Chemical Changes Involved in the Oxidation of Lipids in Foods

6 Chemical Changes Involved in the Oxidation of Lipids

Downloaded by UNIV OF PITTSBURGH on October 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0075.ch006

in Foods D. A. LILLARD Food Science Department, University of Georgia, Athens, GA 30602

The oxidation of lipids was recognized at least 150 years ago when Berzelius (1) described experiments illustrating the induction period, oxygen uptake, carbon dioxide formation and polymerization of linseed oil. Since that time there have been numerous research reports on lipid oxidation. The flavors that result from the oxidation of lipids have triggered a large number of these investigations. However, in addition to the formation of these flavorful secondary products, free radicals produced during lipid oxidation are capable of reacting with other constituents of food such as coloring substances, vitamins, enzymes, amino acids, and proteins. These reactions can have a deleterious effect on the nutritional quality of foods. The free radicals have also been implicated in reactions that lead to pathological changes in animal and human tissue (2). This review will cover the chemical reactions involved in the oxidation of lipids in foods with emphasis on the formation of flavor compounds. It is by no means an inclusive review since many aspects of lipid oxidation have been covered by others (2, 3, 4, 5). Mechanisms and Products of L i p i d

Oxidation

Unsaturated l i p i d s a r e almost e x c l u s i v e l y considered the i n i t i a l substrate i n l i p i d o x i d a t i o n . The r e a c t i o n i s autoc a t a l y t i c i n that the o x i d a t i o n products themselves c a t a l y z e the r e a c t i o n and cause an increase i n the r e a c t i o n r a t e as o x i d a t i o n proceeds. The u n i v e r s a l l y accepted f r e e r a d i c a l r e a c t i o n scheme f o r l i p i d o x i d a t i o n can be w r i t t e n as f o l l o w s : Initiation RH RH + 0

2

— R - + H— • R00- + H-

0-8412-0418-7/78/47-075-068$05.00/0 © 1978 American Chemical Society In Lipids as a Source of Flavor; Supran, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

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Propagation R- + 0 ROO- + RH

ROOROOH + R-

2

Termination ROO- + R- — > R- + R—> ROO- + ROO-

ROOR R - R ROOR + 0

2


where RH = unsaturated l i p i d R- = l i p i d radical ROO- = l i p i d peroxy radical Once initiated, the oxidation reaction i s also propagated by the breakdown of hydroperoxides to free radicals which i n turn can accelerate the rate of l i p i d oxidation. Initiation reaction. Although the free radical mechanism of l i p i d oxidation has been well established, the i n i t i a l reaction or the formation of the f i r s t hydroperoxide, especially i n hydro peroxide free l i p i d s , i s s t i l l open for debate. This i s due to the fact that the reaction of lipids with 0 to form free radi cals or hydroperoxides i s very unlikely. Hydroperoxide formation by this reaction would require a change i n total electron spin since the hydroperoxides and lipids are i n singlet states while oxygen i s i n the t r i p l e t state. The spin conservation of this improbable reaction would be satisfied and take place i f singlet oxygen were the reactive species instead of ground state t r i p l e t oxygen ( 6 ) . Using this hypothesis, Rawls et a l . (6) proposed the following mechanism by which singlet oxygen could be formed by photo-chemical reactions i n the presence of a sensitizer, and presented evidence that singlet oxygen reacts with lipids at a rate which i s 1 4 5 0 times faster than t r i p l e t oxygen: 2

S

+ hv

3S*+

3Q

> 2

1υ * + RH ROOH Ω

l

s

*— * 3 *

1 * 0

S

2

+ 1

S

ROOH

2

> free radicals

where lg lg* 3g* -k)

= = = = =

2

singlet state sensitizer excited singlet state sensitizer excited t r i p l e t state sensitizer ground t r i p l e t state oxygen excited singlet state oxygen

The sensitizers required to convert t r i p l e t oxygen to singlet oxygen are normally found i n plant and animal tissues and consist

In Lipids as a Source of Flavor; Supran, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


