Chemistry and Function of Pectins - American Chemical Society

15

Softening o f C o o k e d S n a p Beans a n d Other Vegetables in R e l a t i o n to Pectins a n d Salts Jerome P. Van Buren

Downloaded by CORNELL UNIV on May 14, 2017 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0310.ch015

Department of Food Science and Technology, Cornell University, Geneva, NY 14456

The use of monovalent salt solutions as cooking media caused a greater softening than when vegetables were cooked in distilled water. Removal of endogenous salts by leaching before cooking reduced softening. The additional softening with salts was the result of two separate actions. One action, associated with the solubilization of part of the tissue Ca , took place when the salt was used either before or after cooking. The other action, requiring the presence of salt during cooking, was associated with the appearance of higher liquor pectin concentrations. A blanch temperature of 71°C resulted in snap beans that softened more slowly when cooked in distilled water and had a slower rate of salt-induced softening than was the case with snap beans blanched at 90°C. ++

Two concepts have dominated the interpretation and design of research concerned with the softening of vegetables during heat processing. One idea has been the association of pectin depolymerization and solubilization with the loss of firmness (JJ • The other idea deals with the ability of Ca** to increase firmness and tissue adhesiveness in cooked tissues (£, 3). In well cooked non-fibrous vegetables the firmness is determined largely by the strength of intercellular adhesion localized at the middle lamella. Variations in firmness that are produced in such tissues can be considered the result of reactions and interactions that take place with the pectins and other components of this restricted region. Softening of fruits and vegetables during heating appears to occur through two different pectin degrading reactions depending on the pH of the tissue. Doesburg (Jj has proposed a useful generalization that softening below pH 4 is consistent with an acid catalyzed cleavage of the cell wall polysaccharides, while at pH levels over 5, the normal conditions for vegetables, softening is consistent with a pectin depolymerization reaction that has the characteristics of a 8 -elimination reaction catalyzed by hydroxyl ions (JL) and inhibited by demethoxylation of pectins . 4

0097-6156/ 86/0310-0190506.00/0 © 1986 American Chemical Society

Fishman and Jen; Chemistry and Function of Pectins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

15. V A N B U R E N

191

Softening of Vegetables

A l a r g e body of research indicates that Ca functions l a r g e l y through i t s i n t e r a c t i o n with pectin |£, £, £.) . Removal of a p a r t of the pectin l i m i t s the a b i l i t y of C a to restore firmness to o r i g i n a l l e v e l s i n tissues l i k e potatoes (jl), while e x t r a c t i o n of Ca with chelating agents r e s u l t s i n the s o l u b i l i z a t i o n of a large part of the c e l l wall pectin. +

+

Many studies have demonstrated that the addition of monovalent s a l t s to v e g e t a b l e t i s s u e s b e f o r e cooking r e s u l t s i n a s o f t e r cooked product. Some o f t h e e a r l i e r work was done w i t h p o t a t o e s . I n v e s t i g a t i o n s by Davis and LeTourneau MO) Table I , using higher cooked potato weight as an index o f h i g h e r cohesiveness, showed a decreased cohesiveness when NaCl and KC1 were used. The cooked potato weight, measured a f t e r d r a i n i n g on a s i e v e , was lower when m o n o v a l e n t s a l t s were present i n the cooking s o l u t i o n . CaCl increased cohesiveness. Hughes et a l . obtained l e s s r e s i s t a n c e to compression f o r potatoes cooked with monovalent s a l t . Peas also respond to NaCl (12., H ) , giving a s o f t e r t e x t u r e when higher conc e n t r a t i o n s of s a l t were added before the s t e r i l i z a t i o n process, Table I I . S t e r l i n g ( J i ) found that c a r r o t s were s o f t e r when cooked with monovalent s a l t s or when higher pH l e v e l s were used. Similar r e s u l t s were found by Mattson (15) u s i n g d r i e d peas as the t e s t material.


f

Table I . Effect of s a l t s on the cohesiveness of cooked potatoes

Salt i n cooking solution NaCl KC1 KC1 CaCl

2

0.008 0.008 0.512 0.008

Cohesiveness, cooked potato weight units

M M M M

85 87 10 120

D i s t i l l e d water

101 From Ref (1£L).

U

Cooked f o r 24 min at 100 C .

Table I I .

Effect of NaCl on canned pea firmness

Pea variety

0

Alaska Alaska Perfection

245 236 229 U

OU" -

Shear Dress values. lb $ NaCl i n brine 1 2 222 220 244

Cooked f o r 30 min at 240 F . ~ From Ref

201 203 221

3 197 175 202

(11).


