Comparative active compounds and antioxidant activity between the sweet- and sour-type star fruit (Averrhoa carambola L.) In Vitro

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Background: Star fruit (Averrhoa carambola L.) is seasonal and originates from many Southeast Asia countries, including Thailand. Previous evidence claimed that it has various antioxidative compounds such as phenolics, saponins, flavonoid C-glycosides, tannin and L-ascorbic acid. In Thailand, the sweet-type of star fruit (SF) is cultivated and marketed more than the sour-type, but their different antioxidant and active compounds between both types have not been investigated. Objectives: This study aimed to compare the active compounds and anti-oxidant activity between sweet- and sour-type SF in vitro. Materials and methods: Active compounds such as total phenolic compound, total flavonoids and L-ascorbic acid in extracts were evaluated between sweet- and sour-type SF crude extracts by using Folin-Ciocalteau reagent, aluminum chloride colorimetric assay and high-performance liquid chromatography, respectively. Antioxidant activity on scavenging radicals such as the 2,2'-azino-bis (3-ethylbenzthiazoline-6-sulphonic acid) (ABTS•+) cation and 1,1-diphenyl-2-picrylhydrazyl (DPPH) cation and nitric oxide (NO) was analyzed. Moreover, the protective activity of glutathione (GSH) oxidation from free radicals generated by high voltage (HV)-stimulation in a mixture of plasma micro/nanobubble water; the same as that of protein oxidation in bovine serum albumin (BSA) and malondialdehyde (MDA) from 2,2’-Azobic (2-amidinopropane) dihydrochloride (AAPH), was evaluated in vitro. Results: Sour-type SF extract at 1 gm showed higher total phenolics (1,625±2.3 µg equivalent gallic acid [GAE]), total flavonoid (245±3.6 µg equivalent quercetin), and ascorbic acid (Vit C) (565±4.5 µg) than sweet-type (520±3.5 µg GAE, 187±2.5 µg, and 513±2.6 µg). In addition, sour-type SF showed a lower dose of inhibitory concentration of 50% (IC50) than sweet-type on scavenging DPPH (32.32±2.3 & 58.9±2.4 mg) and NO (23.1 ± 1.1 mg & undetected). However, IC50 on ABTS•+ scavenging of sweet-type was lower than that of sour-type (348.8±2.5 & 511.9±2.6 mg). Sweet-type showed protective effects with a dose response at 0.25-1.0 mg of extract, 125-500 µg of protein carbonyl and 62.5-500 µg of lipid peroxidation. However, sour-type at high doses showed pro-oxidant activity on increased GSH oxidation, protein carbonyl and MDA formation. Conclusion: Sour-type SF showed higher active antioxidants, such as total phenolics, total flavonoids and Vit C as well as radical scavenging of DPPH and NO, than sweet-type SF. However, high concentrations aggravated GSH, protein and lipid oxidation. Whereas, sweet-type SF showed beneficial protective effects.