Thermally induced globular protein gels: peculiarities of formation, melting, and restoration of gels of different structure

Abstract

The results of the calorimetric study of the globular protein heat-set gels performed by the authors during recent years have been generalized in the present paper. The conformational variations have been studied which took place after the denaturation in globular protein concentrated solutions with continuous heating in the temperature range 20–140°C. The effects of pH, the ionic strength of the solution and the protein concentration on the formation of heat-set gels and their melting have been studied in parallel with visual observations of the solution states under the same conditions. Four different types of globular protein gels have been discovered (e.g. lysozyme), and the ranges of their existence have been determined. The results obtained have shown that two types of gel can be formed within the pH range 1.7–9.0 at fixed ionic strength and protein concentration: clear meltable gel (type I) and a turbid one which cannot melt (type II). The heating thermograms of the type I gel demonstrate besides denaturation heat absorption an additional high temperature maximum, which is the calorimetric manifestation of cooperative transformation proceeding at the melting of this gel. The incipient stage of the melting gel formation is described at critical values of pH, ionic strength and protein concentratión. The adjustment of protein aggregation in solution with the help of pH, ionic strength and concentration has been shown to broaden considerably the pH range in which thermoreversible gel is formed and to provide such changes in the structure of the turbid gel which make it meltable (type III). This results in the appearance of a high temperature maximum in the thermograms of such a gel. The melting of such a gel induces cooperative stratification of the system into the collapsed gel (type IV) and water.