Application of microwaves for drying of durian chips

• Combined microwave-vacuum (MWVC), microwave-hot air (MWHA) and hot air (HA) for durian drying were compared.
• The combined microwave techniques provided a higher drying rate but consumed less energy than hot air drying.
• Drying rate of durian chips increased mainly with microwave power rather than a decrease in pressure.
• Both combined microwave techniques provided dried durian chips with similar crispiness and higher than the HA.
• Overall preference of dried durian chips from MWHA was higher than MWVC, but still less than commercial chips.

In this study, alternative processes, i.e. microwave vacuum (MWVC) drying and combined microwave-hot air (MWHA) drying, were evaluated and compared with hot air (HA) drying with regard to producing un-oily crispy durian chips. Fresh durian chips of the Mon Thong variety with an initial moisture content of 197.7–213.9% dry basis (d.b.) were dried by MWVC to a final moisture content of 3.5% (d.b.) under microwave power of 150, 200 and 250 W and a pressure of 10 and 30 kPa. The same microwave powers as for the MWVC process were used in MWHA drying combined with a drying temperature of 65 °C, air velocity of 0.3 m/s and 80% air recycled, as used in the HA drying. The quality of the dried durian chips was considered in terms of colour, percentage of shrinkage, texture and microstructure. The energy utilization of the drying system was evaluated in terms of the specific energy consumption (SEC). The results showed that MWHA drying provided a higher drying rate than MWVC and HA drying. The drying rate increased with an increase in the microwave power and a decrease in pressure, which led to a shorter drying time. However, the microwave power level had more effect on the drying rate than the pressure level. High microwave power also produced dried products with low lightness, shrinkage and hardness values, but more crispy and with large pore sizes. Dried durian chips from the combined microwave techniques had higher lightness and crispness values and more porous structure with larger void area fraction but less shrinkage and hardness than those dried by HA. The combined microwave techniques gave lower specific energy consumption (SEC) than HA drying, and the SEC decreased with an increase in microwave power. The overall preference of dried durian chips from MWHA was higher than that from MWVC but lower than that of commercial fried durian chips.