chemical changes in meat due to processing—a review

CHEMICAL CHANGES IN MEAT DUE TO PROCESSING-A REVIEW

By R. A. LAWRIE

The advantages in terms of length of storage life of the various processes used to preserve meat are contrasted with their disadvantages in terms of chemical change. It is concluded that freezing offers the best combination of factors until the organoleptic disadvantages of sterilising doses of ionising radiation have been overcome.

Introduction Before considering the chemical changes effected by

processing in meat, it seems desirable to outline briefly the nature of its components. Meat is essentially the post mortem aspect of muscle cells embedded, together with deposits of fat, in connective tissue. In broad chemical terms it consists of about 75% of water, 18% of protein, 3% of fat, 1.6% of non-protein organic nitrogenous substances, 1 '2% of carbohydrate and its metabolites, 0.7% of inorganic salts and a small balance of traces of vitamins, etc.

The major portion of the protein-lO% of the meat-is found as actomyosin in the water-insoluble contractile elements or myofilaments (and in their structural attachments). A further 6%, which consists mainly of enzymes, is present as soluble protein in the fluid bathing the myolilaments, in insoluble particles (mitochondria and lysosomes) which are suspended in this fluid phase, and in minute aggregates on the walls of the fine tubular network (sarcoplasmic reticulum) which systematically threads the interior of the muscle cell. This fraction includes the pigment myoglobin, to which meat owes its colour. Much of the remaining 2% of protein is found as the collagen and elastin of the connective tissue in which the muscle cell is embedded, but a small proportion is found as lipoprotein (and other forms) in the walls of the sarcolemma (the membrane by which the cell is bounded), of the cell particles and of the sarcoplasmic reticulum. Most of the fat is extracellular, but a small proportion exists within the muscle cell as lipoprotein, phospholipid and as metabolites such as fatty acids. The nitrogenous substances include creatine, amino acids and nucleotides. The latter, 24 hours after slaughter, which is usually the earliest that meat is consumed, are generally in the form of inosine monophosphate and ribose. By this time also most of the carbohydrate has been broken down by post-mortem glycolysis to lactic acid (pH 5 . 5 ) or by amylolysis to glucose.

Originally the sole purpose in processing meat was preservation and this is still the aim of the more recently devised processes; but several ancient procedures have been deliber- ately retained for organoleptic reasons.

It is convenient to classify processes in five groups. They are those involving: heat removal (chilling and freezing) ; heat addition (cooking and canning); water removal (dehydration and freeze-drying); chemical addition (curing and sausage making); and irradiation.

J. Sci. Fd Agric., 1%8, Vol. 19, May

Chilling and freezing When applied to meat 24 hours post mortem the process of

chilling (i.e. refrigeration above -1 -5" , the freezing point) permits an extension of its edible life by discouraging the microbial growth by which the meat would otherwise be spoiled within 1-3 days. The useful storage or conditioning period may be 2-4 weeks. Some of the sarcoplasmic proteins have become denatured by 24 hourspost mortem,' and a further percentage becomes insoluble during the conditioning period; the denatured portion of sarcoplasmic proteins, together with dipeptides such as carnosine and anserine, are attacked by the muscle's proteolytic enzymes. The level of free amino acids consequently rises. Probably because these increase intracellular osmotic pressure, and because there is a concomitant release of calcium and uptake of potassium by the myofibrillar proteins,2 the water-holding capacity of the meat becomes greater. A more subtle change also affects the myofibrillar proteins, because the chemical bonds linking the actin rods to the Z-lines are broken, probably non- enzymically,a although they still remain firmly linked to myosin as the actomyosin complex which forms during rigor mortis. This change accounts for at least some of the enhanced tenderness long observed to occur in conditioned meat. The electrophoresis pattern of the proteins which remain soluble alters considerably.4 Inosine monophosphate is broken down to inorganic phosphate and inosine, the latter being de-aminated to hypoxanthine. Some amylolysis of residual glycogen to free glucose occurs. The enhanced content of free sugars and of amino acids and hypoxanthine in conditioned meat may possibly explain its stronger flavours since it has been shown that, when these substances are heated together, desirable 'meaty' odours and flavours develop.617 The beneficial changes may be somewhat offset by incipient oxidative rancidity of the fat and a synergistic oxidation of myoglobin to brown metmyoglobin. These chemical changes are not caused by the process of chilling per se, but, since chilling is responsible for prolonging the edible life of the meat sufficiently for them to occur, it is the indirect reason for such altered meat being presented to the consumer.

