Article pp.13-34 du Vol.23 n°1 (2003)

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ARTICLE ORIGINAL ORIGINAL PAPER

Muscle foods: consumption, composition and quality

Joseph Culioli¹, Cécile Berri², Jacques Mourot³

INTRODUCTION

The flesh of mammals and birds has been a significant element in the human diet, both nutritionally and symbolically, since the times of the stone-age hun- ter-gatherers. Foodstuffs of animal origin made up 50% of the energy intake of homo habilis and up to 80% of that of modern man about 10,000 years ago (EATON and KONNER, 1985). Whether obtained by hunting or by ritual sacri- fice, meat has been the most valuable and most sought after foodstuff in most civilizations, but it has also very often been subject to restrictions and taboo (prohibition of pork by Jews and Muslims, fasting periods for Christians, vegeta- rianism in certain castes in India and sects).

However, the introduction of farming resulted in a significant reduction in meat consumption by man, with a transition from game to domestic animals.

The development of agriculture also resulted in human nutrition being mainly based on cereals and dairy products. Then, for some considerable time, meat consumption has been limited for most people and the privilege of a social elite.

In some countries it has remained a socially discriminating factor, whether in terms of quantity or quality. Until the end of the Middle Ages, beef, veal, mutton and chicken were the exception on the common man’s table, whereas pork was frequently eaten. Moreover, the animals and cuts which are today considered to be the greatest delicacies have not always been so considered. Beef was thus considered in the Middle Ages and during the Renaissance to be a common man’s meat used mainly to make stock and reserved for manual laborers. The upper classes preferred poultry, which was more delicate and more digestible (FLANDRIN and MONTANARI, 1990).

1. Station de Recherches sur la Viande, INRA, 63122 Saint Genès-Champanelle, France.

2. Station de Recherches Avicoles, INRA, 37380 Nouzilly, France.

3. Unité Mixte de Recherches Veau et Porc, 35590 Saint Gilles, France.

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It was only during second part of the twentieth century that meat consumption really assumed a significant place in developed countries. Average meat consumption in France tripled from 30 kg (expressed as carcass equivalent) at the beginning of the century to 90 kg by the end of the 1980s (figure 1). Directly related to the higher standard of living, this increase also reflected the reduction in costs of meat production due to farming rationalization and intensification.

Figure 1

Evolution of the meat consumption in France during the two last centuries (cited by DUMONT et al., 1989).

However, this increase in the availability of meat has been accompanied by changes in the perception of meat as a foodstuff. From being initially a source of strength, energy and vitality which was essential for manual laborers, meat has become the subject of debate and criticism in a society which has become more and more sedentary and aware of nutritional, dietary and ‘health’ elements of food intake. Criticism has mainly focused on content and composition of the fat in the meat. Moreover, meat consumption is no longer a question of means and availability but rather an issue of choice in a society where the range of available foodstuffs has become very wide.

It is however necessary to recognize that there is wide range of products referred to as “meat”, depending on many factors such as animal species and muscle origin of a cut, as well as biological factors (age, sex, breed), farming conditions (speed and intensity of growth, feed type) and processing methods, particularly cooking, leading to the final product.

It is therefore difficult to deal with meat generally, whether in terms of consumption, composition or sensory qualities. The aims of this article are to present some of the aspects of diversity in these three areas and the main factors contributing to them.

0 20 40 60 80 100

1800 1850 1900 1950 2000

Year kg/person.year (carcass-equivalent)

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1 – CONSUMPTION OF MEAT AND MEAT PRODUCTS

Meat and meat products represent the most significant outlay in a household’s food budget. The proportion of expenditure on meat, i.e. about 27%, has remained relatively consistent for some years, despite the regular decrease in the household budget spent on food (26% in 1970; 17% in 1999). In addition, the meat sector occupies a significant place in the national economy. Meat production represents about 50% of farming activity, and meat processing is the main sector in the food industry (25% of the total turnover of food industries).

The annual consumption of meat per person in France (expressed as carcass equivalent) has increased regularly for some years (78 kg in 1970; 94 kg in 1988) and appears to have now reached a plateau at about 93 kg. The main meat consumed in 2000 (figure 2) was pork (36.1 kg), followed by beef and veal (25.6 kg), poultry (24.8 kg), lamb (5.1 kg) and horseflesh (0.6 kg). Despite this apparent stability there have been considerable differences in consumption between the meats of different animal species. Although beef was for many years the main meat consumed in France, with a peak of 33 kg in 1985, there has been a regular decrease of 1-2% per annum. Beef consumption has thus been reduced by about 20% in 15 years. Parallel with this significant tendency to decrease, there have been wide transitory variations since 1996 with the occurrence of bovine spongiform encephalopathy in French herds. Pork consumption is now stable after a considerable increase between 1970 (31 kg) and 1988 (38 kg), whereas poultry meat consumption has consistently increased, doubling since 1970 (from 12 to 25 kg).

Moreover, there have been wide inter-country differences in meat consumption in the European Union, in which mean consumption, which has long been lower than that of France, is now reaching similar levels (88 kg in 2000). Some countries, such as Finland (66 kg), Sweden (70 kg) and the United Kingdom (76 kg), have considerably lower meat consumption. Others such as Spain, Denmark, Austria and Ireland have much higher consumption (113, 108, 98 and 96 kg, respectively). In some countries such as Spain, Denmark and Austria pork, is the main source of meat products (66, 65 and 60 kg, respectively).

