(I) Rice

  1. Rice storage characteristics  

　　Rice has a relatively hard hull that protects the kernels and, to a certain extent, resists pests and the effects of external temperature and humidity. Therefore, rice is easier to store than ordinary processed grain. However, rice germinates easily and is not resistant to high temperatures, requiring special attention.

　　Most rice varieties, such as indica rice, have no post-ripening period. They are physiologically mature at harvest and capable of germination. At the same time, rice requires little water absorption for germination. Therefore, if continuous rainy weather occurs during harvest and the rice cannot be harvested, threshed, or dried in time, it may germinate in the fields or on the drying grounds. Rice in storage can also germinate easily if condensation, moisture regain, or rain leakage occurs. If threshing and drying are delayed and the rice is piled together with straw, it is prone to yellowing and fermentation. Rice that has germinated or undergone yellowing and fermentation experiences a significant decline in both quality and storage stability.

　　Rice is not resistant to high temperatures. Rice stored through the summer is prone to aging. Rice exposed to strong sunlight, or rice that is suddenly exposed to cold after sun-drying, is prone to the “broken-waist” phenomenon.

　　Shortly after newly harvested rice is placed in storage, if the temperature drops, condensation often forms on the surface of the grain pile, increasing the moisture content of the surface grain and adversely affecting storage. The moisture content of the surface grain should be reduced promptly.

　　2. Rice storage methods  

　　Ensure the quality of rice entering storage: Rice with high moisture content, excessive impurities, and a high proportion of imperfect kernels is prone to heating and mold and is not suitable for long-term storage. Therefore, improving the quality of rice entering storage is essential for safe rice storage. The safe moisture standard for rice should be determined based on the variety, season, region, and climatic conditions. Generally, the moisture content of indica rice should be below 13%, and that of japonica rice should be below 14%. The less impurities and imperfect kernels, the better. If rice entering storage has high moisture and many impurities, it should be stored separately by grade, dried naturally or mechanically in time, and passed through sieves or cleaned by air separation to remove impurities.

　　Timely ventilation: Newly harvested rice has intense respiration and relatively high grain temperature and moisture content, so it should be ventilated in time to reduce temperature and moisture. Especially when autumn temperatures begin to fall, the temperature difference inside and outside the grain pile becomes large. Ventilation should then be strengthened, combined with deep turning of the grain surface, to release moisture and heat from the pile and prevent condensation. Mechanical ventilation may be used where conditions permit.

　　Low-temperature airtight storage: Make full use of the cold and dry winter weather for ventilation, lowering the grain temperature below 10℃ and the moisture content below the safe standard. Before temperatures rise in spring, cover and seal the grain pile to ensure safe storage through the summer.

(II) Wheat  

　　1. Wheat storage characteristics

　　Strong hygroscopicity: Wheat has a relatively thin seed coat and a loose tissue structure, giving it a strong moisture-absorbing capacity.

　　Long post-ripening period: Wheat has a relatively long post-ripening period. The duration varies by variety, with most varieties requiring from two weeks to two months. Wheat with a suitable moisture content experiences improved quality and greater storage stability after completing post-ripening.

　　Relatively heat-resistant: Wheat has strong heat resistance. Tests show that wheat with a moisture content of 17% can be dried at temperatures not exceeding 46℃; or, when its moisture content is below 13％, it can be exposed to temperatures not exceeding 54℃ without reducing enzyme activity or germination capacity. The processing quality of the resulting wheat flour does not decline but may improve, producing soft and expanded steamed buns. This provides a basis for using high-temperature drying or high-temperature pest control for wheat.

　　Good storability: Wheat’s greatest advantage is its good storability. After post-ripening, wheat has weak respiration, lower than that of other cereal grains. Under normal conditions, wheat with a moisture content within the standard (12.5％) can generally be stored for 3-5 years at room temperature or 5－8 years at low temperature (15℃), with no noticeable change in its edible quality.

