Rheokinetic characterization of maturing collagen particle suspensions in shear flow

MARIEL L. OTTONE; MARTA B. PEIROTTI; JULIO A. DEIBER

Jornada; VI Jornadas Internacionales de Proteínas y Coloides Alimentarios JIPCA VI; 2011

UBA

In the food industry, collagen edible films are produced from precursor collagen particle suspensions (PCPS) having solid concentrations in water within the range 0.5 to 4 % w/w. Therefore the characterization of maturing PCPS in shear flow is required in order to get relevant information concerning the evolution of the suspension viscosity during processing. These suspensions were studied here to evaluate their thixotropic response in sudden imposed shear rates and shear loops. This rheological behaviour is a consequence of particle aggregations leading to particle cluster formations. In general, under static conditions, the maturation process of suspensions may be described through a basic structural parameter evolving to the network percolation point. Within this rheological framework, two asymptotic viscosities are well identified at the asymptotic limit of zero shear rates. One involves the initial solution viscosity that may be associated with the null structural parameter (the microstructure is not formed yet). The other is the percolation zero shear rate viscosity (assuming an infinite value when approaching the gel time) and corresponds to the maximum value of the structural parameter. Under flow, thixotropic theories combined with the knowledge of suspension rheology allow one to convert directly experimental data obtained as shear stress versus time for a given shear rate into the time evolution of the structural parameter. Within this framework, we proposed a rheokinetic model involving both the rates of structure breakdown and build-up processes, where the sizes of clusters are governed by the value of the shear rate at a given maturation time. Here we show that the maturing process of PCPS may be described by the normalized structural parameter evaluated with this rheokinetic model (Deiber et al., Food Hydrocolloids, 2011).     The phenomenological description is carried out in particular for rheometric responses comprised between the primitive structured suspension under static conditions (maximum value of the structural parameter and high viscosity) at a temperature above the percolation value, and the Newtonian limit response of the fully degraded suspension (null structural parameter) for a given shear rate at long shearing times. Thixotropic responses show that PCPS are complex fluids composed mainly of different cluster sizes suspended in the solvent at each shear rate step. At high times, both the equilibrium structural parameter and shear viscosity are achieved, and their values are determined by the shear rate history. Also increasing the shear rate, the equilibrium viscosity value decreases, and the suspension microstructure gets a smaller equilibrated cluster size on the average. Under the usually denominated “quenching hypothesis”, the shear viscosity of PCPS satisfies the Bingham model. In shear loops, PCPS present higher thixotropy for the low particle concentration range where a wide distribution of cluster sizes is observed. It was found that clusters were formed via the “hit and stick” mechanism yielding a rather open architecture (low fractal value) due to rather high particle attractive forces