(1) Background: The oil stability of tree nuts during storage can be influenced by storage conditions such as temperature, humidity, and moisture concentration. However, few studies have assessed how the presence of testa and shell affects the oil stability of tree nuts during storage. We aimed to determine how storage conditions affect oil stability in almond and canarium, in particular, the presence of testa and storage time of nut-in-shell (NIS). (2) Methods: We measured peroxide value (PV), free fatty acid (FFA) and hexanal concentrations of almond and canarium (blanched vs. kernel-in-testa) stored at 45 °C for 24 days. We also measured PV, FFA and fatty acid composition of canarium samples at days 0 and 140 stored as NIS under ambient conditions. (3) Results: The presence of testa in almond and canarium decreased hexanal and PV concentrations at day 24 of incubation. Canarium PV and FFA concentrations increased over 140 days of storage in the shell compared to day 0. However, both PV and FFA concentrations remained within the acceptable threshold during storage. No changes in fatty acid composition were found during NIS storage. (4) Conclusions: Testa and shell could act as a natural coating, slowing down oxidation rates. Hence, long-term storage on nuts in testa or nuts in shell are recommended for tree nuts.

Nut quality is of great economic concern because nuts are rich in oil, which can be rapidly oxidised if the nuts are not properly handled. Tree nuts are often maintained in-shell after harvest so that they can be processed later and marketed out of season. Poor handling procedures prior to kernel storage, and suboptimal temperatures during storage, may compromise quality and lead to colour development, particularly for tree nuts with high oil content that are susceptible to oxidative rancidity. However, there is limited information on the effects of in-shell handling procedures or kernel storage temperatures on the quality and colour development of tree nuts during long-term storage. We determined how the freezing of kernels from the tropical tree, canarium (Canarium indicum), affected their subsequent kernel quality when stored at 25 °C for up to 9 months. Pre-frozen kernels had significantly higher peroxide values than non-frozen kernels (3.90 and 0.35 meq O₂/kg oil, respectively) after 3 months of storage. However, pre-frozen kernels had significantly lower peroxide values than non-frozen kernels after 9 months of storage, most likely due to degradation of peroxides to secondary compounds over time. We also evaluated how the prolonged maintenance of nuts-in-shell prior to processing affected kernel quality, oil composition and colour development during storage at 4 °C or 25 °C for up to 15 months. Storage at 25 °C led to higher free fatty acid levels from 9 months onwards than storage at 4 °C, both for the kernels that had been maintained previously in-shell and for the fresh kernels. Nonetheless, peroxide values and free fatty acid levels of the previously stored in-shell and of the fresh kernels were still below the acceptable quality limits for tree nuts. The concentrations of palmitic, stearic and linoleic acid in fresh kernels stored at 4 °C or 25 °C did not differ between the storage temperatures. However, the oleic acid concentration was higher in refrigerated kernels than unrefrigerated kernels. Kernels stored in-shell had lower reflectance at 25 °C than those stored at 4 °C at 12 months of storage. Our findings demonstrate that freezing of canarium kernels accelerates lipid oxidation and quality loss, but that refrigeration suppresses lipid oxidation. Kernels stored at room temperature were still below the maximum acceptable limits for peroxide values and free fatty acid levels, although they would likely develop rancidity if stored for more than 15 months. We recommend that the kernels of tropical tree nuts with high oil content like canarium not be frozen, but should be freshly processed, kept at 4 °C if shelled, and stored immediately, to ensure high quality.