Impact of transglutaminase treatment on properties and in vitro digestibility of white bean (Phaseolus vulgaris L.) flour

Common beans (Phaseolus vulgaris L.) are rich in nutrients and have significant amounts of proteins and complex carbohydrates, besides to be rich in unsaturated fatty acids and dietary fibres. Consumption of beans could be improved by processing them into flour. In this study the effect of microbial transglutaminase (TG) on the structure, physical (colour parameters, moisture, water holding capacity), thermal properties and in vitro digestion of undehulled (WB) and manually dehulled (SB) flour samples from white common beans (P. vulgaris L.) was evaluated. Flour samples were incubated in the absence and presence of TG (WB/TG and SB/TG). We observed that the enzyme is able to catalyse the formation of polymers, suggesting that the proteins occurring in the bean flour act as TG substrates. Microstructure of samples was examined by Scanning Electron Microscopy (SEM), while thermal properties were studied by Differential Scanning Calorimetry. Microstructural results showed that the TG-treated samples possess a more compact structure, made of starch granules surrounded by proteins that, presumably, contain TG-catalysed polymers. Moreover, TG treatment had a major impact on colour, water holding capacity (WHC) and thermal properties. In particular, WB and SB samples presented a darker colour than WB/TG and SB/TG samples, while the latter showed reduced WHC that was only 30% and 37% of WB and SB samples, respectively. The transition enthalpy (ΔH) in the temperature range from 57 to 70 °C (WB, WB/TG) and from 60 to 68 °C (SB, SB/TG) followed the order: WB/TG > WB and SB/TG > WB, respectively. In vitro digestion experiments indicate that the presence of isopeptide bonds decreased the digestibility of TG-treated flour samples.