Effect of different extraction conditions (cold press and solvent) and different refining processes on physicochemical properties and minor components of black cumin seed oil

Abstract

Abstract Introduction and purpose: black cumin seed with the Nigella sativa L. name has beneficial effects on human health due to its bioactive compounds and strong antioxidant properties. This oil has many medicinal uses. However, due to the high amount of peroxide and the content of free fatty acids in this oil, its use in food applications is limited. In the present research, the feasibility of refining black cumin seed oil extracted using cold pressing from the seeds and then extracting it with solvent from the pressed cake was studied. Procedure: The oil of the cleaned black seeds was extracted using a spiral press machine at 40ºC, and then the remaining oil in the press cake was extracted using hexane solvent. The obtained oils were separately subjected to the refining process (degumming, neutralization, bleaching, and deodorization). Extracted from crude oils, samples were taken in each of the refining stages, and all the samples were stored separately at 4ºC and their qualitative characteristics, including peroxide value, anisidine value, free fatty acid percentage, oxidative stability, the concentration of chlorophyll and carotenoid, color properties, refractive index, fatty acid composition, total phenolic compounds, thymoquinone content, and antioxidant capacity, were investigated by the Rancimet test. Finally, the data were statistically evaluated by analysis of variance (ANOVA) using GraphPad Prism 9 software, and a statistically significant level of 0.05 was used to report the significance of the difference (p<0.05). Findings: The results showed that the stability of black seed oil extracted from press cake using solvent is higher than the oil obtained from cold pressing of seeds (oxidative stability: 8–21 hours). The peroxide value (from 8–32 meq/kg of oil) and anisidine value (from 4.5–8.1 mmol/kg of oil) of the oil obtained from the cake press were lower than the oil obtained from the cold pressing of seeds, while the oil obtained from pressed cake using solvent contained more free fatty acids (15.6-2.16%) than the oil obtained from the cold pressing of seeds. The measurement of color indexes and the concentration of pigments showed that extracting with solvent from pressed cake, compared to extracting with cold press from seeds, incorporated more pigments into the oil and obtained a darker oil (concentration of chlorophyll and carotenoid: 6-23.7 mg/kg of oil and 3.7-9.6 mg/kg, respectively). The profile of fatty acids showed a slight difference between the oil obtained from the cold pressing of seeds and the oil obtained from the cake press. The only major difference was related to oleic acid (26.6–24.9) and linoleic acid (51.5–54.3). Total phenolic compounds (297-964 mg gallic acid/kg oil), thymoquinone content (130-3013 mg/kg), and antioxidant capacity (68-92.2%) of the oil obtained from cold pressing of seeds compared to the oil obtained from the press cake were higher. The qualitative characteristics of oil samples obtained at different stages of the refining process indicated that the oil's stability grew significantly during the refining process (oxidative stability: 5–8.5 hours). Peroxide value (11.56–14.24 mmol/kg oil) decreased during refining, while anisidine value (41.45–35.40 mmol/kg oil) increased. The content of free fatty acids (0.19–0.41% in oil) decreased significantly after the refining process. The color indexes and the concentration of pigments were measured during the refining process, and the results showed the oil became lighter after refining and the quantity of pigments decreased during the refining process (chlorophyll content: 1–7.2 mg/kg oil, carotenoid content: 1.5–0.5 mg/kg). The fatty acid profile changed little during the refining process. During the refining process of black seed oil, total phenolic compounds (94–900 mg gallic acid/kg oil) slightly decreased, thymoquinone content (330–70 mg/kg), a significant decrease, and antioxidant capacity (95.3–80%) were evaluated without change. Conclusion: The results showed that the oil obtained from a pressed cake using solvent has better oxidative quality than the oil obtained from cold pressing seeds. During the refining process, the quality properties of the obtained oils improved and reached the standard level of edible oil. However, the main healthy and bioactive compound of black seed, thymoquinone, had a significant decrease