Finally, the considerable difference in meat consumption in the USA must be mentioned. It is more than 40% higher than that in the European Union, with poultry (50 kg) and beef (45 kg) predominating (OFIVAL, 2000).

The quantity of meat in fact consumed is, however, much lower than the levels expressed in carcass equivalent. Precise determination depends on the meat yield when carcasses have been deboned. Any such calculation can only be approximate in view of the different yields for each species, and within each species between animals from different types of farming method. If a mean yield of 60% is taken into account, individual consumption is then no more that 55 kg per person per annum, i.e. 150 g per day, which represents approximately 30 g protein. The result using this method of calculation is comparable to that obtained from individual consumer studies. Daily consumption by French adults in 1999 was 135 g per person (VOLATIER et al., 2000). This is slightly less than previously determined amounts. However, it probably is an underesti- mate because it does not include a certain number of meat products used in prepared meals, sandwiches, pizzas and quiches.

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The corresponding quantity of lipids ingested is more difficult to determine because of the wide variations in fat content in butchers’ cuts of meat. The fat content depends on several biological factors (anatomical origin of the cut, species, breed, sex, age, feeding regime) and on the fact that most meat fat is not consumed because it is either removed by butchering before cooking or by the consumer during a meal. If the mean lipid content of meat consumed is 10%, a high level compared to intramuscular lipid value which varies between 1 and 5% but relatively low compared to the quantity of fat on certain butchers’ cuts (20%), lipid consumption can be estimated at about 15 g per day. This corres- ponds to a protein/lipid ratio of 2, but the ratio may vary between 1 and 10 according to the type of cut. Meat and meat product lipids thus provide about 17% of total lipids, 14% of saturated fatty acids, 13% of polyunsaturated fatty acids and about 19% of monounsaturated fatty acids (MALVY et al., 1999).

Although meat is mainly consumed as it is, 35% of meat consumption is in processed products (processed meats, canned, frozen and chilled prepared meals). The latter are mainly made from pork (85%), and 70% of pork production is processed. Manufacture of poultry-based products (chicken and turkey) has however considerably increased in recent years. Processed poultry represents 15% of poultry production, whereas beef and lamb are still largely (95%) consumed in the unprocessed state. Mean daily consumption of processed meat products is 40 g per person (49 g for men and 33 g for women), more than half of which is cooked or dried ham (VOLATIER et al., 2000). Such products provide 8-9% of saturated and polyunsaturated fatty acids and 11-13% of monounsaturated fatty acids (MALVY et al., 1999).

Figure 2

Proportion of meat from different animal species eaten in France (from OFIVAL, estimation for the year 2000).

2 – MEAT COMPOSITION

2.1 Main components (macro and micronutrients)

Only very low levels of glucides are present in muscle tissue (about 1%), mainly in the form of glycogen. Moreover, glycogen is almost completely hydro- lyzed after slaughter during the process of rigor mortis. Proteins and lipids can therefore be considered as the only macronutrients in meat.

6%

39%

27%

1%

Pork Beef/veal Poultry Sheep Horse

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After water (approximately 75% of muscle tissue), proteins are the main component of meat. Protein content, 75% of dry matter at least, is relatively constant in all types of muscle and animal, and only depends on the quantity of intramuscular lipids. Moreover, all muscles have a similar amino acid profile, except those which contain high proportions of connective tissue. In fact, collagen, the main protein of the connective tissue, is particularly rich in glycine (1/3 of amino acids), proline and hydroxyproline, but contains no tryptophane and is low in sulfur containing amino acids: when it is present in large quantities it may therefore reduce the nutritional value of proteins of muscle fibers. The latter are rich in lysine and have balanced levels of essential amino acids which vary little according to type of muscle and animal species (figure 3). These amino acids are stable on cooking and only very high temperatures can change the availability of lysine, methionine and tryptophane (PELLET and YOUNG, 1990).

Figure 3

Contribution in essential amino acid of meat from different animal species in relation with the human s’ requirement corresponding to 1 (from PAUL et al., 1979).

The overall differences in composition reported between species (Table 1) are generally related to metabolic and contractile type of animal muscle: oxyda- tive red muscles are more characteristic of ruminants and waterfowl, whereas glycolytic white muscles are predominant in white meat poultry and pigs.

Red meat, particularly that of ruminants, is usually richer in lipids than white meat, as lipid content increases with the oxidative nature of the muscle. In addition, red meat has lower levels of unsaturated fatty acids because the digestive physiology of ruminants enhances the synthesis of saturated fatty acids.

Moreover, because intramuscular lipid content is inversely related to water and

0 0,5 1 1,5

Histidine Isoleucine

Leucine Lysine Methionine + Cystine Phenylalanine + Tyrosine Threonine Tryptophane Valine

Roasted chicken Lamb : roasted leg

Pork : roasted fillet Beef: steak

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protein content, red meat (beef, lamb) has up to 10% less water than other meats and exhibits the lowest protein content. In comparison, lean meats such as turkey cutlets and the white meat of chicken contain higher protein levels.

These differences in composition, particularly in lipids, have a direct effect on the energy values of meat, which in the raw state may vary from 450 kJ to 900 kJ according to animal species and cut (Table 1).

Table 1

Raw meat composition (/100g) [From FAVIER et al., 1995 and *RABOT, 1998]

Lipid contents of muscles (Table 2) are clearly lower than those of butchers’

cuts (Table 1) which often include a high quantity of lipids from intermuscular and subcutaneous fat deposits. They do not usually exceed 4% and may even be less than 1% in the white muscles of pork and poultry.

