Effect of nut-in-shell storage conditions on volatile profile in macadamia nuts

Abstract

In order to study the effect of storage conditions on the volatile profile of macadamia nuts, both temperature and the presence of oxygen were controlled. Nuts-in-shell of variety 246 were stored at -18, 10 and 15°C. At each temperature, samples were stored in net bags and vacuum packed in EVOH (ethylene vinyl alcohol) for a period of 2 months. Prior to testing, samples were cracked and roasted. Analytical tests included peroxide value, p-anisidine and TBA (thiobarbituric acid number). Volatile compounds were isolated using Head-Space-Solid-Phase-Micro Extraction (DVB/PDMS/CAR). Volatiles were analysed by chromatography/mass spectrometry (GC/MS). The results show that at –18°C peroxide, p-Anisidine and TBA values were lower compared to the other 2 storage temperatures. This indicates that lipid degradation was lowest at this temperature. The main compounds found in volatile components of both fresh and roasted macadamia nut were hydrocarbons, aldehydes, and ketones. Several compounds deriving from lipid degradation and Maillard reaction were detected such as hexanal, thiazole. This indicates that changes in volatiles of macadamia nuts result from compounds present in macadamia nuts which were the precursors of volatile compounds produced during roasting. Storage materials which were net bag and EVOH showed no significant effect on volatile produced during 2 months storage trial. Volatiles generated depend mainly on processing steps such as roasting. The lipid degradation was minimised at low temperature. Therefore storage conditions for nut-in-shell influence the final macadamia kernel quality, especially shelf-life, as peroxide value and other lipid degradation product were used as criteria of shelf-life changes while volatile produced by Maillard reaction indicated roasting quality.

Keywords: Macadamia, Volatiles, Storage conditions, Head-Space-Micro Extraction, Lipid changes