The process of chilling has an additional, and direct, effect if it is applied to meat earlier than about 24 hours post mortem. Post-mortem glycolysis, which dominates the changes in muscle in the period immediately after slaughter,

234 h w r i e : Chemical Changes it1 Meat due to Processing-A Review

proceeds at a slower rate as the temperature falls from that in vivo (37"-39") to about 12"-15". Between the latter and O", however, the rate increases once mores and hexose-6- phosphate tends to accumulate (since phosphofructokinase is relatively more inhibited than phosphorylase in these circumstances).g This phenomenon increases the degree of interdigitation of the actin and myosin rods, whereby the sarcomeres shorten, and there is a concomitant toughening of the meat.lOJ1 It is true that this cold-shortening is mainly manifested if the meat is free to shorten when exposed to these temperatures, but it also occurs to some extent even when the muscles are held on the carcass.

When meat is subjected to temperatures below its freezing point, the chemical changes effected depend on the rate at which freezing occurs, and on the freezing temperature ultimately attained and its duration.

A major effect of freezing, superficially manifested by a reddish exudation or drip which appears on thawing, arises from damage done to the muscle proteins, whereby their water-holding capacity is lowered, and from a translocation of water from within the muscle cells to the exterior. Freezing begins in extracellular spaces, thus increasing the concentration of the extracellular fluid. In turn, this draws water osmotically from within the still-unfrozen cell which adds to the growing ice crystals and denatures muscle proteins. The faster the transition from 0" to 5", the less is the translocation during freezing and the less are protein damage and exudation. If frozen and thawed sufficiently quickly, muscle cannot be distinguished microscopically from the fresh tissue, but such rates are impossible in commercial practice. As drip contains minerals (e.g. iron), vitamins of the B complex and free amino acids, slow freezing can lead to some loss of nutrients unless the exudate is retained for incorporation during cooking.'2,'3 Lower temperatures inhibit, and longer periods of storage enhance, undesirable changes in fat and protein (Table I).13

TABLE I

Approximate times for appearance of distinct rancidity in fat or oxidation of lean in unwrapped meat14

Storage temperature, 'c

-8" - 15" - 22" - 30" Beef 3 months 6 months 12 months - Pork - 3 months 6 months 12 months

That freezing, in addition to causing some denaturation (manifested both by increased insolubility and by a rise in pH), alters muscle proteins in other ways is shown by an increase in the number of electrophoretically more mobile sarcoplasmic components.13

Denaturation is reflected by increasing insolubility of sarcoplasmic proteins,l5 increasing difficulty of extracting actomyosin (especially in fish),'s a loss of myosin ATP-ase activity (especially below -2O")," a decrease in titratable -SH groups18 and pH increment.19 Freezing ruptures lysosomal membranes, thus liberating hydrolytic enzymes and making them accessible to their substrates.20 This may explain the fact that, even at -20°, there is a slow breakdown of protein to amino acids-at least in the liver of meat animals21 and in the muscles of poultry.22 The lower the

temperature of frozen storage, the greater is the percentage of water separated as ice. Although at - 10" 95 % of the total water is frozen,23 Love & Elerian24 have shown that in fish muscle the attainment of a temperature of about -183" causes an irreversible removal of some of the structurally bound water of actomyosin and greater toughness. It is possible that a similar phenomenon operates in meat. Prolonged storage even at - 10" leads to some loss of flavour, possibly involving highly volatile substances such as d ia~ety l .*~

Cooking and canning It is probable that the organoleptic benefits of cooking

were originally incidental to the observation that it had a short-term preservative action. The effects of heating are progressive with increasing time and temperature.

The most obvious change is a loss of water-holding capacity.Zs This reflects denaturation in sarcoplasmic and myofibrillar protein which both become increasingly insoluble as the temperature is raised (Table II).27

TABLE I1

as % of value in fresh tissue Effect of heat on extractability of muscle proteins

temDerature Heating Sarcoplasmic protein Myofibrillar protein

20 100 100 40 86 69 60 23 2 80 1 12*

*Increase may be due to some breakdown of connective tissue

The sarcoplasmic fraction includes many of the enzymes of muscle. Their susceptibility to heat varies.

