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.