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There are two major aspects of meat quality, nutritional quality which is objective and "eating" quality as perceived by the consumer - flavour, juiciness, tenderness and colour - which is highly subjective.
There are considerable differences between the preferences of individuals including preferences for different cuts of meat, lean or fatty, muscle or organ meats, methods of cooking, etc.
In the industrialised countries the demand for what is perceived as eating quality and also the demand for particular qualities for a range of products from the meat processing industry dictate the breed, feed and management of the animals with intensive rearing and specially formulated dietary supplements and a tendency to slaughter earlier.
On the other hand the demand in most developing regions of the world is for more animal products of almost any kind. The animals live under variable conditions often of rough grazing and grow more slowly, yielding older animals for slaughter; when animals are primarily used for draught they are very old at the time of slaughter. Old animals yield meat that is less juicy and of a quality that differs considerably from that demanded in the industrialised countries.
Post-Mortem Changes
The post-mortem changes that take place when muscle is converted into meat have a marked effect on the quality of the meat.
After slaughter the glycogen in the muscle is converted into lactic acid causing a fall in pH from an initial value of pH 6.8 - 7.3 to about 5.4 - 5.8 at rigor mortis. If animals are stressed immediately prior to slaughter as when they are roughly handled or fight one another the muscle glycogen is released into the blood stream and, after slaughter, is rapidly broken down to lactic acid while the carcass is still warm. This high level of acidity causes a partial breakdown of muscle structure which results in pale, soft and exudative meat (termed PSE) - a condition mostly occurring in pigs. The meat losses some of its water-binding capacity which is so important in certain types of meat processing.
Long-term stress before slaughter or starvation uses up the glycogen so that less lactic acid is formed after slaughter resulting in an abnormal muscle condition in which it remains dark purplish-red on exposure to air instead of a bright red colour. This is termed dark, firm and dry (DFD) in the case of pigs and "dark cutting" in beef. The condition is rarer in lambs. Such meat and products made with it have a pH above 6.0 and spoil quickly since the low acidity favours rapid bacterial growth.
PSE and DFD meat are perfectly safe to eat but limited in their processing capacity. PSE meat has higher drip and cooking losses due to the reduced water-binding capacity (WBC). As well as the pale colour the meat has less flavour than usual.
DFD meat has normal or increased WBC and so is suitable for scalded/boiled sausages and other cooked products but it has poor meat flavour. While there is no remedy for these defects in the meat, DFD and PSE meats can be blended with normal meat for the preparation of products of good quality.
After slaughter as the glycogen in the tissues is exhausted rigor mortis sets in and the whole carcass become stiff. This is due to the contraction of the muscle fibres when the actin filaments of the muscle fibres slide inwards between the myosin filaments so shortening the myofibrils.
If the meat is cooked when the muscles are still in rigor it is extremely tough. This condition is prevented by "aging" or "ripening" after slaughter which is achieved by storing the meat until the muscles gradually recover their extensibility and become more tender through partial enzymatic breakdown of the muscles fibres. At this stage rigor mortis is said to be resolved.
Rigor is completed in cattle after 12-24 hours and is resolved by periods that depend on the temperature:- 10-13 days at 0°C, 4-5 days at 10°C, 30-40 hours at 20°C and 10-11 hours at 30°C The process is twice as fast in pork as beef or lamb: it is faster in young animals and slower in "red muscles. that function slowly and continuously in the living animal. "Aging" also leads to improvement of flavour.
Obviously if meat has to be sold within a few hours of slaughter it is still in pre-rigor or rigor, and the tough meat has to be cooked longer with some loss of nutrients.
If lamb, and to a lesser extent beef, are chilled too rapidly after slaughter the muscles may undergo extreme contraction or "cold shortening" which results in very tough meat when cooked. Pork is almost unaffected in this way. Cold shortening does not take place when the carcass is cooled more slowly - the temperature must not fall below 10°C before the onset of the rigor. To achieve this the carcass is kept at ambient temperature for some hours to accelerate rigor and then rapidly chilled or frozen - a process called "conditioning".
Old animals, especially old draught animals, have a high content of tough connective tissue in the muscle and prolonged cooking at a low temperature is needed to soften the meat by hydrolysis of the connective tissue - a fact not always known to consumers.
