64

TKE CHEMISTRY OF WARMED-OVER FLAVOR IN C O O W mATS*

KLJNITO SAT0 AND HAROLD K . HERRING h o u r and Company

INTRODUCTION

When uncured cooked meat i s stored f o r a relativel.. -‘-?rt period of time, it develops what i s commonly referred t o as warmed-over f lavor (WOF) . T h i s objectionable s t a l e , rancid odor becomes noticeable especially when cDoked meat i s refr igerated and t h p n reheated again for consumption. T h i s f lavor and odor problem of cooked meat has assumed much greater significance i n recent years due t o the rapid increase i n fas t food service f a c i l i t i e s (a i r l ines , restaurants, vendors, and franchises) requiring the use of large quant i t ies of precooked o r pa r t i a l ly cooked meats and meat products.

This paper w i l l be devoted mainly t o a review of t h e chemistry involved i n WOF development i n cooked meats and a l so t o methods used t o prevent o r re ta rd i t s occurrence.

OXIDATION OF LIPIDS

O f the chemical components of foodstuffs, the l i p ids a re most prone t o autoxidation. However, the deleterious e f fec ts of t h i s oxidation, which occurs slowly a t normal temperatures, are more widespread since peroxidic products of l i p i d oxidation can a t t ack molecules of other types and many of the unwelcome charac te r i s t ics eventuating from autoxidation a r e due t o secondary reactions (48). peroxides may r e su l t not only i n t he destruction of valuable vitamins (15, 24, 32) but may a l so have a deleterious e f fec t on ce r t a in metabolic processes (19, 24, 32, 34).

For instance, the presence of

Food l i p i d s undergo extensive deter iorat ion on exposure t o atmospheric oxygen. This process of oxidative rancidity, which i s a l so known as autoxidation, r e su l t s i n the development of unpleasant odors and f lavors which render the food unpalatable and sometimes even toxic (15, 31). Peroxides of unsaturated f a t t y acids have been found t o be toxic t o animals (24, 34). and Kaunitz (19) while t o x i c i t y of thermally polymerized fish o i l s was reported by Matsuo (34) .

Toxicity of heated fats was described by Kumnerow (24)

I n l i p ids containing unsaturated linkages, it has been found that ranc id i ty is prFmarily due t o oxygen a t t ack a t , or near, the unsaturated center, while i n saturated fats a t t ack may occur anywhere along the hydrocarbon chain, w i t h a t t a c k a t the beta posi t ion possibly predominating

* Presented a t t he 26th Annual Reciprocal Meat Conference of t he American Meat Science Association, 1973.

65

(17). l i p ids , the individual oxidative reactions a re generally similar t o those occurring during preliminary stages of the autoxidation of drying o i l s and petroleum derived products (17, 48).

Since Oxidation occurs primarily i n the hydrocarbon portion of

MECHANISM OF OXIDATION

Most evidence developed in recent years on oxidation mechanisms in organic matter supports the theory t h a t oxidation of f a t t y substance i s an autocatalyt ic "chain" reaction based on formation of f r e e radicals which serve t o perpetuate the oxidation reaction. Oxidation of fa t ty substances i s believed t o take place In three stages as given by the following simplified schemes (6, 17, 43): ( a ) In t ia t ion : This probably corresponds t o the oxidation induction period of a f a t o r o i l and during th i s stage fat o r o i l molecules convert t o unstable f r e e radicals which can catalyze further free rad ica l formation in the substrate . induction period can be defined as the period of time which elapses e i the r under storage conditions or under cer ta in conditions of tes t ing , before rancidi ty becomes organoleptically detectable. It is a period during which axidation normally takes place ra ther slowly, compared w i t h the r a t e of oxidation subsequent t o the induction period, and during which secondary products accumulate t o a concentration where they may be detected organoleptically (32). following manner :

The

This reaction can be depicted i n the

I n i t i a t o r ( s ) m ) R * +He ( f a t molecule ) ( f a t t y f r ee rad ica l )

