The harm of corn
At present, China is a large corn producing country with a corn planting area of ​​more than 300 million mu and a production of more than 105 million tons, ranking second in the world. Northeast and North China are the main producing areas of China's corn. Corn kernels are rich in nutrients, rich in fat, protein, starch and various vitamins, mineral trace elements, etc., and the content of UPFA (such as linoleic acid) in fat is high. Therefore, corn is also known as "golden food." As a living organism, corn kernels continue to undergo various physiological and biochemical changes during storage and are always affected by the environment. During storage, the corn granule itself's respiratory oxidation and the action of various enzymes, as well as the invasion of microorganisms and insects, cause its storage quality to decline. If it exceeds a certain period of time, it will eventually cause the death of the embryonic cells.
Aging food is not equal to toxic and harmful food. Grain aging is a natural normal phenomenon in the storage and storage of food. The food will breathe and heat. It is a living body. After storage for a certain period of time, the quality and preservation skills begin to decline. When it enters the state of aging, it is often said that the grain will be changed from new grain to grain. We usually put more than the normal storage period, the intrinsic quality of food, especially the taste of food began to decline, and the food that needs to be sold out is called “Chenhua Foodâ€. Aging food is a technical concept used to judge whether food can continue to be stored. Therefore, in this sense, aging food is not equal to toxic and harmful food. Whether the food in stock is toxic or not depends mainly on whether the toxins such as AFB1, DON and ZEA exceed the national health and safety standards, and are more toxic and harmful (Ren Zhengfei, 20160426). Among them, the fat in the aged corn is in contact with oxygen in the air during storage, and it is prone to severe spontaneous oxidative rancidity.
The oxidized components interact with each other, resulting in the auto-oxidation of various components, eventually causing the oil in the corn to produce toxic aldehydes and ketones, resulting in scent, fading, browning, etc., resulting in fat solubility in corn. Vitamins and lutein are destroyed by oxidation, which leads to a decline in feed quality. These free radicals enter the animal body and affect the intestinal mucosa, liver and nerves, causing damage to the animal's feed intake, diarrhea, hepatomegaly, etc. Animal growth and development, and in the case of poisoning and even death. Therefore, it is essential to add antioxidants to the feed. However, the addition of common antioxidants to the feed can only prevent the oxidation of fat in the feed and does not remove the free radicals produced by the oxidized fat.
Reasons for corn aging
The difficult storage of corn is mainly manifested in the following aspects: the original moisture content of corn is high, the maturity is uneven, and many immature grains and more broken grains are mixed at the time of harvest, which are highly susceptible to insects and molds; corn embryos Large part (Table 2), strong hygroscopicity: corn embryos account for almost 1/3 of the whole grain, more than 3% of protein and more soluble sugar in the embryo, strong hygroscopicity, strong breathing; corn embryo More fat, easy to rancid: corn embryos contain high fat, accounting for 77%-89% of the whole corn kernel fat content, the embryo is highly susceptible to pests during storage, rancidity begins with the embryo, so the embryonic acidity The content is always higher than the endosperm, and the rate of rancidity is very fast; the amount of bacteria in the corn germ is large, easy to mold, the nutrient part of the corn is rich, and the amount of microbes is large. After the moisture is absorbed by the corn, the mold will be large under suitable conditions. Breeding, so the corn embryo is very easy to ship when mildew.
Starch in corn is the most abundant in corn kernels (Table 1) and the most important carbohydrate (about 70% of the kernel). Starch is divided into two types, amylose and amylopectin. The former is soluble in hot water, and the solubility of amylopectin in hot water is very low. Nelles (2000) and Barron (2001) reported that under normal conditions, the change of corn starch is not the main aspect, and the change of starch is mainly the change of “qualityâ€, which is manifested in the decrease of viscosity and the increase of gelatinization temperature with storage time. .
The crude protein content in corn is 8% to 14%, with an average of 9.9%. Zhao Tongfang (1983) reported that the qualitative change of protein is mainly alcoholic, salt-soluble, and digested by pepsin and trypsin. The proteins, starch and cell walls in the endosperm themselves and their interactions increase, resulting in the formation of disulfide bonds between proteins. In 1975, Krasnook.NP reported that the normal salt-soluble protein and acid-soluble protein in the normal rice gradually decreased, and many studies have confirmed this. In addition, changes in amino acid composition and content of grain proteins during storage have been of interest to researchers, who have attempted to find the most significant amino acid change as a grain safety indicator. Baldi et al. (1977) reported that in five Italian varieties of rice, the free amino acid content decreased under normal conditions for three years, and the lysine content decreased by 14 to 46%, the most obvious decline. Yanke (1992) and Zhao Baolu (1999) found that the content of sulfhydryl (-SH) decreased, the content of disulfide bond (-SS-) increased, and the content of sulfhydryl group was positively correlated with viscosity/hardness ratio during rice aging. In summary, during storage, the total nitrogen content of the grain protein is unchanged, but the various proteins will change, the sulfhydryl groups of the protein will be converted to disulfide bonds, and the amino acids will also change.
The fat in corn includes starch fat and non-starch fat. Non-starch fat is easy to decompose, which affects the safe storage of corn. It is quite difficult to separate starch fat from starch. It is in the spiral structure of amylose and is quite stable.
The fatty acid composition of corn oil accounts for 15% of saturated fatty acids and 85% of unsaturated fatty acids. The unsaturated fatty acid mainly contains oleic acid and linoleic acid in a ratio of 1:2.5. The fatty acid composition of corn oil is generally stable, linoleic acid content is 55%~60%, oleic acid is 25%~30%, palmitic acid is 10%~12%, stearic acid is 2%~3%, linolenic acid content Very few. The content of vitamin E in corn oil is very high, reaching 100mg/100g oil.
There are two ways to change the fat during storage: one is hydrolysis, hydrolysis produces glycerol and fatty acids; the other is oxidation, which produces peroxides and carbonyl compounds, mainly aldehydes and ketones. So the corn has been extended over time. Its fatty acid value and total acid value will increase and produce scent and other abnormal odors.
Since the discovery of low-quality corn with higher fatty acids in the 1930s, it has been well established that the fatty acid and corn storage quality are well correlated. Generally, the newly harvested corn has a fatty acid value of 15-20 mg KOH/100 g dry basis during reasonable storage. The growth rate is slow, but it rises rapidly under bad conditions. The fatty acid value can reach 250 mg KOH / 100 g dry basis. The acid produced by corn, in addition to fatty acids, also phosphoric acid, lactic acid, acetic acid, amino acids, acid phosphate, etc., many studies have shown that in the process of deepening quality, fatty acids increase faster than any other acid, corn quality After the deterioration has progressed to a considerable extent, the phosphoric acid content is significantly increased, and the increase in amino acid is discovered only after the deterioration has progressed to a considerable degree.
Zhang Yong (2007) mathematical simulation of 15 different regions and storage years of corn, found that fatty acid value, peroxidase activity, malondialdehyde content can be more significant to simulate the degree of corn. Fatty acid value has always been an important indicator for the quality of corn storage. During the storage of corn, the change in fatty acid value is very obvious, thus verifying the correctness of the current fatty acid value as a measure of corn storage quality. Peroxidase is an important antioxidant enzyme in corn, which can reduce the damage of corn membrane lipid by free radicals. Malondialdehyde is an important compound produced by membrane lipid peroxidation, and the malondialdehyde content reflects the degree of membrane lipid damage. In summary, the accumulation of toxic substances, the decrease of antioxidant enzyme activity, and the deepening of membrane lipid peroxidation have an impact on the freshness of corn, which can reflect the degree of corn.
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