The harm of aging corn

　　At present, China is a big corn producer, with a corn planting area of more than 300 million mu and a yield of more than 105 million tons, ranking second in the world. Northeast China and North China are the main corn producing areas in China. Corn grains are rich in nutrition, rich in fat, protein, starch, various vitamins, mineral trace elements, etc., and high in UPFA(such as linoleic acid) in fat. Therefore, corn is also called "gold food". Corn kernels, as a living organism, are constantly undergoing various physiological and biochemical changes even after harvest and during storage, and are always affected by the environment. During the storage period, the respiration and oxidation of corn grains, the action of various enzymes, the invasion of microorganisms and insect pests, etc., reduce the storage quality of corn grains, and eventually lead to the death of embryonic cells after a certain period.

　　Aged grain does not equal poisonous and harmful grain. Grain aging is a natural normal phenomenon in the storage and storage of grain. Grain will breathe and get hot, which is a living organism. After being stored for a certain period, the quality and freshness preservation skills begin to decline and enter the state of aging, which is often said that at this time, grain will become old grain from new grain. We usually put beyond the normal storage life, the internal quality of grain, especially the food taste began to decline, need to sell out of the grain known as "stale grain". Aged grain is a technical concept used to determine the sustainability of grain storage. Therefore, in this sense, aged grain is not toxic or harmful grain. Whether the grain in stock is toxic or harmful mainly depends on whether the toxins such as AFB1, DON and ZEA exceed the national health and safety standard, exceeding the toxic or harmful level (Ren Zhengfei, 20160426). Among them, the fat in aged corn is in contact with oxygen in the air during storage, which is prone to serious spontaneous oxidation and rancidity.

　　Oxidized also interaction between these elements, leading to a variety of ingredients of autoxidation and eventually make corn oil produce toxic substances of aldehydes and ketones, leading to clam demodex flavour, fade, Browning, and so on, make the fat-soluble vitamins and lutein from maize by oxidative damage, light led to the decrease of the feed quality, these free radicals into animals in intestinal mucosa, liver and nerve tissues, animal feed intake down, harm, such as diarrhea, hepatomegaly, affect the animal growth and development, or poisoning or even death caused by accidents. So adding antioxidants to the diet is essential. But the addition of ordinary antioxidants in the feed can only prevent the oxidation of fat in the feed, but can not eliminate the free radicals produced by the oxidized fat.

　　Causes of aging of corn

　　The difficulty of maize storage is mainly reflected in the following aspects: high original water content, uneven maturity, mixed with many immature grains and more damaged grains during harvest, which are easily attacked by insect pests and mold; The maize embryo is very large (Table 2) and has strong hysteretic property: the maize embryo accounts for almost 1/3 of the whole grain. The maize embryo contains more than 3% protein and more soluble sugar. It has strong hysteretic property and vigorous respiration. Maize embryo contains a lot of fat and is prone to rancidity: Maize embryo contains high fat, which accounts for about 77%-89% of the fat content of the whole maize grain. During storage, the embryo is vulnerable to pest invasion. Corn embryo has a large amount of bacteria, which is easy to be mildewed. Corn embryo is rich in nutrients and has a large amount of microbial attachment. After corn embryo is hygroscopic, molds will multiply in large quantities under appropriate conditions, so corn embryo is easy to be mildewed when it is delivered.

　　In corn, starch is the most abundant (Table 1) and the most important carbohydrate (accounting for more than 70% of the corn). Starches are divided into amylose and amylopectin, the former is soluble in hot water, amylopectin is very low solubility in hot water. Nelles(2000) and Barron(2001) and other reports, under normal circumstances, corn starch change is not the main aspect, starch change is mainly the "quality" of the change, specific performance as the storage time is prolonged viscosity in the decline, gelatinization temperature increases.

　　The crude protein content in corn was 8%~14%, and the average was 9.9%. Zhao Tongfang (1983) reported that the qualitative changes of proteins were mainly manifested as reduced solubility in alcohol and salt, and reduced digestibility by pepsin and trypsin. The proteins, starches and cell walls themselves and their interactions in the endosperm increase, resulting in the formation of disulfide bonds between proteins. In 1975, Krasnok. N.P reported that normal rice would gradually decrease in medium salt soluble protein and acid soluble protein, which was later confirmed by many studies. In addition, the changes in the amino acid composition and content of grain proteins during storage have been of interest to researchers, who have tried to find the most obvious amino acid changes as an indicator of grain safety. Baldi et al. (1977) reported that the content of free amino acids in five Italian varieties of rice decreased in three years under conventional conditions, among which the content of lysine decreased by 14-46%, with the most obvious decrease. Bian Ke (1992) and Zhao Baolu (1999) found that the content of thiol group (-SH) decreased while the content of disulfide bond (-S-S-) increased during the aging process of rice, and the content of thiol group was positively correlated with the viscosity/hardness ratio. To sum up, during the storage period, the total nitrogen content of grain proteins remains the same, but various kinds of proteins change. The sulfhydryl group of proteins is converted to disulfide bond, and the amino acid is also changed.

　　The fat in corn includes starch fat and non-starch fat, and non-starch fat is easy to decompose, which affects the safety of corn storage. It is quite difficult to separate starch fat from starch, which is in the spiral structure of amylose and is quite stable.