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LIPIDS AS A SOURCE OF FLAVOR

of p h o t o s e n s i t i v e compounds such as c h l o r o p h y l l , pheophytin and myoglobin. Recently, Aurand ejt a l . (7) reported that s i n g l e t oxygen was the immediate source of the hydroperoxides that i n i t i a t e d milk l i p i d o x i d a t i o n which was c a t a l y z e d by l i g h t , copper and xanthine oxidase. In l i g h t - i n d u c e d o x i d a t i o n , r i b o f l a v i n served as the s e n s i t i z e r by producing s i n g l e t oxygen d i r e c t l y from i t s p h o t o s e n s i t i z e d t r i p l e t s t a t e . In the copper and xanthine oxidase system, s i n g l e t oxygen was formed by dismutation of the superoxide anion which was formed i n these systems. Oxidation was prevented i n a l l three systems by the i n c l u s i o n of a known s i n g l e t oxygen trapper (1,3-diphenylisobenzofuran) or a s i n g l e t oxygen quencher (1,4-diazabycyclo(2-2-2)octane). L i p i d o x i d a t i o n was a l s o prevented i n the copper and enzyme systems by a d d i t i o n of the superoxide dismutase enzyme. T h i s enzyme c a t a l y z e s the superoxide dismutation to ground s t a t e oxygen thereby preventing the spontaneous dismutation of superoxide to s i n g l e t oxygen. This study provided evidence that s i n g l e t oxygen i s the i n i t i a l reactant with l i p i d when o x i d i z e d i n systems other than those that are l i g h t - i n d u c e d . Although evidence i s accumulating to i n d i c a t e that s i n g l e t oxygen i s a very l i k e l y i n i t i a l r e a c t a n t i n s e v e r a l types of l i p i d o x i d a t i o n , there i s evidence that s i n g l e t oxygen does not p a r t i c i p a t e i n the o x i d a t i o n of l i p i d s that c o n t a i n no s e n s i t i zers (8). Cort (8) found that the quenching agent, 3-apo-8 c a r o t e n a l , had no e f f e c t on the o x i d a t i o n of s a f f l o w e r o i l , o l e i c a c i d and l i n o l e i c a c i d . In these experiments, no s e n s i t i z e r was present and i t was concluded that s i n g l e t oxygen was not i n v o l v e d i n the a i r o x i d a t i o n of these s u b s t r a t e s . Recently, Terao and Matsushita (9) i s o l a t e d the hydroperoxides from p h o t o s e n s i t i z e d o x i d a t i o n of unsaturated f a t t y a c i d e s t e r s . They found that hydroperoxide groups of a l l isomers were attached to the carbon atoms which o r i g i n a l l y e x i s t e d at both sides of a double bond and the double bond s h i f t e d to the adjacent p o s i t i o n s . T h i s d i s t r i b u t i o n of hydroperoxide isomers was d i f f e r e n t from the d i s t r i b u t i o n obtained from a i r o x i d i z a t i o n of these f a t t y a c i d e s t e r s (10, 11). The p h o t o s e n s i t i z e d o x i d a t i o n was i n h i b i t e d by 3-carotene (a s i n g l e t oxygen quencher) but was not i n h i b i t e d by b u t y l hydroxytoluene (a f r e e r a d i c a l s t o p p e r ) . These r e s u l t s i n d i c a t e d that s i n g l e t oxygen o x i d a t i o n d i f f e r s from a i r oxidation. I t has yet to be determined i f the i n i t i a l r e a c t i o n of a l l types of l i p i d o x i d a t i o n i n v o l v e s s i n g l e t oxygen. A complete understanding of the s i n g l e t oxygen r e a c t i o n with unsaturated l i p i d s i s d e s i r a b l e s i n c e most foods that are s u s c e p t i b l e to o x i d a t i o n contain components that are capable of inducing the formation of s i n g l e t oxygen. The use of e f f e c t i v e nontoxic s i n g l e t oxygen quenchers i n foods could prove to be very e f f e c t i v e i n i n c r e a s i n g the s t a b i l i t y of t h e i r unsaturated l i p i d s (12). ?


6.

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Oxidation

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in

71

Foods

Secondary reaction products. Lipid hydroperoxides are very unstable and break down to produce many types of secondary reac tion products. This hydroperoxide decomposition proceeds by a free radical mechanism and can be illustrated by the following scheme (13): R-CH-R

>

ι

(A)

I

0-OH

0

R-CH-R


R-CH-R + -OH

>

R-CH + R-

I

fl

0

0

R-CH-R + R ^

1

> R-C-R+ R -

1

I

0

OH 1

R-CH-R + R .

> R-C-R+ R^H

1

II

0

0

R-CH-R + R^O0

(B)

> R-C-R+ ROH

(C)

(D)

(E)

0

In reaction (A), the hydroperoxide i s cleaved to alkoxy and hydroxy free radicals. Reactions (B-E) i l l u s t r a t e the reaction of the alkoxy free radical with other free radicals or molecules to form secondary products. Since hydroperoxides are flavorless (14), i t i s these secondary products that contribute to the oxidized flavor of food l i p i d s . As i s evident from the complex nature of this reaction and the complex composition of food l i p i d s , numerous compounds are formed during the oxidation of l i p i d s . The type of oxidation products include carbonyl com pounds, alcohols, semi-aldehydes, acids, hydrocarbons, lactones and esters (J_3, 15). Many of the compounds identified as oxidation products are produced by the oxidation of the primary scission products (15, 16, 17, 18).