Various agents such as light, heat, and heavy metals (especially copper and iron ) a r e pr incipal i n i t i a t o r s of autoxidation. Fat ty f r ee radicals which have formed can conibine w i t h molecular (atmos- pheric) oxygen t o form peroxide f r e e radicals which can react with the substrate t o form more f r ee radicals and hydroperoxides. This chain reaction may be depicted as:

(b ) Propagat ion :

R. + 02 -.LL. > ROO* (peroxide f r ee rad ica l )

} ROOH + R * ROO. + RH -.. ~ . " (hydroperoxide )

RH r e fe r s t o any unsaturated f a t t y acid i n which the H i s l a b i l e by reason of being on a carbon atom adjacent t o a double bond. t o a f r ee rad ica l formed by removal of a l a b i l e hydrogen. process becomes more complex a f t e r the development of a quantity of ROOH, f o r the ROOH decmposes, e i the r through thermal i n s t a b i l i t y or through reaction w i t h other materials, t o form more f r ee radicals , which then par t ic ipa te fur ther i n t h e chain reaction (6) . During propagation, especial ly i n the presence of ca t a ly t i c agents, decomposition of the

R * refers The oxidation

66

hydroperoxides leads t o formation of a wide var ie ty of aldehydes, acids, e t c . responsible for the obnoxious odor and flavor character is t ics of rancid f a t s and o i l s (19, 32, 43).

Hydroperoxide decomposition may be monomolecular:

catalyst ROOH - 7 R O O + *OH

l i p i d oxyradical hydroxy radica 1

-OH + €?H

or bimolecular:

) R * + H20

2 ROOd----3ROO' + R O O + H20

( c ) Termination: t he free radicals (autocatalysts ) are deactivated or destroyed. occur i n various ways, such as:

Termination of t he oxidation chain reaction occurs when This may

Inactive products, s t ab le non-radical end products

ROOR + ROO. ---+ROOR + O2

[R- may be a f r e e rad ica l or f r ee rad ica l inh ib i tor . I n the latter case, H* converts the peroxy rad ica l t o a hydroperoxide and becomes a resonance s tab i l ized rad ica l incapable of continuing t h e chain (l7)I.

Example: reac t w i t h the chain-carrying free radicals t o form i n e r t products as a termination s t ep i n the chain reaction mechanism (6 ) .

Antioxidants t h a t f'unction as f r ee rad ica l inhibi tors

PRODUCTS OF OXIDATION

It is now widely accepted t h a t hydroperoxides are the primary products The so-called secondary of the reaction of oxygen w i t h unsaturated l i p i d s .

degradation products of l i p i d oxidation are believed t o be formed la rge ly f r D m hydroperoxide decomposition. such as alcohols, aldehydes, ketones, acids, lactones, and unsaturated hydrocarbons a r e highly susceptible t o further oxidation (20, 32, 43). Since hydroperoxides and peroxides a r e odorless, the rancid odors and f lavors occurring i n fa t are considered t o be due chief ly t o aldehydes, ketones, and acids formed from t h e peroxides (20, 26, 32, 43).

These secondary degradation compounds

The complex and dynamic nature of t he secondary degradation products makes quantitative study of their occurrence i n autoxidizing l i p i d s very d i f f i cu l t (20) . For example, aldehydes are notoriously unstable compounds, susceptible t o polymerization and condensation reactions . I n addition, t he ac t ive oxygen i n t h e autoxidative systems may oxidize them t o carboxylic acids . H i g h l eve ls of f r ee aldehydes thus w i l l not accumulate i n oxidizing f a t s . However, from a f lavor standpoint, the aldehydes w i l l always be a problem because some of them (2,4- decadienal) have flavor thresholds of less than one part per b i l l i o n (20).

67

It should be pointed out that many unsaturated and eas i ly oxidized vo la t i l e s a re never detected because they a re l o s t during isolatson and fractionation i n a gas chromatographic column and that compounds issuing from a chromatographic column are not necessarily a l l those that were injected (8).

OXIDATION OF LIPIDS IN MEATS AND DEVELOPMENT OF WOF

The polyunsaturated f a t t y acids which have non-conjugated double bonds occur widely i n nature i n both plants and animals. abundant polyunsaturated f a t t y acids in animal t i s sue a re the cl8, C20, and C22 acids with 2 t o 6 double bonds (15, 16). t o rapid oxidation by atmospheric oxygen, and a r e mainly responsible f o r the rancidity, including WOF, w h i c h develop in f a t t y foods.

