Protein-polysaccharide conjugates as food emulsifiers and steric stabilizers

Protein-based emulsifiers and stabilizers are important in food colloids. The potential of protein-polysaccharide conjugates prepared through the Maillard reaction as food emulsifiers and stabilizers has been well established and reported in the literature. In this work, following a review of previous studies, a preliminary investigation on the conjugates between whey protein isolate (WPI) and maltodextrins (MD) was conducted. The conjugates were prepared through two methods: dry-heating approach and wet-heating method, using the Spinning Disc Reactor (SDR), and their performance compared. The formation of these conjugates was confirmed by the spectrophotometric technique. Furthermore, the physicochemical properties of conjugates such as solubility, emulsifying activity and stability in oil-in-water system were also studied. The results show that WPI-MD conjugates prepared by dry-heating method exhibit enhanced solubility and emulsifying properties compared to native WPI. This improvement is most likely due to the enhanced steric stabilisation provided by the hydrophilic polysaccharide moiety. \nIn the study of competitive adsorption between WPI-MD and unmodified WPI on oil-water interface (Chapter 3), both theoretical calculations from Self-Consistent-Field (SCF) model and results from surface pressures suggest that WPI-MD conjugate can adsorb onto the oil-water interface in the presence of unmodified WPI and not be easily displaced by unmodified WPI. \nThe nature of polysaccharides can significantly influence the stabilizing properties of WPI-MD conjugates prepared via Maillard reactions. Experimental results from Chapter 4 suggest that longer polysaccharides attached to proteins have stronger steric stability than shorter ones in O/W emulsions. Furthermore, when lactose is present in WPI and MD mixtures as an impurity before dry-heat treatment, the stabilizing properties of Maillard-type products are not significantly affected even when the molar ratio between MD and lactose at 1:10. This interesting finding shows a potential to lower the cost of preparing whey protein based conjugates in large scale by using less pure whey protein from cheese manufacturing. \nThe major obstacles for large-scale manufacturing of protein-polysaccharide conjugates are long preparation time (from a couple of days to a few weeks) and dry processing conditions for Maillard reactions. In this PhD project, a novel processing method to prepare protein-polysaccharide conjugates was explored by using Spinning Disc Reactor (SDR). This method can successfully combine WPI and MD in less than 20 mins under controlled processing conditions in an aqueous medium. The SDR-processed conjugates have similar stabilizing properties as the ones prepared through traditional dry-heat treatment. The details can be found in Chapter 5. \nProtein-polysaccharide conjugates have a promising future in food applications as new emulsifiers and stabilizers based on these studies. \n