Thermal gelation of beta-lactoglobulin AB purified from cheddar whey .1. Effect of pH on association as observed by dynamic light scattering

Thermal association of beta-lactoglobulin AB (beta-Lg) purified from Cheddar whey, a major source of commercial whey ingredients, was studied by dynamic light scattering(DLS). The objective was to observe the effect of the process and/or possible micromolecules in this beta-Lg source of commercial relevance. Protein solution (8%, w/v) was heated (25-90 degrees C) at pH 3.5, 7, and 9.0. DLS data were analyzed according to the method of Cumulants for apparent mean diameter and by the Contin method for percentile size distribution. Data indicated gradual change in mean size that started at 35 degrees C, much below the reported denaturation temperature of 70 degrees C. Moreover, the percentile distribution of various micrometer- and submicrometer-sized aggregates differed at the different pH values studied, conceivably indicating conformational alteration limiting spatial freedom for intermolecular association. Monomeric/dimeric form (1-9 nm range) was seen only at pH 3.5 at temperatures below 65 degrees C. The ubiquitous aggregate size range was 100-599 nm (Agg3), and greatest changes in aggregate size and distribution were detected around 70 degrees C. Micrometer-sized (>1000 nm) aggregates were formed at this temperature and above, concomitant with disappearance of Agg3. On the basis of polydispersity data and rate kinetics dependent on the disappearance of Agg3, the association tendency between 25 and 70 degrees C was in the order pH 7.0 > pH 3.5 > pH 9.0.