Behaviour of emulsion gels in the human mouth and simulated gastrointestinal tract : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Manawatu, New Zealand

Food structure greatly impacts the digestion process of food in the human body. Food structure design is a potential strategy to modulate food digestion and develop foods with controlled nutrient digestion and release. This research aimed to understand the dynamic processes of digestion of whey protein emulsion gels from the mouth to the intestine. Therefore, a series of heat-set whey protein emulsion gels, the structure of which was designed by varying NaCl concentration (10, 25, 100 and 200 mM) and oil droplet size (1, 6 and 12 µm), were formed. Mechanical properties in linear viscoelastic region, large deformation and fracture region and microstructure of gels were evaluated. In vivo oral processing and in vitro oral-to-gastrointestinal digestion of whey protein emulsion gels with different structures were investigated with a focus on the effect of gel structure on the gel disintegration and lipid digestion. Results showed that the gel strength increased with increasing NaCl concentration. At the micro-scale, the gel structure became from homogenous to porous with increasing NaCl concentration from 10 to 200 mM. The fragmentation degree of whey protein emulsion gels in the mouth showed a positive linear relationship with the gel strength (i.e. a higher gel strength, the greater gel fragmentation degree). During oral processing, the small oil droplets (~ 0.45 µm) incorporated in the protein matrix were stable without oil droplet release. During gastric digestion, the bolus of the gel containing 10 mM NaCl (soft gel) disintegrated much faster than that of the gel containing 200 mM NaCl (hard gel) in the human gastric simulator (HGS). The disintegration of the soft gel in the HGS was caused by both the abrasion and fragmentation while the abrasion was the predominant mechanism of the disintegration of the hard gel. The larger particle size of the soft gel bolus slowed down the emptying of gels from the HGS. With continued digestion, the emptying of both gels from the HGS was accelerated by gel disintegration. The gel structure greatly influenced the gel disintegration at the micro-scale. The soft gel particles were gradually disrupted into individual oil droplets, with the protein matrix dissolving after gastric digestion for 4 hours while the hard gel particles still retained the oil droplets inside the protein matrix. The colloidal structure of emptied gastric digesta, which generated from original gel structure, still significantly impacted the digestion of whey protein emulsion gels in an in vitro intestinal model. In general, the colloidal structure of the emptied gastric digesta of the hard gel hindered the breakdown of gel particles and hydrolysis of oil droplets more effectively than that of the soft gel. The remaining structure within the hard gel particles limited the free motion of oil droplets, which led to a lower degree of coalescence and breakup of oil droplets. Interestingly, coalescence appeared to occur between neighboring oil droplets inside the emptied gastric digesta of the hard gel during intestinal digestion. The structure of the gels containing 100 mM NaCl became from aggregated particle to emulsion-filled with increasing oil droplet size from 1 to 12 µm. The gel strength also decreased with the increase of droplet size. For the gels containing large oil droplets (6 and 12 µm), oil droplets were released from the protein matrix along with some coalescence during oral processing. During gastric digestion, a higher degree of coalescence of oil droplets occurred and coalesced oil droplets creamed to form a top oil layer. This slowed down the emptying of gels from the HGS. For the gels containing 1 µm oil droplets, most oil droplets still retained in the protein matrix during oral and gastric digestion with minimal instability of oil droplets. In addition, increasing interactions between oil droplets and protein matrix by decreasing oil droplet size hindered the protein hydrolysis. Overall, this research provides an understanding of the way in which food disintegrates in the human body and highlights the role of food structure on the digestion of food in the human body. These findings could assist in designing the functional new foods that deliver health benefits (e.g. lipid regulation, encapsulation and release of nutrients) and improving human health related to food digestion (e.g. dysphagia, dyspepsia)