Vacuum freeze drying of wheat seeds

Vacuum freeze-drying treatment of wheat seed in Journal of Chongqing University (Natural Science Edition) YANG Ying, LI Yu-ning, TONG Ming-wei (Department of Refrigeration, College of Power Engineering, Chongqing University, Chongqing 400044, China) and hot-air drying were used to dry wheat seeds in different environments. Under pressure, the temperature-time profile of seeds was studied, and the effects of these three drying methods on the biological activity of wheat seeds were further studied by analyzing the rehydration rate, germination rate, and seedling growth. The research results show that the drying temperature and drying time must be strictly controlled when hot air drying is used. Otherwise, the seeds will be easily killed. The vacuum drying method can easily weaken the biological activity of wheat seeds and should not be used for seed drying treatment; The biological activity of the seeds was not affected. The average survival rate of wheat seeds under vacuum freeze-drying conditions was higher than the other two methods and could be applied to the drying of wheat seeds. Under vacuum freeze-drying of wheat, the sublimation temperature of water ice crystals is closely related to the drying chamber pressure. When the absolute pressure in the drying chamber is 200~400Pa, the sublimation temperature of ice crystals in wheat is 18~20°Q: drying; vacuum freeze-drying; wheat; seeds Wheat is China's main food source, especially in northern China. It has a wide sowing area and abundant production. Wheat contains more starch and is an energy food. Wheat varieties and yields have a bearing on the lifeline of our national economy.

After the wheat is harvested, if it is not dried in time, it can cause mildew and even germination. Currently, there are two main types of drying methods used in wheat: the traditional drying method using the heat of sunlight to evaporate the moisture in the seeds and forced drying by hot air. The former is vulnerable to natural weather conditions, especially in the rainy season, the lack of sunlight will make wheat unable to dry; the latter has good drying effect, but the control of hot air temperature is the key, otherwise it will affect the activity of wheat seeds. Vacuum freeze-drying technology is a method for obtaining dry products by sublimating ice crystals in a vacuum while keeping the material in a frozen state. It is a multidisciplinary technology 1231 with a high technological content and a wide range of knowledge involved in drying technology. Applied to medicine, biological products, food, flowers, animal and plant specimens and new materials research and other fields. Compared with other methods, the vacuum freeze-drying process is carried out under low temperature and vacuum conditions, avoiding heat-sensitive reaction and oxidation, and can maintain the nutrients and physiological active ingredients of the dried material to the utmost and maintain the original appearance and shape; After the material is dried, it has many microporous structure and excellent rehydration; the inorganic salt carried by water in the material does not migrate with the sublimation of water; the temperature of the heat source when the material is sublimated and dried is not high, and the overheat of the material will not occur. 1231. In addition, vacuum freeze-drying technology is also used for dry preservation of living organisms such as human organs, sperm and bacterins without affecting the activity of dried products. However, whether this method can be used to dry wheat seeds, especially its effect on the activity of wheat seed organisms, has not been studied in detail yet. For this purpose, the author applied the vacuum freeze-drying technique to the drying of wheat seeds. The device consists of a cold source, a fluidized bed, a drying chamber, a cold trap, and a vacuum pump. During vacuum drying, the cold source and the fluidized bed are closed. When the hot air is dried, only the fluidized bed and hot air are used. The cold source adopts high-pressure nitrogen gas as the motive force to radiate liquid nitrogen through the ejector to form low-temperature nitrogen gas14. The drying room is provided with an electric heater, a wire mesh frame and a wire mesh, and the heating is performed by electric heating.

1 Under germination Determine the physiology of each group of seed seeds. The rehydration rate of wheat seeds after vacuum freezing and drying is the highest. This is because the water in the seeds forms ice crystals during freezing and forms microporous structure with the sublimation of ice crystals, which increases the specific surface area of ​​the seeds and absorbs moisture more easily; The rehydration rate of the dried wheat seeds was the lowest, indicating that the hot air temperature and drying time had an effect on the water recovery of the seeds. The higher the temperature and the longer the drying time, the lower the rehydration rate of the seeds. However, after prolonged soaking time, the rehydration rate of seeds dried by hot air can basically reach above 95%.

2.3 Comparison of budding rate Wheat kernels consist of pericarp, seed coat, aleurone layer, endosperm, embryo, etc. The weight ratio of wheat endosperm, germ, and skin is 82:2:16. Wheat aleurone contains more protein and Ash, endosperm contains a large amount of starch, but also contains gluten-forming proteins. In the wheat granules, carbohydrates are the most abundant components. The fiber content is 2%, the starch content is about 70%, and the fat is about 3%. Changes in wheat composition will directly affect the growth and development of wheat.

Wheat seeds have three major characteristics, namely, ripening, heat resistance and hygroscopicity. Postharvest action refers to a long post-harvest period after wheat seeds are harvested. Normal spring wheat is 30-40 days, half winter wheat is 60-70 days, and winter wheat is 80 days. At the stage, the seed body continues to carry out the synthesis of some substances; the heat resistance means that the wheat that has not completed the ripening effect has high heat resistance, but has passed the physiologically ripened wheat seeds, the heat resistance is significantly reduced; moisture absorption Sex refers to the wheat easily absorbs the moisture in the air, especially under the conditions of high air humidity. Therefore, the seeds after the ripening period are particularly harsh to the drying conditions. Among the three drying methods, the average germination rate of seeds after vacuum freeze-drying was the highest. It shows that the vacuum freeze-drying process has little influence on the biological activity of wheat seeds; the average germination rate of vacuum-dried wheat seeds is the lowest, which means that in the vacuum drying process, the moisture in the seeds evaporates in the form of liquid, during the water migration process, Salts, minerals and vitamins that dissolve in water migrate with the water to the surface of the seed, which affects the growth and development of the seed. In the process of vacuum freeze-drying, because moisture is frozen in advance into ice crystals, other substances in the sublimation process will not With the sublimation, therefore, once the rewatering seeds are quickly reduced; the germination rate of the wheat seeds dried by the hot air after drying for 4 h at 48C is only 50%, while the germination rate of the seeds in the other two groups with a drying time of only 0.5 h is higher. This shows that the wheat seeds in the hot air drying process, the longer the drying time, the greater the impact on the activity of wheat seeds; long time high temperature wheat can be seen from Figure i, the absolute pressure in the vacuum freeze-drying process is smaller, complete The shorter the sublimation drying time and the higher the sublimation temperature, the lower the germination rate of the seeds, so the more appropriate absolute pressure is 200Pa or so.

From observable, in vacuum drying experimental data, the germination rate of a set of seeds with absolute pressure of 1300 Pa is relatively high, and the germination rate of a set of seeds with absolute pressure of 400 Pa is relatively low, indicating that the absolute pressure in vacuum drying is higher. The greater the effect on the physiological activity of seeds.

2.4 Growth of Seedlings Germinated seeds continue to be cultivated, and the seedlings grow well to 2 cm. There was no significant difference between the experimental groups in each group, indicating that the physiological activity of the seeds that survived after drying was not affected.

4 Conclusions The first drying of the wheat seeds by vacuum freeze-drying proved that the biological activity of the freeze-dried seeds was not affected.

By comparison of vacuum freeze-drying, vacuum drying and hot-air drying of wheat seeds, the average survival rate of wheat seeds under vacuum freeze-drying conditions is higher than the other two methods. It is a feasible drying method; hot air drying must The drying temperature and drying time are strictly controlled, otherwise it will easily cause the death of the seeds; the vacuum drying method is easy to weaken the biological activity of wheat seeds, and should not be used for the drying of seeds.

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