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MEAT, FAT AND OTHER EDIBLE CARCASS PARTS

(Types, structure, biochemistry)

Sources of meat, fat and animal by-products.

Meat, fat and other carcass parts used as raw materials for the manufacture of processed meat products are mainly derived from the domesticated animal species cattle, pigs and poultry and to a lesser extend from buffaloes, sheep and goats. In some regions other animal species such as camels, yaks, horses and game animals are used as meat animals but play only a minor role in meat processing.

In this context, meat can be defined as “the muscle tissue of slaughter animals”. The other important tissue used for further processing is fat. Other edible parts of the slaughtered animal and often used in further processing are the internal organs1 (tongue, heart, liver, kidneys, lungs, diaphragm, oesophagus, intestines) and other slaughter by-products (blood, soft tissues from feet, head).

A special group of internal organs are the intestines. Apart from being used as food in many regions in particular in the developing world, they can be processed in a specific way to make them suitable as sausage casings (see chapter on Casings, page 249). Some of them are eaten with the sausage; others are only used as container for the sausage mix and peeled off before consumption.

The skin of some animal species is also used for processed meat products. This is the case with pork skin and poultry skin, in some cases also with calf skin (from calf heads and legs).

For more details on the utilization of animal tissues for processed meat products see also chapter “Selection and grading of meat materials for processing” (page 43).


1) With the emergence of BSE (Bovine Spongiform Encephalopathy), some edible animal tissues from ruminants, in particular brain, have been declared “specified risk materials (SRM)” and have to be condemned in BSE affected areas.


Muscle meat

Chemical composition of meat

In general, meat is composed of water, fat, protein, minerals and a small proportion of carbohydrate. The most valuable component from the nutritional and processing point of view is protein.

Protein contents and values define the quality of the raw meat material and its suitability for further processing. Protein content is also the criterion for the quality and value of the finished processed meat products. Table 1 shows the chemical composition of fresh raw and processed meats.

Table 1: Content of water, protein, fat, ash (in percent) and calories
(approximate values for selected raw and processed food products)

 

Product

Water

Protein

Fat

Ash

Calories / 100g

F
R
E
S
H

Beef (lean)

75.0

22.3

1.8

1.2

116

Beef carcass

54.7

16.5

28.0

0.8

323

Pork (lean)

75.1

22.8

1.2

1.0

112

Pork carcass

41.1

11.2

47.0

0.6

472

Veal (lean)

76.4

21.3

0.8

1.2

98

Chicken

75.0

22.8

0.9

1.2

105

Venison (deer)

75.7

21.4

1.3

1.2

103

Beef fat (subcutaneous)

4.0

1.5

94.0

0.1

854

Pork fat (back fat)

7.7

2.9

88.7

0.7

812

P
R
O
C
E
S
S
E
D

Beef, lean, fried

58.4

30.4

9.2

 

213

Pork, lean, fried

59.0

27.0

13.0

 

233

Lamb, lean, fried

60.9

28.5

9.5

 

207

Veal, lean, fried

61.7

31.4

5.6

 

186

Raw-cooked sausage with coarse lean particles (ham sausage)

68.5

16.4

11.1

 

170

Raw-cooked sausage finely comminuted, no extender

57.4

13.3

22.8

3.7

277

Raw-cooked sausage

(frankfurter type)

63.0

14.0

19.8

0.3

240

Precooked-cooked sausage

(liver sausage)

45.8

12.1

38.1

 

395

Liver pate

53.9

16.2

25.6

1.8

307

Gelatinous meat mix (lean)

72.9

18.0

3.7

 

110

 

Raw-fermented sausage (Salami)

33.9

24.8

37.5

 

444

 

Milk (pasteurized)

87.6

3.2

3.5

 

63

  Egg (boiled)

74.6

12.1

11.2

 

158

  Bread (rye)

38.5

6.4

1.0

 

239

 

Potatoes (cooked)

78.0

1.9

0.1

 

72


As can be seen from the table, water is a variable of these components, and is closely and inversely related to the fat content. The fat content is higher in entire carcasses than in lean carcass cuts. The fat content is also high in processed meat products where high amounts of fatty tissue are used.

