Coconut is widely used as a food source in producing countries, and during consumption, the waste that is generated needs to be reduced through by-products processing to ensure environmental sustainability. This study aimed to assess the functionality of by-products (endo- and mesocarp) of coconuts at early and mature stages. The aqueous and ethanolic (50 and 100% ethanol in water) extracts of coconut by-products were evaluated for the DPPH radical scavenging activity and subjected to linoleic acid-β-carotene system assay in contrast with synthetic antioxidants. Ultrasound-producing extract of young coconut mesocarp provided the highest antioxidant activity with a lower IC50 value (117 µg mL−1) than butylhydroxytoluene (BHT, 170 µg mL−1). Based on the linoleic acid-β-carotene system assay, the extract exhibited a higher antioxidant activity (1.25×) than tertiary butylhydroquinone (TBHQ, 200 µg mL−1); and comparable with butylhydroxyanisole (BHA, 250 µg mL−1). Therefore, extracts of coconut by-products, particularly the young mesocarp, can be an alternative natural antioxidant.

Coconut (Cocos nucifera) fruit has attracted consumer interest due to its health benefits, especially the consumption of coconut water produced from 6–7-month-old coconut fruit. Consequently, by-products from young coconut fruit are also being expanded, including coconut mesocarp containing phenolic compounds. Therefore, this study aimed to provide new applications for young coconut mesocarp wastes. Specifically, optimizing the ultrasound-assisted extraction (UAE) of phenolic compounds from coconut mesocarp using a Box–Behnken design in conjunction with response surface methodology (RSM). The effects of three extraction variables, such as temperature (10–70 °C), solvent-to-sample ratio (20:1–10:1 mL g−1), and pulse duty cycle (0.4–1.0 s−1) were examined on the level of total phenolic content (TPC) and antioxidant activity of the extract. Subsequently, the optimum UAE condition was predicted using RSM models with coefficients of determination (R2) higher than 0.94, low prediction errors (less than 2.34), and non-significant lack-of-fit values (p < 0.05) for the two responses. Extraction time was evaluated through kinetic (5 to 25 min) studies applying the optimum extraction temperature (70 °C), solvent-to-sample ratio (20:1 mL g−1), and pulse duty cycle (0.55 s−1). An efficient extraction was achieved within 5 min, resulting in an extract with 47.78 ± 1.24 mg GAE 100 g−1 DW for the total phenolic compounds and high antioxidant activity (87.28 ± 1.01% DPPH). Extraction by ultrasound was then concluded to facilitate a fast extraction rate with high reproducibility (coefficients of variation were less than 3% in the levels of antioxidant activity and phenolic compounds).