Characterisation of macadamia nuts using X-ray microtomography

Macadamia nuts are among the most nutritious and highest in monounsaturated oil content among edible nuts1. They are widely grown in Australia (46 % of total production), United States of America, especially in Hawaii, South Africa and Guatemala2. In practice, there are several steps involved in macadamia processing, including sorting and grading, drying, cracking, roasting, packaging and storage. Drying is a very crucial step as it needs to preserve macadamia quality as well as enhance storage stability through the reduction of water activity. It is obvious that physical properties of the nut contribute to its drying characteristics, and hence its storage stability. Accurate measures such as kernel volume ratio or shell density could help for improving drying efficiency.
We present a methodology to investigate structural differences between varieties of macadamia nuts in order to understand the factors involved in storage stability. Fresh nuts-in-shell are scanned by X-ray microtomography, and the different parts of the nuts (shell, kernel, tracheids) are segmented by a set of classical 3D image operators. After image segmentation, volumes are determined, and additional weighing of the nuts allows density measurements. These quantities are plotted for several nuts from each variety.