processor m u s t b e a b l e to p r o v i d e foods c o n t a i n i n g at least t h e a m o u n t of a n u t r i e n t l i s t e d at t h e t i m e o f p u r c h a s e .

There

fore, t h e d e t e r m i n a t i o n of losses as o u t l i n e d b y L a b u z a et a l . ( 8 5 ) o r L e n z a n d L u n d (72) s h o u l d b e p a r t o f t h e q u a l i t y c o n t r o l p r o g r a m i n a food company.

T h e development

of c o m p u t e r s i m u l a t i o n m o d e l s f o r

p r e d i c t i o n of q u a l i t y losses, i n c l u d i n g n u t r i e n t s , w i l l b e c o m e i n c r e a s i n g l y important

(85-88).

Ascorbic Acid Losses During Industrial Processing B e c a u s e o f v i t a m i n C s l a b i l i t y t o h e a t - i n d u c e d o x i d a t i o n a n d its h i g h s o l u b i l i t y i n w a t e r , m a j o r losses o f t h e v i t a m i n c a n o c c u r d u r i n g food processing

techniques

t h a t u t i l i z e l o n g heat treatments o r t h a t

i n v o l v e large q u a n t i t i e s of s o a k i n g , r i n s i n g , o r c o o k i n g w a t e r

(35,91,92).

T h i s section r e v i e w s t h e effects of different f o o d p r e s e r v a t i o n processes o n the retention of v i t a m i n C i n commercial food products.


514

ASCORBIC

Blanching.

ACID

H o t w a t e r , m i c r o w a v e , or s t e a m b l a n c h i n g is u s e d p r i o r

t o f r e e z i n g , c a n n i n g , or d r y i n g operations p r i m a r i l y to i n a c t i v a t e e n z y m e s t h a t cause d e t e r i o r a t i o n d u r i n g storage of f r o z e n foods.

B l a n c h i n g is

also u s e d to c l e a n a n d r e d u c e t h e m i c r o b i a l p o p u l a t i o n o n the v e g e t a b l e or f r u i t , to soften b u l k y vegetables a n d r e d u c e v o l u m e p r i o r to p a c k a g i n g , to e x p e l gasses t h a t c a n create excessive


m a i n t a i n o r " f i x " color Hot

water

pressure i n the c a n , a n d to

(35).

blanch

and

subsequent

water-cooling

is

undesirable

because of l e a c h i n g of v a r i o u s w a t e r - s o l u b l e n u t r i e n t s , e s p e c i a l l y ascorbic a c i d . Losses of ascorbic a c i d are e s p e c i a l l y h i g h w h e n t h e surface area p e r mass of the f o o d is large ( s m a l l p i e c e s ) , w h e n there is a l a r g e w a t e r to f o o d r a t i o , w h e n t h e contact t i m e is l o n g , a n d w h e n the p r o d u c t is extensively s t i r r e d i n w a t e r . W h e n b l a n c h c o n d i t i o n s are f a v o r a b l e , loss of ascorbic a c i d c a n b e less t h a n 1 0 % , b u t the loss c a n b e 5 0 % o r m o r e u n d e r severe c o n d i t i o n s .

T h e w i d e r a n g e of a s c o r b i c a c i d r e t e n t i o n i n

c a n n e d fruits a n d vegetables m a y reflect the v a r y i n g c o n d i t i o n s of b l a n c h ing,

a l t h o u g h i n some cases h i g h losses r e s u l t f r o m the use of

copper

salts as color stabilizers ( 9 3 ) . L a t h r o p and L e u n g (94)

c o l l e c t e d p e a samples at v a r i o u s p o i n t s

a l o n g a c o m m e r c i a l c a n n i n g l i n e i n W a s h i n g t o n State ( T a b l e I V ) . found a total 8 %

loss of a s c o r b i c

c l e a n i n g , a n d s i z i n g operations d u e to l e a c h i n g ( n o h e a t a p p l i e d ) . w a t e r b l a n c h i n g ( 3 m i n at 8 2 - 8 8 ° C ) c a u s e d a f u r t h e r 1 9 %

Table IV.

They

acid concentration d u r i n g soaking, Hot

reduction

Vitamin C Losses Due to Specific Operations in Commercial Thermal Pea Processing Vitamin C

Content (mg/100 g of Peas) 0

Process Receiving Soaking Washing and sizing Blanching Hot filling still continuous T h e r m a l processing still continuous

Loss After Each Operation (%)

Cumulative

± ± ± ±

0.9 1.5 2.9 1.1

0 2.4 5.8 19.2

0 2.4 8.0 25.7

10.4 ± 10.3 ±

0.1 1.4

43.8 44.3

58.2 58.6

8.2 ± 8.0 ±

1.1 0.9

21.2 22.3

67.1 67.9

24.9 24.3 22.9 18.5

Source: Lathrop and Leung (94). ° Mean ± sd for five or more samples.


21.

ERDMAN AND KLEIN

515

Vitamin C in Foods

i n v i t a m i n C . T h e same authors f u r t h e r s t u d i e d t h e effect of b l a n c h t i m e o n t h e ascorbic a c i d content of 2 0 - 4 0 - g q u a n t i t i e s of peas b l a n c h e d i n a p i l o t - s c a l e steam b l a n c h e r at 1 0 0 ° C o r i n d i s t i l l e d w a t e r ( 4 : 1 , w a t e r : p e a s ) at 8 5 ° C . and

A f t e r 3 m i n t h e r e t e n t i o n w a s 7 8 . 5 % after s t e a m

7 1 . 9 % after w a t e r b l a n c h i n g . A b o u t o n e - t h i r d o r one-half of t h e

v i t a m i n C lost f r o m h o t - w a t e r - b l a n c h e d peas w a s l e a c h e d i n t o t h e b l a n c h


water. S e l m a n , i n h i s r e v i e w of v i t a m i n C losses d u r i n g p r o c e s s i n g of peas ( 9 5 ) , o b s e r v e d that there w a s a l a r g e v a r i a t i o n a m o n g varieties f o r b o t h the v i t a m i n C content a n d r e t e n t i o n d u r i n g p r o c e s s i n g steps s u c h as b l a n c h i n g . T h e losses w e r e n o t r e l a t e d to t h e i n i t i a l ascorbic a c i d content of t h e v a r i e t y . T h e w o r k of M o r r i s o n (96) shows a t h r e e f o l d v a r i a t i o n of v i t a m i n C loss f r o m six c u l t i v a r s of peas b l a n c h e d i n w a t e r f o r 1 m i n at 9 7 ° C . I n g e n e r a l , steam b l a n c h i n g appears to result i n l o w e r loss of a s c o r b i c a c i d a l t h o u g h some studies h a v e s h o w n n o difference

between

steam

a n d h o t w a t e r . L e a c h i n g loss is less w i t h steam b u t i n c r e a s e d o x i d a t i o n m a y o c c u r because of l o n g e r b l a n c h times ( 3 5 ) . M i c r o w a v e b l a n c h i n g has b e e n s h o w n i n some cases to b e s u p e r i o r to steam b l a n c h i n g f o r v i t a m i n C r e t e n t i o n (80,97) p r o b a b l y because of less l e a c h i n g loss. T h e hot gas a n d s u p e r h e a t e d steam b l a n c h i n g p r o c e d u r e s

h a v e y e t to b e

a d e q u a t e l y tested f o r t h e i r effects o n ascorbic a c i d r e t e n t i o n i n a v a r i e t y of p r o d u c t s . Pasteurization

and Commercial

Sterilization

(Canning).

Along

w i t h b l a n c h i n g , p a s t e u r i z a t i o n a n d s t e r i l i z a t i o n are t h e c o m m o n t h e r m a l p r e s e r v a t i o n p r o c e d u r e s i n i n d u s t r i a l processing. t e u r i z a t i o n is to i n a c t i v a t e vegetative organisms.

