Abstract:
Milk powder deposition is an area of importance to the dairy industry as it affects the efficiency of the spray drying process. Deposition is related to the stickiness of the powder amongst other factors. Stickiness is characterised by the inter-particulate and particle-surface forces of attraction, which are defined as cohesion and adhesion respectively. Initially a fundamental comparison of adhesion bond strengths were performed with particular reference to dairy powders. Van der Waals force and the liquid bridge force were of particular interest in this study as these mechanisms were considered more likely to affect dry powder or semi-dry particle deposition in the drying process. This study investigated the adhesion of whole milk powder to a polished stainless steel surface used an established centrifuge technique. Optical microscope and image acquisition techniques were used to acquire data from the centrifuge experiments. Powder composition and size have an effect on adhesion forces and a preliminary investigation into increasing powder temperature from 40°C to 60°C showed increased adhesion between these temperatures. Further experiments showed adhesion appeared to increase considerably from 30 to 45°C however beyond approximately 60°C the adhesion of the powder appeared to decrease slightly. This increase may have been due to liquefaction of constituent fat causing liquid bridge formation. The subsequent reduction in adhesion may have been due to the reduced viscosity of that liquid fat. Increasing consolidation force was found to increase the adherence of whole milk powder to stainless steel especially at very high consolidation velocities. Consolidation velocity was increased from 500 to 10000 rpm and a great difference was found between the detachment particle size distributions which indicated higher adhesion at the higher consolidation velocity. This was probably due to plastic deformation creating a larger contact area which subsequently increases the adhesion force. An unconfined yield test was used to measure the cohesion of dairy powders to determine the influence of powder composition, temperature, moisture content and particle size. As an index of cohesion, values of unconfined yield stress were obtained for whole milk powder (WMP) and skim milk powder (SMP). Milk powder cohesion (using the unconfined yield testing method) was found to increase markedly in the 6-7 wt % moisture content range, which corresponded well with previous research. However very dry powder did not show a higher cohesion as shown in this earlier study. The experimental results were modelled with accepted fundamental expressions and the model was found to describe the experimental results of shear stress when a critical humidity for liquid bridge formation was 70 % RH. The magnitude of the shear stress was underestimated by the model by a factor of approximately 3, this may be due to the agglomeration of the powder. This agglomeration could potentially increase contact points and consequently the magnitude of the bonding forces. Dry WMP was found to be more cohesive than dry SMP with increasing temperature, which indicates the influence of fat in the cohesive mechanism in WMP. Reducing the particle size increased the cohesion of dairy powders. The effect of temperature was investigated by incubating powder for 4 hours to ensure all the powder was heated. A marked transition in the cohesion was noted at certain temperature range and was highlighted by the change in transition temperature between two WMPs with different moisture contents. The effect of humidity on milk powder deposition on to a stainless steel plate was investigated experimentally and the deposition rate was found to reduce markedly when the humidity exceeded 60 % RH. This gives further indication of the changing inter particulate interaction at higher humidity.