(120b) Effect of Agitation Intensity during the Formation of Whey Protein Aggregates on Breakage Rate under Subsequent Turbulent Flow Transportation

It is well known that both the flow field characteristics and the flow intensity experienced during the formation of aggregates and flocs have a major influence on ultimate particle size. Some authors have suggested that varying agitation conditions at this stage can modify the microstructure and perhaps the strength of particles formed; this suggests that aggregates formed under different agitation conditions should have different breakage rates when they are subsequently subjected to the same (comparatively higher) levels of turbulence, for example as they are transported via pipeline during processing. The experimental results of the present study support this last idea. In this work, whey protein aggregates were formed at 60°C in a standard configuration stirred tank reactor by acid addition (2M HCl) until the isoelectric point of the solution (pH 4.15) is reached. The agitation speed during the acid addition (aggregation stage) was varied to form particles with different size and perhaps different microstructure. The tested agitation speeds were: 350, 550, 750 and 950 rpm, which resulted in spatial average shear rates of 357, 702, 1118 and 1591 s^-1 respectively. The dispersion was then diluted (from an original solids content of 10% by weight) to less than 0.1% of solid content in order to negate any particle-particle interactions that could cause significant breakage. The diluted dispersion was propelled trough a 100mm straight pipe (ID 0.736mm) using compressed air at different pressures to get flow velocities between 5.6 to 16.8 ms^-1, corresponding to turbulent flow. The dispersions were recycled 50 times to study the particle size reduction along with exposure time. Dispersion samples were taken after each recycle and the particle size was measured using laser diffraction technique. The experimental results show that particle size is a function of the agitation intensity during the aggregation period, e.g., the mean particle size for the agitation corresponding to 357 s^-1 is about 23µm while for 1118 s^-1 is about 8µm. The particle breakage in the pipeline is different for each particle formed at a particular agitation, suggesting that the agitation during the aggregation time modifies the formed aggregates ″microstructure″. Particles formed at lower agitation have a higher breakage rate especially during the first recycles than those formed at higher agitation, although the apparent ″stable″ size (particle diameter after long exposure time when very little size reduction is observed) is bigger for particles formed at lower agitation. As breakage occurs the aggregates lose their fractal configuration and a strong core is revealed (which can withstand the hydrodynamic disruptive forces), it is suggested that particles with bigger core were formed at low agitation. The breakage rate was calculated using the Weibull model and the results show that the rate is a function not only of the mass flow rate in the pipe but also of the agitation during the aggregates formation. Finally, the particle size after each recycle through the pipeline was predicted using the model proposed by Zumaeta et al (2005). The predictions suggest that it is necessary to modify the Zumaeta model by introducing a new term (φ) that considers the modification in the microstructure and strength of the formed aggregates at different agitation speeds. The model suggested is: Δd= φ*k*Δt*(ε_i -ε_th)/(ε_th)ⁿ and it relates the particle diameter reduction (Δd) to the exposure time (Δt) to a local turbulent eddy dissipation rate (ε_i), the maximum dissipation rate (ε_th) that a particle with a given size can withstand before any breakage occurs and the agitation intensity during the protein precipitation (φ).