Galactose lactic acid bacteria

Another fermentation process is described by Fayolle et al.14 In this work, processes producing lactic acid bacteria were studied. Samples were extracted from the reactor and assayed by conventional methods and scanned in the IR. Equations were generated, using PLS, for lactose, galactose, lactic acid, and biomass. The SEP for each of these constituents was, respectively, 3.4, 1.5, 0.9, and 0.9 g/1. 

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

Barrett, E., Stanton, C., Zelder, O., Fitzgerald, G., and Ross, R.P. (2004) Heterologous expression of lactose-and galactose-utilizing pathways from lactic acid bacteria in Corynehacterium glutamicum for production of lysine in whey. Appl Environ. Microbiol, 70 (5), 2861-2866. 

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.

Lactose, milk sugar a reducing disaccharide. M, 324.3, a-form m.p. 223°C, [a]g-t-89.4°55.5° (water), p-form m.p. 252°C, [a]p-H34.9°- 55.3° (water). L. crystallizes from water as the p-form above 93 °C, and as a-lactose monohydrate below 93 °C. It consists of galactose P-1,4 glycosidically linked to glucose, both monosaccharide residues in the pyranose form. L. is not fermented by ordinary yeasts, but it is fermented by yeasts such as kefir. The souring of milk consists of the eonversion of L. into lactic acid by lactic acid bacteria. 

After completion of alcoholic fermentation, low concentrations of hexose sugars may remain in the wine. These include glucose and fructose with lesser amounts of mannose and galactose. Among the five-carbon sugars (pentoses), arabinose, ribose, and xylose are the most common. Further, there may be sufficient quantities of sugar to support the growth of lactic acid bacteria in dry wines. 

Osmotic laxatives (e.g., lactulose, sorbitol) are poorly absorbed or nonabsorbable compounds that draw additional fluid into the GI tract. Lumen osmolality increases, and fluid movement occurs secondary to osmotic pressure. Lactulose is a synthetic disaccharide that is poorly absorbed from the GI tract, since no mammalian enzyme is capable of hydrolyzing it to its monosaccharide components. It therefore reaches the colon unchanged and is metabolized by colonic bacteria to lactic acid and to small quantities of formic and acetic acids. Since lactulose does contain galactose, it is contraindicated in patients who require a galactose-free diet. Metabolism of lactulose by intestinal bacteria may result in increased formation of intraluminal gas and abdominal distention. Lactulose is also used in the treatment of hepatic encephalopathy. 

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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