Kernel brown centres in macadamia: a review

Abstract

The incidence of kernel brown centres in macadamia (Macadamia integrifolia and M. tetraphylla and hybrids) has increased substantially in Australia. Although the defect amounts to only ~1% of all kernels processed in Australia, it costs the macadamia industry over AU$2 million per annum. Little formal research has been conducted, although the defect is mentioned widely in informal grower journals. Possible causal factors are reviewed in this article. Evidence suggests that kernel brown centres may be associated with exposure of enzymes in cell membranes and are also associated with incorrect nut-in-shell drying regimes. There appears to be an interaction among nut-in-shell moisture content, nut drying regime, and the incidence of brown centres. There is some indication that storage of wet nuts in poorly ventilated silos increases the potential for developing kernel brown centres. It is recommended that future research focusses on these issues.

Maintaining high moisture content of macadamia nuts-in-shell during storage induces brown centres in raw kernels

“…Browning of kernels is an important issue for nut crops and internal browning is an especially difficult problem. It is known as ‘concealed damage’ in almonds, ‘opalescence’ in pecans and ‘brown centres’ in macadamias . Concealed damage of almonds is apparently initiated when kernels are exposed to a warm, moist environment .…”

“…Kernel brown centres, also termed ‘internal browning’ or ‘internal discolouration’, is an internal quality defect of macadamia kernels sometimes accompanied by an ‘off’ odour and flavour. Brown centres affect typically around 1% of total kernel, and costs the Australian macadamia industry around AU$2 million annually . The defect is difficult to detect during kernel processing because it is internal, often with no visible external signs …”

“…Brown centres affect typically around 1% of total kernel, and costs the Australian macadamia industry around AU$2 million annually . The defect is difficult to detect during kernel processing because it is internal, often with no visible external signs …”

“…. Correct on‐farm drying and storage of nuts‐in‐shell is critical in maintaining kernel quality . Nuts‐in‐shell should be partially dried on‐farm to approximately 10% moisture content for transport, then further dried to 3% at the processors before cracking .…”

Internal discoloration of various varieties of Macadamia nuts as influenced by enzymatic browning and Maillard reaction

“…The fundamental factors associated with this defect are still unknown. Since this problem occurs erratically, therefore the basic cause of this defect has been difficult to investigate (Lagadec, 2009). However, the internal discolouration or browning of the kernel center varies from light brown to dark brown depending on the shape of nuts which ranges from oval to unconformity.…”