The value of animal foods is essentially associated with their content of proteins. Protein is made up of about 20 aminoacids. Approximately 65% of the proteins in the animal body are skeleton muscle protein, about 30% connective tissue proteins (collagen, elastin) and the remaining 5% blood proteins and keratin (hairs, nails).

Histological structure of muscle tissue

The muscles are surrounded by a connective tissue membrane, whose ends meet and merge into a tendon attached to the skeleton (Fig. 1(b)). Each muscle includes several muscle fibre bundles which are visible to the naked eye (Fig. 1(c)), which contain a varying number (30-80) of muscle fibres or muscle cells (Fig. 1(d) and Fig. 2) up to a few centimetres long with a diameter of 0.01 to 0.1 mm. The size and diameter of muscle fibres depends on age, type and breed of animals. Between the muscle fibre bundles are blood vessels (Fig. 1(e)) as well as connective tissue and fat deposits (Fig. 1(f)). Each muscle fibre (muscle cell) is surrounded by a cell membrane (sarcolemma) (Fig. 2, blue). Inside the cell are sarcoplasma (Fig. 2, white) and a large number of filaments, also called myofibrils (Fig. 1(g) and Fig. 2, red).

The sarcoplasma is a soft protein structure and contains amongst others the red muscle pigment myoglobin. Myoglobin absorbs oxygen carried by the small blood vessels and serves as an oxygen reserve for contraction of the living muscle. In meat the myoglobin provides the red meat colour and plays a decisive role in the curing reaction (see page 34).

The sarcoplasma constitutes about 30 percent of the muscle cell. The sarcoplasmatic proteins are water soluble. About 70 percent of the muscle cell consists of thousands of myofibrils, which are solid protein chains and have a diameter of 0.001 – 0.002 mm. These proteins, which account for the major and nutritionally most valuable part of the muscle cell proteins, are soluble in saline solution. This fact is of utmost importance for the manufacture of certain meat products, in particular the raw-cooked products (see page 97, 127) and cured-cooked products (see page 97, 171). A characteristic of those products is the heat coagulation of previously liquefied myofibril proteins. The achieved structure of the coagulated proteins provides the typical solid-elastic texture in the final products.


Fig. 1: Muscle structure
(skeletal muscle)

Fig. 2: Entire muscle fibre or
muscle cell, 0.01-0.1mm

Changes of pH

Immediately post-mortem the muscle contains a small amount of muscle specific carbohydrate, called glycogen1 (about 1%), most of which is broken down to lactic acid in the muscle meat in the first hours (up to 12 hours) after slaughtering. This biochemical process serves an important function in establishing acidity (low pH) in the meat.


1) In the live animal glycogen is the energy reserve for the muscles used as fuel for muscle contraction.


The so-called glycolytic cycle starts immediately after slaughter in the muscle tissue, in which glycogen, the main energy supplier to the muscle, is broken down to lactic acid. The build up of lactic acid in the muscle produces an increase in its acidity, as measured by the pH. The pH of normal muscle at slaughter is about 7.0 but this will decrease in meat. In a normal animal, the ultimate pH (expressed as pH24 = 24 hours after slaughter) falls to around pH 5.8-5.4. The degree of reduction of muscle pH after slaughter has a significant effect on the quality of the resulting meat (Fig. 3).

The typical taste and flavour of meat is only achieved after sufficient drop in pH down to 5.8 to 5.4. From the processing point of view, meat with pH 5.6-6.0 is better for products where good water binding is required (e.g. frankfurters, cooked ham), as meat with higher pH has a higher water binding capacity. In products which lose water during fabrication and ripening (e.g. raw ham, dry fermented sausages), meat with a lower pH (5.6–5.2) is preferred as it has a lower water binding capacity (see also page 322).

The pH is also important for the storage life of meat. The lower the pH, the less favourable conditions for the growth of harmful bacteria. Meat of animals, which had depleted their glycogen reserves before slaughtering (after stressful transport/handling in holding pens) will not have a sufficient fall in pH and will be highly prone to bacterial deterioration (see also box page 5/6).