192

CHEMISTRY A N D FUNCTION O F PECTINS

The major c a t i o n p r e s e n t i n f r u i t s a n d v e g e t a b l e s i s K . Its c o n c e n t r a t i o n c a n be a f f e c t e d by e n v i r o n m e n t a l f a c t o r s , p r i n c i p a l l y the amount o f a v a i l a b l e K i n t h e s o i l s o r r o o t media CUD. Different a p p l i c a t i o n s o f K r e s u l t e d i n more t h a n a t w o - f o l d d i f f e r e n c e i n snap bean pod K ( J l ) , T a b l e I I I . H i g h e r pod K l e v e l s w e r e a s s o c i a t e d w i t h s o f t e r b e a n s when m e a s u r e d a f t e r a s t a n d a r d c a n n i n g p r o c e d u r e . F i r m n e s s v a l u e s i n t h i s t a b l e and i n T a b l e s I V - V 1 and i n F i g u r e s 1 and 2 were o b t a i n e d a s k g f o r c e r e s i s t a n c e t o compression i n a back extrusion c e l l (17). +


Table I I I .

E f f e c t s o f K f e r t i l i z a t i o n on t h e f i r m n e s s o f canned snap beans grown i n sand c u l t u r e

Fertilization with K meq/1

Blanch treatment 82

0.3 6.0

Canned bean f i r m n e s s kg f o r c e

Jit Fresh pods

Can liquor

C / 2 min None

59 41

0.98

0.45 0.56

82 ° C / 2 min None

42 33

2.60

1.21 1.17

u

Canning heat t r e a t m e n t was a t 115 C f o r 20 min. Source: Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 17. C o p y r i g h t 1982 I n s t i t u t e o f Food T e c h n o l o g i s t s . Table IV.

E f f e c t o f l e a c h i n g and N a C l on snap bean f i r m n e s s Treatment

Beans Leached Leached Not l e a c h e d Not l e a c h e d Leachings a t 2 °C. Reproduced Institute

Can media

Firmness kg f o r c e

D i s t i l l e d water 0.145 M N a C l D i s t i l l e d water 0.145 M N a C l

53 38 43 34

Liquor Ca, ug/ml 42 63 79 90

were a f t e r b l a n c h i n g and o v e r n i g h t i n d i s t i l l e d water Snap beans cooked 20 min a t 115 °C. Source: w i t h p e r m i s s i o n from r e f e r e n c e 18. C o p y r i g h t 1983 o f Food T e c h n o l o g i s t s .

S i n c e h i g h e r t i s s u e s a l t s caused s o f t e n i n g , i t was o f i n t e r e s t t o s e e i f t h e r e m o v a l o f s a l t s p r i o r t o the c a n n i n g p r o c e s s would r e s u l t i n f i r m e r beans ( 1 8 ) . S a l t s were removed by l e a c h i n g b l a n c h e d b e a n s o v e r n i g h t i n d i s t i l l e d w a t e r . A f t e r c a n n i n g , the f i r m n e s s o f the l e a c h e d beans was h i g h e r t h a n f o r t h o s e n o t l e a c h e d , T a b l e I V . The u s e o f NaCl i n t h e c a n n i n g media l e d t o s o f t e r cooked beans t h a n when t h e c a n n i n g media was d i s t i l l e d w a t e r .

S i n c e t h e f i r m n e s s o f h e a t t r e a t e d v e g e t a b l e s c a n be i n f l u e n c e d by b o t h t h e e x t e n t o f p e c t i n d e p o l y m e r i z a t i o n and p e c t i n i n t e r a c t i o n Fishman and Jen; Chemistry and Function of Pectins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


VAN BUREN

193


50

1

1

1

0.25

1

0.50

0.75

' 1.0

NaCl CONCENTRATION IN SOAK SOLUTION, MOLARITY

F i g u r e 1• The e f f e c t o f s o a k i n g c a n n e d s n a p b e a n s i n N a C l s o l u t i o n s on t h e r e l a t i v e f i r m n e s s o f the soaked beans. Reproduced w i t h p e r m i s s i o n from R e f 1 9 . C o p y r i g h t 1 9 8 4 , ' I n s t i t u t e o f Food T e c h n o l o g i s t s ' *

DISTILLED WATER COOK. NO AFTER C O O K SOAK

KCI, 0.13M, SOAK

CoCU 0.044M, SOAK

KCI,

HEXAMETA-

0.UM,

PHOSPHATE,

THEN

0.002M,

CoC»

