Comprehensive study of acid gelation of heated milk with model protein systems

The effect of heat treatment of milk on the formation of acid gels was investigated using model protein systems. Protein systems contained micellar casein alone (35 g kg(-1)) (systR) or in the combination with either beta-lactoglobulin (betaLG) (4 g kg(-1); systB) or egg ovalbumin (4 g kg(-1); systO). Proteins in a milk ultrafiltrate (UF) were heat-treated at 90degreesC for 24 min. The same heat-treated systems without casein were also prepared: the UF alone (systUFR), with betaLG (4 g kg(-1); systUFB) and with ovalbumin (4 g kg(-1); systUFO). Proteins in pellet and supernatant fractions were analysed using reverse-phase high-pressure liquid chromatography and sodium dodecyl sulphate polyacrylamide gel electrophoresis. The solubility of globular proteins in heated systUFB and heated systUFO was determined at pH 1.6-6.5 by absorbance measurements. Rheological changes during gelation with a starter at 42degreesC were determined and microstructures of gels by scanning electron microscopy. High quantities Of K-casein and betaLG in the form of complexes (disrupted by 2-mercaptoethanol) were found in supernatant of heated systB and not in that of systO. The gelation pH was related to the solubility of the globular proteins, and the gelation pattern for systB and systO resembled that of heated milk and the systR pattern resembled that of unheated milk. The gels of systR were coarse and consisted of large particles. Particles in gels from systB and systO were smaller and less clustered. Although the two globular proteins behave differently during heating, they both confer to the casein micelles their solubility properties as a function of pH. A model is presented where both soluble and colloidal complexes including the globular protein interact at pH 5.5 and initiate the acid gelation. (C) 2003 Elsevier Ltd. All rights reserved.