meat: current developments and future status

INTRODUCTORY EDITORIAL

MEAT: CURRENT DEVELOPMENTS AND FUTURE STATUS

INTRODUCTION

At a time when the world population continues to increase exponentially, it is clearly important to consider the means of satisfying the concomitant demand for nourishment. There is much current interest in the fact that the yield of vegetable proteins is much greater than that of animal proteins for a given acreage (e.g. beans provide 20 times as much protein as beef); and that that of microbial or micro- cellular protein, produced by continuous fermentation, is greater still--by a hundred-fold factor. At such a time it is salutary to reassess the importance of meat as a commodity. This is not merely a sentimental exercise for a departing delicacy. Only 10% of the world's land surface is, or is likely to prove, usable for vegetable production in the foreseeable future. Moreover, the infrastructure required for fermentation plants imposes severe locational constraints. On the other hand, a further 30% of the world's land surface, which is too poor for crop production, can be grazed by herbivores and these can, therefore, directly convert the meagre growth of this vast, but comparatively unproductive, land into very palatable protein of the highest nutritive value--meat.

MORE AND BETTER MEAT ANIMALS

In respect of the conventional meat species--oxen, sheep and pigs--research continues vigorously into means of increasing fertility, feed conversion efficiency and the yield of lean meat from the carcase. Progeny testing, including the use of proven sires, is assisting the selection of those animals having the most desirable

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Meat Science (1) (1977}---O Applied Science Publishers Ltd, England, 1977 Printed in Great Britain

2 INTRODUCTORY EDITORIAL

meat characteristics--although it should be emphasised that the gulf between what the producer, butcher and consumer mean by 'desirable' is considerable.

The duration of the breeding season and the number of conceptions within it are being increased by the injection of hormones into stock, e.g. gonadotrophins. The factors determining sperm mobility, and their capacity to fertilise ova, are being assessed, especially with a view to enhancing the conception rates when employing artificial insemination. Sperm banks ensure that the genetic benefits of males long dead can be preserved and used to produce offspring of desired characteristics, years later. Fertilised ova, of eugenic parents, can be transplanted into females of low genetic usefulness for incubation and rearing. Incidentally, an unexpected aid to breeding has developed from what was an undesirable feature of the meat from boars. T'le substance responsible for boar taint, in a slightly modified chemical form, is present in the submaxillary glands of mature male pigs, and enhances the willingness of sows to mate. When used as an aerosol it can artificially encourage such unions.

In the past the disadvantages of the entire male for meat production have included its tougher flesh. As a consequence, when large families required large, fat joints it was better to obtain them from mature castrates or females. Now that smaller portions for the individual consumer are becoming popular, and frequently form the vehicle for handling display and sale, the undoubted superiority of the entire male in converting feed into flesh can be exploited, provided the animals are killed at a relatively early age. This helps to account for the increasing interest in bull and boar meat.

Another aspect of this trend is the use of breeds of cattle which develop large carcases at early ages. These include the Charotlais, Limousin and Chianina breeds. (The latter, at maturity, stand 7 ft high at the forequarter.)

One of the reasons for the high yields of lean meat from Charollais cattle is the frequent occurrence in this breed of a recessive gene which causes the ratio of muscle/bone to be about twice that normally found. These are referred to as 'doppelender' animals, but in fact the enhanced meat content is distributed throughout the carcase. Such meat, in most respects, is of somewhat better quality than normal, having less connective tissue (gristle), although it tends to be paler. It is true that there are difficulties in calving, but this drawback is being overcome as experience is gained in controlling the cross-breeding pattern. The science of molecular genetics may eventually permit the identification of the gene involved and reveal the means of deliberately inducing the development of the condition. This could substantially increase the capacity of meat animals to convert feed into edible muscle instead of into inedible bone or undesirable fat.

