No abstract

This paper reports on an experimental investigation of the drying characteristics and kinetics of camellia oleifera seeds under microwave intermittent drying conditions. The effects of key parameters such as microwave power, heating time, and the length of the intermittent time are discussed in detail. The energy consumption in the drying process along with the main quality parameters of camellia oil, namely the peroxide and acid values, have also been determined and compared with hot‐air drying. Results show that the microwave drying curve consists of an acceleration rate period, a constant rate period and a falling rate period. As the microwave power and heating time increase, the drying time decreases. While as the length of the intermittent time increases, the drying time increases. The hot‐air drying time was found to be much longer than microwave drying time, and the energy consumed during hot‐air drying was also several hundred times higher than that used during microwave intermittent drying. The acid and peroxide values of camellia oil after hot‐air and microwave drying were both within the allowable range in the National Standard of China. The Midilli and Kucuk model was the best model to describe the kinetics of the drying process.Practical applicationsDrying is the first step in the processing of camellia oleifera seeds, which not only affects the storage of camellia oleifera seeds, but also affects the quality of camellia oil, and has thus become a research hotspot in recent years. However, several disadvantages of hot‐air drying have been identified like the slowness of the drying process and the relatively large energy consumption. Therefore, finding a drying method that is more suitable is necessary. Microwave intermittent drying is a type of discontinuous drying. The moisture and temperature inside the material can be evenly redistributed during the intermittent time, so that the material is not overheated. This feature of preserving the quality of the products to be dried has made microwave intermittent drying attractive and has been successfully applied to agricultural, forestry, and food products.

The processing method of Camellia oleifera fruit used in the industry was investigated, that is, fresh C. oleifera fruit was shelled in the drying equipment first and then the remaining C. oleifera seeds returned until the target moisture content (about 10% d.b.) was reached. The variable-temperature drying of C. oleifera seeds was investigated for the first time and the drying process was optimized and compared with constant-temperature drying. Two independent variables, including hot air drying temperature and air velocity, were studied by central composite design. The responses were drying time and quality of Camellia oil, including acid value and peroxide value. Results showed that the optimal constant-temperature drying conditions were drying at a temperature of 60.5°C and air velocity of 2.1 m/s. Under this condition, the total drying time was 607 min, and the acid and peroxide values were 1.72 mg/g and 0.12 g/100 g, respectively. The optimal variable-temperature drying conditions were drying at a constant air velocity of 2 m/s and a drying temperature of 55°C for 67 min, then 60°C for 213 min, and finally 65°C for 297 min. Under these conditions, the acid value was 1.75 mg/g and the peroxide value was 0.1 g/100 g. The optimal variable-temperature drying efficiency was 16.7% higher than that of constant-temperature drying when the optimization objective of drying time, acid value and peroxide value was 2:1:1.

The drying characteristics and drying quality of fresh white waxy corn were investigated under three different drying methods (sun drying, hot-air drying and far-infrared drying) and different drying temperatures (55 °C, 60 °C, 65 °C, 70 °C, 75 °C, 80 °C). The optimal drying method and drying condition were obtained by comparing the drying time and drying quality including damage rate, rehydration capacity and the contents of protein, fatty acid and starch. The results showed that the drying time of sun drying was the longest, while the drying quality was the best, i.e., the damage rate was the lowest and the nutrient retention was the highest. The far-infrared drying time was shorter and the quality was better than that of hot air drying. The higher the temperature, the faster the drying rate. The optimal drying temperature was 60 °C under far-infrared drying, which can effectively remove the water and keep the food quality. Besides, the drying rate was faster than hot air, and the color of the dried products was brighter. When hot air drying was used, the value of fatty acids did not change much below 65 °C, but it rose sharply above 65 °C. The optimal drying temperature was 65 °C, under which the drying rate was moderate and the quality was relatively good. If the production cost was considered, sun drying was the best, following the 60 °C far-infrared drying. This conclusion can provide a reference for the drying conditions of corn after harvest in agricultural production, and to a certain extent provide suggestions for the later drying treatment methods and treatment temperatures of fresh white waxy corn.