Study of the sodium caseinate agregation induced for glucono-delta-lactone

HIDALGO, MARÍA EUGENIA; NESPOLO, CÁSSIA; RISSO, PATRICIA

e-universitas, UNR Journal

Centro de Publicaciones Periódicas Electrónicas de la Universidad Nacional de Rosario

Lugar: Rosario; Año: 2009 vol. 2 p. 355 - 356

1852-0707

Different fractions of caseins (CN), alphaS1-, alphaS2-, beta- y kppa-CN, are an important source of proteins to the food industry, due its high nutritional value and its potential functionality. Its derived, the sodium caseinate (Na-CAS), is an ingredient widely used to the formulation of foods due its functional properties as its gelation capacity. During the acidification process, Na-CAS form a gel structure, result of the agregation of casein fractions. The protein gel formation by pH reduction can be promoted by the glucono-delta-lactone (GDL) hydrolysis. Objectives of this work were study the Na-CAS aggregation process by acidification and determinate the effect of different variables on this process. Na-CAS aqueous solutions were characterized by polyacrylamide gel electrophoresis in denaturalized media and native conditions. The protein suspensions aggregation by slow acidification on the time (t) was evaluated, after the addition of GDL, by means of turbidimetrics measures based on the turbidity (t) dependence with the wavelength (l) in 400-700 nm range, where there is not absorption of protein chromophor groups, measuring simultaneously the pH of samples. Linear plots of log absorbance (A) vs. log l were obtained, and extracted the b parameter from its slopes that is in a directly relationship to the average size and the compact degree of aggregated particles. Bovine Na-CAS concentration, temperature and % P/P GDL/ % P/P Na-CAS relation (R) were varied, modifying only one variable at the same time and maintained constants the others. The aggregation process also compared to the process for ovine Na-CAS at the same protein concentration, R and temperature. Protein intrinsic fluorescence emission spectra and absorption spectra were realized to detect conformational changes. All the samples showed the existence of two stages well defined. In the first, much slower, was noted a gradual increase of turbidity and a decrease of parameter beta during the time while the pH diminished. This behaviour could indicate the existence of a dissociation process of CN and/or a  conformational change with gradual opening of protein structure, that was confirmed by the variations observed on the UV absorption spectra (shift to the blue spectra band) and on the fluorescence emission (increment of fluorescence intensity). This dissociation could be necessary to the aggregates formation second stage, revealed by a sudden increase of turbidity and beta, to the gel network formation, moment from which both parameters remain constant. During the first stage, the dissociation could be conduce to the increase on the exposition of protein intrinsic fluorophors, indicating that they could be included or could be close to the molecular region that participate in the interparticular interaction. The dissociation rate depended in the direct way of R and temperature, and in the opposite direction of the protein concentration, pointing to the participation of electrostatic interactions in the studied process. The dissociation of ovine Na-CAS, with greater proportion on alphaS-CN in its protein composition, was slower, so this fact could be implicated that these protein fraction start the second stage of aggregation to the gel network formation.