Lactoglobulin solubility

In the absence of calcium, /3-lactoglobulin solubility increases as pH is increased from 6.4 to 7. Addition of calcium at any pH causes a decrease in solubility. At any ratio of pH and calcium ion concentration, the increased charge on the protein induced by pH change is balanced by added calcium ions to hold the level of denaturation constant (de Wit 1981). This suggests that calcium-induced precipitation of /3-lactoglobulin occurs by an isoelectric mechanism. However, Hillier et al. (1979) found that an increase in the calcium concentration up to 0.4 mg/ml slowed heat denaturation of /3-lactoglobulin, but additional calcium had little effect. 

Chemical reactions Polymerization of casein and whey proteins are due to some kind of chemical reactions. The different proteins as found in the supernatant of milk after precipitation at pH 4.6 are collectively called whey proteins. These globular proteins are more water soluble than caseins and are subject to heat dena-turation. Denaturation increases their water-binding capacity. The principal fractions are P-lactoglobulin, a-lactalbumin, bovine serum albumin (BSA), and immunoglobulins (Ig). 

It was recognized 60 years ago that whey prepared by any of the above methods contained two well-defined groups of proteins which could be fractionated by saturated MgS04 or half saturated (NH4)2S04 the precipitate (roughly 20% of total N) was referred to as lactoglobulin and the soluble protein as lactalbumin. 

About 20% of milk protein is soluble in the aqueous phase of milk. These serum proteins are primarily a mixture of /3-lactoglobulin, a-lactalbumin, bovine serum albumin, and immunoglobulins. Each of these globular proteins has a unique set of characteristics as a result of its amino acid sequence (Swaisgood 1982). As a group, they are more heat sensitive and less calcium sensitive than caseins (Kinsella 1984). Some of these characteristics (Table 11.1) cause large differences in susceptibility to denaturation (de Wit and Klarenbeek 1984). 

Proteins tend to show a minimum solubility at their isoelectric pH—a fact that is apparent for /3-lactoglobulin (fig. 6.1). The decrease in solubility at the isoelectric pH reflects the fact that the individual protein molecules, which all have similar charges at pH values away from their isoelectric points, cease to repel each other. Instead, they coalesce into insoluble aggregates. 

Solubility of /3-lactoglobulin as a function of pH and ionic strength. The isoelectric pH (pi) for this protein is about 5.2, which corresponds to the point of minimum solubility. 

Multiple funtional improvements in /f-lactoglobulin (/J-LG) could be achieved by covalent binding to CMD (Mn lOOOOgmol x) using the water-soluble... 

FIG. 4. Solubility profiles of (3-lactoglobulin esterified to different extents with [A] 0%... 

Comparison of the solubility of all three esterified proteins showed that the magnitude of the shift in the isoelectric points decreased in the following order (3-lactoglobulin > a-lactalbumin > (3-cascin. 

Bertrand-Harb, C., Charrier, B., Dalgalarrondo, M., Chobert, J.-M., and Haertle, T. 1990. Condensation of glycosidic and aromatic structures on amino groups of (3-lactoglobulin B via reductive alkylation. Solubility and emulsifying properties of the protein derivatives. Lait 71, 205-215. 

Bouhallab, S., Morgan, F., Henry, G., Molle, D., and Leonil, J. 1999. Formation of stable covalent dimer explains the high solubility at pH 4.6 of lactose-(3-lactoglobulin conjugates heated near neutral pH. J. Agric. Food Chem. 47, 1489-1494. 

Kitabatake, N., Cuq, J.-L., and Cheftel, J.-C. 1985. Covalent binding of glycosyl residues to (3-lactoglobulin effects on solubility and heat stability. J. Agric. Food Chem. 33, 125-130. 

The solubility of a number of proteins in NMA was investigated by Rees and Singer232. Of these insulin and j3-lactoglobulin were found to be soluble to at least one gram per liter while zein is soluble to at least ten grams per liter233). An ORD study of zein in NMA233) led to the conclusion that the helicity (28 °C) is 42%. 

T. Arakawa, S.N. Timasheff, Abnormal solubility behavior of beta-lactoglobulin — salting-in by glycine Mid NaCl, Biochemistry 26... 

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