So it is clear that in many areas conditions militate against good quality meat long transport of animals and poor lairage facilities prior to slaughter reduce the glycogen in the muscles , poor hygiene, high ambient temperature and lack of refrigeration during and after slaughter lead to heavy contamination and growth of microorganisms and considerable losses from spoilage together with dangers of food poisoning. All this can be aggravated by inadequate care of the meat during transport and in the market.
Obviously there is room for improvement in conditions of meat production even for purely local consumption to reduce losses and improve efficiency but if shipment of meat to distant parts is to be considered then it is essential to adopt the sophisticated techniques and methods of refrigeration that are now expected in national and international trade.
Control of Hygiene and Safety
The safety of meat calls for control throughout the food chain from the farm of origin, and inspection before and after slaughter, to the handling and storage of meat and the products until the time it is consumed. The responsibility for the production of safe and wholesome meat is shared by the industry and the controlling authority. This requires a controlling authority that is adequately resourced and has the legal power to enforce the requirements and which should be independent of the management of the establishment where the meat is produced.
The Codex Alimentarius Commission has elaborated (besides meat inpection Codes) the Recommended International Code of Hygiene Practice for Fresh Meat (CAC/RCP 111976) and the Recommended International Code of Hygienic Practice for Poultry Processing (CAC/RCP 14-1976) which describe the minimum requirements of hygiene for meat and poultry production.
The application of these Codes can be an important step towards the targets:
a) that the food will not cause infection or intoxication when properly prepared;
b) does not contain residues (of pesticides, veterinary drugs and heavy metals) in excess of established limits;
c) is free from disease;
d) free from obvious contamination;
e) free from defects generally recognised as objectionable;
f) has been produced under adequate hygienic control;
g) fulfils the expectation of the consumer in regard to composition.
The Codex Alimenarius Commission Guidelines include advice on the construction of abattoirs and the facilities required; control of pests, quality of water for cleaning and disinfection; rules of meat inspection and hygienic practices (including supervision by a veterinary inspector).
The Code of Hygienic Practice for Fresh Meat is currently under revision to include a more systematic approach to sanitation and process control, namely Hazard Analysis Critical Control Points (see next section).
The growing demand for meat, both per capita and due to growing populations, will increase pressure on slaughter-houses. There is obviously a vast gap between the state of the art in industrialized communities and traditional method in some of the more remote areas of the Third World. Custom-built abattoirs which allow separation of the various stages of the process to prevent cross- contamination and sophisticated techniques of quality control are far removed from slaughter under conditions where energy for refrigeration and adequate supplies of hot (potable-quality) water for cleaning purposes are not available.
Such desirable facilities might be made available in densely populated areas where a regular throughput justifies capital expenditure but it is obvious that these standards must be regarded as long-term objectives in remote areas where slaughter and meat production follow tradition rather than scientific principles.
Indeed, the guidelines concede that traditional practices may permit departure from some of the requirements laid down when fresh meat is produced for local trade.
The Codex Commission recognised that small or relatively isolated abattoirs did not warrant the full-time presence of a veterinary inspector if a veterinary assistant is available for meat inspection. But it was also recommended that meat hygiene in general should be under the control of a veterinary inspector.
As regards the Code of Practice for small manufacturers the Commission suggested that this should be left to the discretion of the authorities in each country.
Hazard Analysis Critical Control Points (HACCP)
As is emphasized elsewhere in this publication an important priority in meat production is to minimise contamination with enteropathogenic organisms during slaughter, dressing and subsequent handling of meat.
A recent development to achieve and ensure safety in food production in general is a systematic approach based on an assessment of the various risks associated with each step of the process. The object is to identify the relative seriousness of risk - Hazard Analysis Critical Control Points.
Critical Control Points are any procedures or locations where control can be exercised over factors that, if controlled, prevent or minimise hazard.
With regard to meat production the HACCP concept systematically identifies potential hazards in the entire chain from animal production to consumption and ranks them according to severity and likely frequency. This covers facilities, equipment and operation and is intended to augment and refine the various codes of manufacturing practice undertaken by industry.
The procedure is intended to enable management to take preventive measures rather than depend on intensive testing of the end-products.
