Casein hydration

Mora-Gutierrez, A., Kumosinski, T.F., Farrell, Jr., H.M. (1997). Oxygen-17 nuclear magnetic resonance studies of bovine and caprine casein hydration and activity in deuterated sugar solutions. Journal of Agricultural and Food Chemistry, 45, 4545-4553. 

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

A lower degree of casein hydration in the resultant cheese... 

The low degree of casein hydration is less favorable to moisture retention during baking and results in dehydration, crusting and poor flow (Kindstedt, 1995 Kindstedt and Guo, 1997). Moisture also acts as a lubricant between the protein layers and between protein and fat layers during melting and thereby facilitates heat-induced slippage of different parts of the matrix (McMahon et al., 1993 Prentice et al., 1993). 

A reduction in fat level results in a decrease in the moisture-to-protein ratio, as reflected by the lower content of MNFS (Figure 11.1 Table 11.1). However, the flowability of rennet-curd cheeses is positively correlated with the content of MNFS (Riiegg et al., 1991 McMahon et al., 1993), an effect which may, in part, be due to the concomitant increase in casein hydration and the lubrication effect of moisture. 

Skim milk (35 g/L) and /1-lactoglobulin (5 g/L) solutions were prepared by dissolving skim milk powder and y6-lactoglobulin powder in distilled water and stirring at room temperature for 4 h. /1-Casein (5 g/L) had been hydrated in a phosphate buffer (100 mM, pH 8) and stirred at 4°C for one night [27]. Sodium azide (0.1%, wt/v) was added to prevent bacterial growth. 

Four reconstituted milks were prepared by blending hydrated skim milk powder (35g/L) with four different emulsions (35g/L) differing by composition of the fat-water interface. Whole reconstituted milks were coded MP (milk proteins), BCAS ( 6-casein), and BLG5 (j6-lactoglobulin 5 g/L). 

Figure 6.8 Sketch of proposed molecular mechanism of protein-surfactant interaction for CITREM + sodium caseinate (0.5 % w/v in aqueous medium (pH = 7.2, ionic strength = 0.05 M) at 293 K. Picture (I) shows the water molecules bound with polar groups of the protein and surfactant, as w ell as w ater molecules structured as a result of hydrophobic hydration around the hydrocarbon chain of the surfactant. (For clarity, the free w ater molecules are not shown.) Picture (H) demonstrates the release of bound and structured water molecules resulting Rom the predominantly hydrophobic interactions between protein and surfactant. Reproduced Rom Semenova et al. (2006) with permission.

Changes in hydration. As would be expected from many of the changes discussed above, the hydration of the casein micelles decreases with the duration of heating at 140CC. The decrease appears to be due mainly to the fall in pH - if samples are adjusted to pH 6.7 after heating, there is an apparent increase in hydration on heating. 

In addition to the above, there are emulsion and suspension stabilizers that act as protective colloids and in some cases as thickeners gums (such as acacia and traga canth), alginates, starch and starch derivatives, casein, glue, egg albumin, methyl cellulose, hydrated Mg and Al silicates, etc Refs Same as in previous item... 

Temperature control is important in conductivity measurements, since the conductivity of milk increases by about 0.0001 ohm 1cm 1 per degree Celsius rise in temperature (Gerber 1927 Muller 1931 Pinkerton and Peters 1958). Increased dissociation of the electrolytes and decreasing viscosity of the medium with increasing temperature are undoubtedly responsible for this effect. An investigation (Sudheendra-nath and Rao 1970) of the viscosity and electrical conductivity of skim milk from cows and buffaloes failed to reveal a simple relationship. The authors attributed the lack of linear correlations to variations in casein structure and its hydration. 

Casein exists in milk as a calcium caseinate-calcium phosphate complex the ratio of these components is approximately 95.2 to 4.8. The dispersed casein particles appear to be spherical m shape and of various sizes. The size distribution of the casein micelles is nol constant, hut varies with aging, heating, concentration, and other processing treatments. Processing alters ihe water-binding of casein and this in turn affects the apparent viscosity of products that contain casein Changes in hydration have not been measured quantitatively although the casein panicles of raw milk... 

The ageing at 5°C of whippable emulsions such as ice cream mix will enhance the hydration of milk proteins in the system. This is due to a property of casein micelles in milk. At low temperatures, the hydration or voluminosity of casein increases. The voluminosity is the volume of hydrated protein per gram of protein. This can be studied by analyzing the protein and water content in the sedimented casein pellet after centrifugation of skimmed milk. 

The increased hydration at low temperature is due to lower protein content in the pellet owing to dissociation of protein from the micelle (mainly beta-casein), and corresponds to data from the literature42. 

Figure 14 Effect of low temperature on hydration of bovine casein micelles and of interfacially bound protein in ice cream mix with (+E) and without (-E) emulsifier (saturated mono-diglyceride).

For typical compact proteins this plot has a positive slope, as the hydrophilic residues on the outside of the dissolved protein have a higher scattering density than the hydrophobic residues on the inside. For casein sub-micelles, the slope is negative (Stothart and Cebula, 1982) (Figure 2). This seems surprising at first sight, but the sub-micelles are so highly hydrated that all the constituent protein... 

Fig. 12. Electron micrographs of bovine casein micelles obtained by different techniques, (a) Freeze fractured and etched (L. K. Creamer and D. M. Hall, unpublished). (b) Fixed and rotary shadowed (Kalab et al., 1982). (c) Unstained, embedded thin section [reproduced from Knoop et al. (1979), by permission of the publishers, Cambridge University Press), (d). Unstained, unfixed, hydrated sample (van Bruggen et al., 1986).

Table I shows that the foaming properties of whole casein improved by slight phosphorylation. The lowest phosphorylated form of casein (4 mol P/mol protein) showed higher foam hydration and stability than the native whole casein. However, the highly phosphorylated whole casein (11 mol P/mol protein) showed poor foaming properties. The foam hydration of as-casein deteriorated while that of K-casein improved by phosphorylation. This discrepancy seemed to be caused by a different initial hydrophobic/ hydrophilic balance of the proteins in their native states. However, foam stabilities of all casein fractions were reduced by phosphorylation, with K-casein being only slightly affected.

Korolczuk, J. 1982. Viscosity and hydration of neutral and acidic milk protein concentrates and caseins. NZ J. Dairy Sci. Technol. 17, 135-140. 

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