L i H a r d and Day (15) made one of the f i r s t systematic studies of the oxidative breakdown of i n i t i a l l y formed carbonyl products. They found that the rate of autoxidation depended upon the class of carbonyl being oxidized. When oxidized at 45 C i n an oxygen atmosphere, n-nonanal had an induction period of 12 hours and the only oxidation product was n-nonanoic acid. No induction period was observed for non-2-enal and hepta-2,4-dienal. The degradation products for non-2-enal were ethanal, n-heptanal, n-octanal, propanal, α-ketooctanal, glyoxal, α-ketononanal and α-ketoheptanal. The carbonyl compounds identified from oxidized hepta-2,4-dienal were propanal, ethanal, n-butanal, cis-but-l-en1,4-dial, α -ketopentanal, glyoxal, α -ketoheptanal and a-ketohexanal. Oct-l-en-3-one did not oxidize when held at 45°C for


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52 hours. Recently, Michalski and Hammond ( 1 7 ) confirmed the work of L i l l a r d and Day by using radio-tracer techniques to follow the oxidation of similar classes of carbonyl compounds in oxidizing soybean o i l . Also, other studies have shown that nalkanals can be oxidized under certain conditions to produce aldehydes, alcohols, esters, hydrocarbons and lactones ( 1 6 , 1 8 ) . These investigations have illustrated that continued oxidation of the i n i t i a l secondary products can account for many of the compounds found in oxidized l i p i d s whose origin could not be attributed to the cleavage of the hydroperoxides believed to be present in the lipids under study.


Factors Affecting Lipid Oxidation Lipid composition and structure. In model systems consisting of pure fatty acid esters, resistance to oxidation can be related to the ease with which the i n i t i a t i o n reaction can occur. The hydrogen l a b i l i t y of the methylene carbons on which the free radicals are formed can be grouped according to the number and type of unsaturated bonds in the fatty acid molecule ( 1 9 ) . However, l i p i d s in food are not simple, pure components and are in a close contact with other oxidizable compounds, enzymes, and various types of prooxidants and antioxidants. For these reasons, the oxidative s t a b i l i t y of foods i s not always related to the degree and type of unsaturated lipids they contain ( 2 0 , 2 1 ) . In order to determine i f the oxidative s t a b i l i t y of an o i l is related to the fatty acid composition, Graboski ( 2 1 ) prepared o i l blends composed of sunflower seed o i l - o l i v e o i l sunflower seed oil-linseed o i l and olive oil-linseed o i l to obtain samples of various degrees of unsaturation. These o i l blends were checked for oxidative s t a b i l i t y using the active oxygen method (AOM). There was no correlation between the individual fatty acid content of these o i l blends and oxidative s t a b i l i t y . Using oxidation rate factors of individual fatty acids ( 2 2 ) as correction factors, the ratio of total unsaturated fatty acids to the saturated fatty acids in each o i l blend was calculated according to the following equation: R =

11*(C18:1) +

114*(C18:2) +

179*(C18:3)

1(18)

* = ( ) =

oxidation rate factors proposed by Stirton et_ a l . ( 2 2 ) concentration of fatty acids

The logarithm of this ratio gave a correlation of - . 8 3 with the oxidative s t a b i l i t y of the o i l . This suggested that the oxidation factors could be used in conjunction with fatty acid composition of the o i l to predict the oxidative s t a b i l i t y of the o i l . However, when these factors were applied to the fatty acid


6.

