Citrus sphaerocarpa, commonly known as kabosu, is a lesser-known citrus fruit native to Japan, specifically thriving in the Oita Prefecture. This fruit is a part of the extensive Citrus family and shares botanical characteristics with the more familiar yuzu and sudachi. Kabosu is celebrated for its distinctive spherical shape and the versatile culinary applications of its tart juice, which is a staple in traditional Japanese cuisine for enhancing flavors and acting as a natural preservative.

Botanical Classification

Kingdom: Plantae
Clade: Angiosperms
Class: Eudicots
Order: Sapindales
Family: Rutaceae
Genus: Citrus
Species: C. sphaerocarpa

Plant Characteristics

The kabosu tree is moderately sized, typically reaching 2-3 meters in height. It exhibits dense foliage with glossy, dark green leaves and is equipped with thorny branches, a common trait among citrus trees that provides a certain level of natural defense. During the blooming season, kabosu trees are adorned with fragrant white flowers, which contribute to their ornamental value and precede the fruiting phase. The fruits themselves are initially a vibrant green, maturing into a deep yellow hue as they ripen through the season.

Chemical Composition and Structure

Kabosu fruit is predominantly noted for its high citric acid content, which imparts a sharp, refreshing taste that rivals that of limes and lemons. This acidity is accompanied by a rich blend of vitamins, particularly vitamin C, which is pivotal for immune function and skin health. Additionally, the fruit contains various flavonoids known for their antioxidant properties, which play a role in combating oxidative stress and inflammation. The essential oils extracted from kabosu are aromatic and are utilized not only for their flavor but also for their therapeutic properties.

How to Cultivate It

To cultivate Citrus sphaerocarpa effectively:

1. Soil: It thrives in well-drained, loamy soils that are rich in organic content, which helps in retaining adequate moisture without waterlogging.
2. Light: Full sunlight is ideal to ensure vigorous growth and optimal fruit production.
3. Water: Regular watering is crucial, especially in dry conditions; however, the soil must not remain soggy as citrus trees are prone to root rot.
4. Temperature: Kabosu trees are frost-sensitive. In regions with cold weather, they must be protected or grown in containers that can be moved indoors during winter.

Uses and Benefits

In culinary contexts, kabosu juice is prized for its ability to add a zesty flavor to fish, noodles, and soups, and as a substitute for vinegar in dressings and sauces. It is also employed to prevent the oxidation (browning) of cut fruits and vegetables. Beyond the kitchen, kabosu is valued in traditional medicine for its digestive benefits and potential anti-inflammatory effects, helping to alleviate conditions such as swelling and pain.

Applications

• Culinary: Extensively used to acidulate sashimi, sushi, and marinades, and to enhance the flavors in various Japanese dishes.
• Medical: Consumed to aid digestion and improve overall health due to its high antioxidant capacity (1). The hexane extract of Citrus sphaerocarpa seems to suppress lipid accumulation (2).
• Cosmetic and Aromatherapy: The essential oils are incorporated into skincare products for their nourishing properties and into aromatherapy blends for their soothing effects.

INCI Functions:

Skin conditioning agent. It is the mainstay of topical skin treatment as it has the function of restoring, increasing or improving skin tolerance to external factors, including melanocyte tolerance. The most important function of the conditioning agent is to prevent skin dehydration, but the subject is rather complex and involves emollients and humectants that can be added in the formulation.

Environmental and Safety Considerations

Cultivating Citrus sphaerocarpa aligns well with sustainable agricultural practices due to its low requirement for pesticides compared to other citrus fruits. Sustainable cultivation not only preserves local biodiversity but also ensures the health of the ecosystem. As with any citrus, kabosu should be consumed in moderation due to its high acidity, which can affect those with sensitive stomachs or cause dental erosion if consumed excessively.

References__________________________________________________________________________

(1) Tomiyama K, Aoki H, Oikawa T, Sakurai K, Kasahara Y, Kawakami Y. Characteristic volatile components of Kabosu (Citrus sphaerocarpa Hort. ex Tanaka). Nat Prod Commun. 2011 Mar;6(3):403-8. 

