Comparative chemical analyses of five finger lime varieties by UHPLC-HR-ESI-MS/MS and evaluation of their in vitro antioxidant activity

Citrus australasica F.Muell (Rutaceae), commonly called finger lime or lemon caviar, is a small tree native to Australia.1 The uniqueness of its finger-shaped fruits and odorous caviar-shaped vesicles of the pulp are the reasons for its enormous, recent diffusion in Europe and in Italy. The indigenous peoples of Australia have used finger lime fruits as a food source and for healing purposes for thousands of years.2 Nowadays, these fruits have huge popularity and economic interest in gourmet culinary preparations. The species has several varieties and hybrids differing in peel and pulp color and taste. These macroscopic differences reflect changes in the chemical composition of bioactive secondary metabolites. Many previous studies suggest fruits belonging to genus Citrus as excellent sources of macronutrients and molecules of therapeutic interest such as flavonoids, phenolic acids, anthocyanins, coumarins, and limonoids,3 but to date few chemical and biological studies about C. australasica were performed.1 The aim of this research was to carry out a comparative chemical analysis among peel and pulp extracts of four C. australasica selected varieties (Red, Collette, Pink Ice, and Yellow Sunshine) and the hybrid species faustrime (Monocitrus australasica x Fortunella sp.x Citrus aurantifolia) by UHPLC-HR-ESI-MS/MS quali-quantitative technique. Peel and pulp extracts were also analyzed for their total phenolic content (TPC) and in vitro antioxidant activity on a Balb/3T3 Clone A31 mouse embryo fibroblasts cell line. The chemical analyses showed a higher content of TPC in peels than in pulps and in Red and Collette varieties. All varieties contain hydroxycinnamic acid derivatives in similar quantities both in peels and in pulps, particularly rich in caffeoylisocitric acid. Collette and Pink Ice are the two varieties with the largest number of identified flavonoids such as rutin, luteolin 7'-O-neohesperidoside/rutinoside, poncirin, and isosakuranetin rhamnosyldiglucoside. Collette peels are also the richest in anthocyanins with delfinidin rhamnosylglucoside the most abundant one. All peel extracts showed no cytotoxicity in the range of 5-120 μg/mL, while the pulp extracts within 30-300 μg/mL. Red and Collette varieties exhibited the maximum protective activity against oxidative damage induced with H2O2 1500 μM, capable of reducing cell viability to 50%. In conclusion, the edible parts of fingerlime fruits are good sources of health-promoting phytocomplexes with antioxidant activity, due to their rich composition in polyphenols, being Red, Collette, and Pink Ice varieties the most promising ones.