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Nuts and argan oils undergo various processing operations that have an impact on their oxidative stability. Supercritical carbon dioxide (SC-CO ) extraction of oil is associated with greater thermal oxidative instability of the oil than solvent or extruder extraction methods (extracted walnut oil is less stable than pressed walnut oil during accelerated storage. During accelerated storage, SC-CO 2 darkness, determined by PV, solid-phase microextraction headspace analysis and sensory methods.

However, SC-CO extracted oils exhibited higher photooxidative stability than pressed oils, which may be due to the presence of chlorophyll in pressed oils. Specialty oils, such as walnut oil, are typically stored in clear glass bottles on store shelves, making them susceptible to photo-oxidation. Therefore, it is important to improve the stability of SC-CO extracted oils to light-induced oxidation. However, the oxidative stability index and tocopherol content were significantly lower in SC-CO (p < 0.05), however, SC-CO 2 pressed walnut oil.

Drying temperature affects the sensory characteristics of pistachios and their roasted flavor increases during high temperature drying (116-138°C) ( Kader et al., 1979) studied the effect of various drying methods (sun drying, bin dryer, vertical continuous dryer, vertical cylinder dryer and funnel vertical dryer) on the quality of pistachios. The different drying methods used did not have any significant (p > 0.05) effect on the free fatty acid (FFA), PV and TBA values of lipids in pistachios. Sun-drying of hazelnuts was considered to be the best, as artificial heating would lead to acidification. Drying hazelnuts at temperatures between 30 and 70°C reduces the initial activity of endogenous lipases, peroxidases and polyphenol oxidases).

Drying to an appropriate moisture content (4-6% wet basis (wb)) is an important factor to ensure a good quality product. Nuts dried to 4% (wb) moisture content are more brittle and sweeter with less bitterness and sourness than nuts dried to 6 or 11% (wb) moisture content. Nuts with 6% (wb) moisture content were sweeter and less bitter and sour than nuts with 11% (wb) moisture content.

Drying affects the composition of pistachios, but less so than blanching and roasting. Drying hazelnuts at temperatures higher than 50°C is not recommended because the rate of sourness increases with temperature, leading to a decrease in hazelnut quality. The optimal drying air temperature for hazelnuts is in the range of 40-50°C, since lower temperatures require longer processing times, while temperatures above 50°C favor lipid oxidation, especially in hazelnuts in the shell.

Higher enzyme activity was observed in shelled hazelnuts than in shelled hazelnuts, probably due to the higher water activity of the former than the latter at the same moisture content. The shell and kernel color of hazelnuts was influenced by the drying method (drying on lawn, concrete, plastic and wooden boards). The best method is drying in concrete because nuts subjected to this treatment dry in a short time and therefore have a high epidermal removal rate and good shell and kernel color.