Activated-rate theory: Effect of protein inhibition and the temperature dependence of crystallization kinetics for lactose-protein mixtures

Previous work to develop an Activated-Rate Theory for lactose has been extended to assess the solid-phase crystallization process for lactose-protein mixtures. In a work by Wang et al. (2010), mixtures of pure lactose and casein in two different weight proportions of 99.5:0.5 and 98:2, as well as pure lactose, were spray dried using a Buchi B-290 laboratory-scale spray dryer. The spray-dried samples were analyzed using Water-Induced Crystallization (WIC) at a controlled temperature and a relative humidity of 75.3%. The temperature dependence of the crystallization kinetics for lactose was analyzed using Water-Induced Crystallization with the Activated-Rate Theory at three different temperatures of 15 °C, 25 °C and 40 °C. Higher moisture content peaks of 13%-14% were found for the different proportions of lactose-casein mixture as compared with pure lactose, which exhibited a peak of 9% (moisture content). The presence of protein with lactose increased the onset time for nucleation and formation of the activated complex by twice its original value for pure lactose particles. It was noted that a higher concentration of casein in the lactose-casein mixture increased the moisture content peak, showing greater protein inhibition and a higher energy barrier to be overcome for the formation of the activated complex. It has also been found that the variation with the moisture content in enthalpy, entropy and free energy of lactose-protein mixtures fully overlaps with that of pure lactose, suggesting that the inhibiting presence of protein only affect the nucleation stage and not the crystal-growth stage of solid-phase crystallization.

► Protein increased onset time for nucleation and formation of activated complex. ► Protein inhibition only affects the nucleation and not the crystal-growth stage. ► Crystallization kinetics is dependent on both temperature and moisture content. ► Crystallization-rate constant varies with temperature as well as moisture content.