Significance of powder breakdown during in-plant transport at industrial milk powder plants
Instant whole milk powder (IWMP) is designed to rapidly dissolve in water, which depends on the particle size distribution (PSD) and agglomeration. The warm and delicate milk powder exiting the dryer is transported via either pneumatic conveying or bucket elevators to packing. The gentleness of this powder transport process is important for IWMP, as it can break down the agglomerates, generating excess fines, which leads to poor dissolution properties. This work looked at the breakdown of milk powder at two different, geographically separate, industrial IWMP plants, using the Malvern Mastersizer, a laboratory laser diffraction instrument, and sieving, to evaluate the importance of breakdown on the final product properties given different conveying methods. It was found that the method of measurement affected the results, with sieves showing a larger powder size reduction during transport as compared with the Mastersizer. PSDs with a larger average size at the start of powder transport showed more breakdown, with a greater decrease in the average particle size. However, the larger decrease was not enough to compensate for the initially larger average particle size, and powder that started out with larger agglomerates at the fluidised beds still had a larger average particle size at packing. The Mastersizer appeared to break the large agglomerates during measurement, especially with powder that had not been through the entire transport line, thus masking the extent of the size reduction, however this could only occur to weaker agglomerates. Thus in order to produce IWMP with the desired functionalities, the focus should be on improving agglomeration as oppose to reducing transport breakdown to achieve the desired particle size distribution.