PSE and DFD (see Fig. 3)

In stress susceptible animals pH may fall very quickly to pH 5.8 – 5.6 while the carcass is still warm. This condition is found most often in pork. It can be recognized in the meat as a pale colour, a soft, almost mushy texture and a very wet surface (pale, soft, exudative = PSE meat). PSE meat has lower binding properties and loses weight (water) rapidly during cooking resulting in a decrease in processing yields.

A reverse phenomenon may arise in animals which have not been fed for a period before slaughter, or which have been excessively fatigued during transportation and lairage. In these cases, most of the muscle glycogen has been used up at point of slaughter and pronounced acidity in the meat cannot occur. The muscle pH24 does not fall below pH 6.0. This produces dark, firm, dry (DFD) meat. The high pH cause the muscle proteins to retain most of their bound water, the muscle remain swollen and they absorb most of the light striking the meat surface, giving a dark appearance.


Dark meat has a "sticky" texture. Less moisture loss occurs during curing and cooking as a result of the higher pH and the greater water-holding capacity but salt penetration is restricted. Conditions for growth of microorganisms are therefore improved resulting in a much shorter "shelf life". DFD conditions occur both in beef and pork.

DFD meat should not be confused with that resulting from mature animals through the presence of naturally dark pigmentation. PSE and DFD conditions can to a certain extend be prevented or retarded through humane treatment and minimization of stress to animals prior to slaughter.

PSE and DFD meat is not unfit for human consumption, but not well suited for cooking and frying (PSE loses excessive moisture and remains dry due to low water binding capacity while DFD meat remains tough and tasteless due to the lack of acidity).

Nevertheless, for meat processing purposes, PSE and DFD meat can still be utilized, preferably blended with normal meat. PSE meat can be added to meat products, where water losses are desirable, such as dry-fermented sausages, while DFD meat can be used for raw-cooked products (frankfurter type) where high water binding is required.


Fig. 3: Changes of pH

Meat colouring

The red pigment that provides the characteristic colour of meat is called myoglobin. Similar to the blood pigment haemoglobin it transports oxygen in the tissues of the live animal. Specifically, the myoglobin is the oxygen reserve for the muscle cells or muscle fibres. Oxygen is needed for the biochemical process that causes muscle contraction in the live animal. The greater the myoglobin concentration, the more intense the colour of the muscle. This difference in myoglobin concentration is the reason why there is often one muscle group lighter or darker than another in the same carcass.


Fig. 4: Fresh meat cut (beef) with intense red meat colour

Myoglobin concentration in muscles also differs among animal species. Beef has considerably more myoglobin than pork, veal or lamb, thus giving beef a more intense colour (Fig. 4). The maturity of the animal also influences pigment intensity, with older animals having darker pigmentation. The different myoglobin levels determine the curing capability of meat. As the red curing colour of meat results from a chemical reaction of myoglobin with the curing substance nitrite, the curing colour will be more intense where more muscle myoglobin is available (see “Curing”, page 34).

Water holding capacity

The water holding capacity (WHC) of meat is one of the most important factors of meat quality both from the consumer and processor point of view. Muscle proteins are capable of holding many water molecules to their surface. As the muscle tissue develops acidity (decrease of pH) the water holding capacity decreases (Fig. 5, 429, 430).


Fig. 5: Compression test
1, different water holding capacity of muscles. Left: Sample with low WHC. Right: Dark meat sample with good WHC (less water pressed out)


1) Compression instrument see page 325


Water bound to the muscle protein affects the eating and processing quality of the meat. To obtain good yields during further processing including cooking, the water holding capacity needs to be at a high level (except for uncooked fermented and/or dried products which need to lose water during processing, see page 115, 171).

Water holding capacity varies greatly among the muscles of the body and among animal species. It was found that beef has the greatest capacity to retain water, followed by pork, with poultry having the least.

Tenderness and flavour

Meat tenderness plays an important role, where entire pieces of meat are cooked, fried or barbecued. In these cases some types of meat, in particular beef, have to undergo a certain ripening or ageing period before cooking and consumption in order to achieve the necessary tenderness (Fig.6). In the fabrication of many processed meat products the toughness or tenderness of the meat used is of minor importance. Many meat products are composed of comminuted meat, a process where even previously tough meat is made palatable. Further processing of larger pieces of meat (e.g. raw or cooked hams) also results in good chewing quality as these products are cured and fermented or cured and cooked, which makes them tender.


