Protein

Protein is the major nitrogen-containing component of cereal grains, and most protein data are based on nitrogen determination, followed by multiplication by a nitrogen-to-protein conversion factor. The general factor is 6.25, but factors vary from 5.7 to 6.31 for cereal products. Representative values for protein content (Tables 3-6) show that levels are lowest in rice, barley, and finger millet and highest in wheat, oats, pearl millet, and proso millet. However, the protein content of cereals can vary substantially, and greater than 2-fold ranges in protein content are found in crops of the same species. This variation is due partly to genetic differences, but agronomic factors are of greater importance. This variation is of little significance when crops are handled in bulk, as in modern milling and processing, but it may be important in less-developed regions. Although not usually considered a good protein source, many cereals provide an adequate amount, relative to energy, for adults. However, protein quality must also be considered, since cereal-based diets tend to be deficient in one or more essential amino-acids (see below).

Protein quality Cereal protein consists predominantly of endosperm storage proteins, which are low in dietary essential (indispensable) amino-acids. These amino-acids are required in differing amounts, and thus quality can be considered only in relation to requirements. Furthermore, requirements can differ. The most notable differences are related to age: the young have higher requirements for both protein and essential amino-acids than do adults. The first limiting essential amino-acid in cereals is generally lysine. However, there are variations between cereals. In oats, rice, and finger millet the deficiency in lysine may be only

Table 7 Amino-acid composition of rice, maize, wheat, barley, oats, and rye; representative values in grams per 100g protein

Amino-acid

Rice

Maize

Wheat

Barley

Oats

Rye

Essential

Histidine

2.4

2.6

2.3

2.1

2.1

2.2

Isoleucine

3.8

3.6

3.5

3.5

3.8

3.5

Leucine

8.2

11.1

6.7

6.7

7.2

6.2

Lysine

3.7

2.3

2.7

2.6

3.7

3.4

Methionine

2.1

1.6

1.2

1.6

1.8

1.4

Cysteine

1.6

2.0

2.5

2.2

2.7

1.9

Phenylalanine

4.8

4.4

4.6

5.1

5.0

4.5

Tyrosine

4.0

3.5

1.7

3.0

3.4

1.9

Threonine

3.4

3.3

2.8

3.4

3.4

3.4

Tryptophan

1.3

0.7

1.5

1.6

1.3

1.1

Valine

5.8

4.0

4.3

5.0

5.1

4.8

Non-essential

Alanine

5.8

8.2

3.5

4.2

4.5

4.3

Arginine

7.5

4.4

4.3

4.8

6.2

4.6

Aspartic acid

9.6

7.2

4.9

5.6

7.7

7.2

Glutamic acid

19.2

18.6

32.1

23.5

21.0

24.2

Glycine

4.3

3.9

4.0

3.8

4.6

4.3

Proline

4.6

8.8

10.7

10.9

5.1

9.4

Serine

4.6

4.6

4.5

4.0

4.6

3.8

marginal, while in sorghum, maize, and other millets it is more pronounced (Tables 7 and 8). Tryp-tophan is also limiting in maize and some millets, while threonine and methionine may also be limiting in some cereals. Protein quality must be considered in relation to total protein content. Furthermore, as protein content is increased, for example by the use of nitrogenous fertilizer, the relative amounts of the essential amino-acids tend to decline as percentages of the protein. High-lysine types of maize, barley, and sorghum have been identified. However, their lower grain yield precludes their wide use.

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