Abstract:
Pollen is the male reproductive material of plants and contains abundant nutrient substances, such as protein, polysaccharide and lipid of pollen, which are good for health-promoting. Drying is an essential processing step for pollen preservation since it can prevent the growth of microorganisms and decrease a lot of the moisture-mediated deteriorative reactions, and thus prolong the shelf-life of pollen. Currently, the traditional drying method for pollen is the natural sun drying, due to its low capital investments and operation simplicity. However, traditional natural sun drying possesses severe disadvantages, such as long drying time, rewetting or rotting caused by bad weather, contamination by dust and insects, non-uniform drying, and color deterioration due to long exposure to solar radiation. Furthermore, there are also some other drying technologies for pollen, such as hot air drying, vacuum freeze-drying and microwave drying. These drying methods all have some advantages and disadvantages which influence the drying rate and quality. Therefore, in order to improve the drying process and enhance the quality of pollen, the traditional drying technique should be replaced by more efficient, safer and more controllable industrial drying methods. Vacuum pulsed drying is a novel drying technology which can increase the drying rate and improve the product quality compared to traditional hot air drying. In current work, the vacuum pulsed drying technology was employed to dry lotus pollen and the effects of vacuum keeping time(15, 12, 9, 6, and 3 min) and drying temperature(45, 50, 55, 60, and 65 ℃) on the drying characteristics and quality of pollen were explored. It was found when the vacuum keeping time was 12 min, it obtained the least drying time. The drying time decreased with the increasing of drying temperature. The Weibull distribution model was applied to the drying process and the results were analyzed. The scale parameter α and shape parameter β were used to calculate moisture diffusion coefficient and drying activation energy. The results showed that the moisture diffusion coefficient varied from 2.154 2×10
-11 m2/s to 6.254 3×10
-11 m2/s, and the drying activation energy was 20.88 kJ/mol, which meant that removing 1 kg water needed about 1 160.00 kJ energy. In terms of dried quality, there was no color deterioration after vacuum pulsed drying. No significant change of b was found between the color of fresh pollen and dried pollen. It was found that the protein content of dried pollen initially increased and then decreased gradually with the decrease of vacuum keeping time. When the drying temperature was 45 ℃, and the pulsation ratio of vacuum keeping time to atmosphere keeping time was 12 min∶3 min, it obtained the most protein content of 18.43%. The vacuum keeping time and drying temperature all had significant influence on the protein of pollen. With the increasing of drying temperature, the microstructure became more porous which could significantly influence the texture. Larger number of pores and large pore size could enhance the moisture migration rate. It was also found that the integrity of pollen grain decreased with the increasing of drying temperature, for higher temperature caused more damage to the microstructure. The current work provides theoretical and technical reference for applying vacuum pulsed drying technology to pollen drying process.