Chemical differences between brown centre and white macadamia kernels

Abstract

Nuts are susceptible to many quality issues, including discoloration defects in kernels. These discolouration defects can affect their market value and result in significant economic losses to both growers and processors. In macadamia, this quality issue is called brown centres and has been related to storage at high temperatures and moisture concentrations. However, the chemical differences between brown centre and white macadamia kernels have not previously been studied. We aimed to determine the chemical differences between macadamia kernels with brown centre defects and kernels that were white and cream coloured. We used brown and white kernel samples from 20 commercially processed batches of macadamia. From each batch, we selected brown centre samples and compared their chemistry with white kernel samples from the same batch. We compared the moisture concentration, sugar concentration, nutrient concentration, fatty acid profile and peroxide value of brown centre defect and white kernel samples. Our results showed that brown centres had a higher moisture concentration compared with white kernel samples (5.16 % vs 2.91 %, respectively, P < 0.05). Reducing sugars of fructose (0.61 %) and glucose (0.53 %) concentrations were only detected in brown centre, although the sucrose concentration was not significantly different. Most nutrient concentrations were higher in brown centres compared with white kernels. Nitrogen concentration was significantly higher in brown centre than white centres (2.10 % vs 1.82 %, respectively, P < 0.05). Calcium concentration was also higher in brown centre than white macadamia kernels (608.44 mg/kg vs 419 mg/kg, respectively, P < 0.05). Our study suggests that high moisture concentrations in macadamia kernels could have triggered the hydrolysis of sucrose into reducing sugars (fructose and glucose). In addition, reducing sugars and proteins can react in the Maillard reaction to form brown colouration in nuts. This is the first study to comprehensively compare the nutritional composition of brown centres and white macadamia kernels. We recommend appropriate drying and storing of macadamia nuts to prevent sucrose degradation into reducing sugars and decrease the formation of brown centres through the Maillard reaction.