Lactic acid bacteria lactose metabolism

The primary function of cheese starter cultures is to produce lactic acid at a predictable and dependable rate. The metabolism of lactose is summarized in Figure 10.12. Most cheese starters are homofermentative, i.e. produce only lactic acid, usually the L-isomer Leuconostoc species are heterofermentative. The products of lactic acid bacteria are summarized in Table 10.4. 

Figure 10.12 Metabolism of lactose by lactic acid bacteria many Lactobacillus species/strains can not metabolize galactose (from Cogan and Hill, 1993).

During cheese production lactose is converted to lactic acid by starter lactic acid bacteria (LAB). Any unfermented lactose is converted to d- and L-lactate by nonstarter lactic acid bacteria (NSLAB) and racemization, respectively. Lactate can be oxidized by LAB in cheese to acetate, ethanol, formic acid, and carbon dioxide at a rate dependent on oxygen availability (McSweeney, 2004). Other pathways include conversion to propionate, acetate, water, and carbon dioxide by Propionibacterium spp. carbon dioxide and water by Penicillium spp. yeasts and butyric acid and hydrogen by Clostridium spp. The rate of lactose metabolism influences proteolysis and flavor formation (Creamer et al., 1985 Fox et al., 1990). 

Lactic acid bacteria and bifidobacteria are preferred as protective and probiotic cultures, and have been used since the beginning of history as starter cultures. They have a long history of being safely used and consumed. LAB are widely used for fermentation of milk, meat, and vegetable foods. In fermentation of dairy products, lactose is metabolized to lactic acid. Other metabolic products, hydrogen peroxide, diacetyl, and bacteriocins may also play inhibitory roles and contribute to improving the organoleptic attributes of these foods, as well as their preservation (Messens and De Vuyst, 2002). 

The significance of some of these functions is discussed further below. The metabolism of lactose by lactic acid bacteria is well understood but will not be discussed here the interested reader is referred to reviews by Cogan and Daly (1987), Fox et al. (1990), and Cogan and Hill (1993). 

Figure 13.1 Lactose metabolism in lactic acid bacteria. GalK, galactokinase GalT, galactose-l-phosphate uridyltransferase GalE, UDP-galactose-4 -epimerase GalU, glucose-l-phosphate uridyltransferase UPD, uridine diphosphate.

Aleksandrzak-Piekarczyk, T. (2013) Lactose and p-glucosides metabolism and its regulation in Lactococcus lactis A review. In Lactic Acid Bacteria - R Dfor Food, Health and Livestock Purposes, ed Marcelino Kongo, J. InTech. Available at http //cdn.intechopen.com/pdfs-wm/42338.pdf [accessed 30 January 2013]. 

Acid production by yogurt cultures is a complex biochemical process. For the purpose of this problem, assume that acid production follows first-order kinetics with respect to the consumption of lactose in the yogurt to produce lactic acid. At the start of acid production the lactose concentration is about . 5%, the bacteria concentration is 10 cells/dm. and the acid concentration at which all metabolic activity ceases is 1.4% lactic acid. 

The galactose produced in this process is subsequently converted by the liver into additional glucose, which is then further metabolized to produce energy. Many people do not produce a sufficient amount of lactase and are incapable of hydrolyzing large quantities of lactose. Instead, lactose accumulates and is ultimately broken down into CO2 and H2 by bacteria present in the intestines. Bacterial degradation of lactose produces several by-products, including lactic acid. 

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