Beef Veal Lamb Pork Chicken Duck Turkey

Flank Fillet Leg Fillet Breast Thigh Meat^§ Breast Thigh meat

Energy (kJ) 814 458 898 475 525 555 447 454

Water (g) 66,4 75 65 74,4 74,7* 74,2* 73,3 74,2 75,7

Proteins (g) 19,6 20,4 18 21 22,3* 18,4* 19,6 23,4 20,4

Lipids (g) 13 3 16 3,2 1,3* 4,5* 6 1,3 2,9

Cholesterol (mg) 65 80 74 65 50* 91* 85 55 78

Fatty acids (saturated / unsaturated)

0,89 0,65 1,14 0,73 0,54*^§ 0,97 0,57 0.59

Iron (mg) 2,5 0,8 2,6 1,2 1*^§ 2,1 0,7 1,8

Niacin (mg) 4,1 8,6 5,4 4,3 7,7*^§ 5,4 7,9 3,9

Vitamin E (mg) 0,3 0,15 0,15 0,1 0,22*^§ 0 traces traces

Thiamin (mg) 0,08 0,08 0,14 1 0,08*^§ 0,07 0,08

Vitamin B6 (mg) 0,3 0,54 0,25 0,45 0,45*^§ 0,34 0,58 0,34

Vitamin B12 (µg) 2 1,2 2 0,7 0,4*^§ 1,3 0,73 1,7

Folate (µg) 9 14 4 4 10*^§ 30 8 20

Sodium (mg) 70 92 70 125 76*^§ 90 63 71

§ Mixing of red and white meat.

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Table 2

Effect of animal species and muscle type on total lipid content of muscle (g/100 g of muscle) [From (1) MOUROT and HERMIER, 2001;

(2) RABOT et al., 1996 ; (3) CULIOLI et al., 1990; (4) GANDEMER, 1992]

Red meat also contains high levels of iron, mainly in the heme form. Certain beef cuts thus contain three times more iron than most other meats. The heme form of iron is much more easily assimilated (15-25% bioavailability) than non- heme iron (2-5% bioavailability). Thus 100 g of cooked beef may provide up to 40% of the recommended daily allowance of iron for men and up to 25% for women. Other minerals present in meat include zinc, which is particularly inte- resting due to the high content of 1 to 5 mg/100g of muscle, which is much higher than in other animal and vegetable products.

The content of certain microelements varies according to animal species.

The meat of ruminants is thus richer in vitamin B12, which is synthesized by the microflora of the rumen, than that of monogastrics. It is, however, less rich in niacin (vitamin B3 or PP) than white meats (veal, chicken and turkey) and in thiamin (vitamin B1), which is found at ten times higher rates in pork than in the meat of other animal species. The level of vitamin E is largely determined by the quantity ingested by the animal. However, the muscles of certain species such as the turkey have low capacity for vitamin E storage. Although vitamin E in meat is valuable from a nutritional point of view, it is also important technologi- cally because it considerably retards lipid oxidation and hence preservation- related damages (MERCIER et al., 1998).

Amongst the micronutrients, selenium (a constituent of glutathione peroxi- dase) play an essential role in control of cellular oxidation events, probably by preserving vitamin E. Selenium has a level of approximately 0.05-0.25 mg/kg in meat, with the concentration being dependent on the quantity present in feed and also affected by animal species. With regards to species the concentration varies in the order: chicken>pork>beef>veal (WENK et al., 2000). In contrast, the carotenoids have a more controversial role. Carotenoids influence the color of tissues, especially fat, but their role as anti-oxidants has not been clearly demonstrated. Muscle also contains a number of other anti-oxidants with both enzymatic and non-enzymatic properties (DECKER et al., 2000) ; including the

Pork (1) Chicken (2, 3)

Muscle Lipid content Muscle Lipid content

Longissimus 1.3 + 0.3 Pectoralis 1.0 + 0.2

Adductor femoris 2.0 + 0.5 Sartorius 2.8 + 0.3

Biceps femoris 1.4 + 0.4 Biceps femoris 2.5 + 0.5

Psoas major 1.3 + 0.3 Beef (4)

Semimembranosus 1.7 + 0.4 Longissimus dorsi 1.9 - 3.4

Semitendinosus 3.5 + 0.5 Psoas major 2.2 - 3.9

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histidine containing dipeptides, anserine and carnosine. These dipeptides, exhi- biting a high buffering capacity, are present in muscle in concentrations of 0.2 to 1.2%, higher in white glycolytic than red oxidative muscle. They have an anti- oxidant effect by chelating certain pro-oxidant metals and by blocking free radi- cal formation (DECKER and MEI, 1999).

2.2 Lipid fraction

Among the various components of muscle tissue, it is without doubt the lipid content which is the most subject to variation, both quantitatively and qualitatively. As mentioned above, these variations depend on the species, the cut and the muscle concerned and also on many biological and farming factors.

2.2.1 Adipose tissue and intramuscular lipids

Among the factors influencing the quantity of lipids in the carcass and the muscles, the energy level of the feed administered, the age and sex of the animal, and the use of growth factors (sex steroids, β-agonists, growth hor- mones, which are not permitted in the European Union) have a significant effect on animal fattening and on the distribution of muscle mass and adipose tissue.