Much is known about the effects of hormones on the growth of muscles. Oestrogen implantation--chemical castration--has been quite extensively used to prod uce beefcarcases with more lean meat and less fat: progesterone implantation also leads to increased muscle growth--but concomitantly to increased intra-muscular

MEAT" CURRENT DEVELOPMENTS AND FUTURE STATUS 3

fat. FAO recognises the importance of such anabolic agents and is actively encouraging research on their mode of action and on the significance (if any) of residues for human consumers.

Other hormones have more specific effects. Thus the growth hormone from the pituitary gland increases the relative proportions of the muscles of the head and neck region. A more desirable change in the carcase of meat animals would be the preferential growth of the muscles at the top of the hind limb. No doubt a hormonal agent capable of effecting such changes will be identified one day. The existence of the kangaroo shows what nature can already do in this respect!

It is desirable to introduce a word of caution into the realm of manipulation. During the past two decades, there has been increasing concern about pale, soft watery muscle (PSE) in pigs. This has been associated with the concomitant desire to select pigs intensively for their ability to convert feed into lean meat- -or perhaps, more correctly, for a decreased ability to deposit fat. It is evidently important to appreciate that intensive selection for a desirable quality, such as leanness, may also signify an unintentional and simultaneous concentration of undesirable traits.

It is equally important to emphasise that there has been heightened interest in developing sources of meat other than those of the familiar domestic species. The difference between mammals lies more in the distribution of meat on the carcase than in its intrinsic properties. Thus the more mobile species have relatively more meat on the limbs: the more aggressive have more around the neck region.

Of course, all kinds of animals, from ants to elephants and whales, have been-- and still are--eaten in various parts of the world, but on a relatively small, local scale. In a number of countries, however, systematic control of the breeding and growth of such hitherto unexploited species as deer, buffalo and antelope is progress- ing. Deer, to give a local example, are better feed converters than sheep in Scottish hill country.

An animal which deserves particularly serious consideration for meat production, especially in the wet tropics, is the water buffalo. It thrives well in conditions which the normal domestic meat animals find intolerable--yet it yields meat which is less fat and more tender than beef. Already the world population of water buffalo is about one-tenth that of cattle and, in the Amazon region, for example, numbers are increasing at the rate of 10~o per year.

The Saiga antelope and the eland are among wild species which yield excellent meat. They can be successfully reared under controlled conditions, and since their diet is complementary to, rather than competitive with, that of other animals, mixed herds can be successfully maintained in a given area.

One of the aspects of the so-called game animals which hitherto limited their general popularity as meat was their 'gamey' flavour. There is some reason to believe, however, that this is mainly due to the means by which they have been killed, i.e. by shooting in open country, when bleeding is much less effective than


under organised abattoir conditions. When the latter have been available, the flesh of such game animals resembles that of the domestic meat species.

PRESLAUGHTER AND ABATTOIR FACTORS DETERMINING MEAT QUALITY

It is only relatively recently that the importance of stress in determining meat quality has been appreciated.

Variable degrees of physiological stress--fasting, fatigue, fear, fighting, expo- suremarise in transporting animals to the point of slaughter. Severity of stress is influenced by distance and duration of journey, mode of transport, loading density, climatic conditions, effectiveness of rest after travel and inherent stress susceptibility.

Losses due to transport stress represent a significant wastage and are reported to have increased markedly in recent years. This may reflect larger scale operations and centralisation of slaughtering facilities (involving fighting between animals of differing origin, more complex or longer transport chain). A high proportion of the stress sustained arises at loading and unloading.

Manifestations of stress include weight loss, bruising, suffocation, bacteraemia, premortem depletion of muscle glycogen, accelerated postmortem breakdown of glycogen and faulty bleeding. It is important to note that the rate and the extent of postmortem conversion of muscle glycogen to lactic acid (i.e. postmortem glycolysis) are major determinants of meat quality.

Fatigue and/or fasting normally fail to reduce glycogen sufficiently premortem to foster the 'dark-cutting' character in cattle and sheep. Tension and tremor due to excitability and fear (entire males, rough handling), and shivering (sudden exposure of pampered stock to temperature drops of 15°C, and 'snow stress" in sheep), do so more readily. Restoration of normal glycogen reserves in muscle may take 3 to 4 days.