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composition of f i v e sunflower seed o i l s , no c o r r e l a t i o n was obtained with the o x i d a t i v e s t a b i l i t y of the o i l s (21). Perhaps the e f f e c t of prooxidants and a n t i o x i d a n t s i n the o i l s had more i n f l u e n c e on the r a t e of o x i d a t i o n than d i d d i f f e r e n c e s i n f a t t y a c i d composition of the o i l s . Raghuveer and Hammond (23) found that the p o s i t i o n of the unsaturated f a t t y a c i d i n the t r i g l y c e r i d e could i n f l u e n c e the o x i d a t i o n r a t e of the o i l . An o i l would be more s t a b l e to o x i d a t i o n i f more of the unsaturated f a t t y a c i d s were l o c a t e d i n the 2 - p o s i t i o n of t r i g l y c e r i d e s than i f they were l o c a t e d i n the 1 and 3 p o s i t i o n s . Recent work at Georgia has shown that the o x i d a t i v e s t a b i l i t y of peanut o i l i s not r e l a t e d to the amount of l i n o l e i c i n the 2 - p o s i t i o n of the t r i g l y c e r i d e s (24). Oxidation c a t a l y s t s . Heavy metals, p r i m a r i l y those having two valency s t a t e s w i t h s u i t a b l e o x i d a t i o n - r e d u c t i o n p o t e n t i a l s , increase the r a t e of l i p i d o x i d a t i o n . They may be e f f e c t i v e as secondary c a t a l y s t s of o x i d a t i o n where they act as e l e c t r o n donors to hydroperoxides to produce RO- f r e e r a d i c a l s (25). The W can be converted back to by r e a c t i n g with hydroperoxides to form ROO* r a d i c a l s . 2

ROOH + M*"

>

ROOH + V&

RO-

>

3

1

3

+ M"

1

ROO'

+ 2

+ M"

OH" + tf*"

These two r e a c t i o n s can e s t a b l i s h a d d i t i o n a l chain sequences i n the o x i d i z i n g l i p i d . There i s evidence, however, that metals may be involved as primary c a t a l y s t s and can i n i t i a t e l i p i d o x i d a t i o n (26) as discussed p r e v i o u s l y . Aurand et a l . (13) i l l u s t r a t e d that C i i c a t a l y z e d milk took place by the formation of s i n g l e t oxygen. The source of s i n g l e t oxygen was the dismut a t i o n of superoxide anion which was formed by the a c t i o n of Cu on oxygen. The impact of heme pigments on l i p i d o x i d a t i o n has been reviewed r e c e n t l y (4, 27, 28). I t i s a known f a c t that heme p r o t e i n s can c a t a l y z e l i p i d o x i d a t i o n but the question yet to be answered s a t i s f a c t o r i l y i s the exact s t a t e of the heme i r o n when c a t a l y s i s of l i p i d o x i d a t i o n occurs (27). Several i n v e s t i gators (29, 30) b e l i e v e that the i r o n of heme groups must be i n the o x i d i z e d form ( F e ) i order to f u n c t i o n as a c a t a l y s t of l i p i d o x i d a t i o n while other workers (31, 32) i n d i c a t e that both F e and F e forms are e q u a l l y a c t i v e as o x i d a t i o n c a t a lysts. Oxidation of myoglobin to metmyoglobin has been shown to accompany l i p i d o x i d a t i o n i n model systems (33). However, i n t h i s case, was i t necessary f o r the myoglobin to be i n the o x i d i z e d form i n order to be an o x i d a t i o n c a t a l y s t or was metmyog l o b i n formed as a s c i s s i o n product of a m y o g l o b i n - l i p i d complex. 1-2

+ 2

+ 3

n

+ 2

+ 3

Antioxidants. Chemicals when added to foods to prevent or delay the onset of l i p i d o x i d a t i o n are c a l l e d a n t i o x i d a n t s . Phenolic type compounds such as b u t y l a t e d hydroxy a n i s o l e (BHA),



74

LIPIDS AS A SOURCE O F

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butylated hydroxy toluene (BHT) and tocopherol can act as free radical stoppers by donating hydrogen to the free radicals. Chelating agents such as ethylenediaminetetracetic acid and c i t r i c acid can be classified as free-radical-production preventers by tying up metal catalysts (4). Certain compounds such as ascorbic acid have the a b i l i t y to provide a synergistic effect when used with phenolic type antioxidants. This synergistic action of ascorbic acid i s attributed to i t s a b i l i t y to regenerate antioxidants by supplying hydrogen to the phenoxy radical (34) or to i t s a b i l i t y to function as an oxygen scavenger in some systems (8). Cort (8) reported that ascorbyl palmitate, an ester of ascorbic acid, has better solvent properties than ascorbic acid and i s more effective at 0.01% concentration than BHT or BHA at 0.02% concentrations in delaying the oxidation of peanut, safflower, sunflower and corn o i l s . Tocopherols are classified among the natural antioxidants. Although they have been shown to be effective i n animal fats, the activity of tocopherols added to vegetable o i l s has always been low. The reason usually given for this has been that tocopherols naturally present in vegetable o i l s mask additional activity. However, Cort (8), using molecular d i s t i l l a t i o n , removed tocopherols from soybean and safflower o i l and found that the addition of tocopherols did not effectively improve the s t a b i l i t y of the o i l s . After demonstrating that tocopherols were more active i n oleic acid than in l i n o l e i c acid, and that tocopherol activity i n animal fats was similar to the activity in oleic acid, Cort concluded that the variations in tocopherols activity in different substrates may be due to the type of unsaturated fatty acids in these substrates. Carlsson et a l . (12) demonstrated that free radical scavenger type antioxidants had no influence on the formation of hydroperoxide i n o i l s oxidized by near UV and v i s i b l e l i g h t . Nickel (II) chelates, which are singlet oxygen quenchers, retarded l i p i d deterioration under their experimental conditions. Antioxidants often exhibit variable degrees of efficiency in protecting food systems. They usually give excellent protection i n unsaturated fats and o i l s but only moderate protection in most other foods. Lipid Oxidation in Various Flesh Foods As indicated earlier, most studies on mechanisms of l i p i d oxidation have been done on model systems consisting of unsaturated fatty acids or their esters. Although i t may be assumed that the same type of reactions occur i n food systems, due to the complex nature of foods many types of additional interactions and reactions may occur. In this section, several types of flesh foods and their oxidative deteriorations w i l l be discussed.