Breed also influences lipid content. Conventional pigs (Large White, Pie- train, Landrace and crosses) have lipid levels in the longissimus muscle of between 1.5 and 1.8% whereas Duroc pigs, which are more often used in the USA, have lipid levels of about 2.5%. Local breeds, corresponding to non- selected pigs, have lipid levels higher than 3% (3.4 for Limousin pigs and 3.9 for Basque pigs) (LABROUE et al., 2000). In beef animals, British breeds such as early maturing Angus and Hereford usually have carcass fat levels and intramuscular lipid levels which are clearly higher than continental European breeds such as Charolais, Limousin and Blonde d’Aquitaine. There is a relationship between the quantity of total fat in beef carcasses and the quantity of intramuscular fat, with a 5% increase in carcass fat corresponding on average to a 1% increase in intramuscular lipids. Intramuscular lipid contents of around 2%

are thus found in carcasses containing 10-15% fat depot and lipid contents of 5-7% are found for levels of fat depot higher than 30% (GOUTEFONGEA and DUMONT, 1990).

In addition, selection based for more than 30 years on criteria of improved growth performance has also greatly influenced the body composition of animals. It has resulted in increased meat yield and reduced carcass fat. This has been particularly marked in the case of species which have been intensively far- med (mainly pigs and chickens). In the case of pigs, overall carcass fat levels have been reduced by half, from nearly 40% adipose tissue to the current level of 20% (SELLIER, 1989). These changes have led to reduction in the levels of intramuscular lipids, estimated at 30% when current figures are compared to those of non-selected pigs such as local breeds (LABROUE et al., 2000). Howe- ver, this beneficial effect from a nutritional point of view has led to disadvanta- ges in the processing and preservation of meat products such as increased susceptibility to oxidation of fatty acids and sometimes affects taste (LEBRET and MOUROT, 1998; LEBRET et al., 1999). Studies on the attitudes and beha- viour of consumers have showed that a minimum intramuscular lipid content is

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necessary for sensory qualities not to be affected (2-3%, according to the type of product – raw or processed meat such as cooked ham) (FERNANDEZ et al., 1999a, b, 2000).

Selection against fatness in the chicken has been as effective as in the pig (LECLERC and WHITEHEAD, 1998). However, the value of this is much more limited for the consumer because the reduction has mainly been in vis- ceral tissue, which is highly developed in birds but eliminated by gutting. The main benefits are for the producers because it has increased yields in terms of marketable carcasses. Comparison of experimental strains of chickens selected for or against abdominal fatness has demonstrated that this type of selection results in reduction of intermuscular fat in the thigh without affec- ting the lipid contents of the muscles themselves (RICARD et al., 1983). Ana- lysis of sensory traits has shown that the meat of fatter chickens (particularly thigh meat) is slightly more tender (RICARD et al., 1983), juicy and flavorful than the meat of lean chickens (CHAMBERS et al., 1989). However, despite this it has been shown that reduction in adipose tissue in this species had a limiting effect on sensory properties due to the consequences of the faster growth rate which has lowered the slaughter age of such birds. Thus chickens from modern strains, which are slaughtered at a young age, provide meat which is clearly more tender and juicy but has less flavor than earlier strains slaughtered at older ages (TOURAILLE, 1981a, b). In certain poultry (chicken, guinea fowl) more intense production methods have sometimes led to a 15-30% increase in lipid content of meat (CULIOLI et al., 1990, BAÉZA et al., 2001).

2.2.2 Characteristics of meat lipids

The lipid content of animal feed strongly influences the fatty acid composition of adipose tissue and muscles, particularly in monogastrics. It is thus possible to propose improving the nutritional quality of meat via the composition of the fatty acids ingested (Table 3). The unsaturated fatty acids ingested are deposited in the tissues, thus reducing the proportion of saturated fatty acids considered to be less beneficial for humans. Programs have therefore been developed in various animal species to increase the n-3 fatty acid content in meat products which contribute to the prevention of cardiovascular diseases (GUALLAR et al., 1999). This increase is mainly based on the introduction of extruded flax seed in animal feed. Similar effects can be obtained with fish oils which are very rich in n-3 fatty acids, but this is costly and regulated or even forbidden in pig finishing because of the unpleasant odors emitted upon cooking. Meat and egg products are already commercially available but production is still limited. Further development of such products will depend on the results of current studies.

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Table 3

Effect of dietary fat on fatty acid composition of Longissimus (pig) and Pectoralis (chicken) muscles (%) [From AYUDAH et al. (1991),

MOUROT and HERMIER (2001) and RATNAYAKE et al. (1989)]

Although the effects of the nature of the lipids ingested are less obvious in ruminants, it is possible to modify the degree of saturation of intramuscular lipids by adapting animal feed. Higher proportions (x1.5-3.5) of n-3 series PUFAs, particular linolenic acid (C18:3), and the long chain derivates such as stearidonic acid (C18:4), eicosapentaenoic acid (EPA C20:5) and docosahexae- noic acid (DHA C22:6) are obtained by feeding grass compared to cereal-based feedstuffs (GEAY et al., 2001). The addition of fish oil also enhances the incor- poration of long chain n-3 PUFAs which cannot be biohydrogenated in the rumen. Similarly the addition of oil seeds rich in PUFAs to cereal-based feedstuffs increases the levels of unsaturated fatty acids in adipose tissues. Such effects can be improved by protecting feed lipids using formaldehyde-treated proteins.

The intramuscular lipids which are often considered together in fact com- prise two main categories, i.e. stored lipids (mainly triglycerides (TG)) and structure lipids (i.e. mainly phospholipids). In contrast to phospholipids, the content of which is low and relatively constant (0.5-1%), intramuscular TG content may vary considerably (0.2 to more than 5%). The main factors in such variations are the type of muscle, animal genotype and feed, whereas age and sex are less significant. However, standardization of production systems, especially pig and poultry farming, have tended to reduce considerably the effects of such factors.

