A sedimentation test to measure heat stability of milk protein beverages

A small-scale oil bath immersion-heating system using sealed stainless-steel process tubes was developed that has the flexibility to evaluate a wide range of holding temperatures and times. After heat treatment and rapid cooling, the liquid product was removed from each stainless-steel tube and a centrifugal protein sedimentation test and laser light scattering particle size analysis were done to determine differences among milk protein beverages in heat stability and casein micelle aggregation before and after thermal processing. The sedimentation and particle size analysis method was able to determine differences in protein sedimentation and casein micelle aggregation of milk protein concentrate (MPC) solutions with 85% protein on a DM basis (MPC85) that were rehydrated at 4°C overnight and heated to 55°C and smoothed by homogenization or not smoothed. The cold rehydrated homogenized MPC85 solutions had a casein micelle size particle distribution similar to skim milk. Smoothing reduced particle size before heat treatment and reduced heat-induced protein aggregation versus no smoothing. The oil bath method was applied to cold rehydrated and smoothed MPC85 high-protein beverages over a range of temperatures typical of retorting and UHT-direct steam injection processing with and without added dipotassium phosphate (DKP). The oil bath heat stability method was able to detect differences in sedimentation and casein micelle aggregation with increasing temperature and the large effect of added DKP in preventing protein sedimentation and casein micelle aggregation. Eight different lots of rehydrated commercial MPC85 powders were rehydrated and smoothed; the method was able to identify 2 out of 8 MPC85 powders that had lower heat stability and more casein micelle aggregation than the others. The heat stability method could be a practical tool to screen heat stability of small volumes of different protein beverage formulations and help avoid pilot plant or preliminary production runs on products that will not be heat stable.