　　In the fatty acid composition of corn oil, saturated fatty acid accounted for 15% and unsaturated fatty acid accounted for 85%. In the unsaturated fatty acid mainly contains oleic acid and linoleic acid, its ratio is 1:2.5. The fatty acid composition of corn oil is generally stable, the content of linoleic acid is 55%~60%, oleic acid is 25%~30%, palmitic acid is 10%~12%, stearic acid is 2%~3%, the content of linolenic acid is very little. Corn oil is high in vitamin E, up to 100mg/100g of oil.

　　There are two ways in which fats change during storage: one is hydrolysis, which produces glycerol and fatty acids; Dioxidation produces peroxides and carbonyl compounds, mainly aldehydes and ketones. So the corn gets longer and longer. Its fatty acid value and total acid value will increase, and produce rancid taste and other abnormal odor.

　　Since the discovery of high fatty acid content in inferior corn in the 1930s, it has been clear that fatty acid has a good correlation with the storage quality of corn. Generally, the value of fatty acid in newly harvested corn ranges from 15 to 20mg KOH/ 100g dry base, and its growth rate is slow during the reasonable storage period, but increases rapidly under adverse conditions. Fatty acid value up to 250mg KOH/ 100g dry base. Acid from the corn, in addition to the fatty acid, and phosphoric acid, lactic acid, acetic acid, amino acid, acid phosphate, etc., many studies have shown that in the process of deepening of quality deterioration, fatty acid increase faster than any other acid, corn to much of quality deterioration, phosphate content is significantly higher, and the amino acid is higher in metamorphic has developed to the point of serious discovered.

　　Zhang Yong (2007) conducted mathematical simulation on 15 kinds of corn in different regions and storage years, and found that fatty acid value, peroxidase activity and malondialdehyde content could significantly simulate the freshness of corn. Fatty acid value has always been an important index for evaluating corn storage quality. During the storage period of corn, fatty acid value changes significantly, thus verifying the correctness of using fatty acid value as an index for evaluating corn storage quality. Peroxidase (PERoxidase) is an important antioxidant enzyme in corn, which can reduce the damage of free radicals to the membrane lipid of corn. Malondialdehyde is an important compound produced by membrane lipid peroxidation. The content of malondialdehyde reflects the damage degree of membrane lipid. In summary, the accumulation of toxic substances, the decrease of antioxidant protective enzyme activity, and the deepening of membrane lipid peroxidation all have an impact on the freshness of corn, which can reflect the new age of corn.

　　Corn embryo of a big, high lipid content, rancid fat easily result in higher fatty acid value, and a high content of unsaturated fatty acids in corn fat, the degradation of unsaturated fatty acids can cause malondialdehyde concentration, malondialdehyde has strong crosslinking, and peroxidase protein, lead to protein denaturation, peroxidase activity. As the aging degree of corn deepens, fatty acids in corn grains will increase, and their degradation will produce more malondialdehyde. Increased malondialdehyde accumulation may result in decreased or inactivated activity of more antioxidant protective enzymes. The continuous accumulation of acid substances, the increasing content of malondialdehyde and the denaturation of more proteins will lead to the destruction of the cell membrane system and the internal metabolic dysfunction of corn grains, which will eventually lead to the decrease of physiological activity and storage quality of corn.

　　Scheme and suggestion for improving aged maize

　　Retrogradation of corn due to its low price and stable supply, become a group company and the first grain of specialization of the raw material, but given the retrogradation of corn led to the decrease of the energy and vitamin content decrease, all kinds of digestive rate of nutrients, reducing, and all kinds of free radicals and mycotoxin the harm to animals, the invisible loss, need to cause enough attention, therefore, to explore a comprehensive solution.

　　1. A large amount of aged corn will lead to the increase of amylose content with the increase of its storage life. Therefore, it is necessary to increase the pelleting temperature during the granulation process to make the starch easier to gelatinize and thus improve the digestibility of starch.

　　2. When using aged corn, the adjustment of amino acids in the formula should be considered.

　　3. In order to improve the utilization rate of oxidized fat, reduce the damage of free radicals such as peroxides to lipid of animal cell membranes, and improve the permeability and integrity of cell membranes, it is suggested to add endogenous antioxidants, so as to increase the feed intake and daily weight gain of animals, and to improve the feed conversion efficiency or shorten the time of storage.

　　4. As for the vitamin content in corn, the vitamin content in aging corn, especially the fat-soluble vitamin E content, is reduced. Therefore, when aging corn is used in large quantities, it should be considered that VE level in the formula should be kept at an appropriate level. When used together with endogenous antioxidant, the synergistic effect of VE level and VE level will be played to achieve the goal of 1+1>2.

　　5. For consumers who are concerned about the yellow color of poultry meat, additional colorings are needed when aging corn is used in large quantities.

　　6. Customers should treat "aged grain" into "feed grain" cautiously, and pay attention to the following points in the process of feed production and use:

　　▪ Obviously mildewable feed raw materials must not be used in feed production;

　　▪ Raw materials that cannot be mildewable should be tested for mycotoxin when purchased. If the toxin exceeds the limit, it cannot be used in feed production.

　　▪ Although mycotoxin was detected in raw materials, but not exceeding the standard, mycotoxin adsorbent should be added in the production of feed, in order to reduce or eliminate the impact of mycotoxin.