Extracts from strawberry by-products rich in phenolic compounds reduce the activity of apple polyphenol oxidase

The ultrasound-assisted extraction of phenolic compounds from strawberry by-products was studied varying the solvent type (water, ethanol 80%, methanol 80%, and acetone 80%), the formic acid concentration (0 and 0.5%), and the number of extraction steps (1 and 2). Total phenolic and total flavonoid compounds were determined spectrophotometrically and analyzed by PAD-HPLC. The antioxidant capacity (DPPH and FRAP assays) and the ability of extracts to inhibit apple (Malus domestica cv. `Red Delicious') polyphenol oxidase (PPO) were also investigated. Extracts with acidified methanol in two-steps yielded the highest phenolic compound concentration (15.01 g/kg), and the highest antioxidant capacity. Agrimoniin was the major polyphenol found, and the extraction with acetone in two-steps produced the highest yield (2.45 g/kg). This ellagitannin was the only polyphenol that correlated (R-2 > 0.80, p < 0.05) with the antioxidant capacity. Water and ethanol showed the lowest phenolic compound yields. However, extraction with these green solvents (water or ethanol) in two-steps showed polyphenol contents similar to those obtained with methanol or acetone in one-step (approximate to 9 g/kg). Additionally, extracted polyphenols (0.24 g/L) produced 30% apple PPO inhibition, in a reversible `uncompetitive' inhibition. Results showed the high revalorization potential of strawberry by-products as a low-cost source of polyphenols, with antioxidant and anti-browning effects.