Surface tension of milk

Besides changing the natural flavor of milk, lipolysis may produce a variety of other effects. One of the most noticeable of these is the lowering of surface tension as lipolysis proceeds (Schwartz 1974). Fatty acids, especially their salts, and mono- and diglycerides, being good surface-active agents, depress the surface tension of milk (see the discussion Methods for Determining Lipase Activity ). Milk fat ob-... 

The surface tension of milk is on the order of 50 dynes cm-1 at 20°C, compared to that of water, which is 72.75 dynes cm-1 at the same temperature. The milk proteins, milk fat, phospholipids, and free fatty acids are the principal surface-active components determining the surface properties of milk. 

Free fatty acids released by lipolysis of milk fat greatly depress the surface tension of milk. In fact, surface tension has been used to some extent as an objective index of the development of hydrolytic rancidity (Dunkley 1951 Herrington 1954 Hetrick and Tracy 1948 Tarassuk and Smith 1940). Its value for this purpose is somewhat limited by... 

Parkash, S. 1963. Studies in physico-chemical properties of milk. XIV. Surface tension of milk. Ind. J. Dairy Sci. 16, 98-100. 

Tarassuk, N. P. and Smith, F. R. 1940. Relation of surface tension of rancid milk to its inhibitory effect on the growth and acid fermentation of Streptococcus lactis. J. Dairy Sci. 23, 1163-1170. 

As temperature is raised in the range 10 to 60 °C, the surface tension of skim milk and whole milk decreases (Mohr and Brockmann 1930 Watson 1958). This decrease is comparable in magnitude to that observed in the surface tension of water, which decreases about 10 dynes cm-1 over this range. 

Homogenization of raw whole milk or cream stimulates lipolysis and thus leads to a decrease in surface tension, but if the product has been previously pasteurized, the effect of homogenization is an increase in surface tension (Trout et aL 1935 Watson 1958 Webb 1933). The reason for such an increase is not known, but suggestions have been made that it results from denaturation or other changes in the lipoprotein complex or from a reduction in the amount of protein available to the milk-air interface because of adsorption on the extended fat surface. The latter explanation seems unlikely in view of the very slight effect of fivefold dilution on the surface tension of skim milk. Another possible suggestion is that homogenization reduces the amount of free fat in the product. 

Calandron, A. and Grillet, L. 1964. Measurement of the surface tension of certain milks... 

Igarashi, Y. and Saito, Z. 1972. Milk components affecting the surface tension of bovine milk. Bull. Faculty Agr. Hirosaki Univ. 18, 43-48. 

Whitnah, C. H., Conrad, R. M. and Cook, G. L. 1949. Milk surfaces. I. The surface tension of fresh surfaces of milk and certain derivatives. J. Dairy Sci. 32, 406-417. 

The effects of homogenization on milk components have been summarized by Walstra and Jenness (1984) and Harper (1976). Homogenization disrupts fat globules and results in an increase in fat surface area (about 4-10 times). Casein micelles adsorb on the fat surface and constitute part of the fat globule membrane. The curd tension of milk is thus lowered. Walstra and Jenness (1984) have described the effect of homogenization on rennet coagulation. 

The /1-casein (jS-CS) and /J-lactoglobulin (jS-LG) from bovine milk were obtained from SIGMA. Molecular weights of 24000 for fi-CS and 18 400 for fl-LG were used. The protein samples were used without further purification. The solutions were prepared with phosphate buffer made by mixing appropriate stock solutions of Na2HP04 and NaH2P04. The surface tension of a 10 mM buffer at pH = 7 was 72.7 mN/m. All measurements were performed at room temperature of 22 °C. 

Lecithin (qv), a natural phosphoHpid possessing both hydrophilic and hydrophobic properties, is the most common emulsifier in the chocolate industry (5). The hydrophilic groups of the lecithin molecules attach themselves to the water, sugar, and cocoa soflds present in chocolate. The hydrophobic groups attach themselves to the cocoa butter and other fats such as milk fat. This reduces both the surface tension, between cocoa butter and the other materials present, and the viscosity. Less cocoa butter is then needed to adjust the final viscosity of the chocolate. 

Interfacial tension may be measured by a number of techniques, including determining how far a solution rises in a capillary, by measuring the weight, volume or shape of a drop of solution formed at a capillary tip, measuring the force required to pull a flat plate or ring from the surface or the maximum pressure required to form a bubble at a nozzle immersed in the solution. Ring or plate techniques are most commonly used to determine y of milk. 

Reported values for the interfacial tension between milk and air vary from 40 to 60 Nm-1, with an average of about 52 Nm-1 at 20°C (Singh, McCarthy and Lucey, 1997). At 20-40°C, the interfacial tension between milk serum and air is about 48Nm-1 while that between sweet cream, buttermilk and air is about 40Nm-1 (Walstra and Jenness, 1984). Surface tension values for rennet whey, skim milk and 25% fat cream are reported to be 51-52, 52-52.5 and 42-45Nm-1, respectively (Jenness and Patton, 1959). 

An inhibitory effect of rancid milk on the growth of Streptococcus lactis has been reported. Early reports (Schwartz 1974) claimed that rancid milk significantly inhibits the growth of bacteria in general and of Streptococcus lactis in particular. It has been stated that rancidity in milk may reach such a degree as to actually render the product sterile. (Schwartz 1974). Tarassuk and Smith (1940) attributed the inhibitory effect of rancid milk to changes in surface tension, but Costilow and Speck (1951) believe that the inhibition is due to the toxic effect of the individual fatty acids. 

In milk, the important interfaces are those between the liquid product and air and between the milk plasma and the fat globules contained therein. Studies of the surface tension (liquid/air) have been made to ascertain the relative effectiveness of the milk components as depressants to follow changes in surface-active components as a result of processing to follow the release of free fatty acids during lipolysis and to attempt to explain the foaming phenomenon so characteristic of milk. Interfacial tensions between milk fat and solutions of milk components have been measured in studies of the stabilization of fat globules in natural and processed milks. 

The effectiveness of surface and interfacial tension depressants can be compared by plots of concentration versus tension. Various dilution studies of milk, skim milk, wheys, and solutions of milk proteins reveal that casein and the proteins of the lactalbumin fraction (/3-lactoglobu-lin, a-lactalbumin, and bovine serum albumin) are powerful depressants, while the proteins of the immunoglobulin fraction are somewhat... 

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