Optimization of ultrasound‐assisted enzymatic extraction of walnut kernel oil using response surface methodology

Abstract In this study, ultrasound‐assisted enzymatic technique was investigated using n‐hexane solvent for walnut oil extraction. To achieve this goal, ultrasonic pre‐treatment time (0–70 min) and enzymatic extracting conditions; namely, cellulase concentration (0–2 %) and incubation time (60–240 min) were optimized using response surface methodology. The optimal ultrasonic‐enzymatic extraction treatment using n‐hexane solvent was 47.37 min ultrasonic time, 2.00 % cellulase concentration and 110.91 min incubation time. This treatment resulted in significantly more EY (p≤0.001), TPC (p≤0.001) and IV (p≤0.01) and low level of PV (p≤0.01) of extracted walnut oil, compared to stirring incubation extraction using n‐hexane solvent, ultrasound‐assisted aqueous enzymatic extraction, and aqueous stirring incubation extraction treatments. Gas chromatography–mass spectroscopy showed higher content of unsaturated fatty acids of extracted oil; and scanning electron microscopy showed more pores and interspaces of walnut powder residues of optimal ultrasonic‐enzymatic extraction using n‐hexane solvent than those of stirring incubation treatment using n‐hexane solvent. Practical applications The ultrasonic‐enzymatic technique using n‐hexane solvent can improve the extraction yield, total phenolic content and iodine value of extracted walnut oil. The extracted walnut oil had more poly‐unsaturated fatty acids (linolenic acid and linoleic acid) than that obtained by stirring incubation method using n‐hexane solvent. Thus, the results illustrated that this method was a promising extraction method for more extraction yield of walnut oil with high quality.