T h e objective of p a s

cells of p a t h o g e n i c

or spoilage

O t h e r t h a n m i l k , most p a s t e u r i z e d p r o d u c t s h a v e a l o w p H

to r e d u c e t h e rate of m i c r o b i a l g r o w t h .

M a n y orange a n d other f r u i t

juices a n d d r i n k s a r e p a s t e u r i z e d . P a s t e u r i z a t i o n is u s u a l l y f o l l o w e d b y other s p e c i a l treatments s u c h as r e f r i g e r a t i o n o r f e r m e n t a t i o n . T h e c o m m e r c i a l s t e r i l i z a t i o n p r o c e d u r e uses sufficient heat to i n a c t i v a t e spores of p a t h o g e n i c or spoilage organisms. S t e r i l i z a t i o n u s u a l l y is u s e d i n c o n j u n c t i o n w i t h a n a e r o b i c storage c o n d i t i o n s

(68,80).

P a s t e u r i z a t i o n r e q u i r e s c o n s i d e r a b l y less t h e r m a l i n p u t t h a n does s t e r i l i z a t i o n . T h e r e f o r e , t h e t h e r m a l losses d u r i n g p a s t e u r i z a t i o n process i n g are q u i t e l o w . H o w e v e r , o x i d a t i v e losses c a n b e h i g h i f care is n o t t a k e n to deaerate a n d i f h i g h - t e m p e r a t u r e - s h o r t - t i m e ( H T S T ) c o n d i t i o n s are n o t u s e d ( 8 0 ) . Thompson

(98) i n v e s t i g a t e d t h e loss of n u t r i e n t s f r o m m i l k after

pasteurization a n d sterilization.

A p o r t i o n of h i s results a r e f o u n d i n

T a b l e V . H T S T ( 7 2 ° C f o r 15 s) p a s t e u r i z a t i o n of t h e fluid m i l k p r i o r


516

ASCORBIC

Table V .

Loss of Nutrients in M i l k D u r i n g Processing Pasteurized


Nutrient Thiamine Vitamin C Folic acid Vitamin B

ACID

1 2

Sterilized

HTST

Holder

UHT

In Bottle

(%)

(%)

(%)

(%)

10 10 0 0

10 20 0 10

10 10 10 20

35 50 50 30

Source: Thompson (98) as noted by Lund (80).

to b o t t l i n g is s u p e r i o r to p a s t e u r i z a t i o n of t h e p r e v i o u s l y b o t t l e d m i l k . U l t r a h i g h temperature

( 1 4 4 ° C for a f e w

s e c o n d s ) is f a r s u p e r i o r

to

s t e r i l i z i n g i n the bottle for r e t e n t i o n of f o u r m e a s u r e d v i t a m i n s . C o m m e r c i a l s t e r i l i z a t i o n of

peas i n a retort r e s u l t e d i n a

r e t e n t i o n of the i n i t i a l a s c o r b i c a c i d i n one s t u d y ( T a b l e V ) 38-60%

r e t e n t i o n i n another

study

(99).

33%

(94)

T h e latter s t u d y

and

reported

c o n s i d e r a b l y l o w e r v i t a m i n C loss d u r i n g the b l a n c h i n g o p e r a t i o n a n d thus h i g h e r t o t a l r e t e n t i o n . F i n a l r e t e n t i o n of v i t a m i n C i n other v e g e tables after c a n n i n g w a s 4 0 %

for c o r n a n d 2 3 % f o r beets ( 9 9 ) .

The

l o w t o t a l r e t e n t i o n of v i t a m i n C for beets w a s l a r g e l y because of b l a n c h i n g a n d p e e l i n g operations.

L a r g e b l a n c h i n g losses for w a x beans a n d g r e e n

beans w e r e also r e p o r t e d

(99). A s c o r b i c a c i d is the most difficult of

Moisture Removal ( D r y i n g ) .

the v i t a m i n s to preserve d u r i n g the d e h y d r a t i o n of foods. Losses of 2 0 % have been reported i n spray-dried m i l k a n d 3 0 % (93).

Those d r y i n g procedures

i n roller-dried milk

t h a t r e q u i r e shorter d r y i n g times

improve vitamin C retention (35).

will

R e t e n t i o n c a n also b e i m p r o v e d b y

a p p l i c a t i o n of v a c u u m or b y use of n i t r o g e n atmosphere d u r i n g d r y i n g ( 9 7 ) , or b y t h e a d d i t i o n of s u l f u r d i o x i d e to fruits a n d vegetables p r i o r to d r y i n g ( 9 3 ) .

S u l f u r d i o x i d e m u s t be u s e d w i t h c a u t i o n , h o w e v e r , as

i t destroys t h i a m i n e . T h e rate of a s c o r b i c a c i d d e s t r u c t i o n a n d the a c t i v a t i o n e n e r g y are v e r y sensitive to w a t e r a c t i v i t y . T h e r e a c t i o n rate constant varies o v e r three orders of m a g n i t u d e for the entire w a t e r a c t i v i t y range (100).

At

h i g h w a t e r a c t i v i t i e s , ascorbic a c i d is r a p i d l y d e s t r o y e d , b u t i t is q u i t e stable at l o w w a t e r a c t i v i t y . T h e s e n s i t i v i t y of a s c o r b i c a c i d to t e m p e r a ture at h i g h w a t e r a c t i v i t y suggests t h a t v i t a m i n C r e t e n t i o n is d e p e n d e n t o n t h e w e t b u l b t e m p e r a t u r e at the b e g i n n i n g of d r y i n g a n d o n t e m p e r a t u r e of e v a p o r a t i o n .

C o n t r o l of d r y i n g rate a n d c o n d i t i o n s

the is

essential for r e t e n t i o n of v i t a m i n C , a n d is p a r t i c u l a r l y i m p o r t a n t f o r d r y i n g of f r u i t slices or halves.


21.

517

Vitamin C in Foods

ERDMAN AND KLEIN

C o n c e n t r a t i o n of f r u i t juices s h o u l d n o t result i n m a r k e d loss of ascorbic a c i d i f t h e pressed j u i c e is d e a e r a t e d a n d a n d e v a p o r a t e d at l o w temperatures (100). A s c o r b i c a c i d retentions i n excess of 9 0 % h a v e b e e n r e p o r t e d f o r c o n c e n t r a t i o n a n d f r e e z i n g processes (38,101)

and can be

expected for freeze c o n c e n t r a t i o n processes (100). G e e (102) d r i e d slices of carrots a n d tomatoes

a n d whole spinach


leaves at 47 ° C f o r 1 6 - 2 4 h i n a f o r c e d - d r a f t o v e n to a w a t e r a c t i v i t y o f 0.33.

N o p r e - b l a n c h c h e m i c a l t r e a t m e n t w a s used.

D u r i n g dehydration

no loss i n t o t a l ascorbic a c i d w a s d e t e c t e d f o r carrots a n d less t h a n 2 0 % loss w a s n o t e d f o r tomatoes.

A s c o r b i c a c i d loss f o r s p i n a c h w a s 6 2 % .

A s c o r b i c a c i d content r e m a i n e d r a t h e r stable i n carrots a n d tomatoes for a b o u t 1 m o n t h stored i n a i r , b u t losses a c c e l e r a t e d r a p i d l y thereafter. N o benefit of n i t r o g e n or v a c u u m p a c k a g i n g o n v i t a m i n C r e t e n t i o n during

storage

was noted.

These

a r e s u r p r i s i n g results.

The high

r e t e n t i o n of v i t a m i n C u n d e r d r y i n g c o n d i t i o n s s h o u l d b e v e r i f i e d . Dehydration

of potatoes c a n result i n h i g h l y v a r i a b l e

acid retention depending

u p o n t h e process t e c h n i q u e s

ascorbic

used.

