Effect of the NaOH concentration and temperature on the dissolution mechanisms of ß-lactoglobulin gels in alkali

The dissolution mechanisms of model protein gels (made from a single protein, ß- lactoglobulin) in alkali were studied at different NaOH concentrations and temperatures. A recent study has shown that the free volume within the swollen gel is a key parameter that determines the dissolution rate (Food Hydrocolloids (2007) 21:1275). This suggests that proteins 'diffuse' in the swollen gel next to the solution before they are dissolved. We extend this hypothesis here by showing that only small oligomers (trimers and smaller), such as those formed after the cleavage of large aggregates, are likely to diffuse throughout the gel. Large aggregates (> 200 kDa) are considered to disentangle at the gel/solvent interface. Size exclusion chromatography has been used to determine the size distributions of the aggregates released from the gel matrix, and to follow the breakdown kinetics of soluble aggregates in alkaline conditions. Analysis of the dissolution process at different temperatures reveals that at low NaOH concentrations (< 0.1 M), the breakdown of large aggregates in alkali exhibits a temperature dependency different from that of the overall dissolution process. At high NaOH concentrations (> 0.3 M), where swelling is inhibited, the increase in the dissolution rate seems to be related to the destruction of cross-linked aggregates, in particular via the beta-elimination of intermolecular disulfide bonds. A previously unobserved type of dissolution behavior at 0.87 M NaOH and at high temperatures is reported.