MAJOR CHEMICAL COMPONENTS OF CEREAL GRAINS

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Compositionally, cereals consist of 12-14 percent water, 65-75 percent carbohydrates, 2-6 percent lipids and 7-12 percent protein. Cereals are quite similar in gross composition being low in protein and high in carbohydrates (Table 2). Oats and maize however contain relatively large amounts of lipids. Oats contain at least 10 percent lipids, one-third of which are polar lipids (phospho- and galacto-lipids). The lipid content of maize ranges between 0.4 percent and 17 percent, most of which are triacylglycerides (Eliasson and Larsson 1993). Different cultivars of a given type of cereal exhibit compositional variability.

The chemical components of cereals are not uniformly distributed in the grain (Table 3). Hulls and bran are high in cellulose, pentosans and ash. The aleurone layer of wheat contains 25 times more minerals than the endosperm; whereas the lipids are generally concentrated in the aleurone and germ. The endosperm, which contains mostly starch, has a lower protein content than the germ and the bran, and is low in fat and ash.

Proteins

Early workers divided the proteins of wheat into four solubility classes called Osborne fractions: albumins, which are water soluble; globulins, which are soluble in salt solutions, but insoluble in water; gliadins, which are soluble in 70-90 percent alcohol; and glutenins, which are insoluble in neutral aqueous solutions, saline, solutions, or alcohol. The respective protein fractions from wheat are also applicable to other cereals and are generally known as albumins, globulins, prolamines, and glutelins. The distribution of these protein fractions varies among different cereals (Table 4). There is considerable variation in the solubility classes among the cereals and also to some extent within each species of cereal. Albumins range from 4 percent in maize to 44 percent in rye, globulins from 3 percent in maize to 55 percent in oats, prolamins from 2 percent in rice to 55 percent in maize, and the glutelins from 23 percent in oats to 78 percent in rice.

Table 2. Proximate composition of cereal grains1

CEREAL

CRUDE
PROTEIN

CRUDE
FAT

ASH

CRUDE
FIBRE

AVAILABLE CARBOHYDRATE

Brown Rice

7.3

2.2

1.4

0.8

64.3

Sorghum

8.3

3.9

2.6

4.1

62.9

Rye

8.7

1.5

1.8

2.2

71.8

Oats

9.3

5.9

2.3

2.3

62.9

Maize

9.8

4.9

1.4

2.0

63.6

Wheat

10.6

1.9

1.4

1.0

69.7

Barley

11.0

3.4

1.9

3.7

55.8

Pearl Millet

11.5

4.7

1.5

1.5

63.4

1 percent dry weight; from Alais and Linden (1991)  

Table 3. Distribution of major components of wheat grain 1

FRACTION

PROPORTION

PROTEIN

LIPID

MINERALS

Whole grain

100

12

2.0

2.0

Endosperm

80

10

1.2

0.6

Aleurone

8

18

8.5

15.0

Seed coat

8.5

6

1.0

3.5


1 percent dry weight; from Alais and Linden (1991)

Among the Osborne fractions in cereals, the prolamin fraction has been the most studied (Eliasson and Larsson 1993). This fraction is called gliadin in wheat, secalin in rye, hordein in barley, avenin in oats, and zein in maize. The fraction includes several protein bands when analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis under both reducing and non-reducing conditions. The high molecular weight subunits of prolamins constitute a higher percentage of the total in wheat than in other cereals (Shewry and Mifflin 1985). The baking quality of wheat flour from different varieties is influenced by the glutelin content (Eliasson and Larsson 1993); however, rice flour, with it’s high glutelin fraction, does not form gluten. The albumin and globulin fractions of cereals are also a complex mixture of proteins; however, they are of relatively low molecular masses and remain unchanged in size following reduction of their disulfide bonds. It is now recognized that cereal proteins exhibit biochemical polymorphism and can be distinguished through electrophoresis of the gliadin fraction (Alais and Linden 1991).

