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<Fcrit; 2,003<5,987).
The thesis has 2 main parts, namely a Literature review and an Experimental part. The Literature review gives an overview of milk, milk serum, milk serum proteins, spray drying process, protein adsorption, Pendant drop and oscillating Pendant drop method, and how the surface rheology is affecting the morphology of the spray dried powder particles. The Experimental part covers material and methods, description of experiments, results, discussion, conclusion and a summary.
The thesis consists of 58 pages, 14 figures, 11 tables, and 3 appendices.

Popular Abstract

Nowadays, the usage of dairy powders is very common and widespread. Even the milk components are separated from milk and dried to get specific product. Milk serum proteins are one example of milk compounds that can be separated from milk and marketed as a new product. Milk serum is the liquid part of the milk that does not contain fat nor caseins – another group of milk proteins. Milk serum contains specific proteins that are called milk serum proteins. They are separated directly from skim milk (milk without fat) using filtration. Obtained milk serum concentrate can thereafter be dried. The most common method for this is spray drying. Spray drying is a method where liquid is sprayed into a chamber that is filled with hot air. The small droplets dry very quickly when they come into contact with the hot air. The obtained powder is collected in a product collector. Depending on the powder composition and the drying parameters, powders can have very different properties. For instance, powders can form lumps and thereby cause several problems for consumers and producers.
Systems that mainly contain lactose, a milk sugar, and proteins, the proteins diffuse to the droplet surface during spray drying, which may cause poor wettability of the powders. The diffusion of the proteins to the droplet surface is called protein adsorption. This study focuses on this kind of system, involving milk serum proteins and lactose. The protein diffusion to the surface is connected to the surface activity of proteins, rheology of (surface tension and elasticity) of milk serum protein solutions are under investigation. Surface tension phenomenon can be seen when water droplet is formed on some surface. Water droplet is formed because the water molecules try to reduce to the surface area of the liquid surface. Proteins have to effect of lowering the surface tension and increasing the surface area. Elasticity gives the solution its ability to stretch and contract. In liquids it is not usually visible with naked eye and measurements are needed.
The aim of this master’s thesis was to evaluate the surface rheology of diluted milk serum protein systems, and observe how rheological properties are affected by diluting the system with lactose solution in various ratios. In addition, the relationship between the rheological properties of the milk serum protein-lactose system and the shape and appearance (morphology) of the spray dried particles, was evaluated. Another objective involved the evaluation of added salt solution in milk serum protein system has an effect on rheological properties of the milk serum protein solutions. This was evaluated because it was thought that adding salts (electrolytes) may affect how quickly proteins adsorb to the surface.
In the current work rheological properties of diluted milk serum protein samples and milk serum protein-lactose systems (with and without added salts) were evaluated using Pendant drop and oscillating Pendant drop method. Pendant drop method involves automatic formation of a droplet and measuring the surface tension of the sample. Oscillating Pendant drop method is similar to Pendant drop method, however, in this method the surface area of the formed drop is changed by automatic expansion and compression of the drop. Oscillating Pendant drop generates the modulus of elasticity of the sample. The modulus of elasticity shows the stiffness of the solution. After the surface rheology measurements, different milk serum protein-lactose systems were spray dried and the morphology of the particles was captured by Scanning Electron Microscope (SEM). In this method, an electron beam focuses on the sample and interacts with the atoms in the sample. The signals gained through this interaction gives information about the morphology of the sample.
The results showed that adding salt solution did not affect milk serum protein-lactose system significantly. There was certain difference between water and lactose solution when they were used as a matrix for the milk serum proteins. However, it was not possible to conclude, apart from increasing in the surface tension of the solution, how exactly the lactose affects the protein adsorption to the surface.
A certain connection between the surface tension, the surface pressure and the spray dried particle morphology was found. Samples that had the highest surface tension at 0.3 second had almost a smooth particle surface. The lower the surface tension and surface pressure were, the deeper dents and larger ridges did the spray dried particles tend to have. It was showed that the protein concentration had the main impact on the morphology of these spray dried particles.

• author: Kraft, Kadri
• supervisor: Björn Bergenståhl; Ida-Marie Andersson; Maria Glantz
• organization: Food Technology and Nutrition (M.Sc.)
• course: KLT920 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: surface pressure, surface tension, particle morphology, rheology, spray drying, milk serum proteins, whey, milk serum, milk, dairy powders, dairy products, powder, proteins, food engineering, livsmedelsteknik
• language: English
• id: 8887117
• date added to LUP: 2016-08-01 17:01:10
• date last changed: 2016-08-01 17:01:10