Coupled bulk and surface mechanisms lead to whey protein deposits under shear

Understanding the mechanisms behind fouling deposit formation remains an unsolved challenge for the dairy industry. While the effect of temperature is well-established, few studies have explored the impact of shear rate, a key parameter in dairy processing. Therefore, this study investigates the growth of surficial deposits and bulk aggregates originating from whey protein solutions under shear. Whey protein solutions with different concentrations (5-12 wt%) were sheared at fixed rates (0-175 s^-1), using a cone-and-plate rheometer, at 65 °C to minimize thermal denaturation. Results show that the area covered by the deposits increases by up to fivefold under shear, particularly in concentrated conditions, compared with that observed at rest. This is accompanied by the complexification of deposit morphology, from sparse globular clusters to branches structures interconnecting into gels at 12 wt%. In contrast, solution dynamics are mostly affected by protein concentration rather than shear, resulting in the formation of submicronic aggregates. Two interpretive hypotheses are discussed: i) heat favors whey protein denaturation and the formation of aggregates increasingly colliding with the surface under shear and ii) active surface deposits, formed by these collisions, interconnect with flowing aggregates, particularly at high protein concentrations. The examination of surface and bulk dynamics presented in this study provides deeper insights into fouling mechanisms. They offer potential applications in other areas where protein denaturation/aggregation plays a crucial role.