Hydrolysis in milk

Other analytical steps such as hydrolysis in milk samples, in addition to completeness of analyte extractions — which is not always achieved with conventional methods. 

Lactose hydrolysis in milk (milk products for lactose intolerant people) Lactase... 

One of the most interesting fields of application of MAE in food analysis is the extraction of lipids. This step, traditionally performed with conventional Soxhlet extraction, has been performed with the focused microwave-assisted Soxhlet extractor prototype of Figure 2B. Extraction of oil from olives, srm-flower seeds, and soyabeans extraction of the lipid fraction of dairy products (milk and cheese) and extraction of fatty acids from precooked and sausage foods have significant advantages over conventional methods, including dramatically reduced extraction times, lower degradation of thermolabile analytes, and acceleration of other analytical steps such as hydrolysis in milk samples, in addition to completeness of analyte extraction, which is not always achieved with conventional methods. 

Two other practical appHcations of en2yme technology used in dairy industry are the modification of proteins with proteases to reduce possible allergens in cow milk products fed to infants, and the hydrolysis of milk with Hpases for the development of Hpolytic flavors in speciaHty cheeses. 

The most common food matrices analyzed include meat, fish, milk, egg, and honey. The first step usually employed prior to analysis is protein precipitation, which is usually done with organic solvents [59, 60, 62, 194, 195], defatting, usually with hexane [60], and acid hydrolysis in the case of honey [190, 191]. 

Engfer, M. B., Stahl, B., Finke, B., Sawatzki, G., and Daniel, H. (2000). Human milk oligosaccharides are resistant to enzymatic hydrolysis in the upper gastrointestinal tract. Am. ]. Clin. Nutr. 71,1589-1596. 

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). 

Milk contains about 0.1 mg niacin per 100 g and thus is not a rich source of the preformed vitamin. Tryptophan contributes roughly 0.7 mg NE per 100 g milk. In milk, niacin exists primarily as nicotinamide and its concentration does not appear to be affected greatly by breed of cow, feed, season or stage of lactation. Pasteurized goats (0.3 mg niacin and 0.7 mg NE from tryptophan per 100 g) and raw sheep s (0.4 mg niacin and 1.3 mg NE from tryptophan per 100 g) milk are somewhat richer than cows milk. Niacin levels in human milk are 0.2 mg niacin and 0.5 mg NE from tryptophan per 100 g. The concentration of niacin in most dairy products is low (Appendix 6A) but is compensated somewhat by tryptophan released on hydrolysis of the proteins. 

The concentration of ascorbic acid in milk (11.2-17.2mgl-1) is sufficient to influence its redox potential. In freshly drawn milk, all ascorbic acid is in the reduced form but can be oxidized reversibly to dehydroascorbate, which is present as a hydrated hemiketal in aqueous systems. Hydrolysis of the lactone ring of dehydroascorbate, which results in the formation of 2,3-diketogulonic acid, is irreversible (Figure 11.2). 

Downey (1980) reasoned that although milk lipoprotein lipase is present in sufficient amounts to cause extensive hydrolysis and potential marked flavor impairment, this does not happen in practice for the following reasons (1) the fat globule membrane separates the milk fat from the enzyme, whose activity is further diminished by (2) its occlusion by casein micelles (Downey and Murphy 1975) and by (3) the possible presence in milk of inhibitors of lipolysis (Deeth and Fitz-Gerald 1975). The presence in milk of activators and their relative concentration may also determine whether milk will be spontaneously rancid or not (Jellema 1975 Driessen and Stadhouders 1974A Murphy et al. 1979 Anderson 1979). 

Luhtala, A., Korhonen, H., Koskinen, E. H. and Antila, M. 1970A. Glyceride synthesis and hydrolysis caused by cells in milk. 18th Int. Dairy Congr. Proc. IE., 80. 

Willart, S. and Sjostrom, G. 1959. The effect of sodium chloride on the hydrolysis of the fat in milk and cheese. 15th Int. Dairy Congr. Proc. 3, 1482-1486. 

Glycopeptides have been found in milk at temperatures above 50 °C (Hindle and Wheelock 1970), and peptides similar to macropeptides from chymosin (EC 3.4.23.4) hydrolysis are produced in milk heated to 120°C for 20 min (Alais et al 1967). Under severe ultra-high-temper-... 

Two types of enzymes in milk are important those useful as an index ol heat treatment and those responsible tor bad flavors. Phosphatase is destroyed by the heat treatments used to pasteurize milk hence its inactivation is an indication of adequate pasteurization. Lipase catalyzes the hydrolysis of milk fat which produces rancid flavors. It must be inactivated by pasteurization or more severe heat treatment to safeguard the product against off-flavor development Other enzymes reported to have been found in milk include catalase, peroxidase, protease, diastase, amylase, oleinase. reductase, aldehydrase. and lactase. 

Enzymatic hydrolysis is a nondestructive alternative to saponification for removing triglycerides in vitamin K determinations. For the simultaneous determination of vitamins A, D, E, and K in milk- and soy-based infant formulas and dairy products fortified with these vitamins (81), an amount of sample containing approximately 3.5-4.0 g of fat was digested for 1 h with lipase at 37°C and at pH 7.7. This treatment effectively hydrolyzed the glycerides, but only partially converted retinyl palmitate and a-tocopheryl acetate to their alcohol forms vitamin D and phyllo-quinone were unaffected. The hydrolysate was made alkaline in order to precipitate the fatty acids as soaps and then diluted with ethanol and extracted with pentane. A final water wash yielded an organic phase containing primarily the fat-soluble vitamins and cholesterol. 

One of the first cases of the application of membrane bioreactors in food processes was the production of milk with low lactose content. (3-galactosidase was entrapped into cellulose acetate fibers to carry out the hydrolysis of milk and whey lactose [2] recently the system was improved by the use of microfiltration and by UV irradiation of the enzyme solution to avoid growth of micro-organisms [45]. 

Journal of the Association of Official Analytical Chemists Another method has been developed for Maretin (I) in milk and eggs (54). This procedure requires hydrolysis of the pesticide in solution and extraction of the fluorescent hydrolysis product which is then chromatographed on a thin-layer. A method for Maretin had previously been developed by measuring its native fluorescence (12). However the fluorescence was not stable which is undesirable. The fluorescence of the hydrolysis product of Maretin, namely, naphthostyri1 (XVIII) is very stable. By this way Maretin was determined at the 0.01 ppm level with over 90% recoveries (Table IX). 

Mannheim, A. and Cheryan, M. 1990. Continuous hydrolysis of milk protein in a membrane reactor. J. FoodSci. 55, 381-385. 

A simple procedure has been described for preparing 14C-methyl-labeled proteins (1,2). Such derivatives and their fragments still may be detected and measured after the proteins have been denatured or hydrolyzed. In this chapter, we describe experiments that illustrate the use of 14C-methyl-labeled milk proteins to study protein interactions in heated skim milk and to follow enzymic hydrolysis of milk proteins in various milks. 

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