Influence of Cation Sequestering and pH on Quiescent Thermal Association of .BETA.-Lacto-globulin NB from Fresh Cheddar Whey: An Insight into Gelation Mechanism.

Abstract

Freshly fractionated β-lactoglobulin AB (β-Lg) from Cheddar whey was dispersed at pH 3.5, 7, and 9 buffers containing 20 mM EDTA, and circulated at 25°C through a closed loop containing a 200 nm pore size membrane, to remove traces of dust and large aggregates, and an water jacketed cuvette placed within a Dynamic Light Scattering (DLS) device for real-time data acquisition. Filtered samples were step-wise heated from 25°C to 90°C with continuous data acquisition to study dynamic changes in mean aggregate diameter (MAD). Data were analyzed by cumulant method for apparent MAD and CONTIN for size distribution. Initial MAD (IMAD) of about 200 nm, reflecting the pore size of the filter used, was observed for all experiments prior to heating. Mid-range aggregate, Agg3 (100–599 nm), was ubiquitous in all distributions and Agg1 (monomer-dimer) was only seen at pH 7 and 25°C. Increased temperatures gave rise to larger aggregates (>micron) (Agg4 and 5) with concomitant disappearance of Agg3. The greatest increase in MAD was at pH 3.5 and the lowest was at pH 9. Pre-gelation (−4 s−1 for pH 3.5, 7.0, and 9.0, respectively, giving MADs that were 3.5, 2.2 and 0.8 fold compared to IMAD. Above 70°C (apparent gelation range), relative aggregate enlargement was greatest at pH 3.5 being 55, 21, and 4.2 fold of IMAD, respectively, for pH 3.5, 7, and 9 at 90°C. An apparent gel formed at pH 3.5 and a turbid gel/coagulum formed at pH 7 and none at pH 9. It is conceivable that increased association at pre-gelation temperatures is required for gelation. A mechanism has been postulated.