In vitro colonic fermentation of red kidney beans depends on cotyledon cells integrity and microbiota adaptation.

In the present study we investigated the effect of cellular integrity on microbial utilization of proteins and carbohydrates by gut microbiota. Cotyledon cells from red kidney beans with different levels of structural integrity were fermented in-vitro by microbial communities previously adapted to the conditions of ascending, transverse and descending colon. The effect of bacterial adaptation to substrate was also assessed by using microbiota exposed to a diet rich in bean cells. Microscopy analyses indicate that cell integrity was maintained during fermentation. The amount of gas generated and the rate of total gas production was higher in broken cells compared to intact cells which suggest a faster and more extensive utilization of nutrients when cell wall is broken. A significantly higher butyric and propionic acid level was detected in broken cells at the end of the fermentation. Moreover, adapted bacterial communities were more efficient in fermenting bean cells where higher amounts of butyrate were produced in all colon regions independently of sample integrity. Bacterial communities of the distal colon appeared to be the most efficient in carbohydrate and protein fermentation as witnessed by the higher levels of gas, and short chain fatty acids. It was also found that cell integrity and adaptation to bean cells modulate the hierarchy of nutrient utilization, with non-starch polysaccharides preferred over starch and proteins by microbiota exposed to bean cells. Our results demonstrated that structural aspects of foods, such as cell integrity in plant tissues, may modulate nutrients utilization by gut microbiota.

A mechanistic model to study the effect of the cell wall on starch digestion in intact cotyledon cells.

The role of the plant matrix is recognized as the main factor restricting starch digestibility in beans. Several authors have provided insights about the mechanisms behind the reduced starch digestibility in plant matrices. In this study, by means of a mathematical model, we provide a mechanistic explanation of the role played by the cell wall. It was confirmed that starch entrapped within intact cells could only be hydrolysed after α-amylase diffusion through the cell wall. This process is limited by the pores naturally present in the cell wall and the adsorption of α-amylase to the cell wall surface. These factors restrict the concentration of α-amylase available within the cells. The model assumptions are valid under controlled laboratory conditions and were validated with in-vitro digestion data giving very accurate results. The proposed approach provides new information to understand the digestibility of starch, and possibly other macronutrients, in complex food matrices.

The effect of cell wall encapsulation on macronutrients digestion: A case study in kidney beans.

Cotyledon cells in kidney beans naturally encapsulate starch and proteins limiting the access of digestive enzymes to their substrates. In this study, we investigated the effect of cell wall on bean protein digestibility and its relationship with starch digestion. Results showed that proteins contained in the cytoplasmic matrix influence the rate at which starch is digested in-vitro. Confocal laser scanning microscopy revealed that storage proteins in the cytoplasm act as a second encapsulation system preventing starch digestion. This microstructural organization only affected starch since no changes in protein digestion rate or extent were observed due to the presence of starch granules. Fourier transform infrared spectroscopy revealed that cellular entrapment limited protein denaturation induced by thermal treatments. High concentrations of a fraction resistant to digestion were found in proteins that were heated when entrapped within intact cotyledon cells, compared to those thermally treated as bean flour.

A closer look to cell structural barriers affecting starch digestibility in beans.

Isolated bean cells were used to understand the contribution of cell wall and cytoplasmic matrix on starch digestibility. Cotyledon cells were treated enzymatically and mechanically to reduce the level of cell intactness. SEM and chemical characterization revealed that enzymatic treatment modified cell wall thickness and porosity without altering the cytoplasmic matrix, whereas mechanical treatment completely disrupted cell structure. Decreasing cell intactness increased the rate but not the extent of starch digestion in-vitro. It was concluded that cell wall serves as a permeable barrier limiting the access of digestive enzymes. Cytoplasmic matrix, on the other hand, reduced further the accessibility of amylase to starch affecting its hydrolysis rate. In addition, it was proven that cell structural changes, if any, occurring during digestion had no effect on starch hydrolysis.