Mass Transport and Structural Changes of Sweet Potatoes during in Vitro Gastric Digestion as Influenced by Cooking Metho

Knowledge of mass transport processes during digestion is important to understand the mechanisms of food digestion, but there is limited knowledge of this subject in the literature. Acid and enzymes penetrating into the ingested food from gastrointestinal secretions may cause structural modifications in the food matrix. The objectives of this study were to estimate the effective diffusivity of acid and water through sweet potatoes of varying cooking treatments, determine the influence of cooking on the mass transport processes, and elucidate the structural changes that occur as a result of these mass transport processes during simulated gastric digestion of sweet potatoes.;To achieve this goal, two studies were conducted using sweet potatoes as a test food. In the first study, sweet potatoes were cooked (boiled, steamed, microwave steamed, and fried), and in the second study, sweet potatoes were boiled and steamed for different times resulting in mild or severe heat treatments. In both studies, sweet potatoes were cut into cubes, cooked, and mixed with simulated saliva, simulated gastric juice was added (100 mL), and samples were incubated in a shaking water bath (37°C at 120 rpm) for up to 240 min. Sweet potato cubes were removed from simulated gastric fluids at different time points to be analyzed. Acidity and moisture content were measured, and effective diffusivity was modeled using the experimental acidity and moisture content values. Structural changes were assessed by monitoring sample texture and conducting light microscopy. Hardness was measured by single compression of the cubes using a Texture Analyzer. Microstructure was examined via light microscopy before and after 240 min of digestion.;In both studies it was found that the effective diffusivity of acid into sweet potato matrices was different than water effective diffusivity, and that mass transport was significantly influenced by both cooking method and cooking severity. In study one, fried sweet potatoes had a higher acid effective diffusivity (1.6x10-9 m2/s) compared to moisture effective diffusivity (0.2x10-9 m 2/s). On the other hand, steamed sweet potatoes had a higher moisture effective diffusivity (2.1x10-9 m2/s) than acid effective diffusivity (1.3x10-9 m 2/s). Textural softening was observed after 240 min of digestion in both studies. In study one, fried sweet potatoes had the greatest decrease in hardness due to softening of the initial crust formed during frying. In the second study, mildly cooked sweet potatoes had a greater decrease in hardness than severely cooked sweet potatoes for both boiled and steamed cooking treatments. In study one, cell wall disruption and starch degradation was observed in boiled, steamed, and microwave steamed samples as a result of cooking and/or digestion. Fried sweet potatoes did not show cell wall breakdown, possibly due to the oil content in the sample. In study two, both boiled and steamed, mildly cooked sweet potatoes showed relatively uniform cell shapes before in vitro digestion. After 240 min of in vitro gastric digestion, cell walls and starch appeared to be degraded from all cooking treatments. Overall, it was observed that cooking method and severity influences how the acid and moisture reacts with the food structure, resulting in structural changes in the sweet potato matrix.;Knowledge of the structural changes in sweet potatoes due to enzymatic and acid hydrolysis during simulated digestion is important to help understand the mechanisms of the food breakdown during digestion. Food breakdown will ultimately influence nutrient release and absorption (e.g. beta-carotene bioaccessibility). In order to optimize food nutritional properties, the relationship between food processing/cooking and food breakdown must be understood.