Behaviour of complex emulsion systems during in-vitro gastric digestion : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Manawatū, Palmerston North, New Zealand

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Understanding the relationship between food structures and their digestion is important in understanding the association between diet and health. However, this becomes difficult as food structures keep varying and evolving through different stages of manufacture, storage, oral processing, and digestion. Mixed biopolymer protein-polysaccharide emulsion gels are simplified models for multicomponent structures present in real food systems. The pH and ionic strength of these systems play an important role in determining the interactions between proteins and anionic polysaccharides. In the present study, heat-set whey protein emulsion gels (d ₄,₃ ~ 0.5 µm) with and without low methoxyl pectin (LMP) were formed at acidic (pH 4) and neutral pH (pH 7) favouring associative or segregative interactions. Additionally, the impact of low and high ionic strength (achieved by 10 mM and 100 mM NaCl addition) on emulsion gel structure was studied. The resultant gels were called CO and PE gels where CO refer to control gels and PE refer to gels with added low methoxyl pectin (LMP). These systems were characterized in terms of rheology, microstructure, and fracture properties. The breakdown kinetics of these gels were assessed using dynamic in vitro gastric digestion model (Human Gastric Simulator, HGS) and the physicochemical characteristics of the gastric digesta were determined. For gels made at pH 7, LMP addition led to a decrease in the gelation temperature of CO emulsions, and this effect was more pronounced at low ionic strength. For emulsion gels formed at low ionic strength, addition of LMP caused the gels to become stronger and less elastic whereas addition of LMP under conditions to emulsion gels formed high ionic strength caused the gels to become weaker and less brittle in comparison to CO gels. Results from in vitro gastric digestion of CO and PE gels formed at pH 7 showed that emulsion gels made at low ionic strength broke down at a faster rate as compared to gels made at high ionic strength. The stability of the emulsion droplets within the gel during gastric digestion was also dependent on the stability of the microstructure. For emulsion gels made at low ionic strength, CO gels were least stable to gastric digestion compared to PE gels. For gels made at high ionic strength the extent of this was discernibly less for PE gels as compared to CO gels. This indicates that pectin addition along with ionic strength played a significant role in the stability of oil droplets during gastric digestion. The extent of gel disintegration and oil released was found to be highly positively correlated. Heat-set whey protein emulsion gels formed at acidic pH (pH 4) with and without low methoxyl pectin (LMP) gels appeared to be particulate in nature irrespective of ionic strength. LMP addition led to an earlier onset of gelation for PE emulsions as compared to CO emulsions. Increase in ionic strength also led to a decrease in gelation temperature for both CO and PE emulsions. Addition of LMP at low ionic strength, caused the gels to become stronger and more elastic whereas under conditions of high ionic strength, the emulsion gels were stronger but also more brittle. Results from in vitro gastric digestion showed significant differences (P < 0.05) in the solid emptying rates, pectin release rates and the extent of lipid release. PE gels made at high ionic strength disintegrated to a lesser extent than PE gels made at low ionic strength. Similarly, CO gels formed at high ionic strength disintegrated to a higher extent as compared to gels formed at low ionic strength. Despite these differences, the oil droplets remained stable during heat-induced gelation, physical shear/ionic changes during simulated oral processing and gastric digestion. A linear positive correlation was found between the rate of the solid emptying from the stomach and the rate of lipid release. Overall, this research provided new insights on the role of ionic strength and pH in structure formation of protein-polysaccharide emulsion gels. The study also demonstrated how the dynamics of these structural transformations affects in vitro gastric digestion. The outcome of this study has the potential to fill the vast knowledge gap of understanding how food structure affects the disintegration and digestion kinetics of complex multicomponent food matrices.
Figure 2-4 is reproduced with permission.