Muscle glycogen reserves in pigs are lower than in cattle or sheep, and more readily lowered further by even moderate fasting and/or fatigue before slaughter (cf. 'glazy' bacon). Restoration of glycogen levels requires feeding, as ~ell as rest. Stress susceptibility can also cause acceleration of postmortem glycolysis in pigs (leading to PSE). This appears to be inherent, being more prevalent in some breeds (e.g. Pi6train). High load density, air temperature greater than 10~C, faulty ventilation (CO2 accumulation) and fighting lead to enhanced stress in transport. Overcrowding and prolonged holding of animals enhances general stress, fighting (and bruising) and the spread of infection from diseased animals and healthy carriers (e.g. with Salmonella spp.). A closer matching of rate of arrival and rate of slaughter is thus beneficial. If animals are fatigued or starved they are more easily infected by extraneous organisms: and more susceptible to parasitic and bacterial invasion from their own digestive tract. Yet feeding can cause bacteraemia; if intermittent, it can increase infection from exogenous sources. There are conflicting views

MEAT: CURRENT DEVELOPMENTS AND FUTURE STATUS 5

on the duration of holding time which is optimal for restoring the condition of already stressed animals, on the one hand, and for avoiding the development of stress and endogenous or exogenous infection of the tissues, on the other.

Research to establish, firmly, for each species, the preslaughter nutritional status and period of holding required to minimise stress, and microbial and parasitic infection, is thus currently being undertaken.

Delay in bleeding causes bacteraemia even in healthy animals. Residual blood promotes spoilage, 'gamey' flavours and discoloration. Its level is increased by stress. It is noteworthy, however, that the majority of carcase condemnations on inspection after dressing are due to non-microbial factors, e.g. emaciation and bruising.

All abattoir operations require the strictest application of hygiene: the avoidance of extraneous contamination is a dominant factor in prolonging useful storage life in meat. This is facilitated, whether operations are mechanised or not, by avoiding contact between the carcase and the abattoir environment using a suspension system.

Curtailment of waste of meat requires better communication of known causes (and of means of their avoidance) to those handling animals. Humane handling at all stages to moment of slaughter is essential for both microbial and eating quality.

INTRINSIC FACTORS INFLUENCING MEAT QUALITY

Muscle is an engine for converting chemical into mechanical energy and, like other engines, its modes of operation--and hence its components and Lhe type of fuel it uses--differ somewhat according to the particular kind of movement to be effected. Because the engine is edible these differences are reflected by variation in the eating quality and, nutritive value of the meat it becomes.

Such intrinsic factors as species, breed, sex, age and the anatomical location of muscles have long been recognised as having an effect on the composition of the meat which muscles become during postmortem glycolysis. Many examples of their influence could be given. Thus, the pale meat of pork and rabbit contrasts with the dark meat of cattle, sheep and whales; the meat of zebu cattle is coarse compared with that of Aberdeen Angus; as has been mentioned already, the meat of males is usually much leaner than that of females; veal is more tender than old beef and fillet steak is less tough--as well as less tasty--than chuck steak.

Even differences between the muscles of adult animals (of a given species, breed, age and sex) are many. They include the proximate composition--water (73-79 ~), intramuscular fat (0.1-17 ~) and total nitrogen (3-3-3.8 ~). But there are also more subtle differences. These include the component fatty acids of the fat (especially the ratio of unsaturated to saturated fatty acids), the amounts and types of protein (within each of the three main groups, sarcoplasmic, myofibrillar and connective


tissue), the amount and type of carbohydrate (mainly glycogen), the quantity of energy-rich phosphate stored and numerous minor chemicals.