As isolated, hexokinase is inactivated at about 40" and creatine kinase requires 60", whereas adenylic kinase is said to withstand temperatures of the order of 100". The reactions of the enzymes are somewhat modified by the environment in meat, however, especially by the pH/temperature interaction? Again, wheras the pigment myoglobin, which is mainly responsible for the red colour of meat, can withstand being heated in solution at 65", it is denatured below this temperature in situ.29 Denaturation is followed by oxidation to the brown insoluble pigment, globin myohae- michromogen. Other factors contributing to the brown colour of cooked meat include the caramelisation of carbo- hydrates and Maillard-type reactions between reducing sugars and amino groups.

The shrinkage and most of the loss of water-holding capacity in cooked meat is due to changes in the myofibrillar proteins, however, which also tend to cause a toughening to the palate. According to Hamm30 heating to 40" causes some unfolding of the polypeptide chain, and the fracture of unstable crosslinks. Indeed at the ultimate pH of normal meat (5 *4-5.5) denaturation of isolated myosin is relatively speedy even at 35";31 and it can be presumed that some denaturation of actomyosin in situ occurs during post-mortem glycolysis.

Between 40-50" these alterations are more marked and are reflected by a rise in pH, some loss of Ca++ and Mg + + binding power, and a fall in acidic groups. The ATP-ase is completely inactivated. Between 55" and 80" all these changes are sharply accentuated; beyond 70" the formation of

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Lawrie: Chemical Changes in Meat due to Processing-A Review 235

disulphide bonds by oxidation of the SH groups of actomyosin begins; above 90" H2S is split from the myofibrils;30 and at the temperatures employed for canning, 110"-120", the evolution of HzS by breakdown of the myofibrillar proteins can be considerable. This may cause unsightly discoloration through the formation of metal sulphides, quite apart from representing a loss of S-containing amino acids.

Elevated temperatures also alter connective tissue proteins. The initial heat-induced changes in collagen result from the disruption of hydrogen bonds. These are superficially manifested by a shrinkage which occurs at 60" in intact meat or at 65" in extracted collagen. Progressively greater changes, with increasing solubility, occur as the temperature is raised further. It may be noted that, whereas collagen can remain virtually unchanged after long periods at 37" under aseptic conditions, prior exposure of meat to 70" for only 15 min will initiate a non-enzymic proteolysis such that considerable amounts of soluble hydroxyproline are produced on subsequent storage at 37". On prolonged heating at 100" collagen is converted to soluble gelatin, but this process is not swift unless meat is pressurecooked at 125".33 Elastin, the other major type of connective tissue, is unlike collagen in being most resistant to breakdown by heat, although it tends to shrink and harden. It will be clear that the method of cooking will largely determine whether the toughening effect of heat on myofibrillar proteins and on elastin where this is present will be offset by its tenderising action on collagen; and the method of cooking, in turn, will be determined by the relative amounts of connective tissue in the meat.

The pressurecooking of meat, however desirable from the point of view of tenderness, has disadvantages. Thus, when autoclaved at 112" for 24 hours, 45% of the cystine of pork may be destroyed and other amino acids (e.g. tryptophan) made unavailable during digestion.34 In normal cooking the temperature of the meat will not reach 100" except in outside layers but the reaction of amino acids, especially lysine, with reducing substances or with carbonyls from fats can be appreciable at high cooking temperatures. During cooking, also, meat will usually lose about one-third of its content of vitamins BI, Be, BIZ and pantothenic acid, and about one-tenth of its vitamin B2, and nicotinic acid.35 Despite the marked shortening of cooking time by microwave heating, there are no appreciable benefits in retention of vitamins.35

During cooking the interaction of ribose or other mono- saccharides with hypoxanthine or inosine and various amino acids is responsible for some of the flavour which develops.7.36 Indeed it has been suggested that the only essential components for producing flavour in heated meat are an aliphatic aldehyde and cysteine.37 Excessive heating may cause breakdown of amino acids and yield HzS and ammonia.

