Effects of structural rearrangements on the rheology of rennet-induced casein particle gels

Post-processing of concentrated fermented milk: influence of temperature and holding time on the formation of particle clusters

“…In addition, especially the larger particle clusters (∼100 μm) seemed to be composed of several smaller particle clusters (10-20 μm), which in turn were composed of the building blocks (1-5 μm), supporting the assumption that smaller particles are the subelements of larger particle clusters. This is in accordance with Mellema et al (2002) who stated that building blocks consisting of fused casein micelles aggregate to particle clusters. Developing this concept further for our microgel suspensions, we propose that after stirring microgel particles (particle clusters) are generated, which potentially may aggregate during post-processing.…”

“…1). The latter mentioned growth behavior of the particles was also observed for supersaturated solid solutions (Lifshitz and Slyozov 1961), hardly soluble precipitates (Wagner 1961), the aggregation of colloidal particles (Smoluchowski 1917), and for rennet-induced casein particle gels (Mellema et al 2002). Lifshitz and Slyozov (1961) postulated a two-stage mechanism for the particle growth: the formation of nuclei due to concentration fluctuations leading to a minor degree of supersaturation, followed by diffusion-driven particle growth.…”

“…6). Milk gels are particle gels (Horne 1999) that show a fractal gel structure (Horne 1999;Mellema et al 2002). This involves that the fractal clusters formed during undisturbed aggregation are self-similar and, thus, resemble each other on different length scales (Mellema et al 2002).…”

“…A predominantly diffusion-controlled process was at least proposed for higher dilutions. In the network consisting of aggregated particle clusters, the protein bonds are dynamic (van Vliet et al 1991), which could lead to rearrangement of particles and clusters on all length scales resulting in particle fusion and an increase of the size of the building blocks (Mellema et al 2002). Higher temperatures may enhance both extensive particle rearrangement during gel formation and the formation of dense clusters of aggregated particles (Lucey et al 1997b).…”

“…These findings are in agreement with the acidinduced aggregation of casein micelles at different temperatures (Haque et al 2001;Lee and Lucey 2004;Lucey et al 1997b;Vétier et al 1997), and the temperatureinduced aggregation of casein submicelles (20-80°C; Panouillé et al 2004) or the growth kinetics of nanocrystals (Lee et al 2006). Thus, temperature plays an important role in the rearrangement of particles which was proposed to be related to the increased mobility due to lower bond energy (Mellema et al 2002). This, in turn, should increase the collision frequency between particles and, consequently, the number of successful collisions leading to extensive particle rearrangement with increasing temperature (Lucey et al 1997b).…”