Bioaccessibility and digestion of nutrients in Bambara groundnuts as affected by postharvest storage and processing
Indigenous pulses are a locally adapted and sustainable solution that can improve food security in developing countries. In addition, malnutrition remains an unresolved challenge, which continues to be addressed. Pulses are commonly stored for long after harvest. Hence, they are a reliable food source in times of famine. However, upon postharvest storage of pulses, several biochemical reactions can occur as described by the hard-to-cook (HTC) theories. The development of the HTC phenomenon in pulses is inconvenient and increases the cost of preparation, reducing consumer acceptability. Besides, the HTC phenomenon has potential negative implications on the nutritional quality of pulses. Therefore, this PhD work aimed to evaluate the influence of postharvest storage and processing on induced structural changes and their consequences on the micro and macronutrient nutritional quality of Bambara groundnuts.
The first part of the work provided proof of the development of the HTC phenomenon in Bambara groundnuts, which were stored under typical sub-Saharan environmental conditions (35 °C/78 % relative humidity). Longer storage (ageing) resulted in prolonged cooking times, a slower reduction of hardness during cooking and delayed cell separation. This was attributed to cell wall changes that could occur during ageing, for instance, the increased strengthening of the cell wall through crosslinks involving pectin and minerals or polyphenols with other cell wall polymers.
In the second part of the work, the hypothetical consequences of the HTC theories on the retention and bioaccessibility of minerals, and starch digestibility were assessed. Soaking and extended cooking time caused high Mg, Fe and Zn losses through leaching. This result was associated with the loss of membrane integrity that could occur in pulses during ageing. On the contrary, Ca was mostly retained in the seed and only leached marginally during soaking and cooking. Based on the result of minerals leaching to different extents, the complexation of minerals by the matrix was assessed. For this, the percentage of soluble minerals present in the aqueous phase when the flour from raw fresh and aged seeds was dispersed in water was determined. Of the four minerals studied, the percentage of soluble Ca decreased remarkably with ageing. To investigate this further, fluorescence images of cross-sections from raw fresh and aged Bambara groundnuts, revealed the presence of more labelled Ca-pectin crosslinks for the aged samples. The increase in Ca complexation with ageing could be associated with certain aspects of the pectin-cation-phytate theory. The knowledge of the amount of minerals retained is insufficient to understand the fate of minerals. Therefore, mineral bioaccessibility was determined. Different trends were observed for each mineral. Ca bioaccessibility decreased with ageing but improved with cooking time. Mg bioaccessibility decreased with ageing and cooking time, whereas the bioaccessibility of Fe and Zn was not greatly influenced by the treatments applied.
Cotyledon cell wall integrity is an important feature that determines starch digestibility. Differences pertaining to the cell wall structure e.g., more pectin-Ca crosslinks and delayed cell separation were observed in aged Bambara groundnuts. Accordingly, starch digestibility was assessed for fresh and aged Bambara groundnuts. Faster starch digestion occurred in aged compared to fresh Bambara groundnuts that were cooked for similar times. Indeed, due to the increased cell rupture in aged samples, amylase had more rapid access to the substrate. The starch digestion behaviour of fresh and aged samples with comparable hardness and microstructures (mostly individual cells) was highly similar. These results ruled out a direct influence of the development of the HTC phenomenon on starch digestion.
Individual cells were the characteristic microstructure of seeds with edible hardness. Therefore, in the final part of the work, the effect of processing intensity and hardness sorting on starch and protein digestibility was evaluated on isolated individual cells. Isolated individual cells from longer cooking times had faster starch and protein digestibility. Considerable diversity in hardness was present in seeds after cooking. Seeds were sorted into high and low hardness classes. Therefore, the effect of hardness differences of seeds from the same cooking time could be assessed. Individual cells from high hardness classes had (s)lower starch and protein digestibility. The estimated kinetic parameters of starch and protein digestibility were highly correlated.
From this doctoral thesis, it can be concluded that postharvest storage is an important but understudied stage of the pulse chain that affects the nutritional quality of pulses taking into account subsequent processing. Additionally, the hardness parameter and its linked microstructure can be used as an overall variable to fine‑tune starch and protein digestibility.