María Eugenia Hidalgo

A study on bovine kappa-casein aggregation after the enzymatic action of chymosin

Kappa-casein (kappa-CN) is a milk phospho-glycoprotein that plays an important role in the electrostatic and steric stabilization of casein micelles suspensions. kappa-CN characteristics such as its aminoacidic composition and its high denaturalization temperature make this protein an excellent nutrient. The enzymatic destabilization of kappa-CN is the basis of cheese and yoghurt manufacture. In this paper, modifications on the kappa-CN aggregation process by hydrolysis with chymosin derived from the initial protein state and the presence of sucrose and lactose were studied. Our study shows that, during its self-association, kappa-CN opens its conformation by raising the exposition of hydrophobic regions to the medium. Hence, the initial polymerization or association state of kappa-CN affects on the aggregation stage after the enzymatic action of chymosin. The inhibition of aggregation when the protein was previously associated suggests that the region which participates in the aggregation of p-kappa-CN particles, would be the one involved in the self-association during the heating of kappa-CN samples. Sucrose and lactose also affect the aggregation of proteolized particles of kappa-CN. Furthermore, the nature and the concentration of the added sugars had significant influence on the protein aggregation process.

Acid-Induced Aggregation and Gelation of Sodium Caseinate-Guar Gum Mixtures

The aim of this work was to study the formation of bovine sodium caseinate (NaCAS) acid gels induced by addition of glucono-δ-lactone (GDL) in the presence of guar gum (GG). At low biopolymer’s concentrations, a one-phase system was observed, whereas at higher mixture concentrations two-phase systems were formed. Aggregation (at low NaCAS concentrations) and gelation (at high NaCAS concentrations) processes were analyzed through the use of full and fractional factorial experiment designs, using turbidimetric and rheological techniques. Finally, the gel images were obtained by confocal laser scanning microscopy and the images were analyzed. Results showed that at low NaCAS concentrations, the presence of GG affects the pH at which aggregation begins but was not significant for the time at which aggregation begins. On the other hand, at high NaCAS concentrations, the concentration of GG only affected significantly the elastic character of acid gels. As polysaccharide concentration increases, the gels obtained were weaker and with larger pores. Also, the formation of NaCAS droplet-shaped structures at certain biopolymer ratio was observed. The presence of GG affects both the rate of gelation and phase separation, which, in turn, determine the type of gel microstructure. Phase separation seems to occur prior to protein gelation because the protein gel network is discontinued, hindering the gel compactness and reducing gel strength. In summary, GG modifies NaCAS stabilization (self-association and phase separation) and the viscoelasticity and microstructure of NaCAS acid gels. The control of such processes and properties would allow obtaining mixture gels with different textures.

Acid-Induced Aggregation and Gelation of Bovine Sodium Caseinate-Carboxymethylcellulose Mixtures

Maria Eugenia Hidalgo1, Bibiana D. Riquelme1,2, Estela M. Alvarez1, Jorge R. Wagner3 and Patricia H. Risso1,2,4 1Facultad de Ciencias Bioquimicas y Farmaceuticas, Universidad Nacional de Rosario 2Instituto de Fisica Rosario (IFIR), CONICET-UNR, (2000), Rosario, 3Departamento de Ciencia y Tecnologia, 4Facultad de Ciencias Veterinarias,Universidad Nacional de Rosario, Rosario Universidad Nacional de Quilmes, Buenos Aires, Argentina

Effect of Xanthan Gum on the Rheological Behavior and Microstructure of Sodium Caseinate Acid Gels

The aim of this work was to study the effect of xanthan gum (XG) on the gelation process of bovine sodium caseinate (NaCAS) induced by acidification with glucono-δ-lactone (GDL) and on the mixed acid gel microstructure. Before GDL addition, segregative phase separation was observed in all the NaCAS-XG mixtures evaluated. The gelation process was analyzed by using a fractional factorial experimental design. The images of the microstructure of the mixed acid gels were obtained by conventional optical microscopy and the mean diameter of the interstices was determined. Both the elastic character and the microstructure of the gels depended on the concentrations of XG added. As XG concentration increased, the kinetics of the gelation process was modified and the degree of compactness and elasticity component of the gel network increased. The microstructure of gels depends on the balance among thermodynamic incompatibility, protein gelation and NaCAS-XG interactions.