Food Chemistry

Module 9. Cereals and cereal products

Lesson 22
INDIVIDUAL CONSTITUENTS – PROTEINS, LIPIDS, CARBOHYDRATES AND VITAMINS IN CEREALS FLOUR AND THEIR RELATIONSHIP IN DOUGH MAKING

22.1 Introduction

Cereal products are amongst the most important staple foods of mankind. Nutrients provided by bread consumption in industrial countries meet close to 50% of the daily requirement of carbohydrates, one third of the proteins and 50–60% of vitamin B. Moreover, cereal products are also a source of minerals and trace elements. The major cereals are wheat, rye, rice, barley, millet and oats. Wheat and rye have a special role since only they are suitable for bread-making. Generally cereals belong to species of the wild grasses.

22.2 Structure of Cereal Grains

The main structural features of cereal grain are (Fig. 22.1)

1. The embryo or germ: The embryo is small and is attached to the base of the seed. It is the embryo from which the root and leaf of the new plant are formed. The germ is rich in lipids and high in total nitrogen and ash.
2. Endosperm: The endosperm makes up the major portion of the seed. It supplies the sprouting embryo with food in the period before the root and leaf begin to function. The endosperm cells consist of mainly of starch and proteins. The starch is form of spherical granules embedded in a matrix of protein. It is the main source of white flour and semolina.
3. Bran: It is dark coloured and consists of several layers of fibre. The layer of cells surrounding the endosperm is known as aleurone. In milling the aleurone layer separates with the bran. The bran has unusually high percent of crude fiber and ash. Crude fibre includes cellulose, hemicelluloses and lignins.

22.3 Composition of Cereal Grains

The composition of the cereals varies depending on variety, geographical and other conditions. The major compositional features are as follows:

• 80% of the dry matter of cereals is carbohydrates mainly starch and dietary fibers.

• Cereals generally contain 10-12% of protein. Proteins are found in all tissues of the cereal. Lysine content is low in all cereals. Methionine is also low, particularly in wheat, rice, barely and corn.

• Lipids are present to the extent of 1-2% in wheat and rice and 3% in maize, while oat contains as much as 5.7%. More lipids are present in germ and bran. Linoleic acid is the predominant fatty acid in cereal lipids.
• About 95% of the minerals are the phosphates and sulphates of potassium and magnesium. Cereals are poor sources of calcium and iron.
• Whole grain cereals are important sources of B vitamins. Since most of these vitamins are in the bran, refining or polishing the grains reduces vitamin B content.

• Cereals grains contain many enzymes of which amylases, proteases, lipases and oxido-reductases are of importance in cereal processing.

22.4 Individual Constituents of Wheat and their Importance in Baking

22.4.1 Proteins

In 1907 T. B. Osborne separated wheat proteins on bases of solubility into four fractions. Designation in wheat % Total wheat

Albumins: as leukosin in wheat 14.7

Globulins: as edestein. 7.0

Prolamins: as gliadin 32.6

Glutelins: as glutenin 45.7

Wheat flour contains soluble and insoluble protein fractions, the soluble proteins comprise 20% of total proteins and include albumins and globulins and certain minor glycoproteins. These proteins do not contribute to dough forming properties of wheat flour.

The insoluble wheat proteins are prolamins and glutenins that exist in ratio 2:3. They are also referred as gluten proteins. Gluten is the major storage protein in wheat. It is the heterogeneous mixture of gliadin and glutenin. Gluten is formed when water flour mixture is kneaded to form dough. Gluten proteins are responsible for formation of viscoelastic dough capable of entrapping gas during fermentation.

Both fractions of insoluble wheat proteins in hydrated forms have different effect on rheological characteristics of the dough. The prolamins are responsible mainly for viscosity of glutelins is responsible for dough elasticity. The gluten proteins in association with lipids are responsible for cohesive and viscoelastic flour properties of the dough that is suitable for making bread and other bakery products. Gluten consists of 90% of protein, 8% lipids and 2% carbohydrates.