2 /

SOAK

0.044M SOAKS

Figure 2. The e f f e c t o f s o a k i n g c a n n e d s n a p b e a n s i n K C I , C a C l , o r Na h e x a m e t a p h o s p h a t e s o l u t i o n s o n t h e i r f i r m n e s s . Reproduced w i t h p e r m i s s i o n from Ref 1 9 . C o p y r i g h t 1 9 8 4 , ' I n s t i t u t e o f Food T e c h n o l o g i s t s ' ,


194

CHEMISTRY A N D FUNCTION O F PECTINS

with Ca , both these mechanisms could have been involved t o produce the f i r m n e s s e f f e c t s seen with monovalent s a l t s present during the cooking. This p o s s i b i l i t y was tested i n experiments where s a l t s were added either before (salt cook) or a f t e r ( s a l t soak) canning, Table V (18.)• Snap beans canned without NaCl were f i r m e r , and t h e i r can l i q u o r had l e s s s o l u b l e p e c t i n and C a , than was seen f o r beans where NaCl was present during canning. When beans, previously canned without added s a l t s , were soaked f o r an extended period i n t h e i r own can l i q u o r augmented with NaCl the r e s u l t i n g soaked beans had an i n t e r m e d i a t e firmness and liquor C a concentration between those of samples cooked with and without NaCl. No change was seen i n the l i q u o r p e c t i n c o n c e n t r a t i o n from that seen when the snap beans had been cooked without NaCl. + +


+ +

Table V.

Effect of NaCl, added before or after cooking, on snap bean firmness

Treatments Cooking media After r e t o r t NaCl, 0.20 M Distilled water Distilled water

Held i n can liquor Held i n can liquor Held i n can liquor p l u s NaCl to give 0.20 M NaCl

Firmness kg force

Liquor Ca Liquor pectin jig/ml mg/ml

23

94

2.6

42

49

2.1

30

86

2.1

a

^ S o l i d s a l t s were d i s s o l v e d i n l i q u o r decanted from opened cans, then t h e l i q o u r was r e t u r n e d t o t h e beans. Beans were then h e l d 3 days a t 1 °C w i t h o c c a s i o n a l g e n t l e m i x i n g p r i o r t o t e x t u r e measurement and l i q u o r s a m p l i n g . Source: Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 18. C o p y r i g h t 1983 I n s t i t u t e o f Food T e c h n o l o g i s t s . It appears that the presence of monovalent s a l t d u r i n g cooking enhanced both p e c t i n s o l u b i l i z a t i o n and C a displacement into the l i q u o r , and that the use of the same c o n c e n t r a t i o n o f s a l t a f t e r cooking i n d i s t i l l e d water d i s p l a c e d Ca^"* but did not increase the liquor pectin concentration. Salt-enhanced p e c t i n s o l u b i l i z a t i o n r e q u i r e d cooking, but enhanced C a displacement took place apart from cooking. Both consequences of the presence of s a l t s were a s sociated with decreased canned bean firmness. + +

1

+ +

The e f f e c t s of adding s a l t s a f t e r cooking The above r e s u l t s showed that two s a l t e f f e c t s on firmness could be separated from each other. The i n f l u e n c e on f i r m n e s s o f the Ca displacement a c t i o n c o u l d be measured by determining the r e l a t i v e firmness, compared to unsoaked beans given a normalized value of 100, when cooked snap beans were e q u i l i b r a t e d with a monovalent s a l t solution (19.) by soaking at 1°C f o r 3 days. As the concentration of NaCl i n the soak s o l u t i o n was increased, Figure 1, the pod firmness decreased. But even a t r a t h e r high c o n c e n t r a t i o n s o f N a C l t h e


15.

VAN B U R E N


195

f i r m n e s s of the beans was s t i l l appreciable. The l o s s i n firmness was mirrored i n increased soluble Ca , Figure 3. After soaking i n 1 M NaCl about 25% of the o r i g i n a l pod Ca remained bound to the pod tissue; t h i s may r e p r e s e n t s t r o n g l y bound C a t h a t makes a part i c u l a r l y e f f e c t i v e contribution to pod firmness. The displacement e f f e c t of KCI i n decreasing f i r m n e s s of snap beans cooked e a r l i e r i n d i s t i l l e d water can be seen i n the bar chart of Figure 2. Bars not surmounted by the same l e t t e r s were s i g n i f i c a n t l y d i f f e r e n t a t the P

Chemistry and Function of Pectins - American Chemical Society

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