It is obvious if livestock production is to be expanded to meet demand and to export that financial arrangements, the use of on-farm resources, production in-puts, veterinary and health services, marketing facilities and resources for research will all need to be improved and expanded. Programmes will need to be suited to local conditions. So far as ruminants are concerned large-scale expansion depends on the availability of land.
In contrast with the developed world much of the meat production in the developing countries is in the hands of small-holders with herds of 2-5 animals, with low productivity and it is improvement in their productivity that will help to fulfil most of the demand rather than the development of large-scale production. It is the opportunistic strategy of resourcepoor farmers to give the animals better feed only when at good milk-yielding age or to increase their strength as draught animals at certain periods of the year. It is then that they tend to compete more directly for food with human beings. Increasing the productivity of each animal is the most efficient way of producing more meat products - doubling the yield (of milk or meat) per animal requires less feed than doubling the number of animals.
A large number of technical policies must be defined in order to establish appropriate livestock development programmes. These involve subjects such as breeding, pasture development, use of non-conventional animals, veterinary programmes, improved farming systems, transport and economic policies including financial production incentives and marketing structure.
Meat, and other animal foods such as milk, can make a valuable contribution to the diets in developing countries. It has less nutritional importance in industrialised countries where a wide variety of foods of all kinds is available.
Many diets in developing countries are based on cereals or root crops and are relatively bulky, especially where fats are in short supply, and this can limit the total energy intake. This is especially true of infants after weaning and young children.
The importance of meat in the diet is as a concentrated source of protein which is not only of high biological value but its amino acid composition complements that of cereal and other vegetable proteins. It is also a good source of iron and zinc and several B vitamins, and liver is a very rich source of vitamin A.
The animal carcass consists of muscle, connective tissue, fat and bone and some 75% water in proportions depending on species, breed, size, age, etc. The muscle (lean meat) is relatively constant in composition in a given species (Table 2-1). The greatest variable in the carcass is the amount of fat, which can range from 2% in some free-living animals to 1540% in domesticated animals intensively reared.
Since the major source of variation in meat composition is the proportion of lean to fat it is useful to consider the composition of lean and fat separately and then to calculate the nutrient composition of the product from the proportions of the two. (Table 2-2).
With the variations between individual animals of the same species under the same management, together with different managements and errors arising from differing sampling and analytical techniques, there are many and considerable differences in the composition of meat as reported in the scientific literature; table 2-3 provides reasonable average or typical values (see also Table 2-12).
It will be noted that the lean meat of various species has similar values for macronutrients and inorganic constituents. The same is true of the vitamins with the exception of pork which has very high levels of thiamin.
Although lean meat has a high water content, about 75%, it is a good source of protein - 20% on a wet-weight basis compared with 8-12% in cereals.
Influence of Diet
The limited effect of diet on the nutrient composition of lean meat is illustrated in a trial in which the composition of intensively-reared beef (fed barley and protein supplements with grazing ad lib) was compared with extensively-reared (grazing alone) as two extremes of husbandry practice (Harries et al 1968). Analysis of the same muscles from animals on the two systems showed no significant differences in their content of protein, fat, iron, thiamin, riboflavin and niacin. There were greater differences between animals fed on the same system on different farms than between different feeding systems, showing that management practices had the greater effect.
With the exception of vitamin A stored in the liver, diet has little effect on vitamin content, but it has been shown that the addition of thiamin to the diet of pigs can double or treble the amount of thiamin in various muscles (Pence 1945).
Influence of Age at Slaughter
As animals grow the proportions of total nitrogen and fat, and also the amounts of iron, increase as the animals approach maturity, and more slowly after that.
At the same time the ratio of polyunsaturated to saturated fatty acids (P/S ratio - see Chapter 3) falls. Collagen (connective tissue) becomes less soluble and less digestible so animals that are poorly fed and so take several years to reach a useful size, provide meat of lower eating quality. Animals killed after a lifetime of work provide even tougher meat. Older animals have a high proportion of water-soluble extractives in the muscle and animals reared on poor pasture, and which are therefore relatively old by the time they reach a size suitable for slaughter, have long been used for the preparation of meat extract.