Table 4. Distribution of Proteins in Osborne Solubility Classes1

CEREAL

ALBUMIN

GLOBULIN

PROLAMIN

GLUTELIN

Wheat

9-15

6-7

33-45

40-46

Rye

10-44

10-19

21-42

25-40

Barley

12

8-12

25-52

52-55

Oats

10-20

12-55

12-14

23-54

Rice

5-11

10

2-7

77-78

Sorghum

4

9

48

37

Maize

4-8

3-4

47-55

38-45

1 Data from Eliasson and Larsson (1993); Alais and Linden (1991)

Osborne fractions from different cereals exhibit similarities in the proportions of the amino acids glutamine (Glx), proline (Pro), glycine (Gly), and cysteine (Cys) (Wiesner et al. 1980) (Table 5). Glutamine, proline and glycine are principal amino acids in all cereal protein fractions. Differences in amino acid composition from wheat cannot explain the poorer baking performance of cereals such as rye and barely.

Sulfhydryl-disulfide interchanges are the major reactions responsible for the formation of wheat dough. The gliadin and glutenin fractions of wheat, represent 80-85 percent of the wheat endosperm protein and these fractions together make up the gluten. It appears that a specific pattern of interaction between low molecular weight glutenin (<90 KDa) and high molecular weight glutenin (> 90 Kda) is important for the development of a viscoelastic gluten (MacRitchie 1992).

Table 5. Partial amino acid composition (mole per cent) of Osborne fractions from different cereals1

 

WHEAT

RYE

BARLEY

OATS

RICE

MAIZE

ALBUMINS

AMINO ACID

 

21

 

23

 

14

 

13

 

15

 

13

Glx            

Pro

10

12

8

6

5

9

Gly

7

7

10

13

10

10

Cys

3

2

4

7

2

2

Lys

3

3

4

5

5

4

GLOBULINS

AMINO ACID

           

Glx

16

17

13

16

15

11

Pro

7

8

7

5

6

6

Gly

9

9

10

10

10

11

Cys

4

2

3

2

4

3

Lys

4

4

5

5

3

5

PROLAMINS

AMINO ACID

           

Glx

38

36

36

35

20

20

Pro

17

19

23

10

5

10

Gly

3

5

2

3

6

3

Cys

2

2

2

3

1

1

Lys

1

1

1

1

1

trace

GLUTELINS

AMINO ACID

           

Glx

31

20

25

19

16

16

Pro

12

10

15

6

5

12

Gly

8

9

7

8

8

7

Cys

1

1

1

1

1

2

Lys

2

4

3

3

3

2

1 Data from Wiesner et al. (1980)

Carbohydrates

In general, carbohydrates constitute about 75 percent of the solid content of cereals. In cereals, as in other plant tissues, carbohydrates are localized in (1) the cell wall, (there are especially thickened walls in supporting tissues of husk and seed coat) (2) plastids, where starch constitutes the largest proportion of carbohydrates in all cereals, and (3) in vacuoles or the cytoplasm.

The principal constituents of cell walls are cellulose, hemicelluloses, pectins, and lignin. The hemicelluloses are a heterogeneous group of polysaccharides that contain numerous kinds of hexose and pentose sugars and in some cases residues of uronic acids. These polymers are classified according to the predominant sugar residue and are individually referred to as xylans, arabinogalactans, etc. Cell walls are the main components of "dietary fiber". The biological availability of protein, minerals and other nutrients, such as vitamin B1 in rice, may be reduced by its fibre constituents (Torre and Rodriquez 1991). On the other hand, there is now considerable evidence for the beneficial role played by fiber in health and disease (Anderson et al. 1990). Dietary fiber absorbs water and provides roughage for the bowels, assisting intestinal transit. The crude fiber content of cereals varies a great deal, ranging from as low as 0.5 percent for brown rice to as high as 10.9 percent for oats (Chaven and Kadam 1989; Eliasson and Larsson 1993).

