br Acknowledgments The authors gratefully acknowledge the su

Acknowledgments The authors gratefully acknowledge the support provide by National Natural Science Foundation of China (Number 31301415).
Introduction β-Galactosidases (EC 3.2.1.23), frequently known as lactases, are widely used in lactose hydrolysis. This disaccharide is present in mammalian milk in concentrations up to 10% (w/w), depending on the species (Fox and McSweeney, 1998). The sweetness of lactose corresponds to approximately 20% of that of sucrose and its aqueous solubility is low compared with other sugars (Gänzle et al., 2008). Additionally, lactose is a hygroscopic sugar which frequently suffers undesirable crystallization. The hydrolytic activity of β-galactosidase is commonly applied in the food industries aiming to reduce the lactose content of dairy products, preventing lactose crystallization problems and increasing sweetness, flavor and solubility (Gänzle et al., 2008). Furthermore, the hydrolysis of lactose into d-glucose and d-galactose allows the production of lactose-free products suitable for lactose-intolerant people. A significant percentage of the global adult population is affected by inhibitor of catalase due to β-galactosidase deficiency. This deficiency results in malabsorption of lactose which can cause several undesirable symptoms such as abdominal cramps, bloating, diarrhea or flatulence (Bhatnagar, 2007). Some β-galactosidases can also catalyze transgalactosylation reactions and have been successfully applied in the synthesis of lactose-based prebiotics, such as galacto-oligosaccharides (GOS) (Torres et al., 2010), lactulose (Silvério et al., 2016) and lactosucrose (Li et al., 2009). These prebiotics are enzymatically obtained through the hydrolysis of lactose followed by the transfer of the galactosyl residue to a suitable acceptor, namely fructose for the disaccharide lactulose; sucrose for the trisaccharide lactosucrose; and lactose for GOS (degree of polymerization from 2 up to 8). The concept of prebiotic was introduced about twenty years ago to define a class of compounds with ability to promote specific changes in the gastrointestinal microflora with benefits upon host well-being and health. The evidence that several beneficial microorganisms at extra-gastrointestinal sites can directly or indirectly be affected by prebiotics stressed the need for an updated concept. Therefore, prebiotic is nowadays considered “a substrate that is selectively utilized by host microorganisms conferring a health benefit” (Gibson et al., 2017). Currently, only few compounds are well-accepted as prebiotics by the scientific community. This restrict group includes fructo-oligosaccharides (FOS), inulin, GOS and lactulose (Gibson et al., 2010, Gibson et al., 2017). Besides these compounds, several other non-digestible oligosaccharides have claimed the status of prebiotics, namely lactosucrose, xylo-oligosaccharides (Seesuriyachan et al., 2017), gluco-oligosaccharides (Sharma et al., 2016), pectic oligosaccharides (Babbar et al., 2016) or isomalto-oligosaccharides (Goffin et al., 2011). Prebiotics have gained nutraceutical and pharmaceutical relevance due to their recognized contribute to maintain and restore gastrointestinal microflora, to prevent colitis and constipation, to reduce the incidence of colon cancer, to promote positive modulation of the immune system or to decrease cholesterol and obesity risk (Patel and Goyal, 2012). The prebiotic market size surpassed USD 3 billion in 2015 and it is expected to reach USD 7.5 billion by 2023 (Global Market Insights, Inc., 2017). To keep up with the growing market trend, efficient production processes ought to be developed, implemented and optimized. The enzymatic synthesis is one of the strategies used at the industrial level for the production of some prebiotics. However, the success of this methodology is in part dependent on the use of effective biocatalysts. In the synthesis of lactose-based prebiotics, such as GOS or lactulose, different microbial sources of β-galactosidase can be used (Torres et al., 2010, Silvério et al., 2016). The search for new and effective biocatalysts represents not only a potential improvement in the production process, but also an opportunity to synthesize novel compounds with enhanced or differentiated biological activity.