Homogenization, milk

The importance of between-laboratory variability on the results of an analytical method can be determined by having several laboratories analyze the same sample. In one such study seven laboratories analyzed a sample of homogenized milk for a selected alfatoxin. The results, in parts per billion, are summarized in the following table. 

Some detergents and surfactants are used as emulsifying agents. An emulsifier keeps oil droplets and water droplets from joining together, so a thick mixture of oil and water will not separate. Examples of emulsions are mayonnaise, butter, cream, homogenized milk, and salad dressings. 

These assumptions were confirmed by the electrophoresis study of the washed creams. Electrophoresis of purified fat globules is a convenient method to characterize and quantify proteins adsorbed at the oil-water interface [35]. Electrophoretic data indicate that no casein, nor whey proteins, were adsorbed at the surface of raw-milk fat globule. Upon homogenization, caseins adsorbed preferentially at the lipid-water interface. In this case, bound a-lactalbumin accounted for 16% of the total interfacial proteins. Heat treatment also induced the interaction of proteins with the fat globules. The amount of bound proteins (per mg of lipids) for heated raw milk was half that for homogenized milk. 

Barcarolo R, Tealdo E, Tutta C. 1988. Multiresidue determination of organochlorine and triazine pesticides in homogenized milk. J High Resolution Chromatography Chromatography Comm 11 10746-10748. 

It has been suggested that XO from homogenized milk enters the vascular system and may be involved in atherosclerosis via oxidation of plasmalo-gens (Appendix 3B) in cell membranes. However, the experimental evidence in support of this view is very weak and the hypothesis has been disclaimed (see Farkye, 1992). 

Figure 9.5 Changes in titratable acidity (O), lactic acid ( ) and lactose ( ) on heating homogenized milk in sealed cans at 116°C. Titratable acidity expressed as mg lactic acid/100 g...

Many variations on this theme that allow repetitive, direct injection and chromatographic analysis of untreated sample matrices including tissue homogenates, milk, plasma, and saliva (107-112) have appeared since the original publication. These packings have been described generically as restricted-access media because they are generally characterized by a limited accessibility of macromolecular compounds to the adsorption sites of the porous supports. 

Milk containing fat globules with a natural fat globule membrane can be activated, deactivated, and reactivated by proper changes in temperature. However, some loss of activity will occur upon repeated activation (Wang and Randolph 1978). The phenomenon of temperature activation is found only when the fat globules have their natural layer of adsorbed materials. Neither homogenized milk, nor emulsions of tributyrin, nor butter oil emulsified in skim milk can be activated in this manner. 

Average ADVs for six pasteurized-homogenized milk samples after storage for 7 days at 5°C. Average raw milk ADV was 0.7. 

A number of salts inhibit lipolysis, the most effective being sodium chloride (Gould 1941 Pijanowski et al. 1962 Willart and Sjostrom 1959 Egelrud and Olivecrona 1972). Lipolysis in cream was found to be insignificant in the presence of 4% sodium chloride and in homogenized milk containing 5 to 8% of this salt (Gould 1941). 

Dimick, P. S. 1973. Effect of fluorescent light on the flavor and selected nutrients of homogenized milk held in conventional containers. J. Milk Food Technol. 36, 383-387. 

Hegenauer, J., Saltman, P., Ludwig, D., Ripley, L. and Bajo, P. 1979B. Effects of supplemental iron and copper on lipid oxidation in milk. I. Comparison of metal complexes in emulsified and homogenized milk, J. Agr. Food Chem. 27, 860-867. 

Tarassuk, N. P. and Koops, J. 1960. Inhibition of oxidized flavor in homogenized milk as related to the concentration of copper and phospholipids per unit of fat globule surface. J. Dairy Sci. 43, 93-94. 

Recently, increased publicity has been given to this XO hypothesis, in particular by promoters of imitation milk products. Imitation milk is alleged to be superior to real homogenized milk because it lacks the enzyme XO. It is important to keep in mind, however, that the XO hypothesis has never been proven (Deeth 1983 Clifford et al 1983 American Heart Association 1981 Carr et al. 1975 Bierman and Shank 1975). 

American Heart Association, Nutrition Program Committee. 1981. Advisory statement concerning the claims that consumption of homogenized milk increases the risk of heart disease. Supplement to guidelines for the development of nutrition programs. American Heart Association. 

Bierman, E. L. and Shank, R. E. 1975. Editorial Homogenized milk and coronary artery disease Theory, not fact. JAMA 234, 630-631. 

Deeth, H. C. 1983. Homogenized milk and atherosclerotic disease A review. J. Dairy Sci. 66, 1419-1435. 

Mangino, M. E. and Brunner, J. R. 1976. Homogenized milk Is it really the culprit in dietary-induced atherosclerosis J. Dairy Sci. 59, 1511-1512. 

Ashworth, U. S. 1969. Turbimetric methods for measuring fat content of homogenized milk. J. Dairy Sci. 52, 262-263. 

Caffyn, J. E. 1951. The viscosity temperature coefficient of homogenized milk. J. Dairy Res. 18, 95-105. 

In undisturbed milk, lipid globules rise and form a cream layer. In the past, this was so commonly known that it could have been left unstated. Since the preponderance of milk today is marketed as homogenized milk, most of the people in urban populations will not have observed the creaming phenomenon. Since creaming is one of the most readily observable physical properties of the lipid globules of milk, this phenomenon has been widely studied. Brunner (1974) reviewed literature in this area through 1971, and Mulder and Walstra (1974) have discussed some of the more recent studies. 

Brunner, J. R., Duncan, C. W. and Trout, G. M. 1953. The fat-globule membrane of nonhomogenized and homogenized milk. I. The isolation and amino acid composition of the fat-membrane proteins. Food Res. 18, 454-462. 

Buchheim, W. 1970A. Distribution of butterfat and casein in completely and partly homogenized milk. Kieler Milch. Forsch. 22. 323-327. (German)... 

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