Lactase sources

The second important application of immobilized enzymes is the hydrolysis of lactose to glucose and galactose in milk and milk products by lactase (Sprossler and Plainer 1983). Several lactase sources are available from yeast, Saccharomyces lactis and S. fragilis, or from fungi, Aspergillus oryzae or A. niger. The enzymes vary in their optimum pH and optimum temperature, as well as other conditions. 

An important source of galactose in the diet is the disaccharide lactose present in milk. Lactose is hydrolyzed to galactose and glucose by lactase associated with the brush border membrane of the small intestine. Along with other monosaccharides, galactose reaches the liver through the portal blood. 

Lactase The disaccharide lactose is the only carbohydrate present in milk, which is essential for survival of an infant. Consequently, the enzyme lactase is essential for babies. Caucasians retain lactase activity into adulthood, whereas many Asian or African groups progressively lose its activity in adult life. This could, therefore, be described as an adult deficiency disease. Ingestion of milk in these individuals causes nausea, diarrhoea and stomach cramps. Symptoms disappear if milk is excluded from the diet or if a source of lactase is ingested along with or before ingestion of milk. The bacteria that are involved in the production of yoghurt contain the enzyme lactase. 

Lactose may be hydrolysed to glucose and galactose by enzymes (fi-galactosidases, commonly called lactase) or by adds. Commercial sources of / -galactosidase are moulds (especially Aspergillus spp.), the enzymes from which have acid pH optima, and yeasts (Kluyveromyces spp.) which produce enzymes with neutral pH optima. /S-Galactosidases were considered to have... 

Source and kinds of disaccharidases The final digestive processes occur at the mucosal lining of the small intestine. Several disaccharidases [for example, lactase (p-galactosidase), sucrase, maltase, and isomal-tase] produce monosaccharides (glucose, galactose, and fructose). These enzymes are secreted by and remain associated with the luminal side of the brush border membranes of intestinal mucosal cells. Absorption of the monosaccharides requires specific trans porters. 

Figure 10-23 Flow Diagram of an Immobilized Enzyme System (Column Operation of Lactase Immobilized on Phenol-Formaldehyde Resin with Glutaraldehyde). Source From W.L. Stanley and A.C. Olson, The Chemistry of Immobilizing Enzymes, J. Food Sci., Vol. 39, pp. 660-666, 1974.

In humans, the enzyme in the small intestine responsible for the hydrolysis of the P (1, 4) glycosidic bond is called lactase. This enzyme is not present in all persons, particularly those whose ancestors did not rely upon cow s milk (and milk products) as a food source. 

Many adnlts who have a lactase deficiency develop the ability to ingest small amonnts of lactose in dairy products without experiencing symptoms. This adaptation probably involves an increase in the population of colonic bacteria that can cleave lactose and not a recovery or induction of hnman lactase synthesis. For many individuals, dairy products are the major dietary source of calcium, and their complete elimination from the diet can lead to osteoporosis. 

Microbes have long been used as dependable sources of such industrial enzymes as amylase, invertase, lactase, protease, pectinase, catalase, and glucose oxidase. The interest in fermentation enzymes seems, to be rapidly expanding as demonstrated by the current activity in the following areas ... 

When society transitioned from hunting to farming, domesticated animals provided milk for adults (and dairy products, with a little bacterial help). This new source of calories increased ERD and helped humans survive. Any adult with a lactase gene that was accidentally left on could take in more calories and would pass along their turned-on gene to their children. 

This results in a considerable increase in the osmolality of the gut contents, as 1 mol of lactose yields 4 mol of lactate and 4 mol of protons. In addition, bacterial fermentation produces carbon dioxide, methane and hydrogen, and the result of consuming a moderate amount of lactose is an explosive watery diarrhoea and severe abdominal pain. Even the relatively small amounts of lactose in milk may upset people with a complete deficiency of lactase. Such people can normally tolerate yoghurt and other fermented milk products, as much of the lactose has been converted to lactic acid. Fortunately for people who suffer from alactasia, milk is the only significant source of lactose in the diet, so it is relatively easy to avoid consuming lactose. 

Lactose is used as an energy source as it is hydrolysed by lactase in the small intestine. A large part of the human population (as well as of other mammals) produces lactase only in childhood. In adulthood, a number of individuals have the enzyme activity reduced or completely absent, and consumption of milk is then problematic. Lactase is produced by lactic acid bacteria that break... 

Lactose. Dietary lactose (milk sugar) enhances the rate of calcium absorption from the small intestine. In this connection, it is noteworthy that (1) milk is the only source of lactose, and (2) the improved absorption of calcium is dependent on the activity of intestinal lactase, the enzyme that hydrolyzes lactose. 

The disaccharide in milk is lactose, which is broken down by lactase in the intestinal tract to monosaccharides that are a source of energy. Infants and small children produce lactase to break down the lactose in milk. It is rare for an infant to lack the ability to produce lactase. However, the production of lactase decreases as many people age, which causes lactose intolerance. This condition affects approximately 25% of the people in the United States. A deficiency of lactase occurs in adults in many parts of the world, but in the United States it is prevalent among the African American, Hispanic, and Asian populations. 

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