α-Galactosidase-assisted Hydrolysis of Spent Coffee Grounds Reveals a Trade-off Between Manno-oligosaccharide Production and Glucose Release

Abstract

Spent coffee grounds (SCG) are an abundant lignocellulosic biomass rich in galactomannan and cellulose, rendering them a promising feedstock for biorefinery, especially for the production of manno-oligosaccharides (MOS) as high-value prebiotics. This study investigated the role of α-galactosidase (α-G) in the enzymatic hydrolysis of ammonia-pretreated SCG, with particular emphasis on MOS production and cellulose hydrolysis. Enzymatic hydrolysis was conducted at 50 °C and pH 5.0, and the resulting sugars and MOS were quantified by HPLC. Supplementation with α-G effectively removed α-1,6-linked galactose side groups from galactomannan and increased MOS yield. Notably, the addition of α-G enabled a 50% reduction in mannanase dosage without compromising MOS production, indicating its potential to improve galactomannan conversion efficiency. However, α-G treatment altered the properties of the resulting MOS and was associated with stronger inhibition in MOS-containing cellulase systems. Under the tested conditions, α-G supplementation in MOS-containing systems was associated with additional numerical reductions in glucose yield of 15.8% for CBHI and 8.1% for EGII, whereas little numerical change was observed in the βG system. These results reveal a trade-off associated with α-G supplementation: although it promotes MOS generation, stronger inhibition was observed in MOS-containing cellulase systems, which may impair downstream cellulose hydrolysis.