What Is the Equilibrium Moisture Content?

The moisture stored in food changes with the temperature and humidity in the air. When the rate at which grain absorbs water from the surrounding environment is equal to the rate at which grain loses moisture from the surrounding environment, the food is in a state of equilibrium with the environment. The moisture in this state of equilibrium is called equilibrium moisture or humidity balance.

According to the principle of thermodynamics, at a certain temperature and

The moisture content of coal at equilibrium between a specified temperature and a specified relative humidity.

Subjects: coal technology (first-level discipline); coal processing and utilization (second-level discipline); coal chemistry and coal quality analysis (third-level discipline)

The moisture of grain during storage changes with the temperature and humidity in the air. When the rate at which grain absorbs water from the surrounding environment is equal to the rate at which grain loses moisture from the surrounding environment, the grain is in a state of equilibrium with the environment. The moisture in this state of equilibrium is called equilibrium moisture or humidity balance. The relative humidity that is balanced with the air around the grain is called the equilibrium relative humidity. The equilibrium moisture content of various grains varies with temperature and relative humidity, and it can be used

Due to the importance of balanced moisture content in the practice of grain storage, the equilibrium moisture content of many grains has been determined experimentally. According to the characteristic values found in different literatures, they are described in terms of cereals, oils, potatoes, finished grains, processed products and by-products. Due to the different types or different varieties of grains, their organizational structure and chemical composition are more or less The difference. Therefore, the equilibrium moisture achieved under the same conditions is also different. There are several points to summarize.

Because the dried potatoes in cereals and potatoes contain more hydrophilic substances, the oils contain more hydrophobic substances. Its equilibrium moisture obviously shows that oils are smaller than cereals and dried potatoes. For example, at 20 ° C and 70% relative humidity, the equilibrium moisture of wheat is 14.8%, soy is 11.5%, and castor seeds are only 6.10%. The equilibrium moisture is almost half less than that of cereals. However, when the relative humidity was close to 90% or more, the soybean showed almost abnormal equilibrium moisture.

At the same temperature, the greater the relative humidity of the air, the greater the grain equilibrium moisture, and at the same relative humidity, the grain temperature and the equilibrium moisture show a negative correlation. This relationship is related to the thermochemical formula of the adsorption process and can be explained by Le Chatelier's principle.

Grain processed products, under the same conditions, the equilibrium moisture is obviously smaller than that of cereals and processed products such as bread and biscuits. This phenomenon may be related to the decrease in the number of exposed polar molecules of starch. With a correspondingly low relative equilibrium value, it is easy to store. For processed by-products such as bran and linseed cake, under the same conditions, it is clear that the equilibrium value is higher than other grains, especially in the saturated state, it can reach more than 38 to 40, and cereals are generally about 20%. Therefore, when the processed by-product is high in moisture, it tends to generate heat and is difficult to store.

Different grain varieties have slightly different chemical analysis. This may be the reason why foods of different maturity levels are considered the same. Changes in the chemical composition or growth process of food crops have a great impact on the equilibrium moisture content. In the process of drying the grain temperature above 60 ° C, due to slight chemical changes, this artificial dryness will reduce the equilibrium moisture content of corn by 0.5 to 1%. Grain equilibrium moisture content also depends on its previous desorption and adsorption experience. There is a difference between the equilibrium moisture content in the adsorption process and the equilibrium moisture content in the desorption process.

There are often two methods for determining equilibrium moisture. One is the static method, that is, the grain is balanced under a certain state without mechanical vibration of air or grain and is completed. One is the dynamic method, that is, the grain reaches equilibrium in a certain state by mechanical vibration of air or grains. The dynamic method is fast to measure, usually using wet grains, and dried to equilibrium. The data can be obtained in two days or less, but there are still problems in design and equipment. Therefore, the static method has been more widely used. But the static method takes up to several weeks.

When measured by the static method, one is a saturated salt solution and the other is an acid solution. Grains and processed products take a long time to reach equilibrium and therefore when the relative humidity is about 80% or more, the mold develops quickly. Without special treatment, the obtained equilibrium value is often not correct.

The relationship between the equilibrium moisture content, temperature, and relative humidity is very close, and it is actually of great significance in production. Drying food by sun and drying methods, using natural ventilation and artificial ventilation to reduce temperature and dehumidification, the requirements for moisture content of grains with different chemical composition under the same storage conditions are achieved by the principle of moisture absorption balance.

The multi-pipe natural ventilation method is to use natural ventilation in the winter and spring season to introduce cold air to reduce the moisture content of 25% of corn and 18% of sorghum to long-term storage. It has been stored in Liaoning and Heilongjiang in Northeast China Provincial trials were successful. This method has the advantages of reducing labor intensity, saving manpower and material resources, reducing storage costs, and avoiding food pollution. It is based on the fact that the air is dry and cold in winter and spring, and the relative humidity is very low. According to the theory of equilibrium moisture, high-moisture grains will gradually release moisture into the air and reduce the temperature through the ventilation ducts and pores of the grain pile under dry and cold air conditions. Therefore, the moisture of the grain is reduced to a relatively safe standard for a long time, and the grain is kept at a low temperature.

Most grain drying uses air as a medium. Dry air plays two roles: (1) removes water evaporated from the grain; and (2) provides heat to evaporate the moisture in the grain. The evaporation of grain moisture is mainly the surface moisture of the grain. Through desorption, the moisture in the grain gradually diffuses to the surface, and eventually achieves the purpose of uniform drying. ^[1]