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Although animal l i p i d s are considered to be f a i r l y saturated, s u f f i c i e n t amounts of unsaturated l i p i d s are found i n the phosp h o l i p i d f r a c t i o n of the intramuscular l i p i d s to produce o x i d i z e d f l a v o r s i n meat products (35). Furthermore, these phospholipids are i n c l o s e contact with v a r i o u s o x i d a t i o n c a t a l y s t s that e x i s t i n muscle t i s s u e . Recently, emphasis has been placed on i n c r e a s i n g the amount of polyunsaturated f a t t y a c i d s i n ruminant f a t s . This can be accomplished by encapsulating polyunsaturated o i l d r o p l e t s with a l a y e r of p r o t e i n (casein) which i s t r e a t e d with formaldehyde to prevent hydrogénation of the unsaturated f a t t y a c i d s by the m i c r o f l o r a i n the rumen (36). T h i s procedure increases the amount of unsaturated f a t s i n meats. T h i s i n c r e a s e may be d e s i r a b l e f o r n u t r i t i o n a l reasons but may lead to increased problems of o x i d a t i v e d e t e r i o r a t i o n upon storage. S c i e n t i s t s at the Eastern Regional Research Center (37, 38) studied the e f f e c t of protected s a f f l o w e r o i l on the l i n o l e i c a c i d content and o x i d a t i v e s t a b i l i t y of rendered f a t from c a t t l e of d i f f e r e n t ages that were fed the o i l . They reported that the depot f a t of a l l animals on protected d i e t s contained higher amounts of l i n o l e i c a c i d than depot f a t s from c o n t r o l animals. The magnitude of the increase was i n f l u e n c e d by the age of the animal and the amount of p r o t e c t e d s a f f l o w e r o i l i n the feed. In the growing and mature s t e e r groups, the increased l i n o l e i c a c i d l e v e l s decreased the s t a b i l i t y of the rendered f a t s . There did not appear to be a r e l a t i o n s h i p between d e p o s i t i o n of tocopherol i n depot f a t and protected s a f f l o w e r d i e t s when a l l animals r e c e i v e d a 20 mg d-a-tocopheryl acetate per day. In experiments using young c a l v e s , an increased l i n o l e i c a c i d content i n depot f a t was a l s o obtained i n animals fed p r o t e c t e d safflower oil. However, h i g h increases i n l i n o l e i c a c i d d i d not r e s u l t i n decreased s t a b i l i t y of the f a t due to high tocopherol l e v e l s i n the f a t (38). Bremner ejt a l . (39) reported on the o x i d a t i v e changes during frozen storage of meat (mutton, lamb, beef) with high l i n o l e i c a c i d content. Peroxides occurred at a f a s t e r r a t e i n adipose t i s s u e from high l i n o l e i c meat than conventional meat when stored at - 1 0 ° C Taste panel evaluations i n d i c a t e d that high l i n o l e i c meat stored at -10°C developed r a n c i d odors and f l a v o r s 2-3 times more r a p i d l y than conventional meat. At -20°C, the r a t e of peroxide formation i n high l i n o l e i c meat was g r e a t l y reduced but the r a t e was s t i l l equal to that of convent i o n a l meat stored at -10°C. I f production of high l i n o l e i c meat becomes commercially important, conventional methods of packaging and s t o r i n g f r o z e n meats may not be adequate f o r t h i s type of product and a l t e r n a t e methods should be sought. Perhaps the most s e r i o u s f l a v o r d e f e c t a t t r i b u t e d to the o x i d a t i o n of l i p i d i n meat i s the warmed-over-flavor (WOF). T h i s d e s c r i p t i v e term was used by Timms and Watts (40) to describe the r a p i d development of o x i d a t i v e r a n c i d i t y i n cooked meat during short-term r e f r i g e r a t e d storage. O r i g i n a l l y , most research i n d i c a t e d that the WOF was due to metmyoglobin-catalyzed l i p i d