According to Hornstein, Crowe & Sulzbacher,38 full meat flavour, especially species-specific character, requires components arising from the effect of heat on fats. Acetaldehyde acetone, diacetyl and the c1-c4 fatty acids are among the volatiles which are produced generally from oxidised fats on cooking. Additional components characterise the various types of meat, e.g. hepta-and nona-2,Cdienals in pork. Such components are produced in relatively small amounts however and have been detected mainly by g.1.c. In terms of gross breakdown, the fat in meat is relatively stable37 in normal cooking. At very high temperatures, however, development of oxidative rancidity is accelerated considerably.40 Moreover when there is pyrolysis of the fat,


as in severe grilling, traces of carcinogenic hydrocarbons can be produced.41 including 3,4-benzpyrene and dibenzanthracene (Hirom & Lawrie, unpublished results).

The possibility of preserving meat for long periods through the application of heat to the product in a sealed container arose from the observations of Appert at the beginning of the 19th century. From this the canning industry developed. The majority of canned meats are commercially sterile, as they are processed to the point at which most micro- organisms, in particular Clostridium botulinum, and their spores have been killed. This requires a minimum of 3 min at 121". From what has been said already, it will be clear that such effects as denaturation, amino acid destruction, HzS liberation, metmyoglobin formation, collagen solubilisa- tion, Maillard-type browning, and the accompanying changes in flavour, will be more marked in canning than in normal cooking. Prolongation of the time of heating at 121" to the point at which no bacteria, even thermophiles, could survive would produce an unacceptably degraded product. Even 3 min at 121" brings about undesirable changes with pork, especially in many popular products containing curing ingredients, and a less severe canning procedure, pasteurisa- tion, must be accepted, where the temperature does not rise much above 60". Curtailment of the heat treatment in such cases is justified by the presence of curing ingredients which make the growth of pathogens unlikely, but cans of pasteurised and semi-preserved meats are best kept cool.

Dehydration and freeze-drying The efficacy of drying in prolonging the edibility of meat

has long been recognised. The dehydration processes available up to World War 11, however, affected meat adversely, owing to the difficulty in removing moisture uniformly. By first cooking the meat in slices, mincing it and then drying it under carefully controlled conditions below 70" a product can be prepared which, on reconstitution, compares reasonably with cooked untreated mince.42 The degree to which reconstitutability is retained is clearly important. Loss of water from both raw and precooked meat causes a closer packing of the muscle fibres. In general, with hot-air drying procedures, the changes occurring in the proteins are similar to those which arise during heat denaturation. In addition, however, salts tend to accumulate on the periphery of the fibres. There will thus be a greater measure of denaturation on the fibre surface which will tend to oppose re-entry of water.

For long-term storage, such dehydrated meat must be packed to exclude as much moisture and oxygen as possible. But, while reduction of oxygen will prevent deterioration of air-dehydrated meat for twelve months or more at 15", non-oxidative deterioration will develop, even under nitrogen at 30". This is mainly Maillard-type browning, accompanied by bitter flavour development, a change which can be minimised by keeping potential reactants at the lowest possible concentration before commencing dehydration, and by drying the meat to very low moisture content.

In the presence of oxygen the storage of dehydrated meat at high temperatures causes it to become pale yellow due to the conversion of myoglobin to bile pigments. A mealy odour develops and oxidative rancidity of the fat may be marked unless the residual moisture content is less than 1 a 5 %, but, in these circumstances, the proteins suffer further damage, leading to adverse internal changes. In dehydrated raw meat there is still considerable lipolytic activity (Table 111); pre- cooking reduces this but does not eliminate it.

236 Lawrie: Chemical Changes in Meat due to Processing-A Review

TABLE 111

Free acidity of fat (as % of oleic acid) in airdehydrated raw beef, after 12 months storage

thought that water, by hydrogen bonding with peroxides, delays their rapid decomposition and inactivates metal catalysts.4e

Temp. of storage 21°C 37%

7.5 17 36 5.0 12 24 3 . 2 6 13

Moisture content

The moisture content at which residual enzyme activity ceases in uncooked air-dried meat will vary with each enzyme. As with chemically processed meat the content of B vitamins (especially B1) is diminished by air-drying.

