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.status: publishe

Optimising nutrition from indigenous sustainable pulses by targeted processing: Starch digestibility and mineral bioaccessibility of Bambara groundnuts

status: Published onlin

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-1). 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.status: Published onlin

Ageing, dehulling and cooking of Bambara groundnuts: consequences for mineral retention and in vitro bioaccessibility

Postharvest storage of legumes is a strategy to ensure food security. However, the hard-to-cook phenomenon, partly explained by pectin-cation-phytate theory, may develop during storage. Based on this theory, minerals could be redistributed within the matrix, affecting the concentration of free/bound minerals and hence their bioaccessibility. Therefore, this study investigated the influence of storage-induced ageing and subsequent dehulling, soaking and cooking treatments on the concentration and in vitro bioaccessibility of Ca, Mg, Fe and Zn in Bambara groundnuts. ICP-OES was performed to determine Ca, Mg, Fe and Zn concentrations after applying different treatments and after simulated digestion in the context of bioaccessibility determination. This study illustrates that while the seed coat is rich in Ca (51%), it contains only less than 15% of Mg, Fe or Zn. Hence, dehulling negatively influenced Ca, but not Mg, Fe or Zn concentration. During soaking, up to 10% or 56% of the initial Mg concentration leaches into whole or dehulled seeds, respectively. Prolonging the cooking time (300 min) of whole aged Bambara groundnuts (32 weeks), necessary for obtaining palatable textures, decreased the overall Mg, Fe and Zn concentrations by 72%, 57% and 48%, respectively. Storage-induced ageing significantly decreased Ca solubility and bioaccessibility. Bioaccessibility of trace minerals in Bambara groundnuts is low, especially for Fe, and cannot be improved by dehulling or cooking treatments. Ageing did not influence Fe and Zn bioaccessibility. Populations relying on both fresh and aged Bambara groundnuts may still be at risk for mineral deficiencies.status: publishe

Impact of postharvest storage and cooking time on mineral bioaccessibility in common beans

Mineral (Mg, Ca, Fe and Zn) bioaccessibility in common beans was evaluated taking into consideration the common bean food chain from postharvest storage over processing (soaking and cooking) until consumption. Beans were stored under realistic tropical conditions (35 °C and 80% RH) which resulted in significantly different cooking behaviour after 8 weeks compared to freshly harvested beans. Based on postcooking hardness, different storage times were selected: unstored, 8 and 20 weeks. Independently of storage conditions, beans were soaked overnight and cooked for 30, 60 or 120 min. The mineral bioaccessibility decreased with increase in both storage and cooking times. Decrease in mineral bioaccessibility with increasing storage time was proved to be the result of increasing mineral chelation of cell wall polymers (e.g. pectin). Additionally, we hypothesize that by cooking, mineral chelators become more accessible, e.g. through pectin solubilization phenomena, in turn capturing more free minerals leading to a reduced mineral bioaccessibility.status: publishe

Influence of storage-induced ageing, dehulling and cooking on the in vitro bioaccessibility of minerals

status: accepte

Texture and interlinked post-process microstructures determine the in vitro starch digestion kinetics of Bambara groundnuts

edition: Proceedings of the 6th International conference on food digestionstatus: publishe

Pectin nanostructure influences pectin-cation interactions and in vitro-bioaccessibility of Ca2+, Zn2+, Fe2+ and Mg2+-ions in model systems

This study explores the effect of pectin structure on mineral bioaccessibility in model systems. Commercial citrus and sugar beet pectin (CP and SBP, respectively) were demethylesterified by the action of carrot pectin methylesterase and characterisation of the resulting samples revealed different degrees of methylesterification (DM). Additionally, SBP exhibited a higher protein content, degree of acetylation, degree of feruloylation, degree of branching and molar mass compared to CP. In vitro-simulated digestion (stomach and small intestine phases) of mineral-enriched pectin solutions showed that decreasing pectin DM results in a decrease in mineral-bioaccessibility due to pronounced cation-pectin interactions. For a given DM, SBP digests exhibited higher mineral-bioaccessibilities than CP digests possibly due to structural differences of pectin in these sources. Furthermore, increasing the mineral-enrichment prior to in vitro-digestion resulted in a higher mineral-bioaccessibility while increasing the pectin concentration decreased the mineral-bioaccessibility. Generally, Mg2+-ions were the most bioaccessible followed by Ca2+, Zn2+, then Fe2+-ions.publisher: Elsevier articletitle: Pectin nanostructure influences pectin-cation interactions and in vitro-bioaccessibility of Ca2+, Zn2+, Fe2+ and Mg2+-ions in model systems journaltitle: Food Hydrocolloids articlelink: http://dx.doi.org/10.1016/j.foodhyd.2016.07.030 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved.status: publishe