Effect of Impeller Geometry, Physical Properties and Process Parameters on Foam Formation in a Continuous Mechanical Whipper
Linda Indrawati, Zebin Wang, and Ganesan Narsimhan. Agricultural and Biological Engineering, Purdue University, 225 S. University Street, West Lafayette, IN 47907-2093
Mechanical whipping is used extensively for foam formation in many applications. A continuous mechanical whipper with three different impeller geometries (four blade straight, six blade straight and six blade curved) was employed to make foams stabilized by sodium caseinate and whey protein. The effects of whipper speed, flow rate, viscosity, type of protein solution and temperature on foam density, power input, bubble size distribution and apparent viscosity were investigated. The whipper speed was varied in the range of 2500 to 15000 rpm, the inlet flow rate of protein solution was varied in the range of 5 to 15 ml/s, the viscosity of protein solution was varied by the addition of xanthan gum in the concentration range of 0 to 0.1 weight % and temperature was varied in the range of 25 to 80 C. The power input was found to be highest for the lowest liquid flow rate, highest whipper speed and was insensitive to xanthan gum concentrations and the type of protein. The relative foam density was lower at higher whipper speeds because of more incorporation of air, lower at higher viscosity due to slower liquid drainage during foam formation and was lower at smaller liquid flow rate. Six blade curved impeller operated in parallel direction to the vanes was found to produce foams with smallest bubble size. Six curved impeller operated in opposite direction and six straight blade produced more foam than the other two impellers. Sodium caseinate produced more foams than whey protein. Higher temperature resulted in better foam stability for foams formed with whey protein possibly due to partial denaturation. On the other hand, higher temperature had detrimental effect on foam stability for foams formed with sodium caseinate. Based on breakage and coalescence rates of bubbles in the whipping chamber, a simple model for the prediction of average bubble size was proposed. The amount of air incorporation for different whipper speeds, liquid flow rates and types of whippers was measured and fitted to a simple expression based on mechanical energy balance with impeller efficiency as a parameter. The fitted impeller efficiency was found to be a strong function of whipper speed and liquid flow rate. The efficiency decreased at higher whipper speeds and lower liquid flow rates. A correlation for bubble size was developed in terms of whipper speed and flow rate based on the above models which was shown to describe the experimental data reasonably well for different whippers.