The most

These acids a re subject

Although oxidation can occur i n f a t t y acids having a s ingle double bond, such as oleic , the methylene group between two double-bonded carbon is very much more susceptible t o oxidative a t tack than carbons adjacent t o a s ingle double hond (31, 50) . Thus, l i no le i c acid, w i t h one act ive methylene group oxidizes t e n t o twelve times as rapidly as o le ic acid. Linolenic acid, with two such l ab i l e carbons, oxidizes twice as f a s t as l i no le i c (50). I n general, it can be s ta ted that the more unsaturated a l i p i d molecule, the greater w i l l be the r a t e of i t s autoxidation. It is well ham that with unsaturated f a t t y acid derivatives, such as those of o le ic and of l i no le i c and other polyunsaturated f a t t y acids w i t h methylene-interrupted unsaturation, the r a t e of autoxidation increases markedly, and i n an exponential manner, with increasing unsaturation (31).

The most extensive evidence w e have fo r rancidi ty r e s u l t i r g from oxidation of muscle l i p i d is that obtained w i t h cooked meats (49). lean meats (beef and pork) which are heated suf f ic ien t ly t o denature protein undergo rapid l i p i d oxidation, as shown by large increases in thiobarbi tur ic acid (TBA) values and progressive development of rancid odors (49). and f lavors responsible f o r WOF.

Fresh

These rancid odors no doubt include those objectionable odors

S imi l a r increases i n TBA values occur in cooked meats of other animals. i n refr igerated chicken and turkey meats as evidenced by increased TBA (l8).

For instance, pronounced oxidative flavor changes were observed

Similar r e su l t s were obtained by Keskinel e t al . (22) and by Sat0 e t a l . (41, 42) w i t h turkey meats.

It is not necessary f o r the proportion of l i p i d i n the food t o be high fo r autoxidation troubles t o be serious; oxidation i n susceptible foods w i t h low l i p i d content can, i n fac t , be ju s t as troublesome as i n f a t t y foods (26). oxidation-accelerating ca ta lys t s or enzymes in the complex food. Another reason i s the l i p ids of the more metabolically act ive t i s sues and organs contain a high proportion of t h e i r f a t t y acids i n complex l i p ids , par t icu lar ly phospho l i p i d s . Such l i p ids often contain r e l a t ive1 high proportions of readi ly oxidizable polyunsaturated f a t t y acids (26 3 .

One reason f o r t h i s is frequently the presence of

Love and Pearsm (30) s ta ted that though the phospholipid content of meat is r e l a t ive ly small, the suscept ib i l i ty of the phospholipids t o oxidation makes them important i n determining meat quali ty

Younathan and Watts (54) demonstrated tha t the proteolipid fract ion of t o t a l l i p id , ra ther than the t r ig lycer ide fract ion, was responsible fo r oxidative deter iorat ion induced by heating animal t i s sue . e t -- a l . (1) showed t h a t t he phospholipid fract ion was implicated i n the ear ly stages of autoxidation of turkey meat.

Acosta

The l a b i l i t y of the phospholipid f rac t ion is a r e su l t of t h e i r high unsaturated f a t t y acid content (26, 30, 50, 53). For example, 1s of the f a t t y acids i n beef muscle phosiih:~iipids have four or more double bonds, while only Obi$ of the t r ig lycer ide f a t t y acids from beef show t h i s degree of unsaturation (16). Similarly, i n pork muscle phospholipids, a large proportion (16$) of f a t t y acids have four or more double bonds compared t o only about 0.1% f o r t r iglycer ides (16).

The suscept ib i l i ty of ghospholipids t o autoxidation w a s a l so demon- Loss of unsaturated f a t t y acids was s t r a t ed i n freeze-dried beef (4,7).

more pronounced i n the phospholipid f rac t ion than i n the neutral f a t (7 ) .

