The objective of this study was to assess the impact of using heat-denatured whey:buttermilk protein aggregate in acid-set type yogurt production. Whey and buttermilk (25:75) protein concentrate was adjusted to pH 4.6, heated at 90 °C for 5 min, homogenized and freeze-dried. Set-type yogurts were prepared from skim milk standardized to 15% (w/v) total solids and 4.2% (w/v) protein using different levels of powdered skim milk or freeze-dried protein aggregate. The use of the protein aggregate significantly modified yogurt texture, but did not affect the water-holding capacity of the gel. Confocal laser-scanning microscope images showed the presence of large particles in milk enriched with protein aggregate, which directly affected the homogeneity of the clusters within the protein matrix. Thiol groups were freed during heating of the protein aggregate suspended in water, suggesting that the aggregates could interact with milk proteins during heating.
Proteins are widely used as emulsifiers in food formulations. However, emulsifying properties of proteins are weak at pH values close to their isoelectric point resulting in destabilization. Protein-polysaccharide interactions have been proposed to improve the emulsification behaviour of proteins in such conditions. In this work, two different polysaccharides (pectin and gum Arabic) with a range of surface charges were chosen to investigate their interactions with pea proteins. The initial aim was to investigate the effect of heat treatment on the complexation of pea protein isolate (PPI) and the polysaccharide with the ultimate purpose of using them as effective emulsifiers at various pH values for beverage application. The emulsions were prepared, and the emulsification ability was determined with the selected protein-polysaccharide complexes at both basic (pH 8.0) and isoelectric pH (pH 4.5) conditions. Turbidity graphs of gum Arabic-PPI and or pectin-PPI complexes at 1:1 mixing ratio revealed an increase in the pH range of the soluble complexes upon heat treatment of the mixture to 75ºC. The soluble complexes of the protein and polysaccharide were able to stabilize oil-in-water beverage emulsions at the isoelectric pH of the protein. The stabilization effect of soluble pectin-PPI complexes was better than gum Arabic-PPI complexes at pH 4.5. At pH 8, although droplet sizes were similar, pectin-PPI complexes caused depletion flocculation leading to a higher accelerated creaming velocity of the emulsion than the gum Arabic-PPI complexes. The emulsions stabilized by pectin-PPI complexes at pH 4.5 had the highest emulsion stability in terms of lower instability index, lower accelerated creaming velocity and the lowest droplet diameter than all other emulsions. The findings of this study will provide beneficial information on the effect of processing conditions on biopolymer interactions and the emulsification ability of protein-polysaccharide complexes for the application in beverage emulsions.
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