UC Davis

Food breakdown during gastric digestion influences gastric emptying, satiety, and nutrient bioaccessibility. However, the engineering design of foods for targeted breakdown has remained a challenge. This is in part due to a lack of data on the breakdown of solid foods during gastric digestion, particularly regarding the influence of simulated peristalsis on the breakdown of solid food particles. In addition, there is a need for standardized model, solid foods for studying food breakdown which could be used to establish a more in-depth understanding of the physical property changes experienced by solid food matrices during in vitro gastric digestion. Such knowledge on the physical property changes of solid foods during in vitro gastric digestion could help to identify the influence of food properties on the mechanisms of particle breakdown during in vitro gastric digestion that includes simulated peristalsis. However, currently available systems for studying food breakdown in the presence of simulated peristalsis include the potential for interactions between food particles as well as for food breakdown due to the direct effect of the simulated peristaltic wave. Thus, there is a need for a new in vitro digestion system that can apply simulated peristaltic waves to single particles of solid food. In this study, progress toward addressing these gaps in the literature was made by assessing the breakdown of almond particles during in vitro digestion both in the presence and in the absence of simulated peristalsis, developing and characterizing standardized, model solid foods with varying breakdown rates during in vitro gastric digestion, and developing a new peristaltic simulator for digestion studies. First, in vitro digestion of almond particles was conducted using a dynamic model (Human Gastric Simulator) and a static model (shaking water bath). This experiment was carried out to gather data on the breakdown of a solid food product during gastric digestion, and to determine whether the observed breakdown could be attributed to the peristaltic motion of the simulated stomach. Results showed that structural breakdown of particles occurred only in the Human Gastric Simulator, as evidenced by a reduction in particle size during the gastric phase of digestion (15.89 ± 0.68 mm2 to 12.19 ± 1.29 mm2, p < 0.05). Fatty acid bioaccessibility at the end of the gastric phase was greater in the Human Gastric Simulator than in the shaking water bath (6.55 ± 0.85% vs. 4.54 ± 0.36%, p < 0.01). Results indicated that the in vitro model of digestion incorporating simulated peristaltic contractions (Human Gastric Simulator) led to breakdown of almond particles during gastric digestion which increased fatty acid bioaccessibility. These results underscore the importance of simulated peristaltic contractions towards achieving realistic breakdown of solid food particles during in vitro digestion. However, due to the inhomogeneous microstructure and particle size distribution of almond particles, it was not possible to determine changes in mechanical properties of almond particles over time during digestion. This pointed to the need for standardized, model foods whose physical properties could be quantified over time during digestion and then related to particle breakdown during digestion experiments that include simulated peristalsis. To meet this research need, standardized, model solid foods were developed based on the macronutrient composition of the Standard American Diet. The model foods were based on hydrogels, which allowed for controllable particle shapes and sizes to be prepared, allowing for assessment of their physical properties using texture analysis, rheology, and fracture property testing. A strong gel version and a weak gel version were created by modulating the pH at which the model foods were produced. Another class of model foods was developed using whey protein hydrogels to serve as a simpler model system. A strong gel and weak gel version of the whey protein model foods were produced by addition of pectin. The whey protein weak gel had the lowest initial hardness of the model foods (6.2 ± 1.0 N) which was significantly (p<0.05) lower than that of the whey protein strong gel (56.2 ± 5.7 N), which had the highest hardness of the four model foods. The breakdown of model foods was assessed during in vitro gastric digestion using texture analysis, dry solids retention ratio analysis, and modeling of moisture diffusion into the model food matrix. The standard diet weak gel had the fastest softening half-time of the model foods (58 ± 5 min), which was significantly (p<0.05) lower than that of the whey protein strong gel (775 ± 82 min) which had the longest softening half-time of the model foods. Fracture properties of the model foods were assessed using wire cutting and knife cutting. Toughness, yield stress, and stiffness were significantly influenced by digestion time (p < 0.01). Rheological properties of model foods were assessed using parallel plate oscillatory rheology, and it was found that the storage modulus (G’), loss modulus (G’’), and tan δ were significantly influenced by digestion time (p < 0.01). One unexpected finding from the study of fracture properties was that the apparent toughness of the whey protein strong gel increased during the first three hours of in vitro gastric digestion but decreased after 24 hours of in vitro gastric digestion. The initial increase in toughness of this model food was attributed to a loss of brittleness due to moisture uptake, with the eventual decrease in apparent toughness due to the weakening of the food matrix by acid and enzymatic hydrolysis. This suggests that competing mechanisms operating on different time scales can lead to non-monotonic changes in food fracture properties during digestion, specifically, rapid moisture uptake can lead to increased toughness, whereas slower