It is becoming clear that the intrinsic nature of meat is substantially determined by the type of muscle of which it is the postmortem aspect; that the types of muscle reflect the kinds of contractile activity which each muscle has been specially developed (in the course of evolution) to carry out; and that the principal differen- tiating factor is the relative extent to which the muscles are required to perform slow, prolonged aerobic contraction or fast, intermittent, largely anaerobic contraction. The former may be broadly classified as so-called 'red' muscles: the latter as so-called 'white' muscles. There is, of course, a considerable range of intermediate types.

From the biochemical point of view 'red' and 'white' muscles can be differentiated by a large number of criteria. Among these are the concentration of the muscle pigment, myoglobin, which is obviously present at markedly higher concentration in 'red' muscles. The latter also have a much greater capacity to undertake aerobic metabolism, this being reflected, for example, by their containing a large number of particles, mitochondria, which link the uptake of oxygen to the synthesis of high energy compounds. There are few mitochondria in 'white' muscles. On the other hand, 'white' muscles have a greater s to re of high energy phosphates and of the carbohydrate, glycogen, whose conversion to lactic acid under anaerobic conditions permits these muscles to operate in short bursts, followed by restorative rest periods. Since, as has already been mentioned, glycogen is also converted to lactic acid under the anaerobic conditions prevailing postmortem, its relatively high concentration in 'white' muscles means that the final pH of the latter is usually lower than that attained by 'red' muscles.

Although much has yet to be learned of the significance of these differences between 'red' and 'white' muscles, it has become apparent that they are not merely academic or biochemical curiosities. They have a direct bearing on meat quality-- especially in an age when prepackaged cuts for individual consumers are increasingly used as the medium of sale. In such circumstances, portions of individual muscles are sold and their intrinsic biochemical propensities frequently become manifested by differences in colour, water-holding capacity, tenderness and flavour.

SOME RECENTLY APPRECIATED PRACTICAL IMPLICATIONS OF

MUSCLE DIFFERENTIATION

About 15 years ago complaints about the toughness of New Zealand lamb became prevalent. New Zealand lamb is a highly standardised commodity and nothing in the breeding or feeding policies had been altered which could have accounted for this trouble. As far as abattoir handling was concerned, procedures had been improved, if anything. After dressing, lamb carcases were now being placed, whilst


slaughter-warm, into blast freezers--the capacity of xvhich was so effective that no preliminary period of carcase chilling was any longer required. This development obviously saved abattoir space and hence was economical. Most unexpectedly, however, it proved to be the reason for the increased toughness in the lambs.

The elucidation of the phenomenon opened up a new chapter in meat science. It was shown that the increased toughness xvas due to the muscle shortening during postmortem glycolysis and the onset of rigor mortis and that the shortening could be related to an acceleration of this process. It had long been known, of course, that when muscles go into rigor mortis at body temperature they tend to shorten and toughen (and to exude fluid), but this 'cold shortening'/toughening behaviour could not have been anticipated. It was explained by the fact that, when the temperature of muscles can be lowered to less than about 10°C whilst their pH is still about 6.7-6-8 (by effective refrigeration engineering as in the New Zealand case), they are still physiologically reactive. This is manifested by a discharge of calcium ions, in response to the cold, from the sarcotubular system--the network of delicate tubes surrounding each muscle fibre, whereby nerve stimuli quickly exert chemical control of muscular contraction. The calcium ions stimulate the enzyme--myofibrillar ATP-ase--which makes available energy by hydrolysing ATP for the contractile process.

'Cold shortening' occurs only when muscles are free to shorten ; but a significant number of the muscles on the carcase can do so, despite their attachments thereon; and those which are relatively near the surface are liable to be cooled sufficiently during efficient refrigeration of hot carcases (especially small carcases such as those of lamb) to do so. Carcase suspension by the aitch bone, rather than traditionally by the Achilles tendon, changes the pattern of tension on the muscles of the carcase and, in general, prevents 'cold shortening" (and toughening) in some of the commercially important muscles which are normally adversely affected in this way.

