Consolidated Bioprocessing in a Dairy Setting-Concurrent Yoghurt Fermentation and Lactose Hydrolysis without Using Lactase Enzymes

Streptococcus thermophilus is a fast-growing lactic acid bacterium (LAB) used in yoghurt and cheese manufacturing. Recently, we reported how this bacterium could serve as a cell catalyst for hydrolyzing lactose when permeabilized by nisin A. To enhance the lactose hydrolyzing activity of S. thermophilus, we mutated a dairy strain and screened for variants with elevated beta-galactosidase activity. Two isolates, ST30-8 and ST95, had 2.4-fold higher activity. Surprisingly, both strains were able to hydrolyze lactose when used as whole-cell lactase catalysts without permeabilization, and ST30-8 hydrolyzed 30 g/L lactose in 6 h at 50 degrees C using 0.18 g/L cells. Moreover, both strains hydrolyzed lactose while growing in milk. Genome sequencing revealed a mutation in L lactate dehydrogenase, which we believe hampers growth and increases the capacity of S. thermophilus to hydrolyze lactose. Our findings will allow production of sweet lactose-reduced yoghurt without the use of costly purified lactase enzymes.

Automated summary

This study discovered that a mutated strain of Streptococcus thermophilus can efficiently hydrolyze lactose using whole-cell lactase catalysts. Genome sequencing revealed a mutation that affects the growth of the strain and increases its lactose hydrolyzing ability. These findings could potentially lead to the production of sweet lactose-reduced yoghurt without the need for expensive purified lactase enzymes.