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LIPIDS AS A SOURCE OF

FLAVOR

o x i d a t i o n and that the cooking process denatured myoglobin and t h i s increased the c a t a l y t i c a c t i v i t y of the heme p r o t e i n s (30, 32, 41). Recently, i t has been shown that non-heme i r o n may be more important than heme i r o n as a prooxidant i n cooked meat (42, 43). Wilson e t a l . (44) demonstrated that turkey meat was most s u s c e p t i b l e to WOF development, followed by chicken, pork, beef and mutton. A n a l y s i s of these data revealed that unsaturated f a t t y acids of the phospholipids may be the substrate being o x i d i z e d i n a l l the meats except pork. In pork, the t o t a l l i p i d l e v e l s seem to be the major c o n t r i b u t o r to WOF. In cured meats, the lower l e v e l of l i p i d o x i d a t i o n observed has been a t t r i b u t e d to the a b i l i t y of the c u r i n g agent, sodium n i t r i t e , to form a complex with the heme p r o t e i n s thereby maint a i n i n g the heme i r o n i n the f e r r o u s form. The reduced heme i s p o s t u l a t e d to be a slower o x i d a t i o n c a t a l y s t than the o x i d i z e d heme. Upon storage, the cured meat pigment i s converted to the f e r r i c form which r e s u l t s i n increased o x i d a t i o n of the l i p i d s (30). Greene and P r i c e (27) reported that increases i n l i p i d o x i d a t i o n , as measured by TBA t e s t s , had no e f f e c t on cured meat f l a v o r as scored by a t a s t e panel. P o s s i b l y some of the products of l i p i d o x i d a t i o n c o n t r i b u t e to the cured meat f l a v o r . This may be e s p e c i a l l y true i n cured and aged pork products such as c o u n t r y - s t y l e hams. L i l l a r d and Ayres (45) reported that many of the carbonyl compounds i n c o u n t r y - s t y l e hams are the same as the compounds i d e n t i f i e d i n o x i d i z e d pork l i p i d s . Although o x i d a t i o n of f r e s h f i s h t i s s u e has not been considered a major cause of f l a v o r d e t e r i o r a t i o n because m i c r o b i a l s p o i l a g e sets i n f i r s t (46), some f i s h do contain s u f f i c i e n t amounts of unsaturated l i p i d s to undergo r a p i d o x i d a t i o n when stored e i t h e r f r e s h or frozen (47, 48, 49, 50). Recently, Ke and co-workers found that l i p i d s a s s o c i a t e d w i t h the s k i n of mackerel were o x i d i z e d at a r a t e e i g h t times f a s t e r than dark and l i g h t t i s s u e l i p i d s and concluded that t h i s was due to one or more f a t s o l u b l e prooxidants l o c a t e d i n the s k i n (48). Other o x i d a t i v e c a t a l y s t s found i n f i s h appear to be s i m i l a r to those found i n other f l e s h foods; t h e i r concentrations vary w i t h i n muscle t i s s u e s of the same f i s h as w e l l as w i t h i n species of f i s h , l o c a t i o n , and time of year harvested ( 4 ) . Deng and coworkers (50) have retarded the onset of o x i d a t i o n i n f r o z e n mullet by use of a s c o r b i c a c i d and mono-tertiary butylhydroquinone i n combination with vacuum packaging. Ascorbic a c i d was found to act as an a n t i o x i d a n t or prooxidant depending on the concentration used (51). In dark f l e s h of m u l l e t , ascobic a c i d acted as an a n t i o x i d a n t at concentrations above 500 ppm and a prooxidant at concentrations below 500 ppm. The a n t i o x i d a n t to prooxidant s h i f t was observed at 50 ppm i n the white f l e s h . The use of machines to remove f l e s h from bones of p o u l t r y and f i s h has created a source of p r o t e i n s which can be used i n e m u l s i f i e d and processed food products. However, t h i s p r o t e i n has l i m i t e d use f o r food i n g r e d i e n t s because of f l a v o r i n s t a b i l i t y


6.

