Heat-induced complex formation between .KAPPA.-casein and .ALPHA.-lactalbumin.

Abstract

It is considered that heat treatment affects the properties of milk such as heat-stability and chymosin clottability. The heat-induced interaction between K>casein and a-lactalbumin was investigated under various conditions (buffer, pH, ionic strength, temperature). The results obtained, using Sephacryl S-300 gel filtration and sodium dodecylsulfate-polyacrylamide gel electrophoresis, indicated that a complex of the two proteins was formed in 35mMphosphate buffer, pH 7.6, containing 0.4m NaCl on heat treatment at 90°C for 30 min, while no complex was formed in 10mMimidazole-HCl buffer, pH 7.1, containing 70mMKC1 with the same heat treatment. A K-casein-a-lactalbumin complex was formed at high pH by heat treatment, and was dissociated in the presence of 2-mercaptoethanol. Under complex forming conditions, a change in the higher-order structure of a-lactalbumin was observed by ultraviolet absorption and fluorescence experiments. Though the participation of specific amino acid residues in the complex formation could not be clarified by the chemical modification procedure, the rearrangement of the disulfide cross-links did take place in the reaction. An optical study on the complex formation did not indicate the involvement of a hydrophobic bond. Heat treatment of milk is necessary for its preservation and as a prior treatment for processing. The characteristics of milk are almost all due to milk proteins. Mechanisms of changes in proteins on heat treatment and formations of a precipitate and gel in ultrahigh temperature sterilized milk have not been clarified. Bovine milk contains about 4% protein, consisting of casein components and whey proteins. Casein components form casein micelles in milk in the presence of calcium ions. The major constituents of wheyproteins

The effect of pH and temperature pre-treatments on the physicochemical and emulsifying properties of whey protein isolate

“…Although less hydrophobic than WPI at 85 C, it is believed that the higher temperatures resulted in the dissociation of a-LA and b-LG structures leading to reduced hydrodynamic radii. Schokker et al (2000) reported b-LG to be reduced from dimers to monomers at temperatures near 70 C. In addition, although a-LA is reported to denature at relatively low temperatures (64.3 C (Relkin et al, 1993;Boye & Alli, 2000)), at !85 C, free thiol groups are formed (Doi, Tokuyama, Kuo, Ibuki, & Kanamori;Schnack & Klostermeyer, 1980) which makes it highly reactive (BertrandHarb, Baday, Dalgalarrondo, Chobert, & Haertle, 2002;Chaplin & Lyster, 1986;Hong & Creamer, 2002) and prone to aggregation (Livney, Verespej, & Dalgleish, 2003). This combined effect of the reactivity of a-LA and dissociation of b-LG dimers at 85 C and pH 5.0 is suggested to allow for the formation of smaller aggregates.…”

A Comparison of Creep Phenomena in Food Protein Gels

“…Nevertheless, the gels showed increased viscous properties at higher temperatures, perhaps reflecting a decrease in hydrophobic interactions. Doi et al (1983Doi et al ( , 1985 reported the formation of gels upon heating a dispersion mixture of casein and whey proteins. More effective gel formation of heated mixtures of these components was obtained by acidification and re-heating (Doi et al, 1985).…”

“…Doi et al (1983Doi et al ( , 1985 reported the formation of gels upon heating a dispersion mixture of casein and whey proteins. More effective gel formation of heated mixtures of these components was obtained by acidification and re-heating (Doi et al, 1985). The casein + WPC gels showed large plastic deformations which may have reflected the existence of large colloidal particles as components of gel network.…”

“…A better definition, and understanding of the rheological characteristics of protein gels is required (Kinsella, 1982;Hermansson, 1982;Yamauchi, 1986). Many researchers have used various techniques such as viscometry (Babajimopoulos et al, 1983;Umeya et al, 1981), texture analyses (Doi et al, 1983(Doi et al, , 1985Hermansson, 1982;Kuwahata and Nakahama, 1975;Schmidt et al 1979;Harwalkar and Kalab, 1981;Miura et al, 1984), and viscoelastic analyses (Miura and Yamauchi, 1984;Kuwahata and Nakahama, 1975;Tokita et al, 1982;Paulsson et al, 1986;Van-Kleef, 1986;Pour-El, 1976) to characterize the behavior of gels. However, there are limited data comparing the gel properties of different food proteins using a single method.…”

Solubility and aggregation of commercial α-lactalbumin at neutral pH

“…The concentration increased linearly and peaked at 60 min, where it reached a plateau up to 150 min. There is no indication that they measured the reactive thiol content of the unheated samples but, based on their extrapolation to 0 time, there was an approximately 5-6-fold increase in the thiol concentration up to their plateau value (Doi et al, 1983).…”