It had been appreciated that the mildest method for drying meat would be the sublimation of the water from the frozen state. The process became commercially feasible between 1955-1960 with the development of accelerated freeze-drying (a.f.d.) 43 whereby earlier difficulties of heat exchange were overcome. In a.f.d. ice sublimes from the frozen meat under high vacuum, the moisture content reducing to 2% in only 4 hours; moreover raw meat can be used. Because of the low temperature and the high speed of operation and the avoidance of local high concentrations of salt, the water-holding capacity of the proteins is relatively unaffected. The freeze-dried steaks rehydrate easily to give a product closely resembling raw meat especially if pre- freezing has not been too rapid. As opposed to hot-air dehydration, freeze-drying does not affect the iso-electric point of the myofibrillar proteins, but it tends to lower the water-holding capacity in this pH region.44 Moreover, even under optimum operating conditions, there is some evidence that the proteins of the sarcoplasmic reticulum are altered, for, on homogenisation, myofibrils tend to cohere longitudinally.45

If the plate temperature used to aid the sublimation of ice towards the end of the freeze-drying process is 20"-30°, the extractability of the myofibrillar proteins remains similar to that of frozen meat.' As plate temperatures are raised, however, there is a progtessive loss of extractability and loss of water-holding capacity on recon~titution,~6 some of which is also attributable to denaturation of sarcoplasmic proteins.' Electrophoresis on starch gel indicates that subtle changes occur in both sarcoplasmic and myofibrillar proteins even at fairly low temperatures (Parsons & Lawrie, unpublished results). That myofibrillar proteins may survive the a.f.d. process in a substantially unaltered condition is shown by the finding that freeze-dried actomyosin can still contract on the addition of ATP, after reconst i t~t ion.~~ The biological value of the meat proteins is unaltered by a.f.d.43 Although myoglobin on exposed surfaces is susceptible to oxidation and denaturation, the mildness of the process is exemplified by the fact that the bulk of the pigment survives as bright red oxymyoglobin, especially with low plate temperatures.48 The concentration of brown oxidation forms of myoglobin, however, increases with increase in time and temperature of storage. Under these conditions, too, non-enzymic Maillard- type browning occurs, and this, as in the case of air-dried meat, leads to bitter flavours. Off-flavour development, due to oxidative rancidity of the fat, can be marked when freeze- dried meat of very low moisture content is stored, It is

Curing and sausage making Another group of processes used to preserve meat and to

alter its attributes in desired directions involves the addition of extraneous substances.

In curing, the additives, salt or sugar, owe their efficacy mainly to their osmotic action, which deprives micro- organisms of available moisture. Although more modern tastes have led to a lowering of the concentration of the curing agents, this has not always been accompanied by appropriate microbiological safeguards. The biochemical mechanism of curing was extensively studied by Initially there is an osmotic removal of water from the muscle proteins by the 25-30 % solution of sodium chloride employed. As salt diffuses inwards, however, it forms a complex with the proteins which has a higher osmotic pressure than the curing solution itself, causing some reversal of the flow of water. The final concentration of sodium chloride attained with traditional curing is about 4-5 %. As would be expected the electrophoresis pattern of both sarcoplasmic and myofibrillar proteins is appreciably altered by c~r ing .~1 The overall ATP-ase activity decreases and the ratio of Mg++- activated to Ca++-activated ATP-ase increases, suggesting that the myosin enzyme is being preferentially denatured. Salt appears to activate the lipoxidase of muscle, because curing causes an acceleration of oxidative rancidity in the fat greater than can be accounted for by its non-specific action.52

In the brines used for traditional curing 2 . 5 4 % of potassium or sodium nitrate is included principally because of its beneficial effect on colour, although nitrite has also a specific antimicrobial action, especially in acid solution.53 Nitrate is reduced to nitrite by halophilic micro-organisms present in the brine. The latter is reduced to nitric oxide, either by other micro-organisms or by enzymes in the brine; and the nitric oxide combines with myoglobin to form a pink compound, which is stabilised by conversion, on being heated, to pink nitric oxide myohaemochromogen. Most of this fixation of colour takes place over a 10-14 day maturation period, during which, also, the salt becomes more evenly distributed. The biochemistry of formation of cured meat pigment is complex and controversial, however, and the mechanism is not yet defined with certainty. With the traditional cure, excess salt suppressed the micro-organisms responsible for the first stage, leading to brown discolorations; insufficient salt led to excess microbial activity and the development of green pigment, which appears sometimes to be associated with hydroxylamine formation.54 More recently, reliance on micro-organisms has been superseded by slice curing.56 In this process slices of pork muscle 2-5 mm thick are passed for 2-5 min through a brine containing 10% sodium chloride and 0.02% sodium nitrite. The sodium nitrite is reduced in only a few hours to nitric oxide by enzymes in the muscle, and this procedure gives accurate control of the nitrite content. A limit of 500 ppm is set by statutory regulation as the substance can be toxic at higher levels, mainly through its effect on blood pigments. Were all the nitrate added in traditional curing to be converted to nitrite, the level of the latter could rise to 2500 p p ~ n . ~ ~ Because the cured meat pigments are particularly


