Probing the mechanisms limiting dissolution of whey protein gels during cleaning

The chemistry and kinetics of cleaning of whey protein gels in alkaline solutions is investigated by studying the dissolution of heat-induced β-lactoglobulin gels in sodium hydroxide. Below pH 13 dissolution is strongly influenced by the conditions under which the gel is formed. The dissolution rate decreases with extended gelation time and higher gelation temperature, and is strongly correlated to the fraction of crosslinked proteins present in the gel. Analysis of the cleavage kinetics shows that these reactions are not the main controlling step. At high dissolution pH (> 13), relatively low dissolution rates are observed, which are almost independent of the gelation conditions. The dissolution rate decreases markedly in strong ionic solutions, most probably due to the reduction in swelling caused by the screening effect of the cations. The observations suggest that the disengagement of protein clusters is a key step in the dissolution process across the whole range of pH studied, 12.2 < pH < 14; this can explain the occurrence of an optimal NaOH concentration in cleaning. The dissolution of these gels at low temperatures and in the absence of erosion is limited by the disentanglement of protein aggregates through the swollen layer, and the cleavage of the disulphide bridges. The limiting mechanisms under other dissolution conditions are also discussed.