Bioactivity of non-extractable phenolics from lemon peel obtained by enzyme and ultrasound assisted extractions

Lemon peel waste constitutes a natural resource for bioactive phenolics with antioxidant, antihypertensive and antidiabetic activities. Bioactivities of non-extractable phenolics from lemon peel obtained by conventional heat-, enzyme-, ultrasound-, and ultrasound-enzyme-assisted extractions were evaluated in comparison with those of extractable phenolics. The antioxidant, angiotensin-I-converting enzyme (ACE), and α-amylase inhibitory activities and phenolic profile of the phenolic fractions were analysed. While the extractable fraction had higher total phenolic content, ascorbic acid content, and antioxidant activity, phenolic profile analysis indicated that the non-extractable fraction contained higher concentrations of phenolics especially hesperidin and hesperetin. The concentrations of hesperidin and hesperetin in the non-extractable fraction were 270.9 mg/100 g dry weight and 415.9 mg/100 g dry weight, respectively, which were about two-fold higher than those present in the extractable fraction. Moreover, ACE and α-amylase inhibitory activities of non-extractable fraction were stronger than those of the extractable fraction. Total phenolic content, total flavonoid content, and antioxidant activity were increased by enzyme and ultrasound treatments compared to those by conventional heat treatment. However, ACE inhibitory activities of all non-extractable fractions were similar while α-amylase inhibitory activity was higher in ultrasound- and ultrasound-enzyme-treated fractions. While ultrasound-assisted extraction slightly improved the yield of non-extractable phenolics, enzyme-assisted extraction yielded two-to four-fold increases in the amounts of phenolic compounds compared to heat-assisted extraction. Non-extractable phenolic fraction from lemon peel was found to have a significant potential as an antihypertensive and antidiabetic agent.