Luwrie: Chemical Changes in Meat due to Processing-A Review 237

susceptible to oxidation under the action of light, vacuum- packaging or packaging under nitrogen has been developed. In some circumstances when applied to bacon in the traditional way, such packaging procedures can give rise to high nitrite concentrations.57 The balance of micro-organisms in vacuum packs can alter its internal atmosphere. For example, residual oxygen may be absorbed and replaced by carbon dioxide.58 This could inhibit the normal microflora of cured meat and cause its replacement by other micro- organisms capable of changing flavour and odour (and perhaps the safety of the product), e.g. lactic acid bacteria can cause souring.5Q Whereas unpacked bacon goes off organoleptically when the bacterial load has reached only 10% of its final value,60 vacuum-packed bacon, especially if stored at relatively high temperatures, does not become inedible until many days after bacterial numbers have attained a maximum.59 This obviously reflects a chemical difference between the vacuum-packed and non-packed bacon.

Smoking is an additional process which enhances meat preservation. Amongst its 200 or more constituents, smoke contains phenols (derived from the decomposition of lignin)61B62 which have antimicrobial action; the latter may be enhanced by the drying action of the smoking process. Smoking also delays fat rancidity. The flavour of the smoked product depends partly on the reaction between the phenols and polyphenols with -SH groups in the proteins and between carbonyls and amino groups.83 Wood pyrolysis products also include carcinogens such as 3,4-benzpyrene and 1,2,5,6-phenanthracene. Although some workers believe the dangers of carcinogens from smoked meat are extremely small64 several European countries are now endeavouring to use carcinogen-free solutions for imparting smoke flavour. In this context 8 pg/kg of 3,4-benzpyrene has been detected in charcoal-smoked steak;65 and the very high incidence of stomach cancer in Iceland has been attributed to the practice, now less prevalent, of eating heavily smoked muttones (H. PAlsson, personal communication). There has recently been evidence that the carcinogen, dimethylnitrosamine, is found when nitrite is heated in the presence of starch;67 at the moment, however, the significance of this finding, if any, in relation to cured meats is unknown.

In products based on comminuted meats, such as sausages and meat roll, salts are important not only for their antimicrobial action but also because they aid in the retention or improvement of water-holding capacity; indeed the addition of water itself influences the latter, retention being a maximum when the ratio of water to meat is 2 : 1.68 In respect of added salts, it is important in general that they should be derived from strong acids as this enhances the formation of the salt protein complex. Excessive salt (> 8 %) has a dehydrating effect on the comminuted meat. If meat is frozen pre-rigor, before the ATP level has fallen to the point at which actomyosin formation occurs, then thawed whilst being comminuted in 2% salt solution, the high pre-rigor water-holding capacity of the proteins is retained, and this process is used in sausage manufacture. Since sodium acetate has no such effect, it is believed that the binding of chloride ions is responsible. Certain salts of weak acids, in particular phosphates and polyphosphates, enhance water-holding capacity and are used with continental- type sausages in particular. Bendall69 believes that whilst most phosphates are effective by virtue of their action on ionic strength and pH, pyrophosphate has a specific action in

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separating the actomyosin of post-rigor meat into actin and myosin, the latter forming a gel. Much of the success of comminuted meat production depends on the ability of the muscle proteins to hold fat as well as water. The factors determining the stability of sausage meat emulsions are thus important. One of the other functions of salt in these products is to enhance the power of the myofibrillar proteins to emulsify fat. Such emulsions tend to break down if the temperature during mixing rises above about 22"70 and over- chopping increases the surface area of the fat particles to the point at which the water-protein phase cannot hold them. The addition of cooked rind (particularly denatured collagen and elastin) enhances the overall water-holding capacity.

