1Characteristics of walnut oil and the residual cake prepared using various pretreatment and extraction methods

The effects of pretreatment technology (microwave roasting, MR; oven roasting, OR) and processing technology (hydraulic pressing, HP; aqueous enzymatic extraction, AEE) on the walnut oil quality and functional properties of walnut proteins in defatted powder were systematically compared. The results showed that the pretreatment increased the yield of walnut oil and HP was the processing method giving the higher yield of 69.72%. AEE oil provided lower acid value (0.25 mg/g) and higher peroxide value (2.47 mmol/kg). There was little difference in iodine and saponification values of the oils obtained by the two methods, and they all met the standards of walnut oil. Walnut oil obtained by HP without pretreatment had the highest proportion of polyunsaturated fatty acid (76.97%). MR and OR resulted in increased solubility, emulsion stability, and oil holding capacity of the proteins, and OR increased walnut protein foaming ability, but MR reduced it. Confocal scanning laser microscopy results showed that MR and OR led to the aggregation of oil bodies in the cotyledonary cells of the walnut kernel, which made the oil easier to be extracted. The results of this study will help to develop and improve the extraction process of walnut oil.