SURFACE RHEOLOGY OF MILK SERUM PROTEIN-LACTOSE SYSTEM - IMPACT ON PARTICLE MORPHOLOGY AFTER SPRAY DRYING

Abstract: Nowadays dairy powders are common products and their market share is increasing. Dairy powders are produced for various reasons, such as to reduce the transportation cost, handle seasonal variation in milk or to valorise the by-products of the production. However, powder production and consumption can entail several problems, such as inadequate solubility or the formation of lumps during wetting. The wetting properties of the powders are to a large extent dependent on the degree of agglomeration, morphology and surface composition of the powder particles. In addition, there is a hypothesis that the surface morphology is influenced by the surface rheology. In this thesis, the surface rheology of diluted milk serum protein and milk serum protein-lactose system, has been evaluated. Furthermore, the effect of electrolytes in the system, the connection between the surface rheology of the milk serum protein-lactose system and the morphology of spray dried particles, was evaluated. It was found that that the surface rheology of both diluted milk serum protein and milk serum protein-lactose system is dependent on the protein concentration. The higher the protein concentration is, the lower the surface tension and higher surface pressure is. The results regarding the modulus of elasticity were inconclusive. In the milk serum proteins system, the modulus of elasticity was higher for the samples with a higher protein concentration. However, in the milk serum protein-lactose system, the modulus of elasticity was higher for the samples with the lowest protein concentrations, and showed an unexpected pattern. The results obtained for the milk serum protein-lactose system showed a correlation between the surface tension at 0.3 seconds and the spray dried particle morphology. Particles formed from a liquid with a surface tension (78.9 mN/m) near to surface tension of 17.5% lactose solution had almost a smooth surface. If the surface tension decreased (68.1 mN/m), the particles got more dented and ridges appeared on the surface of the particle. Samples with milk serum protein/lactose ratio of 40/60 and 60/40 had the lowest surface tension (respectively 57.9 mN/m and 57.4 mN/m), and the spray dried powder particles were covered with large dents and ridges. Since the decrease in surface tension is caused by proteins, which are the most surface active compounds in the system, the connection between protein concentration and spray dried particle morphology was established. In addition, it was found that electrolytes in used amount did not had significant effect on the milk serum protein-lactose system (F