Abstract. The volatile components of both peel and juice of Japanese citrus, Kabosu (Citrus sphaerocarpa Hort. ex Tanaka) were investigated using SAFE (Solvent Assisted Flavor Evaporation) technique after solvent extraction. In this study, wine lactone, rose oxide, (2E)-4,5-epoxy-2-decenal, mintsulfide, and indole were newly identified from Kabosu. AEDA (Aroma Extract Dilution Analysis) of the oxygenated fraction of the peel extract showed high FD (Flavor Dilution) factors for linalool, (2E)-4,5-epoxy-2-decenal, octanal, (4Z)-decenal, beta-citronellol, geraniol, and wine lactone, while wine lactone, linalool, eugenol, geraniol, and (2E)-4,5-epoxy-2-decenal from the juice extract. The enantiomeric distribution of linalool, cis-rose oxide, beta-citronellol, and wine lactone were also determined using a multidimensional chiral GC/MS.

Akakabe Y, Sakamoto M, Ikeda Y, Tanaka M. Identification and characterization of volatile components of the Japanese sour citrus fruit Citrus nagato-yuzukichi Tanaka. Biosci Biotechnol Biochem. 2008 Jul;72(7):1965-8. doi: 10.1271/bbb.80144.

Abstract. A total of 39 aroma compounds were detected in the essential oil of Citrus nagato-yuzukichi Tanaka (nagato-yuzukichi) by gas chromatography-mass spectrometry (GC-MS). The essential oil was characterized by a high percentage of monoterpene hydrocarbons (12 components, 90.52%). The composition pattern of essential oil in C. nagato-yuzukichi was fairly similar to that of Citrus sudachi Hort. ex Shirai (Sudachi). Principal component analysis (PCA) of data obtained with an electronic nose indicated a variation of each oil along PC1. The oils of Citrus junos Tanaka (Yuzu) and Citrus sphaerocarpa Tanaka (Kabosu) showed a clear upward displacement as compared with those of C. nagato-yuzukichi and C. sudachi. However, in PC2, the oils of C. nagato-yuzukichi and C. sudachi showed a displacement in a negative direction and a positive one respectively.

(2) Zang L, Kagotani K, Hayakawa T, Tsuji T, Okumura K, Shimada Y, Nishimura N. The Hexane Extract of Citrus sphaerocarpa Ameliorates Visceral Adiposity by Regulating the PI3K/AKT/FoxO1 and AMPK/ACC Signaling Pathways in High-Fat-Diet-Induced Obese Mice. Molecules. 2023 Dec 9;28(24):8026. doi: 10.3390/molecules28248026. 

Abstract. Obesity is an emerging global health issue with an increasing risk of disease linked to lifestyle choices. Previously, we reported that the hexane extract of Citrus sphaerocarpa (CSHE) suppressed lipid accumulation in differentiated 3T3-L1 adipocytes. In this study, we conducted in vivo experiments to assess whether CSHE suppressed obesity in zebrafish and mouse models. We administered 10 and 20 μg/mL CSHE to obese zebrafish juveniles. CSHE significantly inhibited visceral fat accumulation compared to untreated obese fish. Moreover, the oral administration (100 μg/g body weight/day) of CSHE to high-fat-diet-induced obese mice significantly reduced their body weight, visceral fat volume, and hepatic lipid accumulation. The expression analyses of key regulatory genes involved in lipid metabolism revealed that CSHE upregulated the mRNA expression of lipolysis-related genes in the mouse liver (Pparα and Acox1) and downregulated lipogenesis-related gene (Fasn) expression in epididymal white adipose tissue (eWAT). Fluorescence immunostaining demonstrated the CSHE-mediated enhanced phosphorylation of AKT, AMPK, ACC, and FoxO1, which are crucial factors regulating adipogenesis. CSHE-treated differentiated 3T3L1 adipocytes also exhibited an increased phosphorylation of ACC. Therefore, we propose that CSHE suppresses adipogenesis and enhances lipolysis by regulating the PI3K/AKT/FoxO1 and AMPK/ACC signaling pathways. These findings suggested that CSHE is a promising novel preventive and therapeutic agent for managing obesity.