Controlling digestion kinetics through food microstructure design
Creators: name: Bayrak, Meltem
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340407
subject: Proteins and peptides
classificationCode: 340602
subject: Chemical thermodynamics and energetics
classificationCode: 401699
subject: Materials engineering not elsewhere classified

description: In order to design personalised food structures for a wide range of food-related diseases or improved health outcomes, the changes of the food structure during digestion in the gastrointestinal tract must be well understood. Casein, a major dairy protein, can serve as the basis for a variety of gel structures with distinct microstructures at various length scales. The structure of casein protein gels impacts their disintegration kinetics, partially due to the accessibility of gastric acid and digestive enzymes such as pepsin during gastric digestion. Understanding the changes in these structures during digestion in the gastrointestinal tract, as well as the diffusion rate of digestive enzymes within gels, are essential for designing personalised dairy food for a wide range of food-related diseases or improved health outcomes. However, to date, the structure of casein gels either during or after digestion has not been investigated over a wide time and length scale. This thesis seeks to examine the structural changes occurring during digestion for different casein gel matrices primarily using neutron scattering techniques and to link this to factors such as pepsin enzyme accessibility.

The investigation of different casein gel matrices was performed using a combination of ultra-small angle (USANS) and small-angle neutron scattering (SANS) for the first time. A novel in situ flow setup for neutron scattering studies was developed in this
thesis to monitor the casein gel matrices on the nano-to-micrometre length scale during gastric digestion in real time. D2O, a solvent commonly used in neutron scattering experiments, was used for the in vitro digestion studies to improve the contrast between the protein network and the solvent phase of the gel. The impact of D2O on both the casein gel microstructure and further digestion kinetics was investigated. An increase in protein hydrolysis was observed in the more rigid gel formed with D2O, attributed to its brittle nature that leads to the formation of smaller particles with a larger surface area, facilitating enzymatic digestion. The gastric digestive enzyme pepsin was also shown to have reduced enzyme activity in D2O; this was related to the lower solubility of pepsin and the strength of the hydrophobic interactions in D2O.

The accessibility of the interior of the gel particles to the digestive enzyme pepsin is another factor that impacts the breakdown of casein gels. Pepsin has a limited penetration depth from the surface of a casein gel. Casein gels with a weaker elasticity can,
therefore, result in smaller gel particles with a larger surface: volume ratio following a mechanical breakdown, thereby improving the accessibility of pepsin. The diffusion of pepsin within casein gels of different structures was measured using Fluorescence-recovery after Photobleaching (FRAP). At a faster time-scale, the movement of water molecules in casein gels was investigated using Quasi-elastic neutron scattering (QENS) to study the evolution of diffusive motions. A comprehensive understanding of the differences in diffusion rates of pepsin within casein gels requires a more in-depth understanding of the diffusive movements over a wide time scale, including water dynamics within casein gels.

This thesis contributes significantly to our understanding of the structural changes of casein gel matrices during gastric digestion. The modulation of food structure can be associated with the gastric emptying rate controlling nutrient absorption and subsequent physiological responses such as satiety. Our findings can improve the experimental design of digestion studies and promote the development of personalised food items.