Improving quality of macadamia nut (Macadamia integrifolia) through the use of hybrid drying process

Macadamia nut is rich in monounsaturated fatty acids and may reduce serum cholesterol when being included in human diet. On the negative side, high content of unsaturated fatty acids leads to oxidative reactions, which result in rancidity that decreases the quality of the nut. Drying is thus needed to reduce the nut moisture content and hence alleviate the above-mentioned problem. This research was conducted to determine a suitable strategy to reduce the moisture content of fresh macadamia nut to a safe level of 1–2% (d.b.). Hybrid drying process, namely, the use of heat pump drying (40 °C) followed by hot air drying (50–70 °C) was chosen to investigate its feasibility to dry macadamia nut in a relatively fast and economic fashion. The effects of the intermediate moisture content (IMC) (8–11% (d.b.)), which is the moisture content at the end of the first-stage heat pump drying, as well as the drying temperature on the drying kinetics and quality of the nut were determined. The quality of dried macadamia nut was assessed in terms of the moisture content, water activity (aw), color, reducing sugar content and peroxide value. The results showed that heat pump drying followed by hot air drying with IMC of 11.1% (d.b.) gave shortest drying time (15–45.5 h). The temperature and moisture content of the nut prior to the second-stage drying had significant effects on the peroxide value (p ⩽ 0.05); higher drying temperatures led to higher peroxide values. However, the interaction between the drying temperature and IMC had no significant effect on the internal and external total color changes as well as on the amount of reducing sugar (p > 0.05).