One curious feature of 'cold shortening', and the toughening it causes, which has now become apparent is the fact that only so-called 'red" muscles are susceptible to it. Two biochemical explanations have been given for this. First, in 'red' muscles the sarcotubular system is less highly developed than in 'white' muscles. This reflects the fact that 'red' muscles, as already indicated, function slowly over prolonged periods. They do not require a speedy control over their contractile machinery and the sarcotubular system is consequently relatively underdeveloped. This means, in practice, that calcium ions discharged from the sarcotubular system by cold shock are less readily reabsorbed by the latter; an increased calcium concentration thus remains to stimulate the muscle to contract, i.e. the 'red' muscle 'cold shortens'.

The second explanation of the susceptibility of 'red' muscles to this behaviour also adduces the biochemical differences from the 'white' type. According to this view, the calcium ions which cause the contraction in 'cold shortening' are discharged from the mitochondria under the anaerobic conditions arising postmortem.


Such mitochondria, as we have seen, are much more prevalent in 'red' muscles and hence the calcium concentration available to stimulate the contractile machinery is also much, higher than in 'white' muscles, leading to shortening and toughening.

Of course the whole problem of 'cold shortening" can obviously be avoided by not holding the hot carcase below 10°C until the pH of the musculature has fallen below 6-7--or at least by not allowing the 'red" muscles to attain this temperature at this pH. This procedure, however, would clearly obviate the operational advantages which the refrigeration of the hot carcases seeks to achieve. A way of retaining these advantages without 'cold shortening' has been devised very recently by the New Zealand meat industry. This involves applying high voltage electrical shocks to the muscles of the warm carcase immediately after slaughter. In this way the process of postmortem glycolysis is greatly accelerated so that the pH is quickly brought below the point at which muscles will undergo cold shock, thus permitting more or less immediate refrigeration of the still hot carcase without 'cold shortening'.

The 'cold shortening' phenomenon basically involves an acceleration of postmortem glycolysis and of ATP breakdown. A related phenomenon which, although much longer known, has also become important because of improved refrigeration facilities is 'thaw-rigor'. This arises when postmortem glycolysis is completely arrested by rapid freezing, whereby the ATP is fixed at the level it had before the onset of rigor mortis. The capacity of the sarcotubular system to remove calcium ions is substantially destroyed by the freezing process. This fact, together with the high ATP levels, causes an exceedingly rapid postmortem glycolysis when thawing permits this biochemical process to resume. There is severe contraction (associated with extreme toughness of the meat) and massive exudation--more than 60~ of the initial weight of the meat may be lost as fluid. These undesirable features of 'thaw-rigor' can be avoided if the prerigor-frozen meat is kept just below the freezing point for a few days. This causes the system to become sufficiently mobile to permit the stored ATP to be broken down; yet it has still sufficient rigidity (because of the remaining ice) to resist shortening and toughening. Such a holding procedure before thawing prerigor-frozen meat, however, would be uneconomic and it has recently been shown that, provided such meat is held for at least three weeks at -10°C (a not uncommon temperature in commercial frozen stores) the high, prerigor ATP concentration will gradually be broken down (in the minute, still frozen, microenvironments); and thus the prerequisites for 'thaw-rigor" will be removed.

In contrast to their behaviour in 'cold shortening', "red' muscles tend to be less susceptible to the phenomenon of 'thaw-rigor'--despite the basic cause of the two phenomena being the same, i.e. an excess of calcium ions stimulating the major contractile ATP-splitting enzyme of the muscle. Nevertheless, this again accords with the biochemical differences between them. Whereas the greater capacity to reabsorb calcium ions of the sarcotubular system of 'white" muscles dominates in 'cold shortening', freezing annuls this advantage, presumably by disrupting the


sarcotubular system. The situation is then dominated by the fact that the intrinsic enzymic capacity of 'white' muscles is much greater than that of 'red'; and whereas in 'thaw-rigor" there is a greatly accelerated postmortem glycolysis in both 'red' and "white" muscles, it is more severe in the latter, and leads to greater shortening.

