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

Abstract

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 β-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 °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.

The Probiotic Enterococcus Lactis SF68 as a Potential Food Fermentation Microorganism for Safe Food Production

“…lactis SF68 was used to inoculate M17 supplemented with 1% glucose, and the cells were harvested by centrifugation (10 min at 10,000 g), washed, and re-suspended in 10 mL of 100 mM K 2 HPO 4 /KH 2 PO 4 buffer (pH 7.4). Different concentrations of the mutagen ethyl methane sulfonate (EMS, Sigma-Aldrich, USA) (i.e., 15, 30, 60, 90, and 120 μL/mL) were added and the cell suspension was incubated at 30 °C for three hours with gentle agitation . Cells not treated with EMS were included as a control.…”

“…Different concentrations of the mutagen ethyl methane sulfonate (EMS, Sigma-Aldrich, USA) (i.e., 15, 30, 60, 90, and 120 μL/mL) were added and the cell suspension was incubated at 30 °C for three hours with gentle agitation. 46 Cells not treated with EMS were included as a control. Aliquots of both treated and control samples were taken immediately and plated onto the modified M17 solid medium (10 g/L glucose, 15 g/L casein peptone, 5 g/L yeast extract, 0.5 g/L magnesium sulfate, 0.5 g/L ascorbic acid, and 10 mL of 10 g/L stock 2,3,5-triphenyltetrazolium chloride (TTC)) to determine lactate dehydrogenase (LDH) mutants.…”