Fig. 6: Aging/ripening of beef hind quarter in cooling room

The taste of meat is different for different animal species. However, it may sometimes be difficult to distinguish the species in certain food preparations. For instance, in some dishes pork and veal may taste similar and have the same chewing properties. Mutton and sometimes lamb has a characteristic taste and smell, which originates from the fat. Even small quantities of fat, e.g. inter- and intramuscular fat, may imprint this typical smell and taste on the meat, particularly of meat from old animals. Feed may also influence the taste of meat (e.g. fish meal). In addition, the sex of the animal may also give a special taste and smell to the meat. The most striking example is the pronounced urine-like smell when cooking old boar’s meat. Meat fit for human consumption but with slightly untypical smell and flavour, which may not be suitable for meat dishes, can still be used for certain processed meat products. However, it should preferably be blended with “normal” meat to minimize the off-odour. Also intensive seasoning helps in this respect.


Beef top round slice

Pork rib chops from loin

Lamb ribs

Chicken leg

The typical desirable taste and odor of meat is to a great extend the result of the formation of lactic acid (resulting from glycogen breakdown in the muscle tissue) and organic compounds like aminoacids and di- and tripeptides broken down from the meat proteins.

In particular the aged (“matured”) meat obtains its characteristic taste from the breakdown to such substances. The “meaty” taste can be further enhanced by adding monosodium glutamate (MSG) (0.05-0.1%), which can reinforce the meat taste of certain products (see page 73). MSG is a frequently used ingredient in some meat dishes and processed meat products in particular in Asian countries.


Animal fats

Fatty tissues are a natural occurring part of the meat carcass. In the live organism, fatty tissues function as

Fatty tissue (Fig. 8) is composed of cells, which like other tissue cells, have cell membranes, nucleus and cell matrix, the latter significantly reduced to provide space for storing fat. Fats, in the form of triglycerides, accumulate in the fat cells. Well fed animals accumulate large amounts of fat in the tissues. In periods of starvation or exhaustion, fat is gradually reduced from the fat cells.


Fig. 8: Fatty tissue (fat cells filled with lipids)


Fig. 9: Intermuscular fat (a) (around individual muscles) and intramuscular fat (b) (inside muscle tissue)

In the animal body there are subcutaneous fat deposits (under the skin) (Fig. 10(a/b)) and Fig. 14(a)), fat deposits surrounding organs (e.g. kidney, heart) (Fig. 10(d) and Fig. 16(a)) or fat deposits between muscles (intermuscular fat, (Fig. 9(a)). Fat deposits between the muscle fibre bundles of a muscle are called intramuscular fat (Fig. 9(b)) and lead in higher accumulations to marbling. Marbling of muscle meat contributes to tenderness and flavour of meat. Many consumers prefer marbling of meat for steaks and other roasted meat dishes.

For processed meat products, fats are added to make products softer and also for taste and flavour improvement. In order to make best use of animal fats, basic knowledge on their selection and proper utilization is essential.


Fatty tissues from certain animal species are better suited for meat product manufacture, fats from other species less or not suited at all. This is mainly for sensory reasons as taste and flavour of fat varies between animal species. Strong differences are also pronounced in older animals, with the well known example of fat from old sheep, which most consumers refuse. However, this aspect is to some extent subjective as consumers prefer the type of animal fat they are used to.

Availability also plays a role when fatty tissues are used for processing. Some animal species have higher quantities of fatty tissue (e.g. pigs), others lesser quantities (e.g. bovines) (Table 1). Pig fat is favoured in many regions for processing purposes. It is often readily available but and has a suitable tissue structure, composition and unpronounced taste which make it readily usable. Fresh pork fat is almost odour- and flavourless. Body fats from other animal species have good processing potential for the manufacture of meat products, but the addition of larger quantities is limited by availability and some undesirable taste properties.