A third feature of meat quality can now be related to muscle differentiation. It has long been known that when meat is held above the freezing point and in the absence of microbial spoilage for a week or so it becomes more tender. This process is referred to as conditioning or ageing. There has long been controversy as to the exact mechanism involved; and this controversy continues. However, one significant feature of ageing, demonstrated 10 years ago, was a weakening of the main contractile proteins of the muscle. This has now been attributed to the action of an enzyme which is activated by calcium ions. This enzyme is present in 'white' muscle at about three times the concentration it has in 'red" and this can explain the observation that 'red' muscles tend to undergo the tenderising changes of ageing to a much lesser extent than do 'white' muscles.

The differing contribution of the myofibrillar proteins to toughness in 'red' and 'white' muscles is also believed to be paralleled by subtle differences in the nature of their connective tissue proteins; but this aspect has still to be fully elucidated.

Exudation has been referred to already as one of the undesirable features of meat in 'thaw-rigor'; but, of course, even meat frozen after the onset of rigor mortis--the more usual circumstance--exudes much fluid or 'drip' on thawing. Indeed, fresh unfrozen meat exudes fluid on standing for a few hours postmortem. This is partly a reflection of the fall of pH postmortem, whereby the muscle proteins attain their isoelectric point, and partly of other changes in the proteins which also lower their water-holding capacity. The latter include the onset of rigor mortis at body temperature--a circumstance which tends to occur, for example, in the deeper muscles of beef hindquarters. These are difficult to cool quickly, because of their bulk. Like most biochemical reactions, postmortem glycolysis proceeds faster at 37°C than at ambient temperature. As a result, the pH falls to acid levels whilst the muscle temperature is still high. The combination of body temperature and low pH denatures the proteins, causing an enhanced degree of fluid loss. More effective cooling slows postmortem glycolysis and diminishes fluid loss. To achieve this with bulky masses of meat without freezing the exterior is virtually impossible, but by separating the individual muscles along their connective tissue fascia--a process referred to as 'seaming out ' - -a much faster rate of temperature fall can be achieved even without ultra-modern refrigeration facilities. Postmortem glycolysis is slowed, exudation and high temperature contraction avoided and commercially important savings in weight and quality achieved, (It should be pointed out that the enhanced rate of cooling of the muscles in such circumstances is still insufficient to cause 'cold shortening" because of the masses of meat involved in beef hindquarters.) As with the other parameters we have been discussing, different muscles show more response to the 'seaming out' procedure than others. To some extent this reflects


their respective capabilities of being cooled swiftly; but it also signifies the fact that the proteins of different muscles have subtly different water-holding capacities and suggests that, to minimise exudation economically, the +seaming out' procedure needs to be applied only to certain muscles.

In general, the lipids---especially the phospholipids--of 'red' muscles are more highly unsaturated than those of the 'white" variety; this would lead one to expect both a greater susceptibility to oxidative rancidity, and a concomitantly high degree of discoloration, on storage. In fact, it has been found that 'red' muscles resist such changes to a markedly greater degree than 'white" during frozen storage at - 10°C and this has been shown to be due to the fact that the final pH attained in 'red' muscles is usually higher than that in 'white'. The higher pH protects the protein of the muscle pigment, myoglobin, from denaturation, thus lowering its tendency to oxidise and, at the same time, diminishing the tendency of the fat to oxidise, since the oxidised form of myoglobin is a pro-oxidant of lipids. This circumstance overrides the inherently greater liability of the 'red" muscles to deteriorate because of their more unsaturated lipids. By the preslaughter injection of adrenalin, muscle glycogen reserves can be deliberately lowered, leading to curtailed production of lactic acid postmortem and a high final pH--whereby not only fat rancidity and discoloration, but also exudation, can be lessened to a commercially useful degree.

