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Finger lime, Citrus australasica, is a thorny understorey shrub or small tree of lowland subtropical rainforest in the coastal border region of Queensland and New South Wales, Australia. It has edible fruits which are grown as a commercial crop. The fruit is a botanical berry, cylindrical and tapered at both ends, slightly curved and about 8 cm long. The skin is rough with numerous oil glands, and greenish yellow to pink. Like many other plants in the genus, the leaves contain numerous oil glands and are aromatic when crushed. Finger lime is also called caviar lime because its spherical pulp sacs give the pulp a caviar-like appearance.

Origin and Production

The finger lime is native to the southeastern corner of Queensland and the northeastern corner of New South Wales, from the area around Brisbane to the Clarence River. It grows in coastal rainforest and extends into neighbouring open forest, often on soils derived from basalt. There is an increasing range of genetic selections which are budded onto citrus rootstock. As of December 2024 the species is assessed as of least concern by the International Union for Conservation of Nature.

Chemistry

In three cultivars grown in Sicily, 84 volatile compounds were identified in the peel and the juice. The cultivars Pink Pearl, Sanguinea and Faustrime have distinct chemotypes, with limonene, sabinene and bicyclogermacrene in Pink Pearl, limonene, γ-terpinene and bicyclogermacrene in Sanguinea, and limonene, β-phellandrene and γ-terpinene in Faustrime. In five Australian cultivars, the predominant volatile compounds in the peel were limonene, γ-terpinene, β-citronellol and citronellal. Limonene was the most abundant monoterpene hydrocarbon in all the samples of peel oil, accounting for up to 73.6% in the Red variety. Bicyclogermacrene was the sesquiterpene hydrocarbon with the highest relative abundance in all the peel oils. In the varieties examined, the peel oils were characterised mainly by monoterpenes, while the pulp oils were rich in sesquiterpenes.

The cold-pressed oil of Faustrime, a hybrid of Microcitrus australasica, Fortunella and Citrus aurantifolia cultivated in Italy, was dominated by limonene (43.2%), citronellal (16.3%), γ-terpinene (4.5%) and α-phellandrene (4.5%). The leaf oil of C. australasica is a different mixture, with bicyclogermacrene (19-28%), germacrene-D (2-8%), delta-elemene (0.5-11%) and limonene (12-24%) as its principal components. The fruits contain the nutrients vitamin C, folate and potassium, as well as the antioxidants vitamin E and polyphenols. Finger limes show promise for commercial development as a functional food.

Culinary Use

The finger lime has become a gourmet bushfood. Its globular juice vesicles, also called pearls, serve as a garnish or go into various recipes, and they give a burst of effervescent, tangy flavour as they are chewed. The juice is acidic and resembles lime juice. Marmalade and pickles are made from the fruit, and the peel can be dried and used as a spice.