Recombinant α-l-rhamnosidase from Aspergillus terreus in selective trimming of α-l-rhamnose from steviol glycosides

Abstract

Although stevioside is the most abundant sweetener in Stevia rebaudiana leaves, other steviol glycosides determine its taste. The steviol glycosides dulcoside A and dulcoside B both feature a terminal α-l-rhamnose residue and are the bitterest compounds in steviol glycoside mixtures. Here we show that Aspergillus terreus α-l-rhamnosidase specifically hydrolyses the glycosidic linkage of dulcoside A, and converts it to rubusoside. The constitutive expression of α-l-rhamnosidase in Pichia pastoris and Kluyveromyces lactis achieved high protein yields of up to 17.6U/mL and 30.6U/mL, respectively. During a 12-h biotransformation, the dulcoside A from crude leaf extracts was completely converted into rubusoside. There was no hydrolytic activity against dulcoside A analogues. This process therefore offers a promising approach for reducing the bitterness of steviol glycoside mixtures.