Recent Advances on Biological Production of a Functional Low-Calorie Sugar d-Tagatose

d-Tagatose, a rare sugar of growing interest, has attracted significant attention due to its low caloric content and associated health benefits. This review summarizes four major biological approaches for d-tagatose production. d-Tagatose can be synthesized from d-galactose either via enzymatic isomerization catalyzed by l-arabinose isomerase or through an oxidoreductive conversion pathway involving galactitol dehydrogenase and xylose reductase. In many cases, β-galactosidase-catalyzed lactose hydrolysis is integrated into the process to provide a cost-effective source of d-galactose. Additionally, several d-tagaturonate 3-epimerases have been identified and engineered to produce D-tagatose from d-fructose via C-4 epimerization. Furthermore, d-tagatose can also be generated from maltodextrin, d-glucose, d-fructose, or sucrose through multienzymatic cascade reactions. This review provides a comprehensive overview of current biotechnological strategies for d-tagatose biosynthesis, offering valuable insights to guide future research and industrial development.