Pig Chicken Tallow Maize oil Rapeseed

oil Tallow Rapeseed

oil Linseed oil Fish oil

C16:0 23.9 24.5 24.6 18.1 14.5 19.1 25.8

C18:0 11.9 11.7 11.6 12.5 8.6 12.4 7.7

C18:1 44.6 41.4 42.8 33.5 38.2 19.0 31.4

C18:2, n – 6 11.1 13.8 11.8 18.4 21.4 23.8 14.2

C18:3, n – 3 0.5 0.4 1.0 1.2 2.9 7.0 0.5

C20:4, n – 6 0.1 0.2 0.2 8.0 5.5 3.4 2.3

C20:5 0.1 0.1 0.2 0.8 0.8 3.6 1.6

C22:5 0.1 0.1 0.1 2.0 2.1 4.7 1.0

C22:6 0.1 0.1 0.1 2.5 3.3 3.9 4.6

Fatty acids (unsaturated/

saturated)

1.27 1.33 1.29 2.03 1.96 2.53 1.99

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Meat is often considered as a source of lipids rich in saturated and poor in polyunsaturated fatty acids (PUFAs). However, this should be interpreted with caution. Triglycerides do in fact contain very variable proportions of PUFAs (2% to more than 30%), mainly C18:2 and to a lesser extent C18:3. The main factor in variations in PUFA content is animal species (Table 4). Although PUFA content of ruminants’ meat triglycerides is usually less than 3%, it ranges from 7 to 15% in pork, to 20-30% in poultry and in rabbit. It is possible to change the proportions of PUFAs by modifying feedstuffs, particularly in non-ruminants, whereas other biological factors such as muscle type, sex and animal age have little effect.

Table 4

Effect of animal species and muscle type on triglyceride, phospholipid and polyunsaturated fatty acid (PUFA) content of meat [from GANDEMER, 1999]

Phospholipid content of meat depends mainly on the metabolic type of muscle fiber: it is higher in slow oxidative red fibers which are smaller and contain more cell and mitochondrial membranes than fast glycolytic white fibers. Phos- pholipids contain high amount of PUFAs (45-55%), particularly C20 and C22 fatty

Beef Pork Chicken Turkey Rabbit

Type Glyc. Oxid. Glyc. Oxid. Glyc. Oxid. Glyc. Oxid. Glyc. Oxid.

Muscle LT D LT M PS S PS S PM SM

Triglycerides (g/Kg of fresh muscle)

Total 21 41 10 9 6 15 9 20 5 35

PUFAs (% of triglycerides)

C18:2 n-6 1.6 2.5 7.2 8.2 17.9 16.9 24.6 24.2 21.2 18.8

C18:3 n-3 0.4 0.5 0.7 1.0 1.2 1.1 5.1 5.3 3.7 3.3

Phospholipids (g/Kg of fresh muscle)

Total 7 11 5 9 5 9 5 9 7 9

PUFAs (% of phospholipids)

C18:2 n-6 13.8 20.1 22.9 24.7 14.8 21.8 18.1 23.5 25.5 14.3

Long n-6 11.6 11.2 8.3 8.5 20.3 19.5 15.8 18.8 15.6 18.5

Long n-3 6.2 4.4 1.1 1.9 5.0 5.1 7.5 5.8 2.8 3.7

Glyc.: Glycolytic; Oxid.: Oxidative;

LT: Longissimus thoracis; D: Diaphragma; M: Masseter; PS: Psoas superficialis; S: Sartorius;

PM: Psoas major; SM: Semimembranosus

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acids with 4 or 6 double bonds. These PUFAs depend little on factors such as species, breed, feed, sex and age (GANDEMER, 1990). Despite their low levels in meat, these components have an essential role in oxidation because of their low degree of saturation and their membrane localization which is highly exposed to cytoplasmic catalysts. The use of antioxidants such as vitamin E in feed is recommended because integration into membranes has a highly protective effect on PUFAs (RENERRE, 2000). Reduction of oxidation phenomena has a marked effect on the stability of meat color and on rancid taste, especially in processed foods. Similarly, greater integrity of cell membranes is believed to be a cause of reduction of water loss. Vitamin E supplementation also reduces during cooking the formation of cholesterol oxides which may have detrimental health effects.

Fatty acids undergo biohydrogenation in the ruminant rumen which results in increased lipid saturation. Although this is incomplete, conjugated linoleic acid (CLA) isomers are formed which pass into the blood, milk and meat at a rate of 3- 6 mg/g of fat (Table 5). In contrast to trans monounsaturated isomers which may increase cardiovascular risk, these conjugated linoleic acids have been shown to be beneficial, with mainly hypolipidemic, antiatherogenic and antitumoral effects.

Table 5

Effect of animal species on conjugated linoleic acid (CLA) content in the lipid fraction of muscles [From CHIN et al., 1982]

Meat, especially the fattiest types, is often considered to be a significant source of cholesterol. In fact, the cholesterol contents of muscle and adipose tissue are similar (50-100mg/kg), apart from poultry in which the skin is richer in cholesterol (100-120 mg/kg). Certain chicken parts are richer in cholesterol (80 mg/kg) than beef or pork (50-65 mg/kg). In addition, the cholesterol content of meat tends to be higher when the muscle is of more oxidative type, which may be related to a larger number of cell membrane structures.

