In vitro starch and protein digestion kinetics of cooked Bambara groundnuts depend on processing intensity and hardness sorting.

When pulse seeds from a single batch are cooked, considerable variability of hardness values in the population is usually observed. Sorting the seeds into hardness categories could reduce the observed diversity and increase uniformity. Therefore, we investigated the effect of processing intensity whether or not combined with sorting into hardness categories on the in vitro starch and protein digestion kinetics of cooked Bambara groundnuts (cooking times 40 min and 120 min). The average hardness values were 89 ± 32 N and 42 ± 20 N for 40 min and 120 min cooking time, respectively. The high standard deviation of hardness for each cooking time revealed a high level of diversity amongst the seeds. Individual cells were isolated from (non-)sorted seeds before simulating digestion. The estimated lag phase describing the initial phase of starch digestion was not significantly different despite the processing intensity or the hardness category, implying that cell wall barrier properties for these samples were not majorly different. However, the rate constants and the extents of starch digestion of samples cooked for 40 min were significantly higher for the low hardness (50-65 N) compared to the high hardness (80-95 N) category (0.71 vs 1.02 starch%/min and 63 vs 77%, respectively). Kinetic evaluation of digested soluble protein (after acid hydrolysis of the digestive supernatant) showed that low hardness samples were digested faster than high hardness samples (0.037 vs 0.050 min). The faster protein hydrolysis in the low hardness samples was accompanied by faster starch digestion, indicating the possible role of the protein matrix barrier. Individual cells of comparable hardness obtained from the two different processing times had similar starch and protein digestion kinetics. Our work demonstrated that, beyond cooking time, hardness is a suitable food design attribute that can be used to modulate starch and protein digestion kinetics of pulse cotyledon cells.