　　Susceptible to pests: Wheat has poor insect resistance and a relatively high infestation rate. Except for a few insect species that feed exclusively on legumes, wheat can be infested by almost all stored-grain pests, with the maize weevil and grain moth among the most harmful. Wheat matures, is harvested, and enters storage in summer, which is the breeding and infestation period for pests. After storage, high temperatures combined with rainy weather create highly favorable conditions for pest activity.

　　2. Wheat storage methods

　　Strictly control moisture: Because wheat is highly hygroscopic, attention should be paid to moisture reduction and protection against dampness during storage. The high summer temperatures after wheat harvest should be fully utilized for sun-drying, so that the wheat moisture content is controlled below 12.5% before storage. After wheat enters storage, moisture-proof measures should be taken, and attention should be paid to possible moisture stratification and the formation of a “capping” layer on the upper surface during post-ripening.

　　Hot-loading airtight storage: Storing wheat in airtight conditions immediately after hot loading is a traditional wheat storage method in China. Sun-drying can reduce the moisture content of wheat, while the high temperatures during sun-drying and airtight storage can control pests and microorganisms. It also promotes completion of post-ripening in newly harvested wheat. The elimination of pests and reduction in moisture and microbial loads greatly reduce respiration intensity, enabling wheat to be stored safely for a long time.

　　The specific procedure for hot-loading wheat is as follows: During the hottest period of summer, select a sunny day with high temperatures and sun-dry the wheat until its temperature reaches approximately 50℃. Maintain the high temperature for 2 hours and reduce the moisture content below 12.5%. At around 3 p.m., pile the wheat, place it into storage while still hot, and seal the entire storage facility. Maintain the grain temperature at approximately 46℃ for about 10 days to kill all pests. The grain temperature will then gradually decrease and reach equilibrium with the storage temperature, after which normal airtight storage can continue.

　　In addition, the storage facilities, equipment, and tools used for hot-loading airtight storage must all be treated for pests in advance.

　　Low-temperature airtight storage: Although wheat can withstand high temperatures, prolonged storage at high temperatures will reduce its quality. Therefore, after the weather cools in autumn, wheat can be thoroughly cooled through natural or mechanical ventilation, and the storage facility can be covered and sealed before spring warmth arrives to maintain a low-temperature condition. Low-temperature storage is a fundamental method for the long-term safe storage of wheat. Maintaining wheat at a certain low temperature helps extend seed life and preserve quality.

　　Wheat can also maintain good quality under frozen conditions. For example, storing dry wheat at a low temperature of－5℃ helps enhance its vitality. Therefore, the severe cold of winter can be used to turn the grain, remove impurities, and freeze it, reducing the wheat temperature to around 0℃. The wheat can then be sealed while cold, which is effective in eliminating overwintering pests in the grain pile. Low-temperature airtight storage allows long-term storage, but contact with moist and hot air must be strictly prevented to avoid condensation on the surface of the wheat pile.

(III) Corn  

　　1. Corn storage characteristics

　　High initial moisture content and uneven maturity: In the main corn-producing regions of northern China, the weather is already cold at harvest. In addition, corn ears are covered by husks and cannot receive sufficient sun-drying on the plant, so the initial moisture content of corn is generally high. The moisture content of newly harvested corn is generally 15-20％ in North China and 20-30％ in Northeast China and Inner Mongolia. Corn maturity is often uneven because pollination occurs at different times at the top and base of the same ear, resulting in insufficient maturity of the top kernels. Unevenly matured corn is not conducive to safe storage.

　　Large germ and intense respiration: The corn germ accounts for almost 1/3 of the total kernel volume and 10%～12% of the kernel weight. It contains more than 30% protein and a relatively large amount of soluble sugars, so it is highly hygroscopic and respires intensely. Tests show that the respiration intensity of normal corn is 8～11 times higher than that of normal wheat. Corn mainly absorbs and releases moisture through the germ.

　　High fat content in the germ and susceptibility to rancidity: The corn germ contains 77-89％ of the fat in the entire kernel. Therefore, its fatty acid value is consistently higher than that of the endosperm, and rancidity begins in the germ first.

