Role of food material properties on the mechanisms of solid food disintegration during gastric digestion : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand

Abstract

The stomach is, after the mouth, the major organ for the breakdown of foods by a complex interaction of biochemical and mechanical mechanisms driven by the diffusion of gastric juice and the peristaltic activity of the stomach. The degree of fragmentation of solid food in the stomach and consequent release of nutrients is largely dependent on the food material properties. Despite extensive research directed at the gastric digestion, the establishment of the proper relationship between the initial material properties of foods and their subsequent breakdown during gastric digestion is still far from being fully understood. To bridge the aforementioned knowledge gap, the aim of this thesis was to characterise the relationship between material properties of solid foods (composition and structure) and their disintegration behaviour in the stomach. Sweet potato (steamed and fried) and egg white gels (pH 5 and pH 9 EWGs) were used as starch and protein based-product models, respectively, to develop experimental models to characterise not only the diffusion of gastric juice into the food matrix, but also the mechanisms underlying the biochemical and mechanical degradation of the food matrix during in vitro gastric digestion. Overall results revealed that the porous network created during frying facilitated a faster gastric acid penetration into the sweet potato food matrix than occurred in the less porous steamed sweet potato. Consequently, the fried sweet potato matrix underwent a faster collapsing and quicker softening time during in vitro gastric digestion than the more compact and denser structure of steamed sweet potato. This led to the faster disintegration and subsequent release of β-carotene in the human gastric simulator from the fried sweet potato matrix. A similar effect was demonstrated with the EWG, where the loose protein network of pH 5 EWG exhibited a significantly higher rate of pepsin diffusion, softening, nutrient release and mechanical breakdown compared to the more tightened gel microstructure found in the pH 9 EWG. In conclusion, gastric disintegration and nutrient release within the solid food structures are mainly controlled by the initial food microstructure and composition. Such knowledge will help to identify key factors for the designing of health-promoting food formulations.