Jadhav

et a l . (103) f o u n d a m a x i m u m loss of 7 8 % of R A A i n a n a d d b a c k , a i r - d r y i n g process a n d a m a x i m u m loss of o n l y 3 0 % i n a f r e e z e - t h a w process f o r p r o d u c i n g

d e h y d r a t e d m a s h e d potatoes.

M a g a a n d Sizer

(104) s t u d i e d v i t a m i n C a n d t h i a m i n r e t e n t i o n i n e x t r u s i o n

processed

potato s n a c k - l i k e flakes. A t h i g h extrusion temperatures, t h e best a s c o r b i c a c i d r e t e n t i o n o c c u r r e d i n t h e l o w m o i s t u r e potato m e a l ( 2 5 % w a t e r ) . H o w e v e r , t h i a m i n e r e t e n t i o n at h i g h e x t r u s i o n t e m p e r a t u r e s w a s g e n e r a l l y best f r o m h i g h - m o i s t u r e m e a l ( 5 9 % w a t e r ) a n d p o o r f r o m l o w - m o i s t u r e meal. Low Temperature Treatment. ucts a r e h i g h i n ascorbic a c i d .

M a n y commercial frozen food p r o d

R e f r i g e r a t i o n is u s e d f o r v a r i o u s f r e s h

f r u i t juices a n d f o r h o l d i n g m a n y fruits a n d vegetables

p r i o r to r e t a i l

sale or f u r t h e r processing. Freezing, i f properly conducted, food

preservation

technique

n u t r i e n t r e t e n t i o n (35,57,105,106). ascorbic

is c o n s i d e r e d

for both

t h e best l o n g - t e r m

o p t i m a l sensory

quality a n d

T h e average r e t e n t i o n of r e d u c e d

a c i d i n different vegetables

that were

blanched, frozen, or

stored f o r 6 - 1 2 m o n t h s at - 1 8 ° C a n d t h a w e d w a s a b o u t 5 0 % (106). Losses d u r i n g t h e entire process are l a r g e l y a t t r i b u t e d to b l a n c h i n g a n d to t h e p r o l o n g e d f r o z e n storage.

Losses differ m a r k e d l y a m o n g p r o d u c t s

a n d w i t h i n varieties of a c u l t i v a r . U n d e r p r o p e r p r o c e s s i n g c o n d i t i o n s , fruits lose less t h a n 3 0 % of t h e i r o r i g i n a l v i t a m i n C content t h r o u g h t h e entire f r e e z i n g a n d f r o z e n storage p e r i o d (106). 90%

of t h e i r a s c o r b i c

F r o z e n c o n c e n t r a t e d f r u i t juices c a n r e t a i n over a c i d (38,106).

P a c k i n g i n s y r u p is g e n e r a l l y

p r o t e c t i v e of ascorbic a c i d .


518

ASCORBIC

ACID

T h e t e m p e r a t u r e of f r o z e n storage is c r i t i c a l f o r o p t i m a l n u t r i e n t r e t e n t i o n . T h e I n t e r n a t i o n a l I n s t i t u t e of R e f r i g e r a t i o n (107)

recommends

that for g o o d r e t e n t i o n of n u t r i e n t s a n d q u a l i t y of foods the m a x i m u m f r e e z i n g storage t e m p e r a t u r e s h o u l d be — 1 8 ° C ( 0 ° F ) .

A t this t e m p e r a

t u r e t h e r e is a s l o w b u t a c c e p t a b l e d e t e r i o r a t i o n of f o o d q u a l i t y ( 3 5 ) . A s c o r b i c a c i d loss i n f r o z e n foods is h i g h l y t e m p e r a t u r e d e p e n d e n t .


F o r e x a m p l e , w h e n peaches, boysenberries, or strawberries are stored at — 7 ° C i n s t e a d of — 1 8 ° C , the rate of a s c o r b i c a c i d d e g r a d a t i o n increases b y a factor of 3 0 - 7 0

(105,106).

K r a m e r et a l . (108) ( ± 1 ° C ) and

fluctuating

m e a s u r e d a s c o r b i c a c i d r e t e n t i o n at constant ( d b 5 ° C e a c h 20 m i n ) t e m p e r a t u r e s i n Salisbury

steaks stored for 3 or 6 m o n t h s ( T a b l e V I ) . T h e y f o u n d g o o d r e t e n t i o n of v i t a m i n C s t o r e d at constant — 2 0 ° C or l o w e r for 3 m o n t h s . A t h i g h e r t e m p e r a t u r e s , o r at fluctuating t e m p e r a t u r e s , t h e r e w a s s u b s t a n t i a l r e d u c t i o n of v i t a m i n C w i t h i n 3-months storage. C r e t e n t i o n u n d e r b o t h constant a n d

N o other studies of v i t a m i n

fluctuating

temperature were found.

P r o p e r storage of c a n n e d a n d b o t t l e d s i n g l e - s t r e n g t h f r u i t juices is essential for m a x i m a l v i t a m i n C r e t e n t i o n . N a g y , i n his r e v i e w of

the

l i t e r a t u r e ( 3 8 ) , c o n c l u d e s that storage at 21 ° C ( t h e t e m p e r a t u r e t h a t m a y be close to the average y e a r - r o u n d n o n r e f r i g e r a t e d c o m m e r c i a l storage conditions)

for u p w a r d s of a y e a r results i n v i t a m i n C retentions of

greater t h a n 7 5 % .

H o w e v e r , storage t e m p e r a t u r e s i n excess of

28°C

cause m a r k e d a s c o r b i c a c i d d e s t r u c t i o n , a n d at 38 ° C l i t t l e v i t a m i n C w a s retained.

R e f r i g e r a t i o n storage

(4-10°C)

r e s u l t e d i n excellent

(90%

or m o r e ) r e t e n t i o n of t h e v i t a m i n , after 1 y e a r .

Table V I . Effect of Constant and Fluctuating Temperatures U p o n the Ascorbic A c i d and Thiamine Contents of Salisbury Steak Ascorbic Acid Content Storage Conditions Initial 3 Months -10°C -20°C -30°C 6 Months -10°C -20°C -30°C

Constant Temperature

Fluctuating Temperature

Thiamine Content Constant Temperature

Fluctuating Temperature

3.0

3.6 1.8 2.8 2.8

1.0 2.0 2.5

2.8 2.9 3.2

1.8 2.1 2.6

1.2 1.6 1.3

1.1 1.1 1.1

1.9 2.7 2.7

1.8 1.7 2.6

Source: Reference 108. Note: Data presented as milligram of vitamin specified per 100 g.


21.

519

Vitamin C in Foods

ERDMAN AND KLEIN

S q u i r e s a n d H a n n a ( 1 0 9 ) c o l l e c t e d samples of seventeen b r a n d s o f commercial

reconstituted

orange

juices

i n plastic-coated

cardboard

containers a n d stored t h e m u n d e r r e f r i g e r a t i o n at 4 ° C . T h e y r e p o r t e d that the average r e d u c t i o n i n R A A w a s a b o u t 2 % / d . Fermentation.

Selective f e r m e n t a t i o n of foodstuffs f o r p r e s e r v a t i o n ,

e n h a n c e m e n t o f n u t r i t i v e v a l u e , i m p r o v e m e n t of


of

alcoholic

beverages has b e e n

flavor,

practiced probably

or preparation

since p r e h i s t o r i c

times b y p e o p l e s of n e a r l y every c i v i l i z a t i o n (110). D u r i n g f e r m e n t a t i o n of foods, o n e often c a n f i n d a net increase i n content of c e r t a i n B v i t a m i n s , s u c h as r i b o f l a v i n a n d n i a c i n , b e c a u s e of m i c r o b i a l synthesis.