　　High microbial load in the germ and susceptibility to mold: The corn germ is rich in nutrients and carries a relatively large amount of microorganisms. Measurements show that after a period of storage, corn carries a much higher microbial load than other cereal grains. The germ is the first part to be damaged by insects and mold. After absorbing moisture, the germ provides suitable conditions for rapid mold reproduction, initiating mold growth.

　　(2) Corn storage methods

　　The key to safe corn storage is improving the quality of corn entering storage and reducing its moisture content. Because temperature and humidity vary considerably from region to region, appropriate storage measures should be adopted according to local conditions.

　　Separate by grade: Store corn with different moisture contents separately. When corn enters storage, it should be stored separately according to moisture content and grade, laying an initial foundation for safe storage. Corn with high moisture content should be dried before entering storage.

　　Low-temperature airtight storage: Based on the storage characteristics of corn, low-temperature and dry storage is suitable. There are two methods: dry airtight storage and low-temperature frozen airtight storage. In southern regions, harvested corn can be thoroughly dried where conditions permit, then screened and placed into airtight storage after its moisture content has been reduced to a safe level. In northern regions, temperature restrictions make it difficult to reduce high-moisture corn to a safe moisture level. Except where mechanical drying is available, low-temperature frozen airtight storage is generally used. The procedure is to use the cold and dry winter weather to spread and cool the corn, reducing the grain temperature below－10℃, then screen it to remove frost and impurities, and place it into airtight storage on a cold, sunny day while the temperature remains low.

　　Storage of corn ears: Storing corn without shelling, in the form of ears, is a relatively well-established method that has long been widely used by Chinese farmers. Because the porosity inside a pile of corn ears is high (up to 51.7%), ventilation is good, and the storage period occurs during the low-temperature season, even though high-moisture corn ears continue to respire intensely, they can maintain a balance in heat metabolism and the pile temperature changes only slightly. With long-term ventilation during winter and spring, the corn gradually dries. When the moisture content falls to 14.5%～15%, the corn can be shelled and transferred to kernel storage.

    When corn ears are stored, the kernel germs are embedded in the cob, providing a certain degree of protection against insects and mold. In addition, nutrients in the cob can continue to be transported into the kernels during the initial period, increasing the nutrient content of the kernels.

    (IV) Soybeans  

    1. Soybean storage characteristics

　　Soybeans are round, with smooth seed coats and hard kernels. They have relatively strong resistance to insects and mold, but damaged soybeans deteriorate easily. Soybean kernels are rich in protein and fat. They readily absorb moisture when air humidity is high and may discolor and develop off-flavors after exposure to high summer temperatures. In severe cases, oil seepage may occur. High temperature and high humidity can also easily reduce the germination rate of soybeans.

 　　At a relative humidity below 70%, soybeans are less hygroscopic than corn and wheat. However, at a relative humidity of 90%, the equilibrium moisture content of soybeans is higher than that of corn and wheat. Therefore, special attention must be paid to moisture protection when storing soybeans.

 　　Susceptible to oil seepage and red discoloration: When the moisture content of soybeans exceeds 13%, increasing temperature first causes the beans to soften. The color at the point where the two cotyledons meet near the germ then turns red, commonly known as “red eye.” The red color subsequently deepens and gradually expands inside the bean, a condition commonly known as “red discoloration.” In severe cases, the cotyledons become waxy and translucent, accompanied by oil seepage and peeling.

    2. Soybean storage methods

 　　Thorough drying: Soybeans should be promptly spread and dried after threshing to reduce their moisture content. The moisture content of soybeans intended for long-term storage must not exceed 12.5%; higher moisture content makes them prone to mold.

 　　Timely ventilation: Moisture is unevenly distributed between newly stored soybean kernels. In addition, post-ripening causes intense respiration, resulting in substantial accumulation of moisture and heat inside the soybean pile. As temperatures are also falling during this period, condensation is highly likely to occur. Therefore, soybeans should be ventilated promptly approximately 3～4 weeks after entering storage to dissipate moisture and heat and improve their storage stability.

　　Low-temperature airtight storage: Freezing soybeans during the severe winter season and using low-temperature airtight storage can not only isolate them from external temperature and humidity and pest infestation, but also prevent oil seepage and red discoloration, helping preserve soybean quality.