THE COMPARATIVE HEAT-STABILITY OF BOVINE BETA-LACTOGLOBULIN IN BUFFERAND COMPLEX MEDIA

The heat stability of beta-lactoglobulin (beta-lg) is usually describe d with reference to a concentration-dependent pseudo-rate constant k. Kinetic and thermodynamic parameters for the irreversible denaturation of beta-lg, in a whey protein mixture dissolved in Tris-HCl buffer, w ere examined over a wide temperature range 75-120 degrees C and for de grees of denaturation [(C-o - C-t)/C-o] up to about 90%. The first-ord er kinetic model best described beta-lg denaturation over the temperat ure range 75-85 degrees C, whereas the second-order model applies in t he range 90-120 degrees C. A comparison between beta-lg thermostabilit y in buffer and literature data pertaining to more complex heating med ia (whey and milk), over the range 75-120 degrees C, was carried out o n the basis of changes in activation free energy (Delta G(#)), which i tself takes into account changes in both activation enthalpy (Delta H- #) and activation entropy (Delta S-#). It was found that the thermal s tability of beta-lg in different media, and irrespective of the kineti c model assumed in the present study, can be ranked: buffer much great er than whey > milk for the temperature range 75-85 degrees C. On the contrary, at the higher temperature range, 90-120 degrees C, the ranki ng is buffer < whey < milk, when using the second-order model to descr ibe the present data. For such comparisons to be valid the initial con centration of beta-lg in various studies, as well as the reaction orde r applied, should be taken into consideration.