The competitive adsorption of surfactants in model dairy emulsions : a thesis presented in partial fulfillment of the requirements for the degree of Master of Food Technology at Massey University, Palmerston North, New Zealand
Emulsion structure is an important factor effecting the properties and stability of dairy products. The formation of new surface area and the adsorption of emulsifiers in the homogenizer influences the final particle size distribution. In addition, this provides insight on what surfactants may be present on the oil droplet surface. This in turn effect what interactions might occur between droplets, the surfactants and other components in the product. Often there are multiple potential surfactants in the system and the ability to predict which will adsorb onto newly created oil droplet surface area is valuable. The objective of this study was to investigate how mixtures of dairy proteins and chemical surfactants stabilize oil droplet surfaces during emulsion formation. Characterizing the surface loading and emulsion surface areas were done in a simple oil-in-water emulsions with WPI and Tween. Initial investigations were carried out to characterize the emulsion structures formed with Tween surfactants. There were several factors limiting the minimal particle size and amount of new surface area created through high pressure homogenization stabilized by Tween. Two different types of Tweens were used to at various surfactant concentrations at 200 bar for 3 passes to determine the particle size distribution and specific surface area. There were no differences between Tween 60 and 80 emulsions. At concentrations above 3% Tween (wt%/oil) minimal changes in particle size and surface area were observed. Similarly, no significant difference was observed when oil concentration was adjusted from 15% to 40% at the same surface area to volume ratio. Further, the effect of temperature from 40°C to 70°C showed minimal differences in specific surface area and particle size. Under these conditions where there is excess surfactant present, the specific surface area created is limited by the capacity of the homogenizer to further break up particles. At high surfactant concentration 5% Tween 60 (wt%/oil) the effect of increasing homogenization pressure was linear and if higher pressure is used, additional surface area may be created. At low surfactant concentration (less than 2% Tween 60 wt%/oil), the limits of new surface area are attained at 100-200 bar (3 passes) due to the availability of surfactant where increasing pressure or number of homogenizer passes showed minimal change in specific surface area and particle size. The surface coverage of emulsion was in approximate agreement with a theoretical CMC model that measures the critical micellar concentration (CMC) which is a specific concentration for surfactants in solution above which forms micelles. The model assumes a theoretical area covered by each molecule of Tween. When Tween emulsions were created in the presence of excess whey proteins, more complex absorption behaviour occurred. At high Tween concentrations, the specific surface area created was similar to what occurred in the absence of whey proteins. At low Tween concentrations there was evidence of coalescence as bimodal particle size distributions were found, and the specific surface area was lower than what could be achieved in the same conditions in the absence of Tween. These observations were extended by measuring the protein loading, adsorbed onto the oil droplet surface. The mass of protein adsorbed per area of oil droplet decreased as Tween concentration increased, until at about 2% Tween, no whey protein adsorption was measured. In the formation of emulsions at different whey and Tween concentrations, Tween outcompetes whey onto the surface even at very low Tween concentration (0.3% (wt%/oil)). The relative size of whey is large compared to monomers of Tween however the rate of adsorption of whey, suggesting it should adsorb faster. However, Tween forms micelles, even at very low aqueous phase concentrations, and the average size of these micelles are similar or slightly larger than the whey proteins. Conformational changes are required for whey unfold and orientate in order to adsorb onto the oil surface. A Tween micelle can quickly spread on impact which exposes the hydrophobic tails and allows spreading on the surface. For these reasons Tween adsorption dominates the surface of the emulsion even at low concentrations. Only when the aqueous phase of the emulsion is depleted of Tween, does the higher concentration whey protein have a chance to adsorb and contribute to stabilizing surface area created in the homogenizer. As the concentration of Tween increases at 2.2% (wt%/oil), whey is outcompeted completely by Tween onto the surface because there at these concentrations there is enough Tween available to stabilize the surface area that can be created by the homogenizer. The findings of the work suggest how the competitive adsorption between surfactants can be explored to identify how to design emulsions with specific components stabilizing the surface of the oil phase.
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