Protein
The protein of typical mammalian muscle after rigor mortis but before post-mortem degradative changes contains about 19% protein: 11.5% is structural protein - actin and myosin (myofibrillar), 5.5% soluble sarcoplasmic protein in the muscle juice, 2% connective tissue (collagen and elastin) encasing the structural protein and about 2.5% fat dispersed among the protein fibres (Table 2-1).
Myoglobin is present in relatively large quantities in heart muscle because of heavy oxygen demand: (the highest amount of myoglobin in mammals is found in the whale to permit prolonged submersion under water).
Collagen differs from most other proteins in containing the amino acids, hydroxylysine and hydroxyproline and no cysteine or tryptophan. Elastin, also present in connective tissue, has less hydroxylysine and hydroxyproline. Hence cuts of meat that are richer in connective tissue have lower protein quality (see Chapter 3). Their content of connective tissue makes them tough and in many regions these cuts of meat bring a lower price. The amino acid composition is given in table 24.
Immediately after rigor mortis there is almost 2.5% carbohydrate present - lactic acid, glucose and derivatives.
As distinct from the average of the whole animal carcass the composition of meat as cut for consumption shows some variation depending on the cut (table 2-5). The types of cut often differ in different regions.
In addition there will be further differences in the amount of water and fat lost in different methods of cooking.
Lipids
Lipids (fats) are found at three sites in the body.
1) The largest amount by far is in the storage deposits under the skin and around the organs. This constitutes the obvious, visible fat in a piece of meat and can be as much as 4050% of the total weight in fatty meat or fatty bacon. This adipose tissue is composed largely of triglycerides contained in proteinaceous cells with relative little water.
Clearly this visible fat can be trimmed off the meat during processing, before cooking or at the table - a growing practice in the western world. (See Chap. 3).
2) Small streaks of fat are visible between the bundles of muscle fibres, intermuscular fat, i.e. in the lean part of the meat; this is known as "marbling" and can amount to 4-8% of the weight of lean meat.
3) There are small amounts of fat within the muscle structure - intra muscular or structural fats - in amounts varying with the tissue. This can be 1-3% of the wet weight of muscle nd 5-7% of the weight of the liver.
Structural fats are largely phospholipids and include long chain fatty acids. Fatty acids are of three types. (1) Saturated fatty acids in which all the carbon atoms in the chain carry their full quota of two hydrogen atoms and the carbons are linked by a single bond; (2) mono-unsaturated fatty acids (MUFA) in which one hydrogen is missing from each of two adjacent carbon atoms which are therefore linked by a double bond; and (3) polyunsaturated (PUFA) in which two or more pairs of hydrogen atoms are missing and there are several double bonds in the carbon chain (Table 2-6).
The physiological significance of these fatty acids in the human diet is discussed in Chapter 3. Species, breed, sex, age, and environment influence the amount as well as the degree of unsaturation of the fat (mainly the ratio between unsaturated oleic acid and the saturated palmitic and stearic acids).
Animals living in woodlands in Uganda and Tanzania (eland, hartebeest, giraffe, buffalo, warthog) and also free-range cattle have only about 2% of lipid in the muscle, of which about 30% is PUFA. Those grazing on grassland have about 3% lipids in the muscle of which about 15% is PUFA.
In contrast lean domesticated cattle (fed on supplemented diets) have about 5% lipids of which only 8% is PUFA, and intensively-reared fatstock have 15-30% lipids Table 2-8 (Crawford 1975).
The cholesterol content of meat is compared with that of some other foods in Table 2-7.
Vitamins
The body content of most vitamins is largely independent of diet. Apart from the thiamin effect on pig meat mentioned above, the exception is vitamin A which is stored in the liver in amounts depending on intake, with small amounts present in the kidney - these are the only tissues to contain significant amounts of this vitamin (there are traces, 10- 60 ug/100 g, in muscle). Under free range conditions of grazing there is a very high intake of carotene (pro-vitamin A) which is mostly converted into retinol (vitamin A). Tables 2-2 and 2-9 list the typical vitamin content of raw meat and offals.
Pig meat is very rich in thiamin compared with all other animals, nine times as much, but has the same content of riboflavin as others.
Liver is by far the richest of animal tissues in all the vitamins, and includes unchanged carotene as well as being the only tissue to contain more than a trace of vitamin D.
