Inducing biochemical changes to simulate after-roast darkening in macadamia kernel

Abstract

After-roast darkening is a defect in macadamia kernel evident only upon roasting that adversely affects kernel quality. After-roast darkening was artificially induced in 3 cultivars by incubating nut-in-shell of high moisture content (about 22% w/w) at elevated temperatures in either sealed or unsealed polyethylene bags before drying to 1.5% kernel moisture. After oil roasting, darkening was more evident in kernel from nut-in-shell incubated for 24 h in sealed bags at temperatures greater than or equal to 47.5°C. At an incubation temperature of 50°C the critical incubation period was 12 h for nut-in-shell treated in sealed bags. In raw kernel induced to exhibit high after-roast darkening upon roasting, the concentrations of the hexoses, glucose and fructose were elevated and levels of sucrose were reduced compared to non-induced kernel. The change in kernel sugar composition increased with increasing incubation temperature. A loss in cellular viability was also associated with kernel susceptibility to after-roast darkening. These results indicate that after-roast darkening might result from reactions, possibly enzymatic, that change the kernel sugar composition as a result of a loss in membrane integrity.