The manipulation and mixing involved with comminuted meats renders them particularly liable to microbial spoilage, which in the presence of salt is only partly overcome. In U.K. sulphur dioxide up to 450 ppm is permitted as a preservative. Continental sausages are often permitted to undergo a controlled fermentation of indigenous or added carbohydrate by non-spoilage organisms, in the course of which lactic acid, which is effective in suppressing the growth of spoilage bacteria, is produced. Much of the flavour desired in continental-type sausages is due to the deliberate introduction of organisms whose metabolism of carbohydrate or fat gives rise to agreeable breakdown products. By their nature, comminuted meats readily lend themselves to the addition of numerous substances which affect odour and flavour. These range from pure substances, such as glutamic acid and sugar, to spices and herbs. The latter can also raise the microbial load.

Ionising radiation The possibility of preserving food by using the anti-

microbial action of ionising radiation appears to have been first considered in 1930 by W U S ~ , ~ ~ but the industrial possibili- ties have developed since World War I1 with the availability of prays (from radioactive elements produced in atomic piles) and of high-energy electrons and soft X-rays (from generators). Their efficacy is achieved through considerable depths of product, even after packaging, with little rise in temperature and usually little total chemical change. Such chemical change as is caused, however, can be very important. The ions and other activated molecules which the rays create are only the first events in a series, and, form, for example, free radicals, polymers and peroxides. In meat, where there is a substantial aqueous phase, destruction of organic molecule takes place through their reaction with H and OH radicals of the irradiated water molecules.

The changes produced in proteins are determined both by their individual nature and by the dose of ionising radiation to which they are exposed. With a dose of 5 Mrad (approxi- mately that required for microbial sterility) there is a marked loss of water-holding capacity72 which is reflected in the behaviour of the isolated myofibrils at all pH values in the physiological range.73 Such irradiated myofibrils synaerese less at low ionic strength and swell less at high ionic strength, on the addition of ATP, than do non-irradiated controls. The oxidation of S H groups may be responsible for these changes. It should be pointed out, however, that histologi- cally no change can be detected in muscle fibres that have received 5 Mrad and subsequent storage at 37" for one year.73 There is concomitantly a steep rise in pH with the time and temperature of storage as in non-irradiated meat. Some of this may derive from the breakdown of soluble amino acids;


there is little destruction of amino acids combined on pro- t e i n ~ . ? ~

Although Tsien & Johnson75 reported that beef proteins subjected to 5.6 Mrad lost 28% of their amino acid components, more recent has questioned whether any loss of amino acids significant in nutrition occurs even at 20 Mrad. Even so, a protein has been isolated from heated meat which, when irradiated at 5 Mrad, loses 13% of its amino acids and yields the 'wet-dog' odour which is a severe disadvantage in the organoleptic quality of irradiation- sterilised meat.77 The principal components of the volatile off-flavours produced on irradiation of meat are methyl mercaptan and HdL78 Increasing quantities of diethyl sulphide and isobutyl mercaptan arise as the dose is increased.

Most enzyme proteins require considerably more than 5 Mrad for inactivation. This can be a serious problem in the storage of irradiated meat, since the high storage temperatures permissible in the absence of bacteria can be associated with continuous enzymic change. The magnitude of the effect

appears to vary with species however, since the structural proteins of rabbit are sufficiently proteolysed to cause the tissue to disintegrate under conditions which have little effect on beef .73.79 In general, the degree of proteolysis of meat is similar in irradiation-sterilised beef to that in non-irradiated sterile samples.

As the dose of irradiation is increased from zero to 40 Mrad, the shrink temperature of collagen falls from 61" to 27",80 an effect interpreted to be disorganisation of the secondary structure of the triple helix and the formation of intermolecular crosslinks.81 Studies on tropocollagen solutions have shown that with doses as low as 30,000 rad there is a decrease in the temperature at which heat-denaturation occurs. Crosslinking began at doses above 30,000 rad, a gel being formed which, on further increase in the dose, produced a completely insoluble material.a2 The presence of oxygen or of a radiation scavenger such as thiourea inhibited the basic crosslinking reactions involved.