TJDLLARD

Oxidation

of

Lipids

in

Foods

77

during storage. Lipid oxidation has been considered the major cause of this flavor deterioration Ç52, 53^, 5*4, 55_ 56, 57.» 03) · Moerck and Ball (55) reported that highly unsaturated fatty acids in the phospholipids were the source of the oxidative attack in mechanically deboned poultry (MDP) . Lee e_t a l . (56) concluded that the hemoproteins were the predominant catalysts of l i p i d oxidation i n MDP and that the relative concentration ratio of polyunsaturated fatty acids to hemoprotein was i n the range where heme-catalyzed oxidation would occur close to the maximum rate. Lee and Toledo (58) investigated l i p i d oxidation i n mullet flesh during mechanical deboning and storage. They illustrated that use of non-stainless steel equipment decreased the oxidative s t a b i l i t y of the fish flesh probably due to an increase i n the iron content of the flesh. Washing the deboned flesh before storage increased oxidative s t a b i l i t y when stored at -16°C but had no effect when stored at 3°C. They also indicated that red muscle was the most susceptible to oxidation possibly due to the effect of heme pigments or a greater concentration of unsaturated l i p i d s i n this tissue. Silberstein and L i l l a r d (59) recently characterized the heme pigments in phosphate buffer extracts of hand and mechanically deboned mullet. Mechanical deboning i n creased the concentration of hemoglobin i n the deboned mullet but had very l i t t l e influence on the myoglobin content. Furthermore, some hand deboned samples contained as much heme protein as the mechanically deboned samples. This v a r i a b i l i t y in the amount of heme protein i n f i s h was also reported by Brown (60) i n his study on heme proteins i n tuna f i s h . Oxidation studies using oleic acid as a substrate, revealed that the catalytic activity of the extracts was dependent on their total heme protein content (59). Using purified hemoglobin and myoglobin as catalysts, Silberstein and L i l l a r d (59) found that myoglobin had a higher catalytic effect than hemoglobin and that samples containing the same amount of heme protein but different ratios of myoglobin to hemoglobin had increased rates of oxygen uptake as the ratio of myoglobin to hemoglobin increased. This indicates that the myoglobin to hemoglobin ratio as well as the total concentration of heme protein should be considered when determining the role of heme pigments as prooxidants i n food systems. Fischer and Deng (61) reported heme iron as the major catalyst i n homogenates of the dark flesh of mullet. They did not determine the amount of hemoglobin and myoglobin i n the extracts, but they did indicate a high percentage of non-heme iron i n the dark muscle of mullet. They suggested that the nonheme iron should be characterized and i t s role i n oxidative s t a b i l i t y be determined.


9

Conclusions A great deal of research has been done i n the area of l i p i d oxidation of foods and a great deal i s known about this complex series of reactions. However, additional information i s


78

LIPIDS AS A SOURCE O F FLAVOR


needed i n order to e l i m i n a t e t h i s problem i n foods. For instance, i s the s i n g l e t oxygen theory the answer t o the i n i t i a t i o n r e a c t i o n i n a l l o x i d a t i o n systems? I doubt that i t i s . A b e t t e r understanding of i t s i m p l i c a t i o n i n l i p i d o x i d a t i o n i s needed and may r e s u l t i n b e t t e r c o n t r o l of some types of l i p i d o x i d a t i o n . A l s o , i f the trend t o produce more unsaturated meats and milk continues f o r n u t r i t i o n a l reasons, conventional processing and storage methods of these products w i l l have to be reevaluated i n terms o f t h e i r e f f e c t on the o x i d a t i v e s t a b i l i t y of these new products. The o x i d a t i v e s t a b i l i t y of foods i s s t i l l a problem and continued research i n t h i s area i s s t i l l needed.

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6.

LILLARD

19. 20. 21.

22.