However,

d e p e n d i n g u p o n o t h e r p r o c e s s i n g steps, t h e f e r m e n t e d f o o d m a y o r m a y not r e t a i n t h e n e w l y s y n t h e s i z e d n u t r i e n t s . S e p a r a t i o n of m i l k i n t o t h e c u r d a n d w h e y fractions p r i o r to cheese m a n u f a c t u r e causes a p a r t i t i o n of w a t e r - s o l u b l e substances ( w h e y ) f r o m the c u r d .

M o r e t h a n 8 0 % of t h e v i t a m i n C a n d t h i a m i n e is r e m o v e d

w i t h the whey fraction

(111).

D e s a l t i n g of t h e b r i n e d u r i n g p i c k l e

m a n u f a c t u r e results i n a 1 0 0 % v i t a m i n C loss.

W h e n d e s a l t i n g is n o t

p r a c t i c e d , losses of a s c o r b i c a c i d are a b o u t 4 0 - 5 0 %

(110).

It w a s r e p o r t e d i n 1939 (112) t h a t d u r i n g t h e a c t i v e f e r m e n t a t i o n p e r i o d of s a u e r k r a u t p r o d u c t i o n , t h e v i t a m i n C content w a s e q u a l t o that of t h e o r i g i n a l c a b b a g e . T h e n , d u r i n g v a t storage, a s l o w , p r o g r e s sive loss of v i t a m i n C o c c u r r e d .

Further destruction ( 2 5 - 3 5 % ) occurred

d u r i n g p r e h e a t i n g a n d c a n n i n g operations.

M a r k e d loss o f v i t a m i n C

w a s f o u n d i n c a n n e d k r a u t stored at e l e v a t e d temperatures (113). recent r e p o r t R o et a l . (114) i n v e s t i g a t e d t h e v i t a m i n B

a c i d content of K i m c h i , a Chinese c a b b a g e f e r m e n t e d w i t h terium freundenreichii creased,

ss. shermanii.

v i t a m i n C content

Propionibac-

Although vitamin B i

decreased

(when

compared

In a

a n d ascorbic

12

2

content i n with

fresh

u n f e r m e n t e d K i m c h i ) d u r i n g t h e first 5 w e e k s o f f e r m e n t a t i o n . F r o m these a n d other studies o n e m u s t c o n c l u d e t h a t most f e r m e n t e d foods a r e n o t g o o d sources o f v i t a m i n C . E x c e p t i o n s w o u l d b e f r e s h c a b b a g e a n d o t h e r f e r m e n t e d vegetables t h a t h a v e n o t b e e n desalted. I o n i z i n g r a d i a t i o n f o r u s e i n f o o d systems c a n c o m e

Irradiation. from

electrons, x-rays, o r g a m m a

rays f r o m

cobalt-60

o r cesium-137.

T h e r e is l i t t l e rise i n t h e t e m p e r a t u r e w i t h i n t h e foodstuff, destruction

of

n u t r i e n t s is m i n i m i z e d .

peroxides are f o r m e d w i t h i n t h e f o o d .

However,

free

so h e a t

r a d i c a l s and

I n t h e U n i t e d States, i r r a d i a t i o n

is classified as a f o o d a d d i t i v e a n d its use i n t h e f o o d i n d u s t r y has b e e n severely r e s t r i c t e d to s u c h areas as p r e v e n t i o n of potato s p r o u t i n g and wheat infestation. A s is n o t e d i n C h a p t e r 3 i n this b o o k , a s c o r b i c a c i d reacts

very

r a p i d l y w i t h free r a d i c a l s f o r m e d f r o m w a t e r . T h e e n d p r o d u c t s of t h i s


520

ASCORBIC

r e a c t i o n c o u l d b e D H A o r other n o n b i o l o g i c a l l y a c t i v e f o r m s

ACID

of t h e

v i t a m i n . V i t a m i n C i n i r r a d i a t e d f o o d systems c a n b e c o n s i d e r e d to a c t as a free r a d i c a l scavenger, b u t as s u c h its o v e r a l l content i n i r r a d i a t e d food w o u l d be reduced. J o s e p h s o n et a l . (115) c o n c l u d e f r o m t h e i r r e v i e w of t h e effects o f i o n i z i n g r a d i a t i o n t r e a t m e n t of foods that n u t r i e n t d e s t r u c t i o n , s u c h as


to v i t a m i n C , i n i r r a d i a t e d foods is no greater t h a n t h a t o c c u r r i n g w h e n f o o d is p r e s e r v e d b y m o r e c o n v e n t i o n a l means. I n a d d i t i o n , i f i r r a d i a t i o n is p e r f o r m e d o n f r o z e n foods, t h e losses of v i t a m i n C are r e d u c e d . Storage

P r o p e r storage

of Preserved Foods.

conditions for pre

s e r v e d foods w i l l extend t h e i r shelf life. E x t e n d e d storage at h i g h t e m p e r a t u r e s , o r at h i g h h u m i d i t y w i t h i n a d e q u a t e p a c k a g i n g w i l l l e a d to d e t e r i o r a t i o n of t h e f o o d p r o d u c t s ' sensory q u a l i t y a n d n u t r i e n t content. A n e x a m p l e of t h e effects of p o o r p a c k a g i n g a n d p o o r storage c o n d i t i o n s f o r c r a n b e r r y sauce is i l l u s t r a t e d i n T a b l e V I I .

Table V I I .

Effects of Packaging Upon Quality of Cranberry Sauce (

P

H

-

2.7)

After Storage for 5 Weeks at 37° C at 80% RH in Packages Made of:

Test M o i s t u r e (Jo) Acceptability (hedonic scale) Ascorbic acid ( m g / 1 0 0 g)

Prior to Storage

Aluminum No. 2 Laminated Lined Tin Can Film

Polyester Film (2 layers)

Polyester Film (1 layer)

56.1 7.3

55.9 6.7

56.0 7.2

54.5 6.3

53.5 5.3

89.2

73.4

71.7

0.4

0.0

Source: Reference 149,

Ascorbic Acid Losses During Food Preparation It has b e e n suggested that, d u r i n g p r e p a r a t i o n of f o o d f o r t h e t a b l e either i n o r o u t of t h e h o m e , losses of n u t r i e n t s o c c u r that surpass those i n c u r r e d d u r i n g p r o c e s s i n g (116).

A n extensive r e v i e w of t h e l i t e r a t u r e

d o n e i n 1960 (117), w h i c h w a s u p d a t e d i n 1975 ( 9 2 ) , r e v e a l e d t h a t f e w n e w d a t a h a v e b e e n r e p o r t e d i n t h e i n t e r v e n i n g years.

E r d m a n (57)

suggested t h a t m o r e i n f o r m a t i o n , p a r t i c u l a r l y o n t h e effects of c o o k i n g p r o c e d u r e s o n n u t r i e n t r e t e n t i o n , w a s necessary. W e d o n o t i n t e n d to p r o v i d e c o m p r e h e n s i v e c o v e r a g e of t h e e a r l y w o r k o n a s c o r b i c a c i d r e t e n t i o n i n h o m e p r e p a r e d foods i n this s e c t i o n ,


21.

ERDMAN AND KLEIN

521

Vitamin C in Foods

b u t to i n d i c a t e t h e c r i t i c a l p o i n t s w h e r e losses m a y occur.

