Lactulose Biosynthesis with Enzymes of Lactose-Fermenting Micromycetes and Bacteria

Lactulose is a well-studied prebiotic popular in medicine and the food industry. β-Galactosidases offer an alternative to traditional chemical methods of lactulose production. The article describes lactulose biosynthesis with crude enzyme preparations of β-galactosidases obtained by separate and joint cultivation of lactose-fermenting yeasts and lactic acid bacteria in whey permeate. The research featured the following producers of β-galactosidases: lactose-fermenting yeasts of Kluyveromyces lactis VKM Y-1333 and Y-1339, Kluyveromyces marxianus VKM Y-459 and Y-1338, and viscous strains of lactic acid bacteria of Lactobacillus acidophilus and Streptococcus thermophilis. The β-galactosidase activity was measured using the method of o-nitrophenyl-β-D-galactopyranoside hydrolysis. The method of high-performance liquid chromatography made it possible to reveal the carbohydrate composition. The lactulose formation by yeast enzymes demonstrated the following pattern: its concentration increased rapidly during the first 30 min of reaction with subsequent decrease or stabilization. Some combined unpurified enzyme preparations of yeasts and lactic acid microorganisms were more active than β-galactosidases obtained by separate cultivation. The patterns of lactulose formation depended on both the yeast strain and the type of lactic acid bacteria. The highest yields of lactulose belonged to the samples that combined β-galactosidases of all yeast strains with Streptococcus thermophilis. Co-cultivation of yeast and lactic acid bacteria in some combinations produced combined crude enzyme preparations with higher activity and greater lactulose yields than separate yeast cultivation. These results can help to obtain purified enzyme preparations of β-galactosidases and lactulose-containing products from secondary dairy raw materials.