Drying Kinetics and Quality　Characteristics of Microwave-Vacuum　Dried Saskatoon Berries

摘要

Saskatoon berry (Amelanchier alnifolia) is a high moisture content fruit crop, harvested at 75-80% moisture content (w.b.) and dried to lower moisture contents for a safe, extended shelf-life and for further usage in food applications. Saskatoon berries were dried from an initial moisture content of 75.5% to around 25%) (w.b.) using microwave-vacuum drying. In this study, saskatoon berries were dried at different microwave power levels P10 (745 W), P7 (514 W) and P5 (374 W) and vacuum pressure levels V20 (67.0 kPa) and V10 (33.5 kPa). The vacuum pressure levels did not greatly influence the drying time of the saskatoon berries compared to the microwave power levels. Increasing the microwave power level produced faster drying times for the samples. After 12 min of drying at power level P10, the moisture contents of the samples at vacuum pressures of 33.5 and 67.0 kPa were 29.74 and 31.24%), respectively. However, after drying for 12 min at vacuum pressure of 33.5 kPa, the moisture contents at power levels P10, P7 and P5 were 29.74, 51.41 and 61.95%, respectively. Polynomial and exponential models were fitted to the drying data. The exponential model provided a better fit for the drying data at the lower vacuum pressure, with higher R2 and lower SE compared to the polynomial model. On the other hand, the polynomial model provided a better fit for the drying data at the higher vacuum pressure. The vacuum pressure and the microwave power levels did not significantly change the yellowness of the samples at indicated by the Δb values. However, microwave-vacuum drying produced significant color changes in the L (white - 100 to black - 0) and a (green = -a to red = +a) values as indicated by ΔL and Δa. Total color difference was dependent on microwave power and vacuum pressure levels. Water activity of the samples after drying ranged from 0.61 to 0.75 for moisture content range of 15.68 to 29.03%.