T h e increased

c o n s u m p t i o n of f o o d a w a y f r o m h o m e , i n a f o o d service f a c i l i t y , s u c h as s c h o o l a n d business cafeterias, restaurants, a n d fast f o o d stands, influence t h e n u t r i e n t s a v a i l a b l e to t h e consumer.

will

Nutrient data collected

i n t h e 1940s h a v e b e c o m e less u s e f u l because of changes i n r a w m a t e r i a l s , f o o d p r o c e s s i n g , t e c h n o l o g y , a n d e q u i p m e n t (118). T h e r e f o r e , o n e m u s t


v i e w t h e i n f o r m a t i o n a v a i l a b l e about t h e influence of v a r i o u s processes a n d p r o c e d u r e s o n v i t a m i n C content of foods as i n d i c a t i v e , r a t h e r t h a n absolute. I n most studies of v i t a m i n C content of p r e p a r e d f o o d , t h e v a r i e t y of f r u i t o r vegetable is r a r e l y r e p o r t e d o r k n o w n . T h e c o n s u m e r , unless s h e / h e is t h e g a r d e n e r as w e l l , has little c o n t r o l over t h e v a r i e t y o f f r u i t or v e g e t a b l e p u r c h a s e d or a v a i l a b l e . I n c o m m e r c i a l r e t a i l i n g of fruits a n d vegetables, t h e decisions r e g a r d i n g v a r i e t y are i n f l u e n c e d b y season, growing

area, a n d q u a l i t y characteristics of t h e p r o d u c t

other

than

D u r i n g preparation for cooking,

most

nutritive value. Preparation for Cooking.

p l a n t materials r e q u i r e some p e e l i n g a n d t r i m m i n g . I n a d d i t i o n , s u b d i v i sion of large w h o l e vegetables is often d o n e to speed c o o k i n g times a n d to p r o v i d e m o r e u n i f o r m size pieces.

T r i m m i n g losses v a r y c o n s i d e r a b l y

d e p e n d i n g o n t h e t y p e of f o o d , its c o n d i t i o n a n d freshness,

a n d the

m e t h o d of c o o k i n g to b e used. A h a n d b o o k of f o o d y i e l d s (119) p r o v i d e s extensive i n f o r m a t i o n a b o u t losses a n d gains i n w e i g h t of f o o d d u r i n g different stages of p r e p a r a t i o n . T h e extent of t r i m m i n g f o r most fruits a n d vegetables w i l l i n f l u e n c e the t o t a l i n i t i a l n u t r i e n t content.

Since a s c o r b i c a c i d is u s u a l l y c o n c e n

t r a t e d i n t h e outer leaves o r layers of t h e f r u i t o r vegetables, r e m o v a l of these portions m a y result i n a c o n s i d e r a b l e loss

(37,67,120,121).

Losses i n a s c o r b i c a c i d content o w i n g to a d v a n c e p r e p a r a t i o n ( c u t t i n g a n d s h r e d d i n g ) d o o c c u r (122-125), b u t the a m o u n t is u n p r e d i c t a b l e a n d inconsistent. T h e losses are i n f l u e n c e d b y t h e degree o f s u b d i v i s i o n , s o a k i n g i n w a t e r , m a t e r i a l of the c u t t i n g i n s t r u m e n t , t i m e of s t a n d i n g , a n d t e m p e r a t u r e of storage

during standing.

D e s t r u c t i o n o f ascorbic

acid

i n c u t c a b b a g e has b e e n r e p o r t e d to b e as l o w as 3 % (121) a n d as h i g h as 4 0 % (122). A c c o r d i n g to V a n D u y n e et a l . (122) r e t e n t i o n of r e d u c e d a s c o r b i c a c i d i n c a b b a g e that w a s s h r e d d e d a n d a l l o w e d to s t a n d f o r 1 h i n a i r , 1 h i n w a t e r , or 3 h i n w a t e r w a s over 8 7 % i n a l l cases.

Similarly,

p e e l e d potatoes, q u a r t e r e d o r w h o l e , d i d n o t lose significant amounts of ascorbic a c i d d u r i n g a 1 - 3 - h s o a k i n g p e r i o d (126).

B a s e d o n t h e sparse

d a t a , i t appears that t h e s o a k i n g of vegetables i n w a t e r f o r short p e r i o d s of t i m e p r i o r to c o o k i n g results i n s m a l l losses o f ascorbic a c i d . I f a v e g e t a b l e is s h r e d d e d o r c u t , t h e losses w i l l b e i n c r e a s e d .


522

ASCORBIC ACID

C o o k i n g of Vegetables. bic acid retention were freezing;

I n t h e 1940s, m a n y of t h e studies o f ascor

concerned

i n t h e next d e c a d e ,

with

processes

studies o f c o o k i n g

of blanching a n d methods

including

pressure c o o k i n g , " w a t e r l e s s " c o o k i n g , a n d s t e a m i n g w e r e m o r e

common.

T h e d e v e l o p m e n t of e l e c t r o n i c c o o k i n g also s t i m u l a t e d r e s e a r c h i n t h e e a r l y 1960s. S i n c e that t i m e , there h a v e b e e n r e l a t i v e l y f e w n e w reports


o n v i t a m i n r e t e n t i o n i n c o o k e d foods. I n assessing t h e c o o k i n g studies, c a u t i o n m u s t b e u s e d i n i n t e r p r e t i n g results.

C l e a r l y , t h e a m o u n t of w a t e r a n d t h e t i m e of h e a t i n g m u s t b e

specified. D i f f i c u l t y i n d e t e r m i n i n g t h e degree of "doneness" m a y i n f l u ence t h e observations. T h e a m o u n t of s u b d i v i s i o n of t h e vegetables m u s t also b e c o n s i d e r e d .

I t s h o u l d b e p o i n t e d o u t that i n a l m o s t a l l of t h e

investigations c i t e d o n l y o n e c o o k i n g t i m e , a n " o p t i m u m " , w a s u s e d . A l t h o u g h there a r e r e c o m m e n d a t i o n s f o r c o o k i n g times a n d a m o u n t s of w a t e r to b e u s e d i n h o m e c o o k i n g , i t is l i k e l y t h a t t h e average p r e p a r e r deviates s u b s t a n t i a l l y f r o m t h e r e c o m m e n d a t i o n s

food

because

of

p e r s o n a l preferences, i n e x p e r i e n c e , or other factors. I t w o u l d b e interest i n g to d e t e r m i n e w h a t

cooking

methods

are a c t u a l l y u s e d ,

a n d to

d e t e r m i n e t h e effects of a r a n g e of h e a t i n g times a n d t e m p e r a t u r e s o n ascorbic a c i d r e t e n t i o n i n vegetables. BAKING,

STEAMING,

A N D PRESSURE

COOKING.

The

most

common

m e t h o d of c o o k i n g vegetables is b y b o i l i n g i n w a t e r . T h e p r o p o r t i o n of w a t e r to v e g e t a b l e u s e d c a n v a r y greatly f r o m a 0 : 1 (waterless to a 5:1 r a t i o ( w a t e r to c o v e r ) .

cooking)

U s u a l recommendations for vegetable

c o o k e r y are t o use a " m i n i m u m a m o u n t " of w a t e r , w h i c h c a n r a n g e f r o m a ratio of 0.25:1 to 1:1 d e p e n d i n g o n t h e f o o d .