23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

Oxidation

of Lipids

in

Foods

79

Farmer, Ε. Η., Koch, H. P., Sutton, D. Α., J. Chem. Soc. (1943) 541. Sattar, Α., Deman, J. Μ., J. Amer. Oil Chem. Soc. (1976) 53, 473. Graboski, Τ. Α.,"TheIsolation and Identification of the Fatty Acids of Phospholipids of Sunflower Seed (Helianthus annuus) Oil", Ph.D. Thesis, University of Georgia, Athens, Georgia (1974). Stirton, A. J., Turner, J., Reimenschneider, R. W., Oil and Soap (1945)22, 81. Raghuveer, K. G., Hammond, E. G., J . Amer. Oil Chem. Soc. (1967)44, 239. Hokes, J., "Factors Affecting the Oxidative Stability of Oils from Various Peanut Cultivars", M.S. Thesis, University of Georgia, Athens, Georgia (1977). Ingold, K. U. in "Lipids and their Oxidation", p. 93, eds. H. W. Schultz, E. A. Day, R. O. Sinnhuber, Avi Pub. Co., Westport, Conn. (1962). Heaton, M. W., Uri, Ν., J . Lipid Res.(1961)2,152. Greene, Β. Ε., Price, L. G., J. Agr. Food Chem. (1975) 23, 164. Greene, Β. E . , J. Amer. Oil Chem. Soc. (1971) 48, 637. Greene, B. E . , J. Food Sci., (1969) 34, 110. Younathan, M. T., Watts, Β. Μ., Food Res. (1960) 25, 538. Brown, W. D., Harris, L. S., Olcott, H. S., Arch. Biochem. Biophys. (1963) 101, 14. Hirano, Υ., Olcott, H. S., J. Amer. Oil Chem. Soc. (1971) 48, 523. Koizumi, C., Nonaka, J., Brown, W. D., J. Food Sci. (1973) 38, 813. Bauerfeind, J. C., Pinkert, D. Μ., in"Advancesin Food Research," eds. C.O.Chichester, E. M. Mrak, G. F. Stewart, Academic Press, New York (1970) p. 219. Hornstein, R. T., Crowe, P. F., Heimberg, M. J., J. Food Sci. (1961) 26, 581. Scott, T. W., Cook, L. J., Mills, S. C., J. Amer. Oil Chem. Soc. (1971) 48, 358. Ellis, R., Kimoto, W. I., Bitman, J., Edmondson, L. F., J. Amer. Oil Chem. Soc. (1974) 51, 4. Kimoto, W. I., Ellis, R., Wasserman, A. E . , Oltjen, R., J. Amer. Oil Chem. Soc. (1974) 51, 401. Bremner, Η. Α., Ford, A. L., Macfarlane, J. J., Ratcliff, N. T., J. Food Sci. (1976) 41, 757. Timms, M. J., Watts, Β. Μ., Food Technol. (1958) 12, 240. Kendrick, J., Watts, Β. Μ., Lipids (1969) 4, 454. Sato, K., Hegarty, G. R., J. Food Sci. (1971) 36, 1098. Love, J. D., Pearson, A. M., J. Agr. Food Chem.(1974)22, 1032. Wilson, B. R., Pearson, A. M., Shorland, F. B., J. Agr. Food Chem. (1976) 24, 7.


LIPIDS AS A SOURCE O F FLAVOR

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45. 46. 47. 48. 49. 50.


51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61.

Lillard, D. Α., Ayres, J. C., Food Technol. (1969) 23, 117. Olcott, H. S., in "Lipids and their oxidation," p. 173, eds. H. W. Schultz, E. A. Day, R. O. Sinnhuber, Avi Pub. Co., Westport, Conn. (1962). Mendenhall, V. T., J. Food Sci. (1972) 37, 547. Ke, P. J., Ackman, R. G., J. Amer. Oil Chem. Soc. (1976) 53, 636. Ke, P. J., Nash, D. Μ., Ackman, R. G., Inst. Can. Sci. Technol. J. (1976) 9, 136. Deng, J. C., Matthews, R. F., Watson, C. M., J. Food Sci., (1977) 42, 344. Deng, J. C., Inst. Food Technol. 37th Annual Meeting (1977) Philadelphia, Pa., June 5-8. Maxon, S. T., Marion, W. W., Poultry Sci. (1970) 49, 1412. Dimick, P. S., MacNeil, J. H., Grunden, L. P., J. Food Sci., (1972) 37, 974. Froning, G. W., Arnold, R. G., Mandigo, R. W., Neth, C. Ε., Hartung, T. E . , J. Food Sci. (1971) 36, 974. Moerck, Κ. E . , Ball, H. R. J r . , J. Food Sci. (1974) 39, 876. Lee, Y. B., Hargus, G. L., Kirkpatrick, J. Α., Berner, D. L., Forsythe, R. H., J. Food Sci. (1975) 40, 964. Babbitt, J. Κ., Law, D. Κ., Crawford, D. L . , J. Food Sci. (1976) 41, 35. Lee, C. M., Toledo, R. T., J. Food Sci. (1977) 42, IN PRESS. Silberstein, D. Α., Lillard, D. Α., Inst. Food Technol. 37th Annual Meeting (1977) Philadelphia, Pa., June 5-8. Brown, W. D., J. Food Sci. (1962) 27, 26. Fischer, J., Deng, J. C., J. Food Sci. (1977) 42, 610.

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Chemical Changes Involved in the Oxidation of Lipids in Foods

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