Minerals (Tables 2-2,2-3 and 2-9)
Meat and offals contain a wide variety of mineral salts. The contents of iron, zinc and copper vary considerably in different species, liver being by far the richest source of these minerals compared with muscle tissue.
High levels of minerals in the feed do not necessarily increase the level of that mineral in the flesh and there is a complex inter-relation between minerals. For example, the molybdenum content of sheep meat increases with dietary molybdenum only when dietary sulphate levels are low. Dietary molybdenum inhibits the accumulation of copper which is partly off-set by increased manganese. Liver copper decreases and molybdenum increases with increasing amounts of molybdenum. Other inter-relations between minerals include calcium and zinc (Byerly 1975).
Copper is used in some feeding systems for pigs as a growth stimulant and can result in levels of several hundred parts per million of copper in the liver.
When pasture is deficient in minerals, especially phosphorus and cobalt, the amounts in the muscle are reduced.
The amount of carcass meat obtained from animals varies with the type of animal only about one third of the total weight of cattle and lambs and half of the pig (Table 2-10A).
The other parts of the animal - liver, heart, brains pancreas (gut sweetbread) thymus (chest sweetbread), tripe, feet (trotters), tail, testes (fries), intestines (chitterlings), cheek meat and head meat and fat (tallow, lard, suet) - are collectively called offal, variety meats, side meats or organ meats in various countries (Table 2-10B). With regard to poultry, the term giblets means liver (with gall bladder removed), heart and gizzard and any other material considered as edible by the consuming country. Not all parts are eaten depending on consumer acceptance, religion and tradition as well as regulations imposed for reasons of hygiene.
Intestines are used as containers for sausages of the different types, blood may be used in sausages, pork skins may be eaten or used as a source of gelatin. In addition, some inedible by-products such as bonemeal can be used as a mineral supplement in animal feed and there are other inedible by-products of economic value such as hides and horns.
Nutritional Value
The nutrient content of offals is given in Table 2-9. In general they are richer than lean meat in iron, copper and certain B vitamins, with liver being a particularly rich source of vitamins A, B1, B2, B6, B12, niacin and pantothenate and even some vitamin C.
Kidney is a rich source of B1, B2 and B12: pancreas is a good source of B1, B2, C and pantothenate.
The vitamin C in lungs, spleen and thymus is usually present in sufficient quantity to allow some to survive cooking.
Other organ meats compare well with lean meat as sources of the vitamins, and all meat products are good sources of zinc and of iron, liver, lungs and spleen being especially rich in iron (Anderson 1988).
Ears and feet have a high protein content but much of this is collagen and so of poor nutritional value, although when consumed this has no significant effect on the quality of the protein of the diet as a whole.
Meat as purchased may include bone, outer layers of fat, gristle and tendons which are removed to differing extents before cooking, so that the composition of meat "on the plate" can vary enormously.
Meat and meat products are considered cooked when the centre of the product is maintained at a temperature of 65-70°C for 10 minutes since the proteins will then be coagulated and the meat tenderised by partial hydrolysis of the collagen. The vegetative form of bacteria, but not spores, will have been destroyed (thermoresistant spores can survive heating above 100°C). The completion of the cooking process is generally indicated by a change of colour from red to brown (red to pink in cured products) and flavours are developed.
Denaturation of red myoglobin and conversion to brown myohaemochromogen starts at 40°C and is almost complete at 80-85°C (Lawrie 1991). Cooked flavour results from a number of reactions including changes in lipids, carbohydrate and protein, with heat breakdown of peptides and amino acids and reactions between proteins and carbohydrates.
Meat from older animals richer in connective tissue requires longer cooking at 5060°C - a temperature at which collagen can be hydrolysed. If heated for long periods at temperatures above 80°C amino acids begin to decompose with the production of unpleasant flavours. (Hydrolysis of collagen is rapid during the canning process when high temperatures are employed for only a short time).
In comminuted meat products, such as sausages, the particles of meat become bound together during cooking through coagulation of extracted proteins. In products that contain pastry this has to be cooked at the same time as the meat.
Water is lost during cooking, the amount depending on time, temperature, method of cooking, size of sample, heat penetration and composition leading to an increase in concentration of the fat and protein. Table 2-11 shows the changes in composition and indicates changes in fat content which depend on the method of cooking.
