Effect of postharvest treatment on macadamia nuts quality

Abstract

The aim of this project was to study possible pathways that lead to the development of brown kernel and postharvest rancidity. Inferior kernel quality results in nut rejection and hence leads to economic loss. Postharvest steps are inter-related to each other, therefore this project has attempted to cover as many aspects as possible to give a full understanding of how postharvest treatments affect macadamia quality. The study was conducted on five macadamia varieties including A38, 246, 816, 842 and Daddow. The study shows that brown kernel can occur through three different pathways including enzymic browning reaction, Maillard reaction and microorganisms infection. To study the enzymic browning reaction, phenolic compounds and polyphenol oxidase (PPO) activities were analysed. The most common phenolic compounds present in macadamia are gallic acid and chlorogenic acid. Brown section had higher level of bound phenolics compared to the white section of the same kernel. This indicated the likely involvement of phenolic compounds in the formation of brown kernel. The possibility of Maillard reaction involvement was studied by determination of reducing sucrose substrate level by HPLC. Sucrose was hydrolysed into reducing sugars when nuts were subjected to drying using either heat pump or cabinet dryers. Reducing sugars then reacted with kernel protein causing brown pigment via the Maillard reaction. The Daddow variety showed the least hydrolysis degree compared to other varieties. Browning could also be a result of microorganisms infection as infected kernels turned into brown. As the kernel is a biological sample, when it was infected with moulds, it could utilise phenols which act as an anti-pathogenic compound. Analysis of the microstructure of kernels by confocal microscopy showed the distribution of possible brown pigment compounds such as lipid, carbohydrate, protein. Some other novel techniques such as X-ray tomography and magnetic resonance diffusion tensor imaging used in this project provide a better understanding of the microstructure. The findings of this study have potential to improve the existing postharvest techniques used in the macadamia processing industry.