Stabilization of milk

Al-Mabruk R M, Beck N F G and Dewhurst R J (2004), Effects of silage species and supplemental vitamin E on the oxidative stability of milk , Journal of Dairy Science, 87, 406 112. 

However, one needs to find the suitable polymer for a given colloidal system. A typical example as found in biology is the stabilization of milk. 

Figure 2.14 Effect of lactose hydrolysis (0, O 5, 10, 15, 20, A 30, A 50-85, x, %) on the stability of milk to freezing (modified from Tumerman, Fram and Comely, 1954).

Casein is low in sulphur (0.8%) while the whey proteins are relatively rich (1.7%). Differences in sulphur content become more apparent if one considers the levels of individual sulphur-containing amino acids. The sulphur of casein is present mainly in methionine, with low concentrations of cysteine and cystine in fact the principal caseins contain only methionine. The whey proteins contain significant amounts of both cysteine and cystine in addition to methionine and these amino acids are responsible, in part, for many of the changes which occur in milk on heating, e.g. cooked flavour, increased rennet coagulation time (due to interaction between /Mactoglobulin and K-casein) and improved heat stability of milk pre-heated prior to sterilization. 

Acid phosphomonoesterase Hydrolysis of phosphoric acid esters Reduce heat stability of milk cheese ripening... 

Milk contains trace amounts of SOD which has been isolated and characterized it appears to be identical to the bovine erythrocyte enzyme. SOD inhibits lipid oxidation in model systems. The level of SOD in milk parallels that of XO (but at a lower level), suggesting that SOD may be excreted in milk in an attempt to offset the pro-oxidant effect of XO. However, the level of SOD in milk is probably insufficient to explain observed differences in the oxidative stability of milk. The possibility of using exogenous SOD to retard or inhibit lipid oxidation in dairy products has been considered. 

SOD is more heat stable in milk than in purified preparations in milk it is stable at 71°C for 30 min but loses activity rapidly at even slightly higher temperatures. Slight variations in pasteurization temperature are therefore critical to the survival of SOD in heated milk products and may contribute to variations in the stability of milk to oxidative rancidity. 

On heating at temperatures above 100°C, lactose is degraded to acids with a concomitant increase in titratable acidity (Figures 9.5, 9.6). Formic acid is the principal acid formed lactic acid represents only about 5% of the acids formed. Acid production is significant in the heat stability of milk, e.g. when assayed at 130°C, the pH falls to about 5.8 at the point of coagulation (after about 20 min) (Figure 9.7). About half of this decrease is due to the formation of organic acids from lactose the remainder is due to the precipitation of calcium phosphate and dephosphorylation of casein, as discussed in section 9.4. 

The heat stability of milk is increased by the Maillard reaction, probably via the production of carbonyls (section 9.7). 

To increase the stability of milk products. Lipoprotein lipase is probably the most important in this regard as its activity leads to hydrolytic rancidity. It is extensively inactivated by HTST pasteurization but heating at 78°C x 10 s is required to prevent lipolysis. Plasmin activity is actually increased by HTST pasteurization due to inactivation of inhibitors of plasmin and/or of plasminogen activators. 

Concentration. Concentration by thermal evaporation markedly reduces the heat stability of milk, e.g. concentrated skim milk containing about 18% total solids coagulates in roughly 10 min at 130°C. The stability of the concentrate is strongly affected by pH, with a maximum at around pH 6.6, but stability remains low at all pH values above about 6.8 (Figure 9.20). Concentration by ultrafiltration has a much smaller effect on HCT than thermal evaporation, due to a lower concentration of soluble salts in the retentate. 

Singh, H. and Creamer, L.K. (1992) Heat stability of milk, in Advanced Dairy Chemistry, Vol. 

Baumrucker, C. R. and Keenan, T. W. 1973. Membranes of mammary gland. VII. Stability of milk fat globule membrane in secreted milk. J. Dairy Sci. 56, 1092-1094. 

Stability. Some discussion regarding stability of milk lipases was presented in the preceding section. Egelrud and Olivecrona (1973) found that the enzyme fractions from heparin-Sepharose can be stored frozen at -20°C with less than 10% loss of activity in 2 weeks. The purified enzyme had only moderate stability at 4°C high concentrations of salt or a pH below 6.5 or above 8.5 increases the rate of inactivation. 

Investigations concerned with variations in the oxidative stability of milk as a result of feeding practices have centered on the transfer to milk of natural antioxidants. Although Kanno et al. (1968) have reported the presence of 7-tocopherol, the only known natural antioxidant of consequence is a-tocopherol. 

Dunkley, W. L., Franke, A. A. and Robb, J. 1968B. Tocopherol concentration and oxidative stability of milk from cows fed supplements of d- or dl-o-tocopheryl acetate. J. Dairy Sci, 51, 531-534. 

Dunkley, W. L., Ronning, M. and Smith, L. M. 1966. Influence of supplemental tocopherol and carotene on oxidative stability of milk and milk fat. 17th Int. Dairy Congr. Proc. A, 223-227. 

Gregory, J. F. and Shipe, W. F. 1975. Oxidative stability of milk. I. The antioxidative effect of trypsin treatment and aging. J. Dairy Sci. 58, 1263-1271. 

Riest, U., Ronning, M., Dunkley, W. L. and Franke, A. A. 1967. Oxidative stability of milk as influenced by dietary copper, molybdenum and sulfate. Milchwissenschaft 22, 551-554. 

Research reports on milk stability have been published frequently since about 1919. Early studies were prompted by the need to ensure sufficient heat stability for evaporated milk to withstand heat sterilization (Sommer and Hart 1919, 1922). Between 1919 and 1960, most attention was directed to the influence of milk salts on heat stability (Miller and Sommer 1940 Pyne 1958 Pyne and McHenry 1955). It was not until the early 1960s that the importance of heating time and pH on coagulation of milk was appreciated (Rose 1961A.B). More recent work has been concerned with factors which affect the stability of milk proteins. 

The variation in lactose concentration in milk is small, but experiments with the adjustment of lactose concentration have shown that lactose plays a part in the stabilization of milk. Addition of lactose increases both denaturation temperatures of /3-lactoglobulin (de Wit 1981). Milk... 

Fox, P. F. 1981B. Heat stability of milk Significance of heat-induced acid formation in coagulation. Irish J. Food Sci. Technol. 5, 1-11. 

Fox, P. F. and Hearn, C. M. 1978A. Heat stability of milk Influence of dilution and dialysis against water. J. Dairy Res. 45, 149-157. 

Holt, C., Muir, D. D. and Sweetsur, A. W. M. 1978. Seasonal changes in the heat stability of milk from creamery silos in south-west Scotland. J. Dairy Res. 45, 183-190. 

Kruk, A. 1979. Relationship between casein hydration degree and thermal stability of milk. Acta Alimentaria Polonica 5, 147-156. 

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