Lactose is the major fermentable sugar of milk at level of 40-50g/L. Glucose, one of the two monosaccharides composing lactose, is used faster than galactose by lactococci. At the end of growth phase, less than 0.5% of lactose has been utilized by lactococci. The fermentation product of lactococci is L(+) lactic acid.
The transport of lactose, glucose, and galactose across the cytoplasmatic membrane can occur by two main mechanisms:
- 1) The permease system: It is common for streptococci and Leuconostoc.
- 2) The PEP/PTS system: Phosphoenol pyruvate-phosphotransferase belongs to lactococci.
The PEP/PTS is a complex system in which lactose is transported into the cell by phosphorylation to lactose phosphate and thus transported across the cell wall. The lactose phosphate is hydrolyzed to glucose and galactose by phospho-p- galactosidase.
The permease system requires energy, using ATP of the cell. Inside the cell, lactose is hydrolyzed to glucose and galactose by p-galactosidase.
The genus Lactobacillus has more p-galactosidase than phospho-p-galactosidase. After transport, the sugars can become lactose phosphate, glucose phosphate, galactose phosphate or be free sugars.
These molecules can be metabolized by three different pathways: The lactose phosphate is hydrolyzed by a p-Plactosidase to give glucose and galactose-6-phosphate. Glucose is catabolized through the glycolysis pathway, while galactose-6-phosphate is metabolized along the D-tagatose-6-phosphate pathway.