3 – INFLUENCE OF COOKING AND PROCESSING PROCEDURES ON COMPOSITION OF PRODUCTS

Cooking, and more specifically the methods of preparation, are the main factors which determine the majority of sensory and nutritional properties of meat. Such treatment causes thermal denaturation of myofibrillar proteins,

CLA (mg/g of lipids) Cis 9, trans 11 (%)

Beef 2.9-4.3 79

Sheep 5.6 92

Chicken 0.9 84

Pig 0.6 82

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leading to a reduction in water holding capacity and to changes in meat color.

Such denaturation of proteins, combined with contraction of the collagen of the connective tissue above 60^oC, results in loss of fluids which may substan- tially affect the composition of a cut. Such losses, which may be as high as 40% of the initial weight of the meat in the most intense cooking conditions, reflect many features including the maximum temperature and duration and method of cooking, factors intrinsic to the muscle tissue being treated such as pH, collagen content and cross-linkage, the initial degree of contraction of myofibers, and size and shape of cuts (LAROCHE, 1988). Such fluid loss may represent more than 10% of the initial dry matter. Depending on the conditions, up to 50% of water, 55% of minerals, 15% of nitrogen and 10-20% of lipids (exclusively triglycerides) may be eliminated in the cooking liquid.

Moreover, thermal denaturation induces additional losses. Although muscle proteins are hardly affected by the traditional heat treatments used in cooking meat, certain constituents are thermo-labile. In particular, phospholipids and their PUFAs undergo greater oxidation the more they are unsaturated. Although linoleic acid (C18:2) is little changed, up to 70% of loss of muscle n-3 C22:5 and C22:6 can be lost in roast chicken (GANDEMER, 1990). Oxidative muscles, which are rich in phospholipids, are more susceptible to oxidation than glycolytic muscles. The consequences on the taste of cooked meat are loss of flavor and even the occurrence of unpleasant flavors during conservation (RABOT et al., 1999). However, the heat stability of PUFAs can be clearly increased when oxidation phenomena are limited during meat storage and when heat treatment is carefully applied (RABOT, 1998).

During cooking, loss of vitamins, especially of group B, are also observed.

Although niacin (vitamin B3) is heat resistant, up to 50% of vitamins B6, B9 (folic acid), and B12 can be destroyed (SAUBERLICH, 1990). Thiamin (vitamin B1) is particularly sensitive, with levels of destruction of 10-70% according to the cooking method. Similarly, 5 to 15% losses of the heme form of iron have been reported in conventional cooking methods, thus reducing its absorption (MARTINEZ-TORRES et al., 1986).

The destruction or elimination of various muscle tissue constituents in cooking liquids results in changes in meat composition which must be taken into account to establish nutritional value (Table 6). As water is the main constituent which is lost during cooking, cooked products have a higher dry matter content than raw meat. They generally contains 1.3-1.5 times more protein, whate- ver the species or heat treatment used. The effects of cooking on minor components vary. Thus, because of the loss of water caused by heating, the proportions of iron and cholesterol increase. In contrast, other constituents have lower contents (folates) or remain more or less at the levels found in raw meat (vitamins B6, B12 and E), thus implying a partial loss of these elements during cooking.

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Table 6

Composition of cooked meat (/100 g) [from FAVIER et al., 1995]

The effects of cooking on the lipid fraction of meat depend both on the cooking method and its duration and on the type of meat. More fatty meats (lamb, beef) have a slightly lower proportion of lipids after cooking, the fat being transferred to the cooking liquid. In such cases, the loss of lipids does not exceed 10-20% of the initial lipid content (GANDEMER et al., 1983, 1985). In contrast, lean meats contain 1.5-2.5 times more lipids after cooking. This increase is due mainly to loss of water during cooking and, to a lesser extent, to transfer of lipids (only triglycerides, GAN- DEMER, 1992) from added fat or from skin in the case of chicken. In the latter case increases in lipids are between 1 and 2.8 g/100g raw muscle (KIM and GANDE- MER, 1988) and increases due to added fat are about 1%.

Generally speaking the addition of fat during cooking and the cooking method have little effect on lipid content in meat compared to the marked effect of the anatomical origin of the cut. For example, the lipid content in a stew made from flank is four times as high as that of a stew containing hock (15.9 vs 3.6%) Similarly the lipid content of grilled rib steak is three times greater than that of rump steak (11.9 vs 3.6%), (CIV, 2000). On the other hand, certain cooking methods, particularly breaded coating, considerably increase the fat content of

Beef Veal lamb Pork Chicken Duck Turkey

Boiled flank

Roasted fillet

Roasted leg

Roasted fillet

Roasted meat^§ and

skin

Roasted meat^§

Breast sautéed

Energy (kJ) 966 675 943 667 678 795 627

Water (g) 57,1 65,1 60 65,2 66,2 64,2 65,8

Proteins (g) 29,4 28,4 25 28,8 26,4 25 29,9

Lipids (g) 12,6 5,2 14 4,8 6,2 10 3,2

Cholesterol (mg) 80 98 84 65 90 120 69

Fatty acids (saturated/

unsaturated) 0,81 0,69 0,98 0,61 0,44 0,41 0,67

Iron (mg) 3,5 1,3 2,2 1,5 1,3 2,7 1,4

Niacin (mg) 3 8,6 6 4,7 7,7 5,1 6,8

Vitamin E (mg) 0,4 0,15 0,1 0,1 0,2 0,02 n.d.

Thiamin (mg) 0,06 0,07 0,07 0,9 0,07 0,26 0,06

Vitamin B6 (mg) 0,27 0,35 0,15 0,4 0,44 0,25 0,54

Vitamin B12 (µg) 2 1,2 2 0,6 0,3 1,2 0,37

Folate (µg) 7 10 3 6 8 10 6

Sodium (mg) 52 93 60 65 80 85 156

§ Mixing of red and white meat.