The principal carbohydrate of all cereals is starch, representing 56 percent (oats) to 80 percent (maize) of the grain dry matter (Eliasson and Larsson 1993). Cereal starches are similar in composition, having 74-79 percent amylopectin, 25-30 percent amylose, and 1 percent lipids. High-amylose and high amylopectin ("waxy") cereal cultivars have also been developed. The baking performance of cereal starches of similar amylose and amylopectin contents (1:4) are however different, with maize starch exhibiting particularly poor qualities (Hoseney et al. 1971) (Table 6). The presence of lipid in cereal starches is a distinguishing feature of these starches (Morrison et al. 1984). Gelatinization temperatures of different cereal starches also show considerable variation. For example, maize and rice starches gelatinize at temperatures 10-200C higher than wheat, rye or oat starches (Eliasson and Larsson 1993). There is also considerable variation in the transition temperatures of starches within species. Interactions of cereal starches with protein and lipids is known to influence physicochemical characteristics such as gelatinization and retrogradation.

Table 6. Bread loaf volume with various cereal starches1

STARCH
SOURCE

LOAF
VOLUME (ml)

Wheat

80

Barley

78

Rye

77

Rice

68

Oats

58

Maize

48

1 Data from Eliasson and Larsson (1993).
Wheat flour was reconstituted with indicated cereal starch

Flours prepared from oats, barley, and rye contain a relatively high percentage (5-25 percent of total carbohydrates) of nonstarch polysaccharides. The pentosan fraction of cereals is a complex mixture of branched polysaccharides with an arabinoxylan backbone containing small amounts of glucose and ferulic acid. Rye flour contains a relatively high content of water soluble pentosans which are capable of absorbing large amounts of water to form gels. Wheat contains fewer pentosans than rye, and has a higher xylose/arabinose ratio than does rye (Eliasson and Larsson 1993). The -glucans of barley play an important role in beer production and those of oats are of interest because of their health benefits as dietary fiber. Chemically, these molecules contain both (13) and (14) linkages of D-glucopyranose. Their high viscosity and slimy consistency can cause wort filtration problems in brewing. The molecular weights of -glucans from rye, oats, and barley are known to differ (Woods et al. 1991).

Lipids

Oats and maize are unique amongst the cereals in that cultivars may contain a relatively high lipid content, e.g., >10 percent for oats and as high as 17 percent for some maize cultivars compared to about 2-3 percent for wheat and most other cereals. The polar lipid content of oats is greater than that of other cereals since much of the lipid fraction is contained within the endosperm. In most cereals the lipid fraction is concentrated in the germ and in the bran milling fractions (Table 7). About one-third of oat lipids are polar (8-17 percent glycolipids and 10-20 percent phospholipid). On the other hand, maize lipids are predominately acyltriglycerides in cultivars having a high total lipid content. The distribution of lipid classes is similar in wheat, barely and rye which contain about 65-78 percent nonpolar lipid, 7-13 percent galactolipid and 15-26 percent phospholipids (Morrison 1978). In wheat, the glycolipids play an important role in gluten development during bread-making (Pomeranz and Chung 1978).

Table 7. Crude lipid contents of rice and wheat milling fractions 1

GRAIN/FRACTION

% CRUDE FAT
DWB

Rice

 

Brown rice

2-4

Milled endosperm

<1

Bran

15-22

Embryo

15-24

Polished

9-15

Wheat

 

Whole grain

2

Pericarp

1

Aleurone

9

Starch endosperm

1

Germ

10

1 Adapted from Haard (1996)

The major fatty acids in cereal grain lipids are linoleic, oleic and palmitic (Haard and Chism 1996) (Table 8).

Table 8. Principal fatty acids (percentage) of some cereal oils 1   

FATTY ACID

CORN

WHEAT

RYE

RICE

C:14:0

-

3

6

1

C:16:0

6

18

11

28

C:18:0

2

7

4

2

C:18:1

44

31

18

35

C:18:2

48

57

35

39

C:18:3

-

4

7

3

1 Adapted from Haard (1996)

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