Oxidation of polyunsaturated f a t t y acids, from whatever source, is accmpanied by the destruction of fat -soluble and water-soluble vitamins (15, 24, 32). The products of oxidation of polyunsaturated f a t t y acids and t h e i r subsequent degradation products impart objectionable f lavors and odors t o t h e point of rendering foods unacceptable (15, 20, 32).

CATALYSTS O F UPID OXIDATION

It has been ra ther wel l documented t h a t hematin compounds-- hemoglobin, qyoglobin, and cytochromes or animal t issues--are catalysts fo r unsaturated fa t oxidation (27). tha t f e r r i c hemochromogen is an act ive ca ta lys t f o r unsaturated f a t oxidation i n cooked meat.

Younathan and Watts (53) hypothesized

Both heme and non-heme iron were reported t o be functioning as ca ta lys t s of l i p i d oxidation of cooked meat (25, 29). t h a t both non-heme iron and hemoproteins were involved i n catalysts of l i p i d peroxide formation.

W i l l s ( 51 ) showed

The most d i r ec t evidence with respect t o the ro l e of non-heme iron was obtained with cooked meat where a f t e r removal of metmyoglobin by t r ea t ing with H202, s igni f icant l i p i d oxidation w a s demonstrated, especial ly a t lower pH where non-heme iron is most act ive (25, 29).

However, heme catalyzed l i p i d oxidation can be and is d ras t i ca l ly checked by the curing process. containing compounds a r e s t ruc tu ra l1 similar t o organic nitroxide f r ee radicals ( inh ib i tors of autoxidation 3 and have an unpaired electron which i s more closely associated w i t h the NO groups than w i t h the i ron . thus achieves i n t h i s way a stereochemical and functional blocking of ca t a ly t i c r eac t iv i ty of heme-containing systems (48).

The n i t r i c oxide complex of heme-

One

69 In beef t i s sue homogenate both hemeprotein and non-heme iron were

However, Sat0

Birano

found t o be act ive catalysts of l ino lea te oxidation (33). and Hegarty (41) found that heme canpounds were found t o have l i t t l e e f f ec t on the WOF d e v e l o p n t in water-extracted beef t i s s u e . and Olcott (11) found that the r a t e of oxidation of a l ino lea te solution w a s catalyzed by low concentrations of heme and heme-proteins and inhibited by higher concentrations. Kendricks and Watts (21) observed that the acceleration o r inhibi t ion of unsaturated f a t t y acid oxidation w a s dependent upon the r a t i o of l i p i d t o heme.

According t o Marcuse and Fredrickkson (33) development of rancidi ty i s influenced considerably by metal ca ta lys i s in emulsions of l i no le i c acid. Heavy metals, par t icu lar ly those possessing two or more valency s t a t e s w i t h a su i tab le oxidation-reduction poten t ia l between them (Co, Cu, Fe, Mn, and N i ) generally increase the r a t e of oxidative deter iorat ion of food l i p i d s (17, 48). autoxidation ca ta lys t s by v i r tue of t h e i r presence i n complex molecules such as hemoglobin, cytochrome, and the prosthet ic group of many enzymes (48). These metals reduce the length of the induction period ( the time during which no measurable oxidation occurs) and increase the maximum rate of oxidation (17). substances a r e such that i n t h e i r lower valency s t a t e s , they can reduce hydroperoxides, but not f r e e oxygen. Their reduction from the higher valency s t a t e requires highly reducing organic compounds which are present i n biological t i s s u e (48).

Compounds of the t r ans i t i on metals can be

The redox potent ia ls of these porphyrin-containing

Barber (2) demonstrated that the ca ta ly t ic ro le of iron and ascorbic acid was an important nonenzymatic mechanism f o r l i p i d oxidation i n t i s sue . found t o accelerate the development of WOF i n water-extracted meat t i s sue by S a t 0 -- e t a l . (41). f e r r i t i n were ac t ive ca ta lys t s of peroxidation i n microsomes i n the presence of ascorbic ac id . Inorganic ferrous iron has a lso been demon- s t r a t ed t o be a ca ta lys t of unsaturated l i p i d peroxidation i n mitochondria (36) and in microsomes (40, 52).

