Competitive Protein Adsorption during Spray-drying
Abstract: The aim of this thesis was to study competitive adsorption of milk proteins during spray-drying. For this purpose, a new surface sensitive method was developed which measures the quantitative amount of specific proteins at the powder surface. The new method is based on steady-state fluorescence measurement of pyrene labelled proteins with oxygen gas phase quenching at the powder surface. The milk proteins studied were bovine serum albumin (BSA), beta-lactoglobulin (beta-Lg), beta-casein and micellar beta-casein. Beta-casein aggregated into micelle-like structures of size 20-30 nm, were assumed to resemble native casein submicelles. The proteins differ in their surface affinity, size and structure, and were therefore expected to show different adsorption properties to the droplet air-water interface during the spray-drying process. Protein adsorption was studied using dextran or lactose as the carbohydrate matrix. The influence of pH on the solutions prior to spray-drying was also studied for whey proteins, sodium caseinate and calcium caseinate using the ESCA-technique. Furthermore, the surface coverage of proteins was correlated with the surface tension of the dried emulsions to elucidate the relationship between the surface tension at the air-water interface and the surface composition of the dried emulsions. Results showed that competitive protein adsorption occurs between the proteins during spray-drying. It was seen that the surface affinity and the adsorption dynamics (protein aggregate complexity and protein flexibility) is important for protein adsorption during spray-drying, particularly at high concentrations. Beta-casein was found to be the most competitive protein, and dominated at the powder surface when spray-dried together with beta-Lg. In an aggregated form the competitiveness of micellar beta-casein was weaker compared to monomeric beta-casein. When micellar beta-casein was spray-dried together with beta-Lg, beta-Lg dominated at a low bulk concentration whereas at higher concentrations the presence of micellar beta-casein at the surface was comparable to beta-Lg. The whey proteins beta-Lg and BSA were found the be equal in their competitiveness during spray-drying. The change of pH in the solutions was shown to not affect the whey proteins with respect to surface coverage. However, both calcium caseinate and sodium caseinate showed an increased surface coverage with an increasing pH. The expected correlation between surface coverage and surface tension, ( i.e. the most surface active protein will give the largest surface coverage) was found for whey proteins and sodium caseinate, whereas for calcium caseinate this correlation was weaker.
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