Tropical fruits and their by-products as a source of natural antioxidants: Phenolic content and bioactivities

The consumption of fruits and vegetables is associated with a myriad of health-promoting benefits, including a lower incidence of cardiovascular ailments, some types of cancer, and type 2 diabetes, among others. These effects in food are commonly attributed to the presence of bioactive compounds with antioxidant and other health-promoting properties, mainly phenolic compounds, which are plant secondary metabolites with more than 8,000 identified compounds. These are divided into sub-groups according to their structural characteristics, including flavonoids, phenolic acids, and proanthocyanidins, among others. Fruits are a major source of phenolics in nature. However, many tropical fruits have an underexplored potential when it comes to phenolics and their bioactivities. Therefore, in this study, the phenolic content of three tropical fruits and their residual fractions, namely guava (pulp and processing waste), jerivá (pomace and seeds), and butiá (pomace and seeds) were investigated. Soluble phenolics were obtained by ultrasound-assisted extraction (30 ℃/20 min) using ethanol/water (30:70, v/v) as a solvent, being further fractionated into free and esterified phenolics, and insoluble-bound phenolics were then obtained by alkaline hydrolysis. The phenolic extracts were analyzed for their total phenolic, total flavonoid, and total proanthocyanidin contents. Their in vitro antioxidant activity was measured by DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)), and FRAP (ferric reducing ability of plasma) assays. Finally, the extracts were tested for alpha-glucosidase, and pancreatic lipase inhibition, LDL-cholesterol peroxidation inhibition, and hydroxyl radical-induced supercoiled DNA strand scission inhibition. The free phenolic fraction from butiá and jerivá seeds showed the highest total phenolic, total flavonoids and total proanthocyanidins contents from all samples. The samples with the highest phenolic contents also exhibited the highest antioxidant capacity, confirmed by all the assays performed. However, the same trend was not observed for enzymatic inhibition, where the free and esterified phenolics from guava, jerivá and butiá (pulp and by-products) were the most efficient at alpha-glucosidase inhibition. Meanwhile, insoluble-bound phenolics from guava pulp showed the highest lipase inhibition. All samples studied demonstrated similar efficiency for inhibiting LDL oxidation (20-25%), while the insoluble-bound phenolics from butiá seeds demonstrated the most significant protection against DNA strand scission (78%). These results demonstrate that tropical fruits and their usually discarded by-products are a valuable source of phenolic compounds with in vitro biological activity. Such fractions can potentially be used in the development of functional foods and/or nutraceuticals, pending in vivo confirmation of these results.