Pork fat


Fig. 10: Pork carcass with backfat (a), belly (b), leafe fat (c) and kidney fat (d)

The subcutaneous fats from pigs are the best suited and also most widely used in meat processing, e.g. backfat (Fig. 10(a), Fig. 12), jowl fat (Fig. 11(b), Fig. 12) and belly (Fig. 10(b) and Fig. 12). These fatty tissues are easily separated from other tissues and used as separate ingredients for meat products. Also the intermuscular fats occurring in certain locations in muscle tissues are used. They are either trimmed off or left connected (e.g. intermuscular fat in muscle tissue) and processed together with the muscle meat. Subcutaneous and intermuscular fats are also known as “body fats”. Another category are the depot-fats, located in the animal body around internal organs. These fats can also be manually separated. In rare cases mesenterical (intestinal) fats of pigs are used for soft meat products (e.g. liver sausage), but only in small quantities as they cause untypical mouthfeel in final products. The kidney fat (Fig. 10(d)) and leafe fat (Fig. 10(c), Fig. 12) of pigs are not recommended for processed meat products due to their hardness and taint, but are used for lard production.


Fig. 11: Jowl fat removed from pig head (a) and cut into strips (b). Behind: Rest of pork carcass with back fat


Fig. 12: All fatty tissues from the pork carcass: Jowl fat, back fat (above); leafe fat, belly and soft fat (below)

Beef fat


Fig. 13: Brisket fat (a) on beef cut (brisket)

Beef fat is considered less suitable for further processing than pork fat, due to its firmer texture, yellowish colour and more intensive flavour. When used for processing, preference is usually given to brisket fat (Fig. 13(a) and Fig. 14(b)) and other body fats preferably from younger animals. Such fats are used for specific processed beef products when pork fats are excluded for socio-cultural or religious reasons. Some tropical cattle breeds have a large subcutaneous fat depot in the shoulder region known as “hump”. Fat is the predominant tissue of the hump together with stabilizing connective tissue and muscle meat. The hump tissue (Fig. 15(a)) is often cut into slices and roasted/barbecued as a delicacy or used for processed products. Buffalo fat has a whiter colour than beef fat and is therefore well suited for processing. The limiting factor for utilization of beef/buffalo fat is its scarce availability, as beef/buffalo carcasses do not provide high quantities of body fats suitable for the manufacture of meat products such as frankfurters, bologna etc., where amounts of fatty tissues in the range of 20% are required. However, for the manufacture of products with a lower animal fat content, e.g. burgers, fresh sausages for frying etc., mixtures of beef and beef fat are well suited.

Fig. 14: Beef carcass, front part with external subcutaneous fat (a) and brisket fat (b) Fig. 15: Hump with fatty tissue (a) of tropical cattle Fig. 16: Kidney fat (a) in beef carcass

Mutton fat of adult animals is for most consumers absolutely unsuitable for consumption due to its typical unpleasant flavour and taste. Fats from lamb are relatively neutral in taste and commonly eaten with lamb chops. Lamb fat can be used as a fat source when producing Halal meat products.

Fat from chicken

Chicken fat is neutral in taste and well suited as a fat component for pure chicken products. Chicken fat adheres as intermuscular fat to chicken muscle tissue and is processed without separating it from the lean meat (see page 56). However, the majority of chicken fat derives from chicken skin (Fig. 17, 84) with its high subcutaneous fat content. For processing, chicken skin is usually minced (see page 56) and further processed into a fat emulsion before being added during chopping.



Fig. 17: Chicken skin to be removed from cuts and used as fat ingredient

The nutritional value of meat and meat products

a. Proteins

The nutritional value of meat is essentially related to the content of high quality protein. High quality proteins are characterized by the content of essential aminoacids which cannot be synthesized by our body but must be supplied through our food. In this respect the food prepared from meat has an advantage over those of plant origin. There are vegetable proteins having a fairly high biological value (see page 431), for instance soy protein, the biological value of which is about 65% of that of meat. Soy protein concentrates are also very useful ingredients in many processed meat products, where they not only enhance the nutritional value but primarily the water binding and fat emulsifying capacity (see page 80).

The contractile proteins or myofibrillar proteins are quantitatively the most important (some 65%) and are also qualitatively important as they have the highest biological value. Connective tissues contain mainly collagen, which has a low biological value. Elastin is completely indigestible. Collagen is digestible but is devoid of the essential aminoacid tryptophan.

