Increasing Antioxidant Activity in Food Waste Extracts by β-Glucosidase

Research background. Food by-products such as onion peels and olive leaves are rich in bioactive compounds applicable as natural and low-cost sources of antioxidants. Still, these compounds mainly exist in glycosylated form. Often, hydrolysis of glycoside compounds increases their antioxidant activity and health benefits. However, not many studies have been done concerning the β-glucosidases effect, specifically from A. niger, on glycosylated compounds within these by-products. Also, changes in the antioxidant activity of the mentioned by-products under the effect of β-glucosidase have not been reported yet. Therefore, this study considered the effect of Aspergillus niger β-glucosidase on glucoside compounds and the antioxidant activity of onion peel and olive leaves extracts. 
Experimental approach. The antioxidant activity of the extracts was assayed by DPPH and FRAP tests. Also, glucose, total phenolic, and flavonoid content were measured. Moreover, TLC and HPLC analyses were applied before and after the enzymatic hydrolysis. 
Results and conclusions. The obtained results showed an increase in the extract antioxidant activity after treatment. Also, an enhancement was observed in the extracts glucose content by β-glucosidase. The TLC and HPLC results showed the β-glucosidase efficacy to hydrolyze quercetin glucosides in onion peel extract, and the quercetin content was increased from (0.479±0.04) mg/mL in the untreated extract to (1.262±0.027) mg/mL in the treated extract (0.5 % m/V) after 3 h enzymatic hydrolysis at 45 °C. Also, the content of quercetin-3-O-glucoside was increased considerably from (0.0018±0.0001) to (0.054±0.009) mg/mL following enzyme treatment. Moreover, oleuropein in olive leaves extract (1 % m/V) was hydrolyzed completely from (0.382±0.016) to 0 mg/mL by β-glucosidase for 24 h at 50 °C. 
Novelty and scientific contribution. This study showed that A. niger β-glucosidase, as a stable enzyme, hydrolyzed quercetin and oleuropein glycosides in onion peel and olive leaves extracts. Thus, A. niger β-glucosidase is a good candidate for processing the food waste and extracting valuable bioactive compounds. Also, the treated extracts with higher antioxidant and biological activity, and without bitterness can be applicable as potent, natural, and cost-effective antioxidants in the food industry.