Effect of extraction methods on physical and chemical properties and shelf life of black cumin (Nigella sativa L.) oil

Abstract

The herbaceous plant black cumin (Nigella sativa L.) has a variety of medical benefits. For therapeutic uses, effective utilization of black cumin oil (BCO) depends upon its functional groups. The objective of the current study was to investigate the physicochemical characteristics, extraction methods' effects, shelf-life, and functional groups of BCO. Mechanical, cold, and Soxhlet extraction methods were used to extract oil from seeds. Proximate analysis of black cumin seeds showed 20.37 % protein, 34.24 % fat, 6.86 % moisture, 7.02 % ash, and 31.51 % carbohydrate. The density of mechanically extracted oil (MEO), Soxhlet extracted oil (SEO), Cold extracted oil (CEO), Locally labeled oil (LLO) and Locally unlabeled oil (LUO) were 0.957 g/cm3, 1.151 g/cm3, 1.170 g/cm3, 1.068 g/cm3 and 1.066 g/cm3, respectively, whereas the refractive indices were approximately similar in all oil samples. In terms of color, CEO and MEO were respectively the lightest and reddest ones among the oils. Initially, free fatty acid (FFA) contents in MEO, SEO, CEO, LCO and LUO were 6.663 %, 6.041 %, 8.214 %, 8.458 %, and 7.573 % respectively, where the highest peroxide value (PV), iodine value (IV) and saponification value (SV) were found as 26.232 mEq O2/kg, 122.275 g I2/100 g and 201.282 mg KOH/g in LCO, CEO and LUO. respectively. By 14 weeks of storage FFA, PV and SV in MEO increased from 6.663 to 14.289 %, 23.508 to 32.229 mEq O2/kg and 196.207–202.629 mg KOH/g respectively while IV decreased from 122.275 to 117.319 g I2/100 g. FTIR analysis reveals the MEO mostly consists of unsaturated fatty acids (cis). The overall findings conclude that MEO remains consumable until 10 weeks indicating a better stability against oxidation and rancidity.