Lactase or p-galactosidase (EC 18.104.22.168) is a hydrolase catalyzing the hydrolysis of the terminal nonreducing p-D-galactosides, releasing p-D-galactose residues.
P-D-galactose (1,4)-a -D-glucose + H20 ^ galactose + glucose
The enzyme is typical of mammals, but also many microorganisms produce beta-galactosidase.
Almost all human infants are born with the enzyme lactase, providing them with the ability to digest lactose, the predominant carbohydrate in milk. However, the ability to digest lactose is physiologically reduced from early childhood on for most people around the world.
The ability to digest lactose is genetically determined. In Europe, only a small percentage of the population is lactose intolerant, but in other continents the majority of people are intolerant. It is estimated that approximately 70% of the world population suffers to a degree from lactose intolerance.
The content of lactose in cow milk is about 5%. If the milk is processed and transformed into yogurt, fermented milks or cheese, the lactic starters use lactose as energy source and release lactic acid. In this way, the content of lactose is reduced to the extent of 3% to 4% for yogurt and fermented milks, app. 4% in fresh cheese, below 3% and often almost 0% in matured soft cheese, and 0% in harder cheeses apart from cheddar that still may contain more than other hard cheeses, especially the young ones.
The main problem with intolerance to lactose is linked to the consumption of milk as a beverage.
By treating milk and milk-based products with lactase, the lactose is broken down to a mixture consisting primarily of glucose and galactose. Lactase ensures the controlled
Figure 22.214.171.124 Influence of temperature on Lactase activity; Influence of pH on Lactase activity.
elimination of lactose to produce low-lactose milk and milk-based products that can be safely digested by virtually everyone without the adverse effects of lactose intolerance. The milk becomes sweeter due to glucose formation, and sandiness in ice cream is reduced because no lactose crystals are formed (lactose is poorly soluble: at 20°C about 19g/100mL water). However, as the molar quantity of reducing sugars in lactase-treated milk is twofold higher than in untreated milk, the UHT processed milk is more vulnerable to the Maillard reaction (see Figure 126.96.36.199).
On the market there are two types of lactase: sweet-neutral and acid lactase. The optimum pH range is between 5.7 and 7.5 for the neutral lactase (some types can be active as low as pH 5.3). Its optimum temperature is approximately 37°C, with high performance between 30°C and 38°C, but still working well down to 20°C and up to 40°C (some may be active up to 45°C). The acid lactase has higher activity at lower pH and at higher temperatures.
Industrially, beta-galactosidase is produced by fermentation. Acid lactase derives from Aspergillus niger, Aspergillus oryzae, and neutral lactase from Bacillus spp., Saccharomyces fragilis, Saccharomyces lactis, and Kluyveromyces spp.