2.2.2  Chemical changes

Cereals are characterized by relatively low protein and high carbohydrate contents contained in kernel. The germ is rich in protein, fat, sugar and minerals whereas, the endosperm is low in protein, fat and ash contents. The various chemical changes that occur during storage are due to increased activity of endogenous and exogenous enzyme responsible for quantitative and qualitative changes in carbohydrates, proteins and fats of the cereals in addition to colour, flavour and texture.

Carbohydrate

In India, the temperature and relative humidity  varies greatly which causes physical   and   biochemical   changes in grains during storage, such as bursting and gelatinization of starch and depending upon the moisture content. Amylases hydrolyse the starch   into   dextrose   and   maltose and significantly increase the content of reducing sugars during storage. Storage  of  wheat  grains  above  12 per cent moisture increased sucrose, glucose, fructose and raffinose contents. Storage of cereals with high moisture content also produces sour odour due to production  of alcohols and acetic acid.

Protein

The high temperature and use of chemicals in grains during  storage (Fig. 2.8) denature the proteins and make them less dispersible in water, deteriorates the gluten quality and increase the free amino acid contents. The formation of certain sulphur containing amino acids impart bad odour. The free amino acids may also undergo mail lard reaction combining with the reducing sugars giving browning of the grains. The type of deterioration is possible at temperature above 20ºC and relative humidity between 60-70 per cent.

Lipid

Oxidation of lipids especially the unsaturated fatty acids results in typical rancid flavour, odour and taste. Hydrolysis of lipids also increases free fatty acid contents, which is considered as a sensitive index for grain deterioration.

Contaminants

Food grains are usually contaminated with foreign materials viz stones, chaffs, poisonous weeds, excreta of insects, pests, rodents etc. which gives poor look to the grains. Limits of weed presence, uric acid and insect excreta should be as described by the Government of India (FCI) for the stored food grains. Before any recommendation is made in this regard, complete evaluation of these materials is required with respect to their chemical composition, toxins, residues of insecticides, pesticides and finally the in vivo feeding value of different categories of livestock