Heat- and cold-setting gels of β-lactoglobulin solutions.: A DSC and TEM study

We extracted data from the differential scanning calorimetric signals obtained with two different solutions of beta-lactoglobulin at pH 3.5 and 7 and at a concentration which favours protein-protein interactions. The transition peaks were partially reversible only with solutions at pH 3.5. The temperatures corresponding to the first increase in heat flow, Theta(onset), and to the amplitude of maximum deviation, Theta(max), and the calorimetric enthalpy changes of heat reaction, Delta(r)H(cal) were higher at pH 3.5 than at pH 7. These differences indicated higher conformational stability at pH 3.5 than at pH 7: the heat transitions from the initial to the final conformational states could process through the reversible 'two-state model' at pH 3.5, while stable intermediate species might be created at pH 7. Isothermal treatments at Theta(onset)<Theta<Theta(max)+5 K, applied to the study of the sol-gel transition showed that solutions which denatured reversibly formed cold-setting gels, while solutions which denatured irreversibly formed heat-setting gels. Furthermore, the curve (Theta-t) which demarcated the diagrams of the sol-gel states intersected at Theta approx. equal to Theta(max), and the gel structures observed by transmission electronic microscope showed that cold-setting gels were constituted by linear aggregates ('string of beads') while heat-setting gels were constituted by very large clumped aggregates. All these results are discussed in terms of protein-protein interaction forces. (C) 1998 Elsevier Science B.V.