Enrichment of two isoflavone aglycones in black soymilk by using spent coffee grounds as an immobiliser for β-glucosidase

Spent coffee grounds, discarded as environmental pollutants, were adopted as enzyme immobilisation solid carriers instead of commercialised solid supports to establish an economical catalytic system. β-Glucosidase was covalently immobilised onto spent coffee grounds for the conversion of isoflavone glycosides into their aglycones in black soymilk. Optimum conditions were determined to be 40 °C and pH 6 using 4-nitrophenyl β-d-glucuronide as an indicator. Operational reusability was confirmed for more than 30 batch reactions and the storage stability was capable of sustaining its highest catalytic activity for 20 days. The kinetic parameters including rate constant (K), time (τ50) in which 50% of isoflavone deglycosylation was reached, and time (τcomplete) required to achieve complete isoflavone deglycosylation, were0.16 ± 0.02 min−1, 4.54 ± 0.32 min, 60 min for daidzin and 0.16 ± 0.02 min−1, 2.28 ± 0.11 min, 60 min for genistin, respectively. The total aglycone content in black soymilk was enriched by 67.14 ± 0.60% in the enzymatic treatment of 60 min duration.