The Effect of pH, Temperature and Alkali Metal Ions on the Hydrolsis of Whey Lactose Catalysed by β-Galactosidase from Kluyveromyces marxianus

Integrated Process for Bioenergy Production and Water Recycling in the Dairy Industry: Selection of Kluyveromyces Strains for Direct Conversion of Concentrated Lactose-Rich Streams into Bioethanol

Dairy industries have a high environmental impact, with very high energy and water consumption and polluting effluents. To increase the sustainability of these industries it is urgent to implement technologies for wastewater treatment allowing water recycling and energy savings. In this study, dairy wastewater was processed by ultrafiltration and nanofiltration or ultrafiltration and reverse osmosis (UF/RO) and retentates from the second membrane separation processes were assessed for bioenergy production. Lactose-fermenting yeasts were tested in direct conversion of the retentates (lactose-rich streams) into bioethanol. Two Kluyveromyces strains efficiently fermented all the lactose, with ethanol yields higher than 90% (>0.47 g/g yield). Under severe oxygen-limiting conditions, the K. marxianus PYCC 3286 strain reached 70 g/L of ethanol, which is compatible with energy-efficient distillation processes. In turn, the RO permeate is suitable for recycling into the cleaning process. The proposed integrated process, using UF/RO membrane technology, could allow water recycling (RO permeate) and bioenergy production (from RO retentate) for a more sustainable dairy industry.

Cloning, purification, and characterization of β-galactosidase from Bacillus licheniformis DSM 13

The gene encoding homodimeric β-galactosidase (lacA) from Bacillus licheniformis DSM 13 was cloned and overexpressed in Escherichia coli, and the resulting recombinant enzyme was characterized in detail. The optimum temperature and pH of the enzyme, for both o-nitrophenyl-β-D: -galactoside (oNPG) and lactose hydrolysis, were 50°C and 6.5, respectively. The recombinant enzyme is stable in the range of pH 5 to 9 at 37°C and over a wide range of temperatures (4-42°C) at pH 6.5 for up to 1 month. The K(m) values of LacA for lactose and oNPG are 169 and 13.7 mM, respectively, and it is strongly inhibited by the hydrolysis products, i.e., glucose and galactose. The monovalent ions Na(+) and K(+) in the concentration range of 1-100 mM as well as the divalent metal cations Mg²(+), Mn²(+), and Ca²(+) at a concentration of 1 mM slightly activate enzyme activity. This enzyme can be beneficial for application in lactose hydrolysis especially at elevated temperatures due to its pronounced temperature stability; however, the transgalactosylation potential of this enzyme for the production of galacto-oligosaccharides (GOS) from lactose was low, with only 12% GOS (w/w) of total sugars obtained when the initial lactose concentration was 200 g/L.