Research On The Combined Drying And Superfine Pulverization Technology Of Raspberries

Drying kinetics and products quality of infrared radiation and hot air dried raspberries were investigated, based on the results of which the hot air-explosion puffing combined drying and infrared radiation-microwave vacuum combined drying were studied, respectively, and optimum drying conditions were obtained. The dried raspberries obtained from single and combined drying methods at optimum conditions were pulverized into powders. The effects of different drying methods on physical quality, nutritional quality and antioxidant activity of raspberry powders were analyzed compared with freeze drying as control. This study aimed at providing theoretical guidance for drying and pulverization of raspberries. Major conclusions were listed below:1. Hot air drying temperature showed significant effect on drying kinetics and quality. With the increase of drying temperature the drying time decreased, but the drying rate and Deff increased. Among five fitting models, Page model was found to well describe hot air drying process. The products became brighter and color lose was observed after hot air drying. The higher redness value and lower total color difference were noted at the temperature of 70°C. When the temperature was 60°C, the rehydration ratio was the highest indicating the best rehydration property. Drying at high temperature and short time(70°C, 540 min) contributed to better anthocyanin retention and antioxidant activity. Evaluated from both drying time and quality, 70°C was suitable for the dehydration of raspberries.2. The effects of infrared radiation temperature and power on the drying kinetics and quality of raspberries were investigated in the infrared radiation drying experiment. Infrared radiation temperature showed significant effect on infrared drying rate and Deff, but drying power did not. Among five fitting models, Page model was found to well describe infrared radiation drying process. The anthocyanin retention increased first but decreased later with the increase of infrared radiation temperature, and the best color and highest anthocyanin retention were obtained at the temperature of 70°C and the power of 675 W. The products were found to present the best rehydration property and the highest antioxidant capacity when the temperature ranged from 70°C to 80°C. Infrared drying power also showed significant effect on the quality of raspberries. Lower power level was observed to be benificial for the color, rehydration property, anthocyanin retention and antioxidant capacity. Overall, when dried at the infrared radiation temperature of 70°C and the power of 675 W, the raspberries presented the best color and rehydration property and the highest anthocyanin retention and antioxidant activity.3. Using quadratic orthogonal rotary regression, the optimum conditions for hot air-explosion puffing drying of raspberries were obtained as follows: predrying temperature of 70°C, moisture content after predrying of 70%, puffing temperature of 97°C, vacuum drying temperature of 69°C, and vacuum drying time of 136 min. The raspberry products obtained from the above drying conditions have attractive color, cristy texture and easy storage.4. The effects of microwave power and vacuum pressure on the drying kinetics and quality of raspberries were investigated in the infrared radiation-microwave vacuum drying experiment. Among five fitting models, Midilli model was found to well describe microwave vacuum drying process. It was noted that high microwave power was good for drying rate, Deff and rehydration property improving. However, the color, anthocynin retention and antioxidant activity increased first and decreased next with the increase of microwave power levels, and the highest values were observed at 600 W. At the same microwave power level, high vacuum pressure showed positive effect on product quality. The optimum condition for infrared radiation-microwave vacuum combined drying of raspberries was at the infrared radiation temperature of 70°C for 60 min, with the microwave power of 600 W under the vacuum pressure of 85 KPa.5. The effect of different drying methods on the quality of raspberry powders was investigated, with freeze drying as control. The results showed that vacuum freeze dried raspberry powders presented the best solubility, soluble sugar content, oil absorption, color and anthocyanin retention, and infrared radiation-microwave vacuum dried powders ranked second with lower moisture and hygroscopicity which facilitates transport and storage as well. The total phenol content, total flavonoid content and antioxidant capacity of raspberry powders after thermal drying methods were noted to be significantly higher than those after vacuum freeze dried powders. Through the comparison of single and combined drying methods, combined drying techniques, especially infrared radiation-microwave vacuum drying was found to be time-saving and efficient with high quality of raspberry powders.