In this study, Australian grown pecans were evaluated for the first time, with a particular emphasis on the composition of the two main cultivars ('Wichita' and 'Western Schley') and the condition described as opalescence in 'Wichita' pecans. Opalescence is an unattractive browning of the interior of pecan kernel meat, compared to the white interior of normal kernels. The Australian pecan industry has identified opalescence as a possible quality problem for pecans.
Total lipid, fatty acid profiles, minerals, sugars, ascorbic acid, and α-, ß-/y- and δ-tocopherol contents were determined in both 'Wichita' and 'Western Schley' cultivars, as were lipase and lipoxygenase activity. Standard analytical and food analysis techniques such as gas chromatography (GC) and high performance liquid chromatography (HPLC) were used for all compositional work. Besides slight differences in the fatty acid profile and tocopherol contents of the Australian grown pecans, the values obtained were very similar to the values for pecans grown in the United States of America (USA).
A subjective scoring system for the degree of opalescence of the kernels (using standard photographs) was developed as part of this study. Approximately 70 % of Australian grown pecan kernels were found to exhibit opalescence to some degree. Evaluation of the kernels for opalescence at each stage during harvesting and processing showed that opalescence resulted from physical damage to the kernels, with it first becoming evident after mechanical cracking. Storage of the pecans (as nut-in-shell) at 0 oC or -18 oC had no effect on the resulting opalescence in the kernels. Differential Scanning Calorimetry (DSC) showed that kernels do not undergo freezing at -18 oC.
Supercritical fluid extraction of opalescent pecans revealed that the brown discolouration was due to the presence of free oil within the kernel, as removal of 25 g 100g-1 of oil from the kernels resulted in a pure white interior. Methodology for the sample preparation of pecans for electron microscopy investigations was developed. This technique revealed that in opalescent pecans, the lipid membranes which surround the lipid bodies in pecan kernels, were disrupted or broken. Chemical investigations of pecans with varying levels of opalescence showed that pecans with high levels of opalescence also have lower levels of calcium compared to non-opalescent pecans. Lower calcium levels in opalescent pecans suggest that cell membranes and cell walls may be weaker, leading to a higher chance of breakage occurring. Opalescent pecans were also found to have a higher oil content than non-opalescent pecans.
USA grown 'Wichita' pecans were also assessed for opalescence after processing in Australia. Minimal opalescence was found in the USA grown pecans. These kernels were also found to contain higher levels of calcium than the Australian kernels. The extra calcium present in the USA grown pecans may provide the additional requirements needed to ensure cell wall maintenance and membrane integrity in these kernels.
The effect of maturity on resultant opalescence was investigated. Pecans harvested six weeks either side of the normal harvest showed little change in composition or opalescence level.
Sensory evaluation of unstored opalescent and non-opalescent pecans showed that opalescent pecans were slightly more bitter and have a lighter testa colour than nonopalescent pecans. Rancid flavours tended to be greater in opalescent pecans than in non-opalescent pecans. Accelerated storage at 25 oC and 60 % RH was found to hasten rancidity development in opalescent pecan kernels, which was highlighted by a darkening in the testa colour. However, minimal rancidity development occurred in opalescent pecan kernels after storage in vacuum packages at 0 oC for up to six months, but some testa darkening was evident.
This study therefore demonstrated that opalescence in pecans is not only a cosmetic problem for the pecan industry, but it has the potential to reduce the quality and shelflife of the product if optimum storage conditions are not maintained.