Relation of glass transition temperature to phase behaviour of whey proteins-hydroxypropylmethylcellulose blends

JARA, FEDERICO L.; PERÉZ, OSCAR E.; PILOSOF, ANA M. R.

Cambridge

Congreso; Amorph - Molecular Basis of Satability in Pharmaceuticla and Food Glasses; 2006

BioUpdate Foundation

Mixtures of protein and polysaccarides are present in foods or used to create new products. The mixing of proteins and polysaccharides above the protein isoelectric point often results in phase separation, and each phase is enriched in one of the polymers. Thermodynamic incompatibility gives same interesting benefits as the reduction in the critical concentration to gel, the formation of non-equilibrium trapped structures that finds application in terms of textures and flavour, thickeners, fat substitutes, carriers of nutritional and taste components, atructural components in food products or matrices for controlled drug release. The aim of the work was to assess the relation of Tg to the compatibility or incompatibility of whey proteins-hydroxypropylmethylcellulose blends and the effect of the degree of phase separation. The binodal curve characterizing phase separation was determined and blends below (single phase) and above the binodal (two phase) were selected. The latter were studied immediately after mixing the biopolymer solutions, after 30 min at room temperature, and after centrifuging to fully separate the segregating phases. The mixture above the binodal was also ajusted to pH5 and 3. The mixtures were quenched by liquid nitrogen freezing, lyphilized and residual water eliminated at 70 ºC for 15 days before measuring Tg in a DSC 822 Mettler calorimeter. Two Tg s were observed only in the fully separated system, but they were higher that defined for each biopolymer. for the protein rich phase, this could be accounted for by the small amount of HPMC present. Nevertheless the strong increase of the corresponding HPMC-rich phase suggests that the polysaccharide is more rigid in segregates phases than in bulk.In addition, due to the high polydispersity of HPMC, molar mass fractionation can take places during phase separation.The high molecular weight fraction of HPMC would stay in the HPMC rich phase rising its Tg. For systems frozen inmediately after mixing the biopolymers of after standing 30 min utes at room temperatures, or adjusted at pH 5, only one Tg was observed, even if the systems were visually segregated, showing a dispersed morphology with large domains formed by WPC. For these systems the high increase in the width of the glass transition region indicates the phase separation. Mixtures adjusted at pH 3 were homogeneous and showed Tg in between the values of single components and the width of the glass transition showed no deviation from that predicted from the widths of the glass transition regions for the individual components indicating this system was fully miscible. The mixture below the binodal showed a single Tg close to the protein Tg. It can be concluded that the degree of phase separation affects the glass transition temperatures of incompatible protein-polysaccharide mixtures. The existence of a single Tg cannot be taken as indicative of miscibility, being the with of the glass transition  region an aditional criterion. For homogeneous systems Tg values reflects different interactions between the biopolymers.