I n making recommenda

tions f o r t h e a m o u n t of w a t e r to b e u s e d , p a l a t a b i l i t y as w e l l as n u t r i e n t r e t e n t i o n are c o n s i d e r e d . E x a m p l e s o f t h e effects of different ratios o f w a t e r t o vegetable u s e d i n v a r i o u s c o o k i n g m e t h o d s o n ascorbic a c i d r e t e n t i o n a r e s h o w n i n T a b l e V I I I . T h e w a t e r to vegetable r a t i o u s e d i n t h e c o o k i n g p r o c e d u r e is i m p o r t a n t because

losses o f ascorbic

a c i d are p r i m a r i l y c a u s e d b y

l e a c h i n g of t h e v i t a m i n . C o o k i n g m e t h o d s that r e d u c e exposure t o l a r g e q u a n t i t i e s o f w a t e r , s u c h as pressure c o o k i n g , s t e a m i n g , a n d m i c r o w a v e c o o k i n g , w o u l d b e e x p e c t e d to result i n h i g h e r t o t a l a s c o r b i c a c i d r e t e n t i o n (10,127-132). W h e n t h e same o r s i m i l a r r a t i o o f w a t e r t o v e g e t a b l e is u s e d i n a n y of t h e c o o k i n g m e t h o d s , ascorbic a c i d losses a r e a p p r o x i m a t e l y t h e same, i f c o o k i n g times are c o m p a r a b l e (131-135). a l l vegetables

cooked

I n almost

i n a m i n i m u m a m o u n t o f w a t e r , ascorbic

r e t e n t i o n i n t h e vegetables

acid

w a s over 7 0 % , regardless of t h e c o o k i n g

m e t h o d u s e d . E h e a r t a n d G o t t (136) n o t e d t h a t w h e n n o w a t e r o r v e r y s m a l l a m o u n t s w e r e u s e d , ascorbic a c i d r e t e n t i o n w a s a b o u t t h e same. H o w e v e r , i f l a r g e q u a n t i t i e s of w a t e r ( 5 : 1 r a t i o ) w e r e u s e d f o r b o i l i n g


21.

E R D M A N AND K L E I N

523

Vitamin C in Foods

Table VIII. Effects of Some Common Cooking Methods on Ascorbic A c i d Retention in Vegetables


Vegetable and Cooking Method Broccoli microwave saucepan (boiling) microwave s a u c e p a n (boiling) saucepan waterless c o o k i n g Cabbage microwave microwave saucepan ( b o i l i n g ) pressure cooker microwave saucepan (boiling) microwave saucepan (boiling) Spinach microwave saucepan microwave saucepan

vegetables,

Water: Vegetable Ratio

Cooking Time (min)

Percent Retention

References

2:1 2:1 0.4:1 5.5:1 0.5:1 0.2:1

3 10 5 6.5 10 8

72 59 87 45 88 82

150 150 130 ISO 132 132

0:1 3.5:1 4:1 1.25:1 0.25:1 5:1 0.5:1 0.5:1

5 4 25 3 4 6.5 12 14

93 85 20 50 80 38 72 69

150 150 150 150 ISO ISO 134 134

0:1 0:1 0.25:1 0.25:1

7 7 6.5 7

56 61 47 50

134 134 135 135

as w a s d o n e b y G o r d o n a n d N o b l e

( 1 3 0 ) , ascorbic

acid

retention was m u c h lower i n conventionally cooked than i n microwave c o o k e d vegetables, e v e n t h o u g h c o o k i n g times w e r e s i m i l a r . Studies i n w h i c h the ascorbic

a c i d r e t e n t i o n a n d c o n t e n t of

the

c o o k i n g l i q u i d w e r e d e t e r m i n e d (130,133,134) s h o w e d t h a t t o t a l a s c o r b i c acid retention was generally about 9 0 % (137) liquid

after c o o k i n g .

S w e e n e y et a l .

r e p o r t e d that t o t a l percent r e t e n t i o n i n b r o c c o l i solids p l u s c o o k i n g d i d not decrease

greatly w i t h time.

Therefore,

ascorbic

acid

a p p e a r e d to b e r e l a t i v e l y heat stable a l t h o u g h i t w a s easily l e a c h e d i n t o t h e c o o k i n g w a t e r . W h e n l a r g e r p r o p o r t i o n s of w a t e r w e r e u s e d ( 1 3 0 ) , a h i g h e r p e r c e n t a g e of ascorbic a c i d w a s f o u n d i n the c o o k i n g l i q u i d , w h i c h emphasizes the i m p o r t a n c e of u s i n g m i n i m u m a m o u n t s of w a t e r i n vegetable

cookery.

It is f r e q u e n t l y stated that s t e a m i n g vegetables is most b e n e f i c i a l i n r e t a i n i n g ascorbic a c i d i n the f o o d . (56)

A n e x a m i n a t i o n of t h e sparse d a t a

i n the l i t e r a t u r e does not s u p p o r t this t h e o r y , h o w e v e r .

cases, the l e n g t h e n e d c o o k i n g t i m e necessary to soften t h e

In many vegetables

c a n result i n d e s t r u c t i o n of v i t a m i n C b y h e a t a n d o x i d a t i o n . I f vegetables


524

ASCORBIC ACID

are extensively s u b d i v i d e d p r i o r to c o o k i n g

(e.g., s h r e d d e d o r t h i n l y

s l i c e d ) , t h e n s t e a m i n g m e t h o d s are satisfactory f o r p a l a t a b l e , n u t r i t i o u s vegetables.

F o r l a r g e r pieces o r w h o l e vegetables,

t h e use of s m a l l

a m o u n t s of w a t e r a n d c o n v e n t i o n a l o r m i c r o w a v e c o o k i n g m e t h o d s w i l l b e q u i c k e r a n d m o r e effective i n r e t a i n i n g a s c o r b i c a c i d . I t s h o u l d b e p o i n t e d o u t t h a t t h e d a t a f o r steamed a n d b o i l e d vegetables f r o m t h e


same

sources

are n o t r e a d i l y a v a i l a b l e .

Thus, the recommendations

r e g a r d i n g s t e a m i n g are i n f e r r e d r a t h e r t h a n k n o w n . MICROWAVE COOKING.

W h e n sales o f m i c r o w a v e ovens i n c r e a s e d i n

the 1970s, t h e n u t r i t i o n a l benefits w e r e a d v e r t i s e d . R e c e n t studies h a v e confirmed the early

findings

(134,136) t h a t m i c r o w a v e c o o k i n g results

i n g o o d r e t e n t i o n of a s c o r b i c a c i d , p r e s u m a b l y because of t h e l o w w a t e r to vegetable ratios a n d short c o o k i n g times u s e d . I n a r e v i e w of m i c r o w a v e effects o n n u t r i e n t r e t e n t i o n , K l e i n

(138)

pointed out that the

a m o u n t of w a t e r u s e d i n c o o k i n g , a n d to a lesser extent, t h e c o o k i n g t i m e , affect ascorbic a c i d losses m o r e t h a n t h e source of energy or t h e t y p e of c o o k i n g . I f short c o o k i n g times a n d s m a l l a m o u n t s o f w a t e r are u s e d , m o r e ascorbic a c i d w i l l b e r e t a i n e d i n a n y c o o k i n g m e t h o d . M a b e s a a n d B a l d w i n (139) f o u n d t h a t f r o z e n peas, c o o k e d w i t h o r w i t h o u t w a t e r i n m i c r o w a v e ovens or c o n v e n t i o n a l l y , v a r i e d i n a s c o r b i c a c i d content.

W h e n t h e same r a t i o of w a t e r to v e g e t a b l e

(1:4) was

u s e d i n the m i c r o w a v e a n d c o n v e n t i o n a l m e t h o d s , ascorbic a c i d retentions were similar ( 7 0 % ) , but lower than w h e n no water was used i n the microwave oven (retention > 9 6 % ). I n a s t u d y of ascorbic a c i d r e t e n t i o n i n f r o z e n c a u l i f l o w e r IX)

(Table

( 1 4 0 ) , t h e ascorbic a c i d c o n t e n t a n d r e t e n t i o n i n t h e vegetables

cooked w i t h no water was significantly higher ( p < 0 . 0 5 )

t h a n those

c o o k e d w i t h i n c r e a s i n g p r o p o r t i o n s of w a t e r ( 0 . 2 5 : 1 , 0.5:1, 1 : 1 ) . H o w ever, t h e c a u l i f l o w e r c o o k e d w i t h o u t w a t e r w a s n o t as p a l a t a b l e .

Table I X .

