关键词: 核桃, 气体射流冲击, 干燥特性, 干燥模型

Abstract: 【Objective】 In order to improve the dried walnut quality, shorten the drying time, the effects of different conditions on the walnut air-impingement jet drying were studied and the activation energy for drying was obtained and the optimum drying model was selected.【Method】 Using the method of energy saving technology of dry heat pipe combined with a self-made gas jet impingement equipment, in 9 groups of an experiment, effects of different jet air temperatures (40℃, 50℃ and 60℃) and air velocities (11, 12 and 13 m·s^-1) on the drying characteristics of materials, effective moisture diffusion coefficient and activation energy were studied, at the same time, through the statistical data selection for the fitting drying models, 5 drying kinetics models were established. The 5 models are the Page model, the Modified Page model, the Logarithmic model, the Herdenson and Pabis model, and the Lemus model. The data were processed by using the DPS, after finishing the fitting, a parametric equation was obtained the final general MR and t. 【Result】 Compared to the most results of food material drying test, air-impingement jet drying of walnut mainly occurred in the falling rate drying period, and there was no constant drying rate stage. Air temperature had large influence on each stage of walnut air impingement. With the increase of the air temperature values, the drying rate was rising and the MR was decreased. The air velocity almost had no influence on the drying time, but had a certain influence on the rate of surface water vaporizing stage by increasing the drying time. The air velocity almost had no influence on the drying rate of internal moisture transfer stage had almost no influence, and by employing this feature, a method of changing the wind temperature and wind speed could be used in different periods, so not only the drying time was shortened, but also the purpose of energy saving was achieved. Overall, for shortening the drying time, the order of the influence was air temperature＞wind speed. The drying rate of the air-impingement jet drying of walnut increased with the increase of air temperature, while had no effect by the air velocity increasing. Fick’s second law was used to calculate the effective moisture diffusivity of walnut in drying process and the values were in the range of 0.9674×10^-11 to 2.2231×10^-11 m²·s^-1. Because of its shell structure so that the effective moisture diffusion coefficient was 1-3 orders of magnitude lower than other food materials. The activation energy was increased with the increase of the air velocity and the lowest activation energy was 27.644 kJ·mol^-1. The five models had a higher fitting degree, and could better describe the walnut air impingement drying. The Modified Page model gave the highest coefficient of determination (R²), the lowest chi-square (χ²) and root mean square (RMSE). Based on the Modified Page model, regression analysis by using the DPS, a parameter equation was established between generally applicable moisture ratio (MR) for walnut air impingement drying and time (t) in the circumstance where the temperature of wind is 40-60℃ and the speed of wind is 11-13 m·s^-1. 【Conclusion】 Air temperature and air velocity had influence on the drying curve, the drying rate curve, effective moisture diffusion coefficient and activation energy. According to the fitting result of the fitted values and the observated values under different conditions, the optimal drying condition was that the air temperature is 50℃ and the wind speed was 13 m·s^-1. The Modified Page and Page models could properly describe the air-impingement jet drying behavior of walnut and could be used when the drying air temperature between 40 to 60℃, air velocities between 11 to 13 m·s^-1. The Modified Page model fitting degree higher, it is the optimal model of the walnut air impingement drying.

Key words: walnut, air-impingement jet, drying characteristics, drying model