Hexose synthesis by cell wall invertase activity and its effects on the roasting behaviour of macadamia kernel

Abstract

After roast darkening (ARD) is a defect concealed in the raw macadamia kernel, only becoming evident upon roasting. Kernels susceptible to some forms of ARD reportedly have a higher glucose and fructose concentration. By developing a procedure to simulate ARD and through the inclusion of effector molecules we have demonstrated that the enzyme invertase is key to this form of ARD. Biochemical analysis of raw mature kernel has shown high invertase activity. Separating the extract into pellet and soluble fractions showed that the high invertase activity occurred in the pellet fraction containing the cell wall isoform and that the soluble fraction had little activity. A broad peak in crude cell wall invertase activity occurred between pH 3.75 and 5.0. Enzyme kinetics of the cell wall invertase from crude extracts assayed at pH 4 indicated a high level of activity (Vmax = 4.11 ± 0.55 mg glucose produced/g fresh weight tissue.h), a high affinity for sucrose (K m = 2.02 ± 0.96 mM), and inhibition by MgCl2 (Ki = 71.2 ± 12.5 mM). We propose that the initial step in the processes leading to ARD in macadamia could involve membrane damage and subsequent modification of the kernel sugar composition by cell wall invertase.