Details are as follows:
Related changes of grain aging
1, physiological changes
No matter whether there is embryo or not, the physiological changes of grain aging are mainly the changes of enzyme activity and metabolic level. In the process of grain storage, physiological changes are mostly carried out under the action of various enzymes. If the activity of enzymes in grains is weakened or lost, their physiological functions will also be weakened or stopped. With the aging process, the vitality of grain gradually loses, and the activities of enzymes related to respiration, such as catalase, tend to decrease, and the respiration also weakens. However, the activities of hydrolases such as phytase, protease and phospholipase increased.
The accumulation of toxic products of self-metabolism in storage also leads to the aging and senescence of grain, such as the accumulation of indoleacetic acid and ferulic acid and the accumulation of some lipid oxidation products, which will accelerate the aging process of grain. It is reported that some unsaturated fatty acids decompose free radicals and react with other lipids, which can destroy the cell membrane structure. In aged seeds, Golgi apparatus loses water, lysosomal membrane breaks, causing cell disintegration, while cell membrane loses integrity and permeability increases. The index of physiological changes of grain stored with embryo is that with the deepening of aging, the viability and germination rate of grain decrease, with the deterioration of cells, the permeability of cell membrane increases, the content of leachate increases and the conductivity increases.
The relationship between grain aging and enzyme activity can usually be reflected by some changes in enzyme activity related to quality. Rice contains catalase and amylase, which have high activity in the early stage of storage. With the extension of storage time, the activity of these enzymes is greatly weakened, and the activity is also reduced. After storage for three years, the activity of catalase decreased by five times, and amylase was equal to zero. During storage, the peroxidase activity of rice lost and the respiration tended to stop. Now people regard catalase activity as one of the indicators to measure the level of grain metabolism.
2. Changes in chemical composition
Generally speaking, the change of the chemical composition of grain, with or without embryo, is faster in fat, followed by protein, and very weak in starch.
2. 1 the change of fat
In the process of grain storage, free fatty acids first appear in the grain because of the easy hydrolysis of fat. Especially when the environmental conditions are suitable, the stored-grain mold begins to multiply in large quantities, secretes lipase and participates in fat hydrolysis, which increases the fatty acids in grain and deepens the aging degree of grain.
2.2 Changes in protein
During grain storage, due to the influence of external physical and biological factors, protein was hydrolyzed and denatured. After protein hydrolysis, free amino acids and acidity increased. After protein denaturation, the spatial structure is loose, the peptide bond is prolonged, the nonpolar groups are exposed, the hydrophilic groups are hidden, the protein changes from sol to gel, the solubility is reduced, and the grain aging is deepened.
2.3 Changes in starch
In the process of grain storage, maltose and dextrin hydrolyzed from starch continue to hydrolyze, reducing sugar increases, dextrin decreases relatively, viscosity decreases, and grain begins to age.
3. Changes in physical properties
The physical properties of grains change greatly during the aging process, which are manifested as follows: grain tissue becomes hard, elasticity and toughness become weak, rice becomes brittle, rice grains become hard, body bones shrink, starch cells become hard, cell membrane permeability increases, gelatinization and water absorption decrease, water retention rate also decreases, rice is broken, viscosity is poor, and taste is "off-flavor".