Acid casein gels

In an acid casein gel (such as set yoghurt), syneresis hardly occurs, and the value of B remains virtually constant during storage. Presumably, breaking of junctions is hardly possible in these gels. Only for gels made at high temperature, say 40° C, some syneresis is often observed. 

FIGURE 17.20 The effect of filler particles on gel properties, (a) Relative modulus (Gm/G0) as a function of particle volume fraction (broken lines are calculated for various values of the ratio Gp/Go, indicated near the curves. The drawn lines are average experimental values for acid casein gels (C) and polymer gels (polyvinyl alcohol, P), with emulsion droplets that are either bonded (B) or nonbonded (N) to the gel matrix, (b) Highly schematic pictures of the gel structure. Shaded area denotes primary gel. Particles are nonbonded (1) bonded (2) bonded but with intermediate layer (3) bonded and aggregated (4). (Adapted from T. van Vliet. Colloid Polymer Sci. 266 (1988) 518.)... 

Roefs, S. P. F. M. (1986). Structure of acid casein gels, a study of gels formed after acidification in the cold. Ph.D. Dissertation, The Agricultural University, Wageningen, The Netherlands. 

Figure 6.14 Effect of sucrose on the storage modulus G at 1 Hz and 25 °C of acid-induced sodium caseinate gels (3 wt%) (A) pH = 5.5 (A) pH = 5.1 ( ) pH = 3.9. Reproduced from Belyakova et al. (2003) with permission.

Dickinson, E., Matia-Merino, L. (2002). Effect of sugars on the rheological properties of acid caseinate-stabilized emulsion gels. Food Hydrocolloids, 16, 321-331. 

Although the gelation properties of whey proteins are of great importance in many foods (Mulvihill, 1992) and it is possible to form a weak gel in creams by the formation of a continuous network of fat globules, most important milk gels are those involving casein micelles which can be made to form a gel matrix either by isoelectric precipitation (acid-induced gel) or by the action of a proteolytic enzyme (rennet-induced gel). Both gel types... 

Electrophoretic Methods. Several electrophoretic procedures have been developed to fractionate or purify the various caseins (McKenzie 1971C Thompson 1971 Whitney 1977). Wake and Baldwin (1961) fractionated whole casein by zone electrophoresis on cellulose powder in 7 M urea and 0.02 ionic strength sodium phosphate buffer at pH 7 and 5°C. Payens and co-workers employed several somewhat different electrophoretic conditions for the fractionation and purification of the caseins on cellulose columns (Payens 1961 Schmidt and Payens 1963 Schmidt 1967). Three fractions, as-, k-, and /3-caseins, were separated at pH 7.5 and 30°C with 4.6 M urea-barbiturate buffer. The purification of asi-casein and the separation of the genetic variants of K-casein were accomplished by altering the electrophoretic conditions. Manson (1965) fractionated acid casein on a starch gel column stabilized by a density gradient at 25 °C. 

Matia-Merino, L., Lau, K., and Dickinson, E. 2004. Effects of low-methoxyl amidated pectin and ionic calcium on rheology and microstructure of acid-induced sodium caseinate gels. Food Hydrocolloids 18 271-281. 

Linearity between log B and log

casein gels. From the slope of the line, D can be calculated, and for acid gels, values between 2.35 and 2.4 are observed, in good agreement with simulation results. For rennet gels, the slope yields D = 2.2-2.25. 

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