There is a loss of water-soluble vitamins, minerals and protein in the juices but this is a small proportion of the total present and, moreover, in most cooking procedures the juices are usually consumed with the meat.
With so many factors that can influence changes on cooking literature data are rarely comparable - unless the work has been carried out in the same laboratory - and cannot be expected to do more than indicate the general effects.
The Massachusetts Data Bank has tabulated the average amounts of nutrients in meats of various types and the large coefficients of variation illustrate the impossibility of attempting to provide precise figures (table 2-12).
Effect on Fat
Even in deep frying there is a loss of fat since lean muscle does not absorb the cooking fat. As an example from one set of food composition tables, raw rump steak containing 18.9% protein and 13.5% fat has a total of 32.4% dry matter (ignoring minerals). Expressed as a proportion of dry matter this is 58.3% protein and 41.6% fat.
When grilled, i.e. cooked by radiant heat with added fat the loss of water and fat reduces the total fat to 30.7% of the dry matter while the protein increases proportionately 693%.
When fried the loss of water is greater than in grilling but the loss of fat is less so that the proportion of protein becomes 66.2% and the fat 33.7% of the dry matter.
Table 2-13 illustrates data for chicken and also shows the differences between dark and light meat and the effect of including the skin in the analysis.
Boiling of chicken causes a greater loss of water than roasting but no loss of fat so that as a proportion of dry matter, fat is highest in the boiled product.
Effect on Protein
Proteins can be damaged from the nutritional point of view when part of an essential amino acid is rendered unavailable. This involves first lysine at temperatures around 100°C; then cystine and methionine at temperatures around 120°C, and other amino acids after prolonged heating (Bender 1978).
At the rather low temperature needed to cook meat there is little loss of available lysine and no loss of methionine and cystine. For example no change in protein quality was found after roasting in an open pan at 163°C when the internal temperature did not rise above 80°C; or when the meat was browned in an oven for 30 min. then sterilised in a can (Mayfield and Hedrick 1949, Rice 1978).
When meat is roasted the outer part reaches a high temperature and turns brown because of a reaction between the lysine and reducing substances present (Maillard or browning reaction) which produces the desired roast flavour. However, since the roasted part is only a small fraction of the total piece of meat and the internal temperature does not exceed about 80°C there is no measurable change in the quality of the protein as a whole.
Effect on Vitamins
Thiamin
One of the most sensitive vitamins is thiamin; it is both water-soluble and heat-labile. It is also damaged by oxygen and at neutral and alkaline pH. It is very susceptible to destruction by sulphur dioxide and sulphites which are used in some countries to preserve comminuted meat products. There is also some destruction during treatment with ionising irradiation but this can be reduced by irradiating in the frozen state. Table 2-14 shows some figures for losses of thiamin.
The juices exuded from meat during cooking include part of all the water-soluble constituents, including mineral salts, proteins and vitamins but except for the heat-labile thiamin these are recovered in the juices consumed with the meat, unless they have been damaged by excessive heat.
In a general review of the subject (Karmas and Harris 1988) losses of thiamin are given as 1540% on boiling, 40-50% on frying, 30-60% on roasting and 50-70% on canning. All figures listed for cooking losses of vitamins must be regarded as rough average values since they will depend on time and temperature and conditions of cooking, the particular product, the size of the pieces of meat and thus heat penetration, to which must be added errors due to sampling and the considerable errors that are unavoidable in vitamin determination. Literature figures can be used only for guidance and if more accurate figures are required they must be determined on the product in question subjected to the specific process - and even then are subject to the problems of analysis.
Riboflavin and Niacin
Cooking losses of riboflavin (Table 2-14) average around 10%. Riboflavin is relatively stable to most cooking practices (excluding the high temperature of roasting) and to canning and dehydration. It is damaged by sun-drying and under any alkaline conditions; dry-curing and smoking lead to about 40% loss, wet curing to about 10% loss.
Niacin is stable to heat, light, oxygen, acids and alkalies and also to irradiation but can, of course, be leached from the food; losses average about 10%.
Other B Vitamins
There is less reliable information about other B vitamins but some reported figures are presented in Table 2-15. On average about one third of the vitamin B6 and pantothenate are lost in cooking.