matrix hydrolysis eventually leads to decreased toughness. Taken together, these findings show that physical property changes in soft solid foods during in vitro gastric digestion are complex. After characterizing the changes in textural, fracture, and rheological properties of the model foods over time during in vitro gastric digestion, it was desired to relate these properties to the breakdown mechanisms experienced by the model foods during in vitro gastric digestion that included simulated peristalsis. However, existing dynamic digestion systems apply simulated peristaltic contractions to numerous food particles simultaneously, meaning that particle/particle grinding could contribute to food breakdown in addition to the direct effect of the peristaltic wave. To address the need for a system that can apply simulated peristaltic contractions to food particles one at a time, a novel peristaltic simulator was designed and constructed. The new peristaltic simulator can apply simulated peristaltic contractions one at a time to a single food particle, and also allows for up to 12 digestion modules to be used simultaneously. This >10-fold improvement in throughput relative to previous state-of-the-art systems could allow for fast screening of food materials for health-promoting properties. The novel simulator was characterized using quantitative video analysis and computational fluid dynamics (CFD) simulations, the results of which were compared to physiological data on the motility of the human stomach and intestine. This new digestion simulator could help future researchers identify the mechanisms leading to food breakdown by studying the effect of digesta rheology or peristaltic wave dynamics on food breakdown. One important characteristic of the novel peristaltic simulator is that it permits simulated peristaltic contractions of adjustable occlusion to be applied one at a time to individual food particles allowing their breakdown to be studied in more detail. The model foods developed with varying breakdown rates during static in vitro digestion were then subjected to static in vitro digestion for varying lengths of time followed by digestion using either the Human Gastric Simulator (HGS) or the novel peristaltic simulator, systems which include simulated peristalsis. The HGS was used to study the breakdown of particles of model, solid foods under physiologically representative conditions which included the potential for breakdown due to the direct effect of the simulated peristaltic wave as well as due to particle/particle grinding. The novel peristaltic simulator was used to isolate the effect of the simulated peristaltic wave without the potential for breakdown due to particle/particle grinding. Results showed that the number and sizes of particles were significantly (p < 0.05) influenced by the type of model food and in vitro digestion time. Results from this study were used to establish the hypothesized breakdown mechanisms for the standardized, model, solid foods during in vitro digestion that includes simulated peristalsis. Specifically, the standard diet strong gel broke down by erosion as well as chipping, whereas the standard diet weak gel broke down by erosion, chipping, and fragmentation. The whey protein strong gel broke down by erosion with only minor breakdown overall, while the whey protein weak gel broke down by erosion, chipping, and fragmentation. Experiments using the peristaltic simulator suggested that the mechanisms of particle breakdown of the model, solid foods were influenced by the number of simulated peristaltic contractions that were applied to the particle. For example, the standard diet strong gel broke down by erosion and chipping until ca. 30 simulated peristaltic contractions had been applied, after which the breakdown could be ascribed to erosion, chipping, and fragmentation. These results demonstrated that the mechanisms which lead to particle breakdown during in vitro digestion that includes simulated peristalsis may be dynamic in time, so that foods which initially break down only by erosion or erosion and chipping may be weakened by the progressive application of contractions, leading to breakdown at later times due to erosion, chipping, and fragmentation. Overall, the model foods, which were shown to fall into distinct classes in the Food Breakdown Classification System (FBCS), experienced differing hypothesized breakdown mechanisms which were related to their initial properties and kinetics of texture change. It was determined that that food particles with hardness < 6 N may break down by erosion, chipping, and fragmentation at the onset of simulated peristalsis but particles with hardness > 9 N may break down initially by erosion but transition to breakdown by erosion and chipping at later digestion times as the particle softens due to contact with simulated gastric fluid. Collectively, the data gathered in these studies show that particle breakdown of solid foods during in vitro gastric digestion was driven by simulated peristaltic contractions, that the physical properties of solid foods influenced their breakdown rates during in vitro digestion, and that the progressive application of simulated peristaltic contractions can lead to transitions in the breakdown mechanisms of solid food particles. These findings could help future researchers develop foods with targeted rates of particle breakdown during gastric digestion, a potential means to modulate gastric emptying rates in vivo. The system of standardized model, solid foods and novel peristaltic simulator could serve as tools to help future researchers develop foods with targeted breakdown rates for controlled bioaccessibility or satiety, for example, by linking the initial values of food physical properties and their changes during in vitro gastric digestion to their expected mechanisms of breakdown during gastric digestion.