Improvement of the Flavor and Oxidative Stability of Walnut Oil by Microwave Pretreatment

Abstract

Walnut oil is in great demand due to its high nutritional value. However, it is easily oxidized and often loses its typical flavor. This study focused on the role of microwave pretreatment in improving the flavor and oxidative stability of walnut oil, and also investigated the effects of microwave pretreatment on unsaturated fatty acids (oleic, palmitoleic, linoleic, and linolenic acids) and antioxidant components (tocopherols and phytosterols). The results indicate that microwave pretreatment is effective in generating pyrazine compounds. The typical ‘roasted’ flavor was present when pretreatment for 2 min or more was applied. Meanwhile, compared with the control sample, only the highly treated sample (microwave‐pretreated for 4 min) showed higher oxidative stability. Only small changes were found in the composition of the unsaturated fatty acids, while the levels of tocopherols and phytosterols significantly decreased with increasing duration of microwave treatment (P < 0.05). The results suggest that the Maillard reaction caused the improvement of oxidative stability, since this reaction can also generate antioxidant products (melanoidins) in addition to pyrazines. Moreover, microwave pretreatment was found to be effective for enhancing the oil yield during pressing. Therefore, despite its adverse effects on tocopherols and phytosterols, microwave pretreatment could be used to improve the flavor and oxidative stability of walnut oil.

Effect of roasting conditions on the composition and antioxidant properties of defatted walnut flour

“…On the other hand, the use of high temperatures of roasting would not be favourable for the walnut flour's antioxidant capacity, as the tannin content could be reduced . On the other hand, the melanoidins formed during roasting have shown some in vitro antioxidant protective effects on lipids, which will be important in preventing oxidative damage of the walnut flour oil . Therefore, melanoidins are probably contributors to the antioxidant activity in the roasted flour samples.…”

“…The production of cold pressed walnut oil is in great demand due to its strong aromatic flavour and high nutritional value . Although pressure systems lead to lower oil recovery, it is the preferred method for obtaining virgin oils with the highest quality .…”

Effect of Blanching Pomegranate Seeds on Physicochemical Attributes, Bioactive Compounds and Antioxidant Activity of Extracted Oil

“…The decrease in β-sitosterol due to the seeds' thermal pretreatment was also reported in previous studies. Zhou et al [55] reported a 7% decrease in β-sitosterol of walnut oil microwaved at 600 W for 4 min.…”

Identification of Volatile Oxidation Compounds as Potential Markers of Walnut Oil Quality

“…The VOCs were analyzed by HS-SPME coupled to GC-MS (7890A-5975C; Agilent Technologies, Santa Clara, CA, U.S.A.) (Zhou, Fan, Chu, & Pei, 2016). Briefly, oil samples (3 mL) were added to 20-mL headspace vials fitted with silicon septa and incubated at 50°C for 10 min.…”