title: Effect of beta-lactoglobulin A and B whey protein variants on the rennet-induced gelation of skim milk gels in a model reconstituted skim milk system.
abstract: The effect of fortifying reconstituted skim milk with increasing levels of the beta-lactoglobulin (beta-LG) genetic variants A, B, and an A-B mixture on rennet-induced gelation was studied by small deformation-sensitive rheology. Free-zone capillary electrophoresis and high-sensitivity oscillatory rheology were used to elucidate the role of potential heterotypic associative interactions between whey proteins and casein in a mixed colloidal system, subjected to moderate heating (65 degrees C for 30 min) prior to renneting, on the gelling properties of the system. Increasing levels of added whey protein, in the concentration range of 0.225 to 1.35% of added protein, led to a concomitant progressive increase in the equilibrium shear storage modulus, G' (recorded after approximately 10,800 s), in the order beta-LG B > beta-LG A and beta-LG A-B, as the general expected consequence of the setup of denser casein gel networks. The preferential effect of beta-LG B over beta-LG A on the mechanical strength of the gels may be due to the formation of cross-links and aggregates involving whey proteins and rennet hydrolysis products or an increase in the size of the casein micelle caused by the grafting of beta-LG B to its surface, or both. The results of free-zone capillary electrophoresis were consistent with the notion that beta-LG B (and not beta-LG A) binds to the casein micelle under an optimal stoichiometry of 1:0.045 (mg/mg), even in the absence of heat treatment. The liquid-like character of the gel networks formed, tan delta, was a parameter sensitive to the level of addition of beta-LG A in particular. At low concentrations (up to 0.45%) of beta-LG A, tan delta increased by almost twice as much, which was interpreted as a result of the increase in the loss modulus, G'', of the sol fraction because of the presence of unbound beta-LG A. At greater incremental concentrations of beta-LG (>0.45%), the formation of smaller whey protein aggregates confined to the sol fraction may have led to a progressive decrease in tan delta. The critical gel time, t(gel), was also affected by the concentration of added whey protein and described 3 zones of behavior, irrespective of the type of whey protein variant. The critical gel time was slightly shorter for beta-LG B than for beta-LG A at 0.45% of added whey protein, but this difference became larger at 0.67%. Even when only beta-LG B was found to associate with casein prior to renneting, both beta-LG A and beta-LG B, either alone or mixed, had a profound influence on the mechanical strength and coagulation kinetics of the rennet-induced casein gels. This knowledge is expected to be useful to exert better control and optimize processing conditions during the manufacturing of cheese and cheese analogs.
authors.affiliation: Laboratory of Dairy Science and Technology, Centro de Investigación en Alimentación y Desarrollo, AC (CIAD, AC), P.O. Box 1735 Hermosillo, Sonora, 83000 Mexico.
authors.forename: M A
authors.initials: MA
authors.lastname: Meza-Nieto
authors.forename: B
authors.initials: B
authors.lastname: Vallejo-Cordoba
authors.forename: A F
authors.initials: AF
authors.lastname: González-Córdova
authors.forename: L
authors.initials: L
authors.lastname: Félix
authors.forename: F M
authors.initials: FM
authors.lastname: Goycoolea
mesh_terms: D000818:Animals; D012085:Chymosin / Q000378:metabolism*; D019075:Electrophoresis, Capillary; D005519:Food Preservation; D005782:Gels / Q000378:metabolism*; D014644:Genetic Variation; D006358:Hot Temperature; D007700:Kinetics; D007782:Lactoglobulins / Q000235:genetics / Q000494:pharmacology*; D008892:Milk / Q000737:chemistry*; D008894:Milk Proteins / Q000494:pharmacology*; D012212:Rheology; D000067816:Whey Proteins
chemical_list: D005782:Gels; D007782:Lactoglobulins; D008894:Milk Proteins; D000067816:Whey Proteins; C025350:rennet; D012085:Chymosin