INTRODUCTION
Qingjinju (Citrus microcarpa ), a cultivar related to citrus, is a hybrid between Citrus reticulata Blanco and Fortunellaspp.1-2 The fruit is primarily distributed across Southeast Asian regions including Hainan, Taiwan, Malaysia, Indonesia, Thailand, and the Philippines. Across these regions, Qingjinju is known by a variety of names: calamondin and calamansi in Taiwan3, ‘jeruk kasturi’ in Indonesia4, kalamondin and limonsito in the Philippines1, musk lime in Malaysia, and ma-nao-wan in Thailand4.
More than 90% of Qingjinju are used for beverage production, with the pulp and seeds discarded after removal of the juice. As Qingjinju seeds comprise approximately 13%4 of the fruit, the juicing process results in a substantial quantity of waste. For example, in the Hainan province of China alone, approximately 23,000 tons of Qingjinju seeds remain following juice production. Although these seeds are generally regarded as agro-industrial waste, studies have found that citrus seeds contain fiber, protein, and oil, as well as a variety of physiologically active substances, including limonoids5, flavonoids6, phenolic compounds7, and tocopherols8, making them a potential source of medically and industrially valuable compounds. Citrus seed oil is particularly important, owing to its potential applications in the food, cosmetic, detergent, and pharmaceutical industries9-10.
There have been an increasing number of studies focused on seeds from variousCitrus spp. Multiple reports describe the fatty acid composition and physical and chemical properties of lemon (C. limon L .)11-14, grapefruit (C. parpadisi )15-16, mussami (C. sinensis )15-16, sour orange (C. aurantium )11, 16, lime (C. aurantifolia )11, kinnow (C. reticulata )15, and mandarin (C. deliciosa )17seed oils. In general, citrus seed oil content ranges between 20 and 40% (by weight) and is a good source of unsaturated fatty acids, including linoleic (18:2 n-6), palmitic (16:0), oleic (18:1 n-9), α-linolenic (18:3 n-3), and stearic (18:0) acid. The total tocopherol, sterol, and phenolic contents in the seed oils are 0.8-21.0 mg/100g, 1310.54-3986.58 mg/kg, and 209.90-287.2 mg gallic acid equivalent (GAE)/kg, respectively18-19. The physicochemical characteristics of citrus seed oils greatly vary with the particular citrus variety: iodine values from 91.4 to 110.0 g I2/100g oil, refractive index (40 °C) from 1.4639 to 1.4681, density (24 °C) from 0.920 to 0.941 g/mL, saponification values from 180.9 to 198.9 mg KOH/g oil, unsaponifiable matter from 0.3% to 0.5%, and acid values from 0.21 to 2.2 mg KOH/g of oil.
Few studies have investigated green kumquant seed oils. Only Manaf et al.4 have studied the musk lime (C. microcarpa ) seed oils in Malaysia. The iodine values, saponification values, unsaponifiable matter, and free fatty acid content of the seed oils were 118.0 g I2/100 g oil, 192.6 mg KOH/g oil, 22 mg/g oil, and 18 mg oleic acid/g oil respectively. The predominant fatty acids of the musk lime seed oils were linoleic, oleic and palmitic acid. To the best of our knowledge, no such quantitative study on the physicochemical characteristics (e.g., refractive index, density) or nutrient substances (e.g., limonin, flavonoids, total phenolic content, β-sitosterol) have been reported for Qinjinju seed oils, nor has there been a thorough qualitative evaluation of its volatile compounds.
The industrial application of citrus seed oils depends on their fatty acid composition, presence and quantity of nutrient substances, flavor component, sensory properties, and processing yield. Studies have found that, along with the citrus variety17 and cultivation area16, 20, the method of oil extraction14, 21 greatly influences these properties.
In this work, we evaluated the physicochemical properties (e.g., iodine value, refractive index, density, saponification value, unsaponifiable matter, acid value, color, and fatty acid composition), nutrient substances (e.g., limonin, nomilin, and total flavonoid, phenolic, β-sitosterol, and α-tocopherol content) and volatile compounds in Qingjinju seed oils. In addition, we evaluated and compared the characteristics of oil isolated by screw press, solvent, and supercritical CO2 extraction.