Blood proteins have a high content of tryptophan but are nevertheless of a lower biological value than meat due to their deficiency of the essential aminoacid isoleucine.

b. Fats

Animal fats are principally triglycerides. The major contribution of fat to the diet is energy or calories. The fat content in the animal carcass varies from 8 to about 20% (the latter only in pork, see table 1). The fatty acid composition of the fatty tissues is very different in different locations. External fat (“body fat”) is much softer than the internal fat surrounding organs due to a higher content of unsaturated fat in the external parts.

The unsaturated fatty acids (linoleic, linolenic and arachidonic acid) are physiologically and nutritionally important as they are necessary constituents of cell walls, mitochondria and other intensively active metabolic sites of the living organism. The human body cannot readily produce any of the above fatty acids, hence they have to be made available in the diet. Meat and meat products are relatively good sources, but in some plant sources such as cereals and seeds, linoleic acid is usually present at about 20 times the concentration found in meat.

In recent years it has been suggested that a high ratio of unsaturated / saturated fatty acids in the diet is desirable as this may lower the
individual’s susceptibility to cardiovascular diseases in general, and to coronary heart disease in particular. There is evidence to indicate that a diet which predominantly contains relatively saturated fats (such as those of meat) raises the level of cholesterol in the blood. To avoid possible health risks from the consumption of the meat, vulnerable groups should reduce the animal fat intake.

In this context, the “hiding” of high fat contents in some processed meat products can be a dietary problem. Improved processing equipment and techniques and/or new or refined ingredients has made it possible to produce meat products with relatively high fat contents, which may be difficult to recognize by consumers. In particular in products like meat loaves, frankfurter type sausages or liver pate, where meat and fat are finely comminuted and the fat particles are enclosed in protein structures, the fat is difficult to detect visibly. Fat contents of up to 40% may be hidden this way, which is profitable for the producer as fat is a relatively cheap raw material. For some consumer groups, such diets are not recommended. On the other hand, there are many physically active hard working people or undernourished people, in particular in the developing world, where meat products with higher fat content may be beneficial in certain circumstances, predominantly as energy sources.


Fig. 18: Meat loaves with different fat contents; Left lower fat (20%) and right high fat (35%)

c. Vitamins

Meat and meat products are excellent sources of the B-complex vitamins (see table 2). Lean pork is the best food source of Thiamine (vitamin B1) with more than 1 mg / 100 g as compared to lean beef, which contains only about 1/10 of this amount. The daily requirement for humans of this rarely occurring vitamin is 1-1.5 mg. Plant food has no vitamin B12, hence meat is a good source of this vitamin for children, as in their organisms deposits of B12 have to be established. On the other hand, meat is poor in the fat soluble vitamins A, D, E, K and vitamin C. However, internal organs, especially liver and kidney generally contain an appreciable percentage of vitamin A, C, D, E and K. Most of the vitamins in meat are relatively stable during cooking or processing, although substantial amounts may be leached out in the drippings or broth. The drip exuding from the cut surface of frozen meat upon thawing also contains an appreciable portion of B-vitamins. This indicates the importance of conserving these fractions by making use of them in some way, for example through direct processing of the frozen meat without previous thawing (which is possible in modern meat processing equipment). Thiamine (vitamin B1) and to a lesser extent vitamin B6 are heat-labile. These vitamins are partially destroyed during cooking and canning.

Table 2: Average content of vitamins in meat (micrograms per 100g)

Food

B1

B2

B6

B12

A

C

Beef, lean, fried

100

260

380

2.7

20

1

Pork, lean, fried

700

360

420

0.8

10

1

Lamb, lean, fried

105

280

150

2.6

45

1

Veal, lean, fried

70

350

305

1.8

10

1

Pork liver, fried

260

2200

570

18.7

18000

24

d. Minerals

The mineral contents of meat (shown as “ash” in table 1) include calcium, phosphorus, sodium, potassium, chlorine, magnesium with the level of each of these minerals above 0.1%, and trace elements such as iron, copper, zinc and many others. Blood, liver, kidney, other red organs and to a lesser extent lean meat, in particular beef are good sources of iron. Iron intake is important to combat anaemia, which particularly in developing countries is still widespread amongst children and pregnant women. Iron in meat has a higher bio-availability, better resorption and metabolism than iron in plant products.

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