Non-homogenized milk

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The increase in phase angle, and thus fluidity, on heating low-fat Cheddar cheese or full-fat Cheddar cheese made from homogenized milk are much lower than that on heating full-fat Cheddar cheese made from non-homogenized milk (Figure 11.16). 

Figure 11.15. Effect of homogenization pressure on the flowability and stretchability of full-fat Cheddar-type cheese prepared from non-homogenized control milk (FFC) or control milk homogenized at respective first and second stages pressures of 25 and 5 Pa (FFCH), and of low fat Cheddar-type cheese from non-homogenized skimmed milk (LFC), after storage at 7°C for 5 ( ), 70 ( ) or 156 ) days (redrawn from data of Guinee et al., 2000c).

Figure 11.16. Phase angle, 8, as a function of temperature for 5 day-old Cheddar type cheese of different fat content, from non-homogenized (A, O) or homogenized ( ) milks. The cheeses were low-fat (1.3%, w/w A) full-fat (30.0%, w/w O), and full-fat homogenized (30.6%, w/w ) (redrawn from data of Guinee et al., 2000c).

The esterases are involved in the hydrolysis of ester linkages of various types. The products formed are acid and alcohol. These enzymes may hydrolyze triglycerides and include several lipases for instance, phospholipids are hydrolyzed by phospholipases, and cholesterol esters are hydrolyzed by cholesterol esterase. The carboxylesterases are enzymes that hydrolyze triglycerides such as tributyrin. They can be distinguished from lipases because they hydrolyze soluble substrates, whereas lipases only act at the water-lipid interfaces of emulsions. Therefore, any condition that results in increased surface area of the water-lipid interface will increase the activity of the enzyme. This is the reason that lipase activity is much greater in homogenized (not pasteurized) milk than in the non-homogenized product. Most of the lipolytic enzymes are specific for either the acid or the alcohol moiety of the substrate, and, in the case of esters of polyhydric alcohols, there may also be a positional specificity. 

Milk acid phosphatase has been purified to homogeneity by various forms of chromaotgraphy, including affinity chromatography purification up to 40 000-fold has been claimed. The enzyme shows broad specificity on phosphate esters, including the phosphoseryl residues of casein. It has a molecular mass of about 42 kDa and an isoelectric point of 7.9. Many forms of inorganic phosphate are competitive inhibitors, while fluoride is a powerful non-competitive inhibitor. The enzyme is a glycoprotein and its amino acid composition is known. Milk acid phosphatase shows some similarity to the phosphoprotein phosphatase of spleen but differs from it in a number of characteristics. 

Liquid/liquid emulsions consist of two (or more) non-miscible liquids. Classical examples for this are oil in water (O/W) emulsions, for example milk, mayonnaise, lotions, creams, water soluble paints, photo emulsions, and so on. As appliances, teeth-rimed rotor-stator emulsifiers and colloid mills, as well as high-pressure homogenizers are used. 

IEF can also be carried out under denaturing conditions, for example in the presence of 9 M urea. For the analysis of hydrophobic proteins, non-ionic detergents such as Nonidet NP-40 or Triton X-100 can be added to the sample and the IEF gel. Analytical IEF is used to determine the pi of proteins, and also as a critical check of the homogeneity of protein preparations. The reprodudbility and resolving power of the technique is exploited in various applications where it is used to establish the identity and complexity of protein mixtures for example in food analysis, the origin of proteins in milk preparations, and in clinical analysis, determining the isoenzyme profile of apolipoproteins. 

As has already been stated, cryogels are not especially suitable for isolation or separation of proteins because of their relatively low capacity. The gels offer, however, the possibility to operate with non-clarified solutions such as cell homogenates, blood containing blood cells, suspensions of microbial cells, and milk. What can be separated using conventional chromatographic media can also be separated... 

It should be noted that the composition of milk protein-stabilized emulsions can change substantially after homogenization due to exchange of adsorbed emulsifiers with non-adsorbed emulsifiers. For this reason, a number of workers have studied preferential adsorption and competitive displacement of milk proteins with each other and with other types of emulsifiers [27,32,52-58]. 

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