PRESERVATION TRENDS

It is desirable to consider, briefly, the means likely to be employed to preserve meat in the future. Frequent reference has been made to freezing in the present discus- sion, and there are a number of reasons why it seems likely to be the preferred mode of preservation--at least in developed countries--in the foreseeable future. When properly applied it permits the retention of much of the character of the raw meat and of its nutritive value--and it is most effective. Provided desiccation is avoided-- and this can be achieved by a sufficiently low temperature of storage, suitable impermeable packaging and by other manipulations such as that of pH already mentioned--frozen storage permits more or less indefinite preservation. There is evidence that the flesh of horses in Alaska, of mammoths in Siberia and of fish in Antarctica has been preserved for 50,000, 20,000 and 1200 years, respectively. Such developments as home freezers, intervention buying and storage of meat in inter- national trade and the frozen pack for institutional catering illustrate the increasing usefulness of freezing for meat preservation.

Freeze-drying of the commodity, although an advance on the older dehydration procedures, remains costly and detracts from the original eating quality to some extent--albeit that it yields a product of light weight which can be stored without refrigeration, provided it is suitably packaged, lonising radiation, in doses sufficient to sterilise meat within impermeable packs, permits years of storage without

MEAT; CURRENT DEVELOPMENTS AND FUTURE STATUS l 1

refrigeration, but such doses cause marked alterations in the eating quality--thus ionising radiation seems only likely to be applied to meat in low doses to prolong the storage life of the chilled commodity.

Canning remains an excellent method of long-term storage of meat--being effective for upwards of 150 years; but it involves cooking the commodity and requires careful control of the heat sterilising process. By canning in rooms at elevated pressure, processing conditions can be effective microbiologically yet cause less damage to the product's eating quality. The use of flexible, retortable packs represents another trend in the same general direction.

Curing has retained its importance for thousands of years as a means of preserving meat products. The high concentrations of salt, together with small quantities of nitrite (which were originally adventitious contaminants of the former), ensure microbial safety. Of course, as with canning, the original state of the meat is considerably altered and the eating quality changed--in this case being valued for its own sake. Recently the tendency has been to use less salt because modern palates are less ready to accept unduly salt products. This has increased the possibility of microbial growth and spoilage--of bacon, for example, especially as the prevalent vacuum-packaging of this product has encouraged domestic mishandling. The development has been concurrent with an appreciation that nitrite--which appears to be necessary for the flavour ofcured meats and for the inhibition of C/. botulinum, even if their colour can be produced by nitric oxide--is able to convert secondary and tertiary amines into carcinogenic nitrosamines. Even although the conditions required are unlikely to arise with cured meat products to any meaningful extent, and indeed nitrites are naturally occurring substances to which mankind has been exposed throughout evolution, some thought is currently being given to the possibility of curing meat without nitrite.

An alternative to a high salt concentration for the inhibition of microbial growth in meat products is to lower the water activity by less unpalatable means. By cooking small cubes of meat in glycerol, an organoleptically acceptable product is obtained which will withstand 3-6 months' storage at 37°C. Requiring no refrigerated storage facilities, such 'intermediate moisture' meats have an application in tropical countries or in other areas where the infrastructure for refrigeration is lacking. So far the process has not been applied in developed countries--except for petfoods, or for military or expeditionary purposes. Its potential is receiving much attention.

NUTRITIVE ASPECTS

It is desirable to allude to the nutritive aspects of meat. Although its eating quality is naturally of importance, the basic purpose of consumption is to acquire the nutrients which the body needs for building and repairing its tissues, and for energy.


Meat is an excellent source of the eight amino acids essential for these purposes-- and which our bodies cannot synthesise. Moreover--and this is a matter of almost equal importance--the relative proportion of these amino acids in meat closely accords with that required by man. The cannibalistic habits of our ancestors--and of human beings in certain remote areas at the present time--are sound, if un- aesthetic.