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meat (x 5). The development of this type of product, particularly in fast-food out- lets, flies in the face of society’s efforts to reduce lipid content and improve fatty acid composition in meat. Similarly, certain food customs result in considerable increase in the quantity of lipids ingested when eating a meat dish, transforming the low calorie raw material into a highly calorific product.

Meat processing (processed meats, ready-cooked meals) is very common for pork (70% of production) and becoming more common for poultry (chicken, turkey). Processing of poultry currently represents 15% of production but this is expected to reach 25% by 2005 (MAGDELAINE and PHILIPPOT, 2000). The composition of processed meats may be very different from that of the meats from which they originate. In the case of pork-based processed products (Table 7), lipid contents are usually increased (except for defatted, skinned ham) with a loss of water and possibly protein (pâtés, sausages). It should however be remembered that with the genetic selection of pigs and changes in manu- factu-ring techniques (less added fat) the lipid content of processed meats has fallen by 25% in the last 30 years. Lipid content is very variable according to the product, but 50% of processed meats consumed in France contain less than 20% lipids. Saturated fatty acids represent on average 39% of the total fatty acids contained in processed meats, and this represents less than 7% of the total saturated fatty acids consumed by 80% of consumers (CIC, 2000). Pro- cessed meats and other meat-based products are also characterized by high sodium levels (generally 1-3%) which are very much higher than those of the corresponding meat, whereas the content of other micronutrients (vitamins and minerals) is about the same overall.

Table 7

Composition of some pork delicatessen products (/100 g) [from FAVIER et al., 1995]

Defatted, skinned

ham

Defatted, skinned dry

ham

Sausage Strasbourg

‘Pâté de campagne’

Poted

mince Dry sausage

Water (g) 72,2 56,3 56,4 52 41,9 33,3

Proteins (g) 18 26,3 12,6 14,3 14,5 26,3

Lipids (g) 4,2 9,5 27,7 29 41,9 34,7

Cholesterol (mg) 50 66 64 134 84 70

Fatty acids (saturated/

unsaturated) 0,6 0,62 0,64 0,67 0,69 0,65

Iron (mg) 0,8 1,4 1 5,7 1 1,3

Niacin (mg) 6 8,7 2,4 8,7 3,7 5,1

Vitamin E (mg) 0,18 0,2 0,25 0,3 traces 0,28

Vitamin B6 (mg) 0,4 0,6 0,1 0,33 0,08 0,35

Vitamin B12 (µg) 0,3 0,5 0,5 6 0,77 1,9

Folate (µg) 30 2 2 160 2 3

Sodium (mg) 900 2700 1000 710 454 2100

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4 – SENSORY AND TECHNOLOGICAL QUALITIES

The relative importance of each sensory (color, texture, juiciness and flavor) and technological (stability during storage, water retention, cooking yield) quality criterion depends on the species and type of muscle. Tenderness and color are particularly important, especially for beef; juiciness for pork and flavor for lamb. The technological qualities of meats destined for processing are particularly important for pork and poultry. Sensory and technological properties depend directly on the characteristics of the muscle tissue, particularly the properties of the myofibers and connective and adipose tissues. These are influenced by several biological factors arising from the farming phase as well as the technological factors which occur during the processing of meat and ready- cooked meat-based products. These factors have either a direct effect on the quality of the meat, by determining the composition of muscles, or an indirect effect by influencing the biochemical mechanisms which are the basis of transforming muscle into meat.

4.1 Influence of type of farming

Breed, age, sex, growth rate, rearing conditions and fattening state have an effect on lipid, pigment and collagen contents, on the metabolic and contractile types of myofiber, on their glycogen storage and on the muscle proteases invol- ved in meat ageing. It is not possible, however, to formulate general recommendations on the control of these factors because their effects on the quality of meat may be either beneficial or harmful according to species. For example, although increasing the slaughter age for animals such as Label Rouge chickens can be an advantage, because the firmness and flavor of the meat increases (CULIOLI et al., 1990, 1994), the same is not true for beef and lamb because delayed slaughter can result in tougher meat and, in the case of lamb, the occurrence of unpleasant taste. Similarly, although increased levels of unsaturated fatty acids due to feed may be valuable for beef, this is not the case for poultry and particularly not for pork, because it can lead to fat of inadequate consistency for transformation and an increase in oxidation phenomena with adverse consequences on color stability and flavor.

Farming conditions which affect animal welfare, particularly animal density, access to open air and physical activity, and stress levels may have an effect on certain muscle characteristics (changes in the respective proportions of different types of myofiber, reduction of collagen thermostability, energy reserves).

However, the effects on meat quality are not usually significant, apart from the known dark cut beef and exudative pork meat, for which the benefits of reduction of stress have been clearly demonstrated. It has been shown in chickens, ducks and geese that, although free-range farming does not influence the sensory quality of the meat, it reduces carcass fat generally (RICARD et al., 1986 (chickens), KNUST et al., 1995 (ducks), BAÉZA et al., 1998 (geese)). In pork the enrichment of the conditions of production (presence of straw, low animal density, access to open air), generally does not have an effect on the sensory quality of meat (LEBRET et al., 1998). Thus, for monogastric animals, the system of production has usually a limited influence on meat quality. However, it is

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noteworthy the impact of extensive rearing on the image of animal production for the consumer.