Changes in the pigment proteins on irradiation are sometimes beneficial. Thus myoglobin may yield a bright red

TABLE IV

Comparison of preservative processes for meat

Process Susceptibility to change Introduction of

extraneous Advantages Disadvantages during storage Immediate Useful storage chemical life (controlled change conditions) (a) Chemical (b) Microbial substances

- - - - - Nil - 3 days

Chilling - 2-4 weeks + + - Substantial Cost; Bulk retention of quality

- - Freezing + '1-2 years' + Retention of Drip: bulk: quality; abuse convenience

Cooking + 5 days - (+) Short-term Degree of storage cooking fixed

Canning + '100 years' (+) ~~

- Convenience; Changed eating long-term quality; bulk storage

Dehydration + 6-12 months + (+I - Convenience; Diminished

Freezedehydrat ion (-1.1 6-12 months (+I - - Convenience ;

lightness eating quality

lightness; substantial Cost retention of eating quality

eating quality possiblity of & storage life toxicity

Curing & smoking + 6 months + + + Enhanced Marginal

Other chemical + variable additives

~~ ~~ + Enhanced Changed eating storage life quality; abuse

(+I (toxicity)

Irradiation (a) Sterilising

doses + several years + - + Prolonged Diminished

storage at eating quality; high temp. cost

(b) Pasteurising doses + - 8-12 Weeks + -I refrigeration

Extended

chill temp. + storage at (? Cost)

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Lawrie: Chemical Changes in Meat due to Processing-A Review 239

compound similar in absorption spectrum to oxymyoglobin but more stable.83 On the other hand some of the meat pigment may be converted to green sulphymoglobin. More generally, brown metmyoglobin is formed.

In the lipids, irradiation causes changes similar to oxidative rancidity. In the absence of oxygen, fatty acids are decarboxy- lated.84 If unsaturated, they are polymerised. In the presence of oxygen, hydroperoxides and carbonyls are formed. Since the carbonyl production does not increase in proportion to increasing fat content, it seems that the oxygen is not attacking neutral fat but the lipid fractions. Carbo- hydrates tend to be oxidised in the 6- position to yield gluconic acids and aldeh~des.~5

Of the vitamins, vitamins C and B1 are particularly affected, destruction of the latter probably representing the greatest nutritional loss from irradiated meat. The potential disadvantage of ionising radiation in meat processing is the possibility of the production of minute quantities of bio- logically potent and toxic chemicals, e.g. carcinogens from sterols, As experiments involving the long-term ingestion of irradiated foods continue, however, this danger appears to be receding, and bacon sterilised by ionising radiation is now permitted for sale in U.S.A.86 From the severe organoleptic changes caused by sterilising doses it is inferred that ionising radiation will be used in very low doses (- 100,OOO rad) at which toxic manifestations are more remote than at 5 Mrad, and will be used in combination with refrigeration. In such a context they will ensure a useful prolongation of storage life at O O - 5 O . 8 7 Nevertheless, a trained taste panel can detect flavour changes in meat after 50,OOO rad, and symptoms of

accelerated fat oxidation can be detected between 25,000- 100,000 rad.88 It has recently been suggested, however, that a useful extension of wrapped beef and lamb carcasses at 1 O can be achieved by surface treatment with up to 400,000 rad before off-flavours devel0p.8~

Conclusion The salient features of the various processes used to

preserve meat are compared in Table IV. Whilst, in terms of useful commercial storage life, canning has a very great advantage over all other processes, this is achieved at the expense of marked modification of the fresh commodity. For long-term storage of meat with a minimum of chemical change freezing has considerable merit. Moreover the 1-2 year period cited is certainly an underestimate of what can be achieved. Adventitious retention of eating quality in the meat of mammoths is reported after a 20,000 year period of frozen storage,B0 and it is clear that, with modern refrigeration engineering, a substantial extension of commercially useful storage life, with virtually no chemical change, can be envisaged. The desirability of dispensing with costly storage facilities after processing remains. In these cases it seems likely that continuing efforts will be made to minimise the disadvantages which still affect the use of sterilising doses of ionising radiation.

Food Science Laboratories, University of Nottingham, Sutton Bonington

Received 8 November, 1967

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chemical changes in meat due to processing—a review

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