Changes in the composition of volatile compounds and lipids during processing of macadamia nuts

Abstract

Macadamia is a subtropical tree originating from Australia. The kernels are rich in monounsaturated fatty acids and may reduce serum cholesterol when included in the diet. It can be assumed that this is due to a particularly high oleic acid content of the nut oil. In general, an average ratio of unsaturated to saturated fatty acids in macadamia nuts is 5.5:1. High content of unsaturated fatty acid leads to oxidative reactions and results in rancidity which decreases quality of macadamia nuts. Moreover, processing methods and storage can have a major impact on changes in quality of macadamia nuts. Therefore, the proposed research aims at investigating the changes of flavour and lipid composition during macadamia nut processing. The latter includes drying, roasting and storage of macadamia nuts. Two methods of drying were employed in this study. Hot air drying (HA) (30°C for 7 days, 40°C for 7 days, 60°C for 3 days) and heat pump drying (HP) (38 °C,1 day, 8% wb) followed by tunnel dryer (TD) (55°C, 2 days) were used to dry macadamia nuts until their moisture content came down to 1.5% wb. This was followed by roasting at 125 °C for 20 minutes until the kernel moisture content reached 1% wb. The final phase of the experiment was storage at 4°C for 3 months. After each processing stage the quality of macadamia nuts was assessed in terms of fatty acid content, volatile compounds, colour, peroxide value, free fatty acids and moisture content. The result showed a decrease of fatty acid content (p[is less than or equal to]0.05) among unsaturated fatty acids such as C16:1, C18:1 and saturated fatty acids such as C14:0, C18:0, C20:0 and C22:0 after drying with HA drying as well as the decreasing of fatty acid content (p[is less than or equal to]0.05) among unsaturated fatty acids such as C18:1 and saturated fatty acids such as C12:0 and C22:0 after drying with HP+TD drying. The decomposition of fatty acids contributed to an increase in the level of total aldehydes, total alcohols and total hydrocarbons. Moreover, free fatty acids and peroxide value increased significantly (p[is less than or equal to]0.05) after drying. HP + TD drying can reduce decomposition of fatty acids during drying more than HA drying. Final content of oleic acid decreased from 59.3 to 45.5 and 62.3 to 57.3 mg/100 g db after HA and HP + TD drying, respectively. The increasing values of hexanal, total alcohols, aldehydes and hydrocarbons after HA drying were more pronounced than after HP + TD drying. The degradation of fatty acids continued during roasting and storage. In addition to that, a small degradation was found during roasting and storage of macadamia nuts at low moisture content after roasting (1% wb). It can be concluded that HP + TD drying showed benefit of time saving and natural quality preservation of the macadamia nuts vs. than HA drying. Although, the changes of fatty acids, volatile compounds, colour and rancidity after drying and roasting continued during storage of macadamia nuts, they occurred at a slow rate. The final values of peroxide value, free fatty acid and colour were within acceptable limits after three months of storage.