Novel in situ evaluation of the role minerals play in the development of the hard-to-cook (HTC) defect of cowpeas and its effect on the in vitro mineral bioaccessibility

Abstract

Highlights• We visualised in situ mineral distribution in cowpea with PIXE.• We evaluated the effect of the hard-to-cook (HTC) phenomenon on mineral distribution.• HTC defect increased Ca and Mg contents in the cell wall-middle lamella of parenchyma cells.• Results confirm phytate-phytase-mineral hypothesis as a probable mechanisms of HTC defect. AbstractCowpea is a nutritionally important drought-resistant legume in Sub-Saharan Africa. It is, however, underutilised, in part due to the hard-to-cook (HTC) defect caused by adverse storage conditions resulting in seeds not softening during cooking. This study introduced a novel evaluation of the potential role that minerals play in the development of the HTC defect. The mineral distribution in the cotyledons of normