Lactose in milk

Nutritional Value of Milk Products. Milk is considered one of the principal sources of nutrition for humans. Some people are intolerant to one or more components of milk so must avoid the product or consume a treated product. One example is intolerance to lactose in milk. Fluid milk is available in which the lactose has been treated to make it more digestible. The consumption of milk fat, either in fluid milk or in products derived from milk, has decreased markedly in the 1990s. Whole milk sales decreased 12% between 1985 and 1988, whereas the sales of low fat milk increased 165%, and skimmed milk sales increased 48% (35). Nutritionists have recommended that fat consumed provide no more than 30 calories, and that consumption of calories be reduced. Generally, a daily diet of 2000—3000 cal/d is needed depending on many variables, such as gender, type of work, age, body responses, exercise, etc. Further, there is concern about cholesterol [57-88-5] and density of fat consumed. Complete information on the nutritive value of milk and milk products is provided on product labels (36) (see also Table 4). 

Another established application in the dairy industry is the hydrolysis of lactose in milk and whey by lactases. Diminished digestibility problems, increased sweetness and prevention of lactose-crystal formation are the results. The lactose hydrolysis is worked out as a case later in this chapter (section 3.6). 

Enzyme in the bacteria changes lactose in milk into lactic acid. Lactic acid transforms milk proteins to produce thicker yoghurt. 

Problems arising from the crystallization of lactose in milk and whey powders may also be avoided or controlled by pre-crystallizing the lactose. Essentially, this involves adding finely divided lactose powder which acts as nuclei on which the supersaturated lactose crystallizes. Addition of 0.5 kg of finely ground lactose to the amount of concentrated product (whole milk, skim milk or whey) containing 1 tonne of lactose will induce the formation of c. 106 crystals ml-1, about 95% of which will have dimensions less than 10/an and 100% less than 15 /an, i.e. too small to cause textural defects. 

Brons, C. and Olieman, C. 1983. Study of the high-performance liquid chromatographic separation of reducing sugars, applied to the determination of lactose in milk. J. Chromatog. 259, 79-86. 

When a lactose solution is dried rapidly, its viscosity increases so quickly that crystallization cannot take place. The dry lactose is essentially in the same condition as it was in solution, except for removal of the water. This is spoken of as a concentrated syrup or an amorphous (noncrystalline) glass. Various workers have shown conclusively that lactose in milk powder (spray, roller, or freeze-dried) is noncrystalline and exists in the same equilibrium mixture of a- and /3-lactose as existed in the milk prior to drying (Zadow 1984). 

Harper (1979) and Doner and Hicks (1982) have reviewed the various methods for analysis of lactose and its derivatives. More recently, Roetman (1981) has described methods for the quantitative determination of crystalline lactose in milk products. The reader should consult these reviews for information on specific procedures. 

Several simple, accurate cryoscopic methods have been developed to measure the percentage of lactose hydrolyzed in whey and lactose solutions. Zarb and Hourigan (1979) have developed a novel method combining enzymatic treatment and cryoscopy for the measurement of lactose in milk and milk products. 

Dahlqvist, A. 1983. Digestion of lactose. In Milk Intolerances and Rejection. J. Delmont (Editor). S. Karger, Basel, pp. 11-16. 

Knoop, E. and Samhammer, E. 1962. Roentgenographic studies on the crystal structure of lactose in milk powder. Milchuiissenschaft 17, 128-131. 

Roetman, K. 1981. Methods for the quantitative determination of crystalline lactose in milk products. Neth. Milk Dairy J. 35, 1-52. 

Biggs, D. A. 1978. Instrumental infrared estimation of fat, protein and lactose in milk A collaborative study. J. Assoc. Official Anal. Chem. 61,1015-1034. 

Wahba, N. 1965. A simple micro colorimetric method for the determination of lactose in milk. Analyst 90, 432-434. 

F. Amarita, C. Rodriguez Fernandez and F. Alkorta, Hybrid biosensors to estimate lactose in milk, Anal. Chim. Acta, 349(1-3) (1997) 153-158. 

D-Galactose appears free in nature in low amounts, but it forms part of lactose in milk and appears in complex biomolecules as glycolipids and glycoproteins. Its content seems to be higher in wines aged in contact with lees (Doco et al. 2003). 

Sugars are linked to one another in disaccharides and polysaccharides by O-glycosidic bonds. Sucrose, lactose, and maltose are the common disaccharides. Sucrose (common table sugar), obtained from cane or beet, consists of a -glucose and P -fructose joined by a glycosidic linkage between their anomeric carbon atoms. Lactose (in milk) consists... 

Lactase (P-galactosidase) is produced commercially from the lactose fermenting KJujvewn cesfmgiHs. The enzyme has a pH optimum of 6—7 and is used ia the hydrolysis of lactose in milk or skim milk. 

In spite of all these difficulties, microdialysis has been successftilly coupled to several analytical techniques for the bioprocess monitoring. HPLC is commonly used and such systems have been apphed in the analysis of oligosaccharides [44,180], ethanol [181], enzymatic digestion of lactose in milk [182], studies on drug dissolution [183], saccharides in wastewater [184], starch hydrolysis products [185]. 

Gravimetric methods have been developed for most inorganic anions and cations, as well as for such neutral species as water, sulfur dioxide, carbon dioxide, and iodine. A variety of organic substances can also be easily determined gravimetri-cally. Examples Include lactose in milk products, salicylates in drug preparations, phenolphthalein in laxatives, nicotine in pesticides, cholesterol in cereals, and benz-aldehyde in almond extracts. Indeed, gravimetric methods are among the most widely applicable of all analytical procedures. 

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