Ascorbic A c i d Retention in Frozen Cauliflower Cooked with Various Ratios of Water to Vegetable Water: Ratio

Cooking Time ( min)

0:1 0.25:1 0.5:1 1:1

5.5 5.5 5 5

Vegetable

U n c o o k e d cauliflower C o o k e d cauliflower

Ascorbic Acid* (mg/100

51.4 52.2 45.5 43.1 42.3

± ± ± ± ±

Percent g)

3.9 8.1 2.5 1.5 0.7

Retention

0

100 99.8 ± 88.0 ± 82.9 ± 81.1 ±

° Mean ± sd for four samples.


7.7 4.9 2.8 1.6

21.

Vitamin C

ERDMAN AND KLEIN

K l e i n et a l . ( 1 3 5 )

525

in Foods

f o u n d t h a t a p p r o x i m a t e l y one-half of the i n i t i a l

ascorbic a c i d i n f r e s h r a w s p i n a c h w a s r e t a i n e d i n the c o o k e d vegetable. T h e a m o u n t of w a t e r a n d v e g e t a b l e

(1:4)

constant, a n d c o o k i n g times w e r e s i m i l a r . ascorbic

a c i d content

between

used i n each method

was

N o significant differences i n

conventionally and microwave

cooked

spinach were found.


I n most f r o z e n vegetables,

sufficient ice clings to the p r o d u c t

p r o v i d e a d e q u a t e m o i s t u r e for c o o k i n g i n t h e m i c r o w a v e o v e n .

to

When

fresh vegetables are p r e p a r e d b y this m e t h o d , some w a t e r is g e n e r a l l y a d d e d to p r e v e n t s c o r c h i n g o r u n d e s i r a b l e p a l a t a b i l i t y characteristics. T h e m i c r o w a v e m e t h o d c a n b e c o n s i d e r e d essentially a waterless m e t h o d , since i t is unnecessary to a d d l a r g e q u a n t i t i e s of w a t e r to the vegetable. FRYING.

T h e d e e p fat f r y i n g t e c h n i q u e is u s e d for r e l a t i v e l y f e w

vegetables w i t h the n o t a b l e e x c e p t i o n of potatoes.

D o m a s et a l .

(141)

r e p o r t e d that r e t e n t i o n of ascorbic a c i d w a s over 7 5 % i n f r i e d potatoes, w h i l e P e l l e t i e r et a l . (142) (143)

n o t e d a b o u t 6 0 % r e t e n t i o n . A u g u s t i n et a l .

o b s e r v e d t h a t w i t h d e e p fat f r i e d p o t a t o p r o d u c t s , a s c o r b i c a c i d

retentions w e r e u n r e a l i s t i c a l l y h i g h .

T h e s e investigators suggested t h a t

this result w a s c a u s e d b y interference w i t h the c o m m o n l y u s e d assay methods

by

browning compounds

formed

during cooking.

Thus,

no

c o n c l u s i o n s c a n b e d r a w n f r o m the a v a i l a b l e studies. A u g u s t i n et a l . (32) suggested that the h i g h p e r f o r m a n c e l i q u i d c h r o m a t o g r a p h i c m e t h o d of d e t e r m i n i n g ascorbic a c i d s h o u l d b e of use i n s e p a r a t i n g the b r o w n i n g compounds and ascorbic acid. T h e r e has b e e n some interest i n t h e o r i e n t a l m e t h o d of s t i r - f r y i n g because this is essentially a waterless m e t h o d of c o o k i n g , u s u a l l y w i t h v e r y short c o o k i n g times.

T h e o r e t i c a l l y , this w o u l d p r o v i d e m a x i m u m

v i t a m i n C r e t e n t i o n . E h e a r t a n d G o t t (136)

found that w h e n broccoli

w a s c o o k e d b y a s t i r - f r y i n g m e t h o d , ascorbic a c i d r e t e n t i o n w a s h i g h e r t h a n w h e n it w a s c o o k e d b y m i c r o w a v e s or i n l a r g e a m o u n t s of w a t e r . U s i n g a l o w ratio of w a t e r to vegetable

(0.5:1) i n conventional cooking

r e s u l t e d i n less loss of ascorbic a c i d t h a n d i d the m i c r o w a v e

cooking

m e t h o d u s e d ( 0 . 9 : 1 ) . I n g r e e n beans, c o n v e n t i o n a l c o o k i n g i n the l o w e s t w a t e r to vegetable r a t i o ( 0 . 5 : 1 ) r e s u l t e d i n h i g h e r a s c o r b i c a c i d r e t e n t i o n t h a n m i c r o w a v e or s t i r - f r y i n g . B o w m a n et a l . (144)

s t u d i e d the effect of s t i r - f r y i n g o n ascorbic a c i d

r e t e n t i o n i n green p e p p e r s a n d s p i n a c h . C o o k i n g times f o r the s t i r - f r i e d vegetables w e r e c o n s i d e r a b l y shorter t h a n those u s e d f o r c o n v e n t i o n a l c o o k i n g m e t h o d s , a n d ascorbic a c i d r e t e n t i o n w a s h i g h e r . T h e s e i n v e s t i gators n o t e d that ascorbic a c i d r e t e n t i o n w a s s l i g h t l y h i g h e r i n m i c r o w a v e t h a n c o n v e n t i o n a l l y b o i l e d vegetables, b u t the differences w e r e significant i n o n l y three of the sixteen vegetables tested.


526

ASCORBIC

ACID

C h a p m a n (145) c o m p a r e d a s c o r b i c a c i d c o n t e n t a n d r e t e n t i o n i n t h r e e vegetables

(broccoli, cabbage, a n d cauliflower)

c o o k e d b y stir-

frying, microwave cooking, a n d conventional cooking.

W i t h the times

and methods used

( T a b l e X ) ascorbic

acid retention was highest i n

c o n v e n t i o n a l l y c o o k e d vegetables, f o l l o w e d b y m i c r o w a v e a n d s t i r - f r i e d . A t least t w o factors c o u l d b e r e s p o n s i b l e f o r l o w values associated w i t h


s t i r - f r y i n g ; size of pieces, l e n g t h a n d t e m p e r a t u r e of c o o k i n g . T o ensure m o r e u n i f o r m h e a t i n g of s t i r - f r i e d vegetables, t h e f o o d w a s c u t i n s m a l l e r pieces, a n d t h e t i m e w a s i n c r e a s e d to p r o v i d e a p r o d u c t w i t h texture comparable w i t h conventional a n d microwave

cooked products.

The

h i g h e r heat u s e d i n s t i r - f r y i n g together w i t h l o n g exposure c o u l d result i n l o w ascorbic a c i d retention.

Table X .

Percent Retention of Ascorbic A c i d in Vegetables Cooked by Three Methods

Method of Preparation Broccoli conventional microwave stir-fry Cabbage conventional microwave stir-fry Cauliflower conventional microwave stir-fry

Water: Vegetable Ratio

Cooking Time (min)

Vegetables

Liquid

Total

0.9:1 0.9:1 no w a t e r

11 11 15

81 76 72

12 14

93 90 72

0.5:1 0.5:1 no w a t e r

6 8 6

83 78 81

10 8

93 86 81

0.5:1 0.5:1 no w a t e r

8 8 14

84 84 79

6 5

90 88 79

Percent Retention

— —

—

Source: Reference 1J+5.

BAKING.

Potatoes a r e a m o n g t h e m o s t c o m m o n l y b a k e d vegetables

M o s t of the studies i n d i c a t e t h a t t h e r e is g o o d r e t e n t i o n of a s c o r b i c a c i d i n b a k e d potatoes (143,146).

P e l l e t i e r et a l . (142) r e p o r t e d t h a t p o t a

toes b a k e d i n t h e s k i n r e t a i n e d 7 8 % of t h e a s c o r b i c a c i d , as c o m p a r e d w i t h boiled i n the skin, 8 1 % ; peeled a n d boiled, 7 3 % ; a n d fried 7 2 % . A u g u s t i n et a l . (143) r e p o r t e d 9 1 % r e t e n t i o n i n b a k e d potatoes. Holding and Reheating of Cooked Foods.