There is growing support for the view that meat is also a source of certain fatty acids which are essential for the development of nervous tissue in the young. These a r e absent from plant foods. This point should be considered simultaneously with the popular view that the relatively saturated fats of meat animals predispose to cardiovascular diseases. This may be true, but one is always deeply impressed by the fact that many healthy centenarians have been consuming meat (and nitrosamines) for much longer than the majority of mankind--apparently with impunity, If there were some reason to wish for softer fats, the means of producing these, even in the meat of ruminants, is available. Thus, if the feed is pretreated with formalde- hyde, the hydrogenation of the double bonds (originally present in ingested tat) by rumen micro-organisms is avoided--and cattle and sheep can then lay down relatively unsaturated fat (although it is more liable to undergo oxidative rancidity after only a short period of storage).

Notwithstanding the desirable organoleptic and nutritive attributes of meat, it is increasingly likely that we shall obtain a substantial proportion of our amino acid requirements from plant proteins in the future--and possibly even from microbial or single-cell proteins. Vegetables, and especially legumes, have obvious quantita- tive advantages over meat in producing protein. Moreover, although there is a tendency for plant proteins to be somewhat deficient in lysine and methionine (or other essential amino acids), such deficiencies can be overcome by using mutually supplementary mixtures of proteins. In any case, some plant proteins, such as soya, have a profile of essential amino acids similar to that of meat. Nevertheless, it should be remembered that, as they naturally occur, the proteins of plants are frequently present in low concentrations and thus their utilisation involves the consumption of an unpalatably bulky food. They are often associated with toxins (either indigenous or derived from contaminating moulds or fungi); and they are almost completely devoid of associated vitamin B, 2--which is prevalent in foods of animal origin. Moreover, such little iron as they may have is much less readily absorbed than the iron of meat. These difficulties, of course, are ~vell recognised and the chemical industry is increasingly involved in preparing protein concentrates from such sources as soya, wheat gluten and field beans, which are free of toxins. Moreover, such concentrates of plant protein are being spun into fibres to form meat-like steaks or extruded to form bite-size pieces.

Increasing numbers of 'meats' simulated from vegetable protein are marketed. Of course, the meat industry can do likewise. Current research is concerned with the fabrication of steaks from meat industry waste such as blood, lungs and


stomachs. These contain good quality protein--indeed better than that of most plant proteins--but in an initially unattractise form. After extraction, they can be fabricated into nutritive 'steaks'--in this way both improving the utilisation of meat animals and serving to limit environmental pollution.

Again, although not yet used in any quantity for human food, the proteins synthesised by micro-organisms by fermentation of petroleum, methane gas and other materials, which are completely inedible per se, can be produced with a hundred-fold greater efficiency than those of animals. Although the need to concen- trate the proteins does not arise in this case, and the content of essential amino acids is frequently as good as that of meat itself, the synthesis of unusual amino acids, high concentrations of nucleic acids, frank toxins or (which is quite feasible, through mutations, in such actively dividing cells) the possible production of carcinogens, and the great expense of constructing fermentation plant and in ensuring that it remains functional, offset the apparent advantages of unicellular protein production.

It should be mentioned that the development of plant (or microbial) proteins as cheap substitutes for meat could lead to abuse, if agreed limits to the degree of substitution were deliberately exceeded. Consequently, efforts to find an unequivocal index of meat protein, which would withstand even severe processing, have been made. A promising substance is 3-methylhistidine, an amino acid which, although absent from the plant and microbial proteins so far examined, is present in lean meat as an integral part of the characteristic contractile proteins of this tissue--and apparently at an invariant concentration.

CONCLUSION

In conclusion, it should be reiterated that only meat animals can convert the rough herbage of the majority of the world's usable land surface--which cannot sustain crops or is too remote for the complex infrastructure required for microbial protein synthesis--into a desirable and highly nutritious commodity. However inefficient, meat animals have no foreseeable alternative in utilising the vast areas concerned; and thus in alleviating the world food shortage. However costly, as much meat as can be afforded will continue to be bought--and enJoyed--even if less frequently and more as a luxury item than as a staple foodstuff, and whether or not the major source of man's protein becomes the plant kingdom.

R. A. LAWRIE

meat: current developments and future status

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