3.2 Nutrient substances of Qingjinju seed oils
Qingjinju seed oils can be utilized as a source of oil for human consumption. Table 2 shows the nutrient substances of the Qingjinju seed oils obtained by different extraction methods.
β-sitosterol is the main sterol in vegetable oils. The β-sitosterol content of the Qingjinju seed oils was 1776-1947 mg/kg, which is higher than observed for oils from C. aurantifolia or C. grandis 18.
Limonoids are unique substances with highly oxygenated triterpenoid backbones and are only found in the Rutaceae and Meliaceae40 families. Citrus limonoids have been reported to display anticarcinogenic41activity and have been suggested as natural chemo preventatives or nutraceuticals42. Limonin and nomilin are the major limonoids and are the primary contributors to the bitterness of citrus fruits and products. The limonin and nomilin content in the Qingjinju seed oils significantly varied across the extraction methods. The limonin content in SO is 722.49 mg/kg, which is 4.22 and 4.90 times less than SCO and SPO, respectively. The nomilin content in SO is 65.02 mg/kg, which is 10.35 and 8.23 times less than SCO and SPO, respectively. This decreased limonoid content may be due to the decomposition of limonin and nomilin during the long exposure to high temperatures in the solvent extraction. Citrus oils are generally characterized by a high percentage of limonin43-44. Interestingly, the content of limonin and nomilin in the Qingjinju seed oils was higher than that of other citrus fruits42.
The flavonoids in the Qingjinju are reported to be 3´,5´-di-C-β-glucopyranosylphloretin, naringin, and hesperidin, and the total flavonoid content in peel ranges from 1168 to 1888 mg/100 g45. The total flavonoid content in the Qingjinju seed oils was greatest for solvent extraction (28.66 mg/mL), followed by screw press (26.25 mg/mL) and supercritical CO2 (24.89 mg/mL) extraction. Notably, the total flavonoid content in the Qingjinju seed oils was higher than that reported for Moringa oleifera seed oil (18.24 mg/g)46, which has been valued for its medicinal properties. This suggests that the Qingjinju seed oils may also display beneficial antioxidant activity and could be applied for the natural management of various chronic diseases.
The α-tocopherol content of the Qingjinju seed oils ranged from 171.68 to 202.70 mg/kg, which was higher than lemon (155-110 mg/kg)14, mitha (26.4 mg/kg)15, and tangerine (116.74 mg/kg)47 seed oil, but lower than grapefruit (380 mg/kg), mussami (220 mg/kg), or kinnow (557.82 mg/kg)15seed oil. The α-tocopherol content varied significantly based on the extraction method, which agrees with the findings of Malacrida et al.47
The total phenolic content of the Qingjinju seed oils ranged from 3.83 to 6.25 mg/kg, which was lower than mandarin, orange, lemon, or tangerine seed oils19, 47. This difference can be attributed to both the chemical composition of the different Citrus spp. and the manner in which the results were obtained. There were significant differences between total phenolic content of the oils obtained using the different extraction methods, which is in agreement with results reported by Mustafa el al.37 The total phenolic content of SCO was 6.25 mg/kg, which was 1.2 and 1.6 times higher than SPO and SO, respectively. Overall, screw press and supercritical CO2 extraction outperformed solvent extraction in obtaining nutrient substances from Qingjinju seeds.