Protein

The diverse physical properties of proteins preclude a simple extraction method. Protein usually is not determined directly, but nitrogen is determined on the totally hydrolyzed sample. The nitrogen concentration of the average protein is 16%; consequently, the nitrogen concentration is multiplied by 6.25 to give the protein concentration. This average factor (6.25) may not be applicable to unique samples that contain a large amount of a specialized protein, e.g., collagen. There are other compounds in the body that contain nitrogen, e.g., nucleic acids, some phospholipids, etc., but these represent a small portion of the total nitrogen and are ignored in body composition determinations.

Usually nitrogen is determined after fat extraction. Heated sulfuric acid digestion of the sample is the most common approach, with clarification of the charred material by addition of hydrogen peroxide or other

Whole animal or part

Nitrogen Heat-muffle furnace

Fig. 2 Chemical determination of body composition.

Table 1 Body composition during growth

Cattlea

Pigsb

Sheepc

Weight, kg

45

364

682

5

27

92

5

32

82

Water, %

72

58

44

68

58

39

76

59

53

Protein, %

20

19

16

12

15

12

15

18

18

Fat, %

4

19

38

13

24

47

4

18

24

Ash, %

4

4

3

3

3

2

5

6

6

aAdapted from Ref. 8 and based on empty body weight. bAdapted from Ref. 9 and based on empty body weight. cAdapted from Ref. 10 and based on carcass weight.

aAdapted from Ref. 8 and based on empty body weight. bAdapted from Ref. 9 and based on empty body weight. cAdapted from Ref. 10 and based on carcass weight.

oxidizing agents. Perchloric acid digestion is sometimes used. The hydrolysis product is ammonia. In the standard Kjeldahl nitrogen method, the ammonia is distilled into weak sulfuric acid that is then titrated. In some cases, the nitrogen is determined using a colorimetric method that can readily be automated.

There are alternative methods to determine protein concentration. These colorimetric methods, e.g., biuret, Lowry, Bradford, etc., are faster than the nitrogen methods, but many interfering substances are a major problem when using complex samples from an entire animal. Consequently, these methods are not used for body composition, but are generally reserved for determination of protein concentration in more specialized samples in the laboratory.

The total amino acid content could be used to determine the protein content, but this approach is cumbersome, complex, and difficult with a heterogeneous sample from an entire animal. Determination of the amino acid composition of particular organs is very important to understand metabolic interrelationships among the diet, the animal, and the individual organs.

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