Low amounts of ascorbic acid i n the presence of Fe(I1) were

W i l l s (52) also showed t h a t inorgan2.c iron and

Under extreme conditions, however, it has been found t ha t heavy metal

Such an antioxidant e f f ec t i n linoleic acid emulsions bas Cu(l1) a t high concentration and low oxygen pressure may behave l i k e an inh ib i tor (33). generally been ascribed t o a ca t a ly t i c e f fec t on chain termination (33). This antioxidant tendency a t low oxygen pressure was a l so shown t o be d r a s t i c a l l y influenced by t h e r a t i o between t h e concentrations of metal and substrate (33) . compounds were ca ta lys t s at low concentration but become inhibi tors a t higher concentration. conversion." oxidat ion i n ground beef w i t h r e l a t ive ly high concentration (150 ppm) of

Betts and U r i ( 3 ) observed tha t cer ta in metal chelate

This phenomenon was cal led "catalyst - inhibitor Sat0 and Hegarty (41) also observed inh ib i t ion of l ip id

Cu(1I).

The e f f ec t of f a t oxidation may be par t icu lar ly c r i t i c a l in systems consisting of a water and a l i p i d phase w i t h metall ic salts or complexes i n the water phase and metal soaps i n the l i p i d phase. Such systems e x i s t

i n most foods (33). i n which they occur i n natural l i p i d s i s therefore one of the major fac tors determining the r a t e of oxidative deter iorat ion of l i p ids (17, 32).

The presence of t r ace heavy metals a t the concentrations

CONTROL OF LJPID OXIDATION

Control of l i p i d oxidation in cooked meats has usually been accomplished t o a varying degree of success in t h e pas t by use of varioiis chemical compounds such as antioxidants and chelatFng agents as well as by exclusion of oxygen.

Most of the antioxidants and chelating agents used i n meats a re chemical substances such as propyl ga l la te , butylated hydroxyanisole, c i t r i c acid (11, 22, 28), polyphosphates ( 9 , 46) and sodium ascorbate (10). Further, most compounds which have a d i rec t antioxidant e f f ec t on unsaturated f a t t y acids or t h e i r glycerides a re phenolic substances.

For use i n meat or meat products today, utmost care is, of course, necessary t o ensure t h a t the antioxidant w i l l not be harmful i n any way t o the consumer.

There is evidence that antioxidative substances a re present i n cer ta in types of foods or can be produced in meats during heat treatment. It is wel l known t h a t tocopherols and plant flavonoids a r e potent phenolic antioxidants (5, 48, 49). influenced by f a t t y acid composition and by the amount of natural antio- xidant, alpha tocopherol, stored i n the fa t . t h a t hot water extracts fran many vegetable sources were effect ive i n retarding t o some extent the oxidation of muscle l i p ids i n cooked meats.

Watts (49) s ta ted t h a t fa t rancidi ty i s

Zipser and Watts ( 5 5 ) found

Zipser and Watts ( 5 5 ) found t h a t production of antioxidative substances by prolonged heat treatment s tab i l ized uncured canned meats against oxidative rancidi ty . It was a l so observed i n our laboratory that ac t ive antioxidative substances were produced in re tor ted beef, pork, and turkey meats (42). isolated i n t h e d i f fusa te f rac t ion of aqueous meat ex t rac ts .

These inhibi tory substances from retor ted meats were

Certain soybean products were found t o be an effect ive antioxidant i n cooked pork (35). heated t o destroy lipoxidase a c t i v i t y possessed appreciable antioxidant a c t i v i t y . protein products, such as textured soy f lours and cottonseed f lour , t o meat loaves provided protection against WOF development.

P ra t t (39) has shown that soybeans which have been

Sat0 -- e t a l . (42) have shown t h a t addition of various vegetable

Browning reaction products obtained from the interaction of sugars and amino acids a l so were found t o inhibit WOF development i n cooked ground beef (42). These browning reaction products were prepared i n model systems wi th dextrose o r lactose as the carbohydrate and glycine, leucine, o r lysine as the amino ac id .