22.4.1.1 Mechanism of dough and gluten formation

Several physical and chemical transformations take place during mixing and kneading of the mixture of water and wheat flour. Under the applied shear and tensile forces, gluten proteins absorb water and partially unfold. The partial unfolding of proteins facilitates hydrophobic interactions and sulphydryl-disulfide interchange reactions that result in formation of thread like polymers. These linear polymers in twin are believed to interact with each other presumably via hydrogen bonding, hydrophobic association and disulfide links to form a sheet like film capable of entrapping gas. Therefore optimum ratio of prolamins and glutelins is necessary to form a visco-elastic dough.

22.4.2 Lipids

Cereals are generally considered to have low lipid content. Germ and bran of the grain contain higher concentration of fat than other parts. In wheat kernels-the germ and aleurone cells are rich in triglycerides, while phospholipids and glycolipids are predominant in endosperm. On an average, the wheat germ contains 6 to 11 % of the lipids, bran 3 to 5% and endosperm 0.8% to 1.5%. Wheat flour contains 1.5 to 2.5% lipids depending on milling extraction rate. Wheat flour lipids are differentiated by their solubility.

Non-starch lipids comprise of around 75% of total lipids of the flour. Remaining 25% lipids are bound to starch. The non-starch and starch bound lipids differ in their composition. In non-starch bound lipids the major constituents are triglycerides and glycolipids, while the starch bound lipids contains lysophosphatides. The non-starch lipids are further fractionated into free and bound form by solvent extraction. Both free and bound lipids contain non-polar glycolipids and phospholipid, the free lipids fractions contains 90% of the total non-polar lipids and 20% of total polar lipids of wheat flour.

By kneading the flour into dough the glycolipids becomes completely bound with gluten, while binding of other lipids is only 70 to 80 %, resulting in a starch-protein-lipid complex matrix. Non-starch lipids affect the rheological properties of dough. While starch bound lipids affect properties of the baked products.
The lipids are enclosed within the amylose helices, the lipids complexed within the starch granules retard swelling and increase their geletinization temperatures; thus they influence the baking behaviour of cereals and the properties of baked product. Polar lipids positively influence the gas holding capacity of the doughs and baking volume while non-polar lipids generally negatively influence the baking results.

22.4.3 Carbohydrates

1) Starch: Starch is the major storage form of carbohydrate in cereals and occurs only in endosperm cells. Cereal starches consist of 25% amylose and 75% amylopectin. Starch granules swell when heated in water suspension. At the end of swelling they loose their native structural form and arrangement i.e. they are gelatinized.
Starch contributes to viscoelastic strength of the semi rigid structure formed along with gluten, during baking Lipids and proteins are heterogeneous constituents of starch granules.
2) Other polysaccharides: Cereals contain polysaccharides other than starch, but in endosperm their content is less than that of starch. They include hemicelluloses, pentosans, cellulose, β-glucans and glucofructans. These polysaccharides are primarily constitutents of cell wall and are more abundant in the outer portions of the kernels.
3) Pentosans: Its content in wheat flour is 2-3%. A portion of pentosans is water soluble. The soluble pentosans are able to absorb 25 times more water and thus can form highly viscous solution. The insoluble form of pentosans swells extensively in water. This portion increases crumb juiciness and chewablity of baked products. Pentosans play an important role in wheat baking quality since they also participate in gluten formation.
4) β-glucan: β-glucan content in wheat flour is only 0.5-2%. They are linear polysaccharides with D-glucopyranose units linked by β (1à3) and β (1à4) linkages. These polysaccharides are known as lichenins. They are slightly mucous and provide high viscosity to water solution.
Sugars: Sugars include mono-, di- and trisaccharides as well as other low molecular weight degradation products of starch. They occur in relatively low concentrations but are important for dough leavening in the presence of yeasts.

22.4.4 Enzymes

Cereal grains contain many enzymes and of these the amylases, proteinases, lipases and oxidoreductases play an important role in cereal processing. Some of them are described below.

1) Amylases: α- and β-amylases are found in all cereals. Their optimum activities are desirable in dough making in presence of yeast.
2) Proteinases: Acid Proteinases are present in wheat flour. It is possible that proteinases are involved in cleavage of gluten bonds, thereby affecting the softening or mellowing of gluten during baking.
3) Lipases: The lipases are responsible for the fatty acids appearing during the storage of cereals and their products.

Table 22.1 Chemical composition of cereal grains and products

(Source: Nutritive value of Indian Foods, National Institute of Nutrition (NIN), Indian Council of Medical Research (ICMR), Hyderabad, 1994)