Finishing of ruminants on grass may also have effects on quality, particularly on flavor and tenderness of the meat and on the color of the fat. Comparison of beef originating from animals finished on grass or trough-feeding has often led to consumers stating a preference for those finished on trough-feeding, the meat being more marbled, the fat whiter, and the intensity of “country” flavor (which is not always appreciated by certain consumers) being less marked (YOUNG et al., 1999). Moreover, when possible finishing animals on grass lengthens the production period and thus increases slaughter age, which can have negative effects on tenderness. On the other hand, a beneficial effect of grass feeding has been demonstrated in the quantity of antioxidants in muscle tissue (GEAY et al., 2001)

4.2 Technological factors

It must not be forgotten that technological factors may predominate over biological and farming factors (CULIOLI, 1999). Muscle undergoes biochemical and physical changes after slaughtering, the kinetics and amplitude of which determine meat quality. The cessation of oxygen provision after bleeding initia- tes anaerobic reactions in cell metabolism, particularly glycolysis which triggers regeneration of ATP from muscle glycogen. These reactions result in accumula- tion of lactic acid and protons which are expressed by progressive acidification of muscles until the biochemical process ceases. When the production of ATP is lower than the threshold necessary for muscle relaxation, muscles loose their elasticity. All these changes correspond to rigor mortis. Their kinetics and amplitude are influenced by several factors, i.e. species, localization and metabolic characteristics of the muscle (oxidative or glycolytic), animal fatigue or stress and technological parameters during slaughter and storage (stunning, boning, refrigeration). For example, animals who have undergone a long period of transportation, stress or fasting which exhaust muscle energy reserves yield meat with high final pH levels, characterized by poor potential for storage and high water retention capacity (DFD (dark,firm,dry) beef, lamb and pork). In contrast, acute stress just before slaughter and slow refrigeration speed hasten the post mortem fall in pH and result in PSE meat (pale,soft,exudative) which is not suitable for processing, and pale and tougher than normal meat after cooking, particularly white meat (pork, turkey, chicken). It has also been extensively demonstrated that poor management of the stages of slaughter and carcass processing (stunning, boning and refrigeration) may lead to unacceptable tou- ghness of meat resulting from abnormal muscle contraction.

Following the onset of rigor mortis comes the ageing phase. This results from proteolysis associated with physico-chemical changes and causes structural changes leading to tenderization of the meat. This phase is crucial for certain types of meat such as beef and lamb, known as slow ageing types, and a little less crucial for white meat such as pork and poultry (figure 4). The speed and intensity of this process depends both on the biological characteristics of the animal and the technological parameters related to treatment and storage of carcasses (OUALI, 1990a and b). Speed of ageing increases with the speed of contraction and glycolytic nature of the muscle. The kinetics of refrigeration of muscles or carcasses also have a significant effect on the process, regulating

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the fall in pH, the muscle contraction state and post mortem proteolytic activity.

Although it appears that control of the pH/temperature combination largely regulates meat changes after slaughter, it remains difficult to define the general treatment conditions ensuring optimal quality because, as for biological factors, the effects of technological factors are very dependent on the animal species.

Figure 4

Relative course of the maturation of meat in relation with muscle type and animal species (from OUALI, 1990b)

CONCLUSION

Meat remains a chosen food in our diet. It is often the basis of meals with which other foodstuffs are associated, particularly vegetables and fruit, contributing to nutritional equilibrium and providing necessary fiber and carbohydrates.

Indeed, in relation to its caloric value, lean meat represents the highest source of the main recommended nutrients apart from glucides. With meat-based products, it thus supplies a major part of our nutritional needs in terms of proteins, group B vitamins (particularly B12) and minerals (iron, zinc, selenium). On the other hand, it only contributes a small proportion (15-20%) of nutritional lipids.

However, meat is a generic term which covers a wide range of processed and unprocessed products with a wide range of sensory and nutritional properties. Although proteins constitute a fairly constant fraction of nutrients, regar- dless of the species, muscle type, animal treatment and meats concerned, the same is not true for lipids and micronutrients. They may vary considerably, both quantitatively and qualitatively. It is therefore not possible to make nutritional recommendations directly on the basis of the characteristics of a single meat type. Similarly it would be inappropriate to generalize the problems encountered with one type of meat to all meat products.

( - ) (+)

1 2 10

Relative speed of maturation

Beef Lamb Pork

Chicken white

Red Color

Glycolytic metabolism and contraction speed of muscle

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It should also be emphasized that the nutritional value of meat is a function of many factors, particularly the processing, cooking practices and alimentary micro-behaviors. Improvements in trimming of muscles, reduction in lipid content of processed meats, elimination of skin before cooking in the case of poultry products, thus eliminate a high proportion of lipids from cooked meat, and at the same time increase protein and mineral content. The caloric contribution of lipid origin may also be considerably reduced if visible fat is removed from prepared cuts and if the fat resulting from the cooking process is not consumed.

Finally, its specific sensory characteristics make meat a highly attractive foodstuff. However these characteristics present wide and poorly controlled variability, particularly in the case of texture which is dependent on many factors, both biological and technological. Maintaining meat and meat-based product consumption at its current level will largely depend on the ability of the industry to meet the demands of the consumer in terms of quality and traceabi- lity of products. The meat chain will also have to pay greater and greater atten- tion to animal welfare (absence of suffering) during both the farming and slaughtering stages, to exclude (or limit the effects of) production and processing methods which might have negative effects on the nutritional and health value of meat products and, more generally, to subscribe to production approa- ches which respect the environment.

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