Further s tudies i n our laboratory have demonstrated tha t reductic acid (2,3-dihydroxy-2-cyclopentene-l-one) and maltol (3-hydroxy-2-methyl- bH-pyran&-one), a gamma-mone, were very effect ive inhibi tors of WOF developnent i n cooked beef. These r e su l t s supported our e a r l i e r findings tha t diffusates from retor ted meats contained reducing-type compounds which were produced during browning reactions and were responsible f o r inhibi t ion of WOF development.

Reductic acid is a browning reaction product (13, 14). Maltol is produced i n typ ica l Maillard-type reactions (13) and is a l so formed i n lactose-glycine systems i n heated skim m i l k (37, 38). Hodge e t -- a l . (14), reported that various reducing compounds cal led reductones including reductic acid were produced during browning reactions. Amino-hexose-reductones have been found t o be effective antioxidants i n soybean, cottonseed, and corn o i l (8). These literature references lend support t o our recent findings t h a t cer ta in reducing and other compounds produced during browning reactions might be responsible f o r WOF inhibi t ion i n cooked ground meat.

Hodge (13) and

The mechanism of action of these browning reaction products as inhibi tors is not c lear ly understood a t t h i s time. presence has prevented WOF developnent i n various meat products during storage. It can be speculated that the absence of rancid odors and of TBA-positive materials indicates that some of these campounds a re f r ee rad ica l inh ib i tors and function by in te r fe r r ing or react ing w i t h the f r e e rad ica l mechanism t o give harmless products and extend the shelf l i f e of the substrate (43). Gamma-pyrone compuunds may e f f ec t i n t e r - ferrence with f r ee rad ica l mechanism by e i the r hydrogen donation o r by electron donation t o the alpha carbon. has been described by Stuckley (44). w a s shown by c i t r i c acid i n t h e presence of gamma-pyrone compounds. enhanced s t a b i l i t y provided by c i t r i c acid can be a t t r ibu ted t o e i the r metal scavenging, peroxide decomposition, or mutual sparing e f fec ts , as i n the case of i t s interact ion w i t h @enolic antioxidants ( 5 , 6, 31).

However, t h e i r

This type of antioxidant action In addition, a synergis t ic e f fec t

The

The effectiveness of reductic acid as an antioxidant i n cooked ground meat system can be a t t r ibu ted t o the presence of the ene-diol portion of t he molecule similar t o that i n ascorbic acid. Reductic acid may have retarded WOF development by preventing myoglobin oxidation. Reductic acid was a l so found t o re ta rd fat oxidation i n frozen minced red salmon and herring flesh (45).

It is c lear that i n meats exposed t o r e l a t ive ly high temperatures the use of natural ly occurring antioxidants, such as inhibi tors obtained from vegetable protein products and those browning reaction products isolated during heating of skim m i l k , sugars-amino acids o r other types of foods, may possess cer ta in advantages over some of the presently-used phenolic-type chemical compounds. Also, the isolat ion, development, and application of these natural antioxidants afford a most promising approach t o control of WOF developnent i n cooked meats and meat products.

72

Evidence has been obtained in our research tha t nonenzymatic browning reactions are not only a source of f lavor i n foods, but they may be used a l so t o synthesize inhibi tor compounds which may be added back t o meats t o control l i p i d oxidation.

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75 KEN BEEHY, WRRC : My question is this. Along the same lines i n

poultry meat, it has been found t h a t i f you pre-cook i n an o i l bath or deep fat f r y and then f i n i s h off w i t h microwave the amount of wamed over f lavor is d i f fe ren t than if you pre-cook t h e m w i t h microwave and finish off with deep fa t P r y i n g . Have you had sbiJ .ar experiences?

HAROLD HERRING: No.

AARON WASSERMAN: Moving along fur ther on the chemistry of meat f lavors , one of the major centers f o r the study of these f lavors and compounds has been a t Rutgers University under D r . Stephen Chang. w e are going t o hear about the Chemistry of Cooked and Canned Meat Flavors. Although D r . Chang is l i s t e d as the speaker, unfortunately he could not make t h e session and i n h i s place w e have D r . Shiu Chi Lee from D r . Chang's laboratory.

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