Biocatalytic approaches using lactulose: end product compared with substrate

Abstract

Lactulose is a lactose-based carbohydrate with well-known prebiotic effect and recognized medical applications. Currently, the commercially available lactulose is chemically synthesized. Nevertheless, the process leads to low yields and high levels of by-products. Alternatively, lactulose can be produced by enzymatic synthesis, which provides a cleaner production under mild conditions. Two different enzymatic routes were reported for lactulose production. Lactulose can be obtained through hydrolysis and transfer reactions catalyzed by a glycosidase. Alternatively, lactulose can be produced by direct isomerization of lactose to lactulose catalyzed by cellobiose-2-epimerase. An interesting characteristic of lactulose is also its capacity to act as substrate in additional enzymatic synthesis which leads to the formation of attractive compounds, such as lactulose-based oligosaccharides and lactulose esters. Besides increasing the interest and potential of lactulose, these lactulose-based compounds can also offer new and promising functionalities and applications. Herein, we review the enzymes involved in the synthesis of lactulose, as well as the reaction conditions and yields. The potential of different enzymes is discussed and it is shown that reaction conditions and composition of products depend on the type of enzyme and its microbial source. The conversion of lactulose into lactulose-based compounds is also covered, describing in detail the biocatalysts involved, the reaction conditions used, and the potential of the final products obtained.