Proximate composition, microstructure, and protein and starch digestibility of seven collections of Jack bean (Canavalia ensiformis) with different optimal cooking times

Abstract

Because of its high protein content, Jack bean (Canavalia ensiformis) is a promising alternative protein source. However, the utilization of Jack bean is limited due to the long cooking time to achieve palatable softness. We hypothesize that the cooking time may influence protein and starch digestibility. In this study, we characterized seven Jack bean collections with different optimal cooking times in terms of their proximate composition, microstructure and protein and starch digestibility. Kidney bean was included as a reference for microstructure and protein and starch digestibility. Proximate composition showed that Jack bean collections have a protein content ranging from 28.8 to 39.3%, a starch content ranging from 31 to 41%, a fiber content from 15.4 to 24.6%, and a concanavalin A content in the range 35-51 mg/g dry cotyledon. Particle sizes ranging between 125 and 250 µm were chosen as a representative sample of the whole bean to characterize microstructure and digestibility of the seven collections. Confocal laser microscopy (CLSM) revealed that Jack bean cells have an oval shape and contain starch granules embedded in a protein matrix similar to kidney bean cells. The diameter of Jack bean cells was measured by image analysis of CLSM micrographs and ranged from 103 to 123 µm, while the diameter of starch granules was 31-38 µm, comparatively larger than that of the kidney bean starch granules. Isolated intact cells were used to determine the starch and protein digestibility in the Jack beans collections. The digestion kinetics of starch followed a logistic model, whereas the digestion kinetics of protein followed a fractional conversion model. We found no correlation between optimal cooking time and kinetic parameters of protein and starch digestibility, implying that optimal cooking time is not predictive of protein and starch digestibility. In addition, we tested the effect of reduced cooking times on protein and starch digestibility on one Jack bean collection. The result showed that reducing cooking time significantly reduces starch digestibility, but not protein digestibility. The present study contributes to our understanding of the effect of food processing on protein and starch digestibility in legumes.

Keywords: Cell wall barrier; Effect of food processing; Intact cells; Optimal cooking times; Protein digestion; Starch digestion.