Recent work b y Augustin

et a l . (44,143) o n h o m e a n d i n s t i t u t i o n a l p r e p a r a t i o n of potatoes e m p h a sizes t h e p o i n t s at w h i c h a s c o r b i c a c i d losses c a n o c c u r .

T h e increased


21.

527

Vitamin C in Foods

ERDMAN AND KLEIN

use of p r e c o o k e d foods i n households a n d i n s t i t u t i o n s suggests t h e n e e d for f u r t h e r i n v e s t i g a t i o n . V e r y f e w studies o f the effects of

cooking,

h o l d i n g , a n d r e h e a t i n g p r o c e d u r e s a p p e a r i n the l i t e r a t u r e . C r o s b y et a l . (147)

r e v i e w e d the effects of i n s t i t u t i o n a l f o o d service

m e t h o d s o n a s c o r b i c a c i d losses i n vegetables h e l d hot, a n d s h o w e d t h a t c o n s i d e r a b l e losses of v i t a m i n C c a n be e x p e c t e d


1975, A n g et a l . (148)

during holding.

In

i n d i c a t e d t h a t ascorbic a c i d i n m a s h e d

potatoes

p r e p a r e d a n d r e h e a t e d a n d h e l d b y c o n v e n t i o n a l f o o d service

methods

was e x t r e m e l y u n s t a b l e , w i t h losses of over 7 5 % r e p o r t e d i n some cases. A u g u s t i n et a l . (143)

r e p o r t e d t h a t the ascorbic a c i d content

of

c o o k e d , c h i l l e d , a n d r e h e a t e d b a k e d potatoes decreased d u r i n g r e f r i g e r a t i o n a n d m i c r o w a v e r e h e a t i n g , r e s u l t i n g i n a net loss of a p p r o x i m a t e l y 3 0 % of the a s c o r b i c a c i d ( d r y w e i g h t b a s i s ) . S i m i l a r results w e r e n o t e d for ascorbic a c i d f o r t i f i e d , r e h y d r a t e d m a s h e d potatoes, s u c h as those c o m m o n l y u s e d i n s c h o o l l u n c h p r o g r a m s , a l t h o u g h the losses w e r e n o t as h i g h as w i t h b a k e d potatoes. A s t u d y d o n e b y C h a r l e s a n d V a n D u y n e (132)

is w i d e l y c i t e d as

e v i d e n c e of losses of ascorbic a c i d d u r i n g h o m e r e f r i g e r a t o r storage a n d r e h e a t i n g of c o o k e d vegetables. I n seven of the n i n e vegetables e x a m i n e d , the concentrations of a s c o r b i c a c i d w e r e s i g n i f i c a n t l y r e d u c e d b y h o l d i n g in the refrigerator for 1 d ( T a b l e X I ) .

A f t e r 1 d of r e f r i g e r a t i o n a n d

r e h e a t i n g , the losses w e r e significant i n a l l p r o d u c t s a n d r a n g e d f r o m 34 to 6 8 % .

F u r t h e r h o l d i n g for 2 or 3 d p r i o r to r e h e a t i n g r e s u l t e d i n

f u r t h e r significant decreases

i n ascorbic a c i d content.

Therefore,

the

p r a c t i c e of r e f r i g e r a t i n g a n d w a r m i n g leftover vegetables is n o t r e c o m m e n d e d f r o m the s t a n d p o i n t of ascorbic a c i d r e t e n t i o n .

Table X I . Effect of Holding and Reheating Cooked Vegetables on Ascorbic A c i d Retention Ascorbic Acid Retention Vegetable

Freshly Cooked

Asparagus Broccoli C a b b a g e , shredded Peas S n a p beans Spinach

86 88 73 88 83 52

Refrigerated Id 82 68° 44 52 41° 48 b

b

Refrigerated 1 d, Reheated 66' 60" 33' 43' 29' 32"

Source: Reference 182 ° Significantly lower (p > 0.05) than corresponding mean for freshly cooked sample. ^Significantly lower (p > 0.01).


528

ASCORBIC

ACID

Conclusions A s c o r b i c a c i d s t a b i l i t y d u r i n g h a r v e s t i n g , h a n d l i n g , storage, or p r o c essing of foods is often q u i t e p o o r unless steps are t a k e n to

prevent

v i t a m i n C d e s t r u c t i o n . T h e m a j o r routes of v i t a m i n C loss f r o m foods are t h r o u g h t h e r m a l d e s t r u c t i o n , w a t e r l e a c h i n g a n d e n z y m a t i c o x i d a t i o n . O n e c a n m i n i m i z e e n z y m a t i c d e s t r u c t i o n b y a v o i d i n g b r u i s i n g of f r u i t Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 13, 2014 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch021

a n d vegetables a n d t h r o u g h p r o p e r storage c o n d i t i o n s .

Water leaching

can also be r e d u c e d d u r i n g s o a k i n g , w a s h i n g , b l a n c h i n g , a n d c o o k i n g b y use of m i n i m a l w a t e r , a n d i n the case of b l a n c h i n g a n d c o o k i n g , the use of steam or m i c r o w a v e p r o c e s s i n g . S o m e t h e r m a l d e s t r u c t i o n of the v i t a m i n is i n e v i t a b l e i n b l a n c h i n g , d r y i n g , c a n n i n g , a n d c o o k i n g operations. H o w e v e r , some types of t h e r m a l p r o c e s s i n g , s u c h as h i g h t e m p e r a t u r e , s h o r t - t i m e c o o k i n g , w i l l u s u a l l y result i n o p t i m a l amounts of v i t a m i n C i n the f o o d "as c o n s u m e d " . Lund

(69)

has p u b l i s h e d a t a b l e c o n t a i n i n g the D

d e s t r u c t i o n of constituents i n foods.

m

values

for

F r o m T a b l e X I I i t is e v i d e n t t h a t

v i t a m i n s are m o r e t h e r m a l l y resistant t h a n are v e g e t a t i v e cells, spores, or

d e s t r u c t i v e enzymes.

I n fact,

most

v i t a m i n s are

more thermally

resistant t h a n are the o r g a n o l e p t i c q u a l i t i e s of color, flavor, a n d texture. A s a g e n e r a l r u l e , one c a n process a f o o d for o p t i m a l sensory q u a l i t y a n d be r e l a t i v e l y sure of o p t i m a l t h e r m a l s t a b i l i t y of the v i t a m i n s c o n t a i n e d i n the foods.

Table XII.

Thermal Resistance of Constituents in Foods T>

Constituents

a

121

(min)

100-1,000 5-500 1-10 0.1-5.0 0.002-0.02

Vitamins C o l o r , texture, flavor Enzymes Spores V e g e t a t i v e cells Source: Reference 69. ° Time at 121° C to decrease concentration by 90%.

U n f o r t u n a t e l y , the t h e r m a l resistance of a s c o r b i c a c i d falls i n the l o w e n d of the scale ( a r o u n d 100, d e p e n d i n g o n a , p H , a n d other f a c t o r s ) i n w

Table X I I .

Therefore, a

final

food product could display

acceptable

sensory q u a l i t i e s b u t s t i l l h a v e lost a significant a m o u n t of v i t a m i n C . If one is c o n c e r n e d w i t h the v i t a m i n C r e t e n t i o n i n foods, t h e n great care m u s t b e t a k e n i n a l l facets of h a n d l i n g , storage, a n d p r o c e s s i n g of t h a t f o o d .


21.

E R D M A N AND K L E I N

Vitamin C in Foods

529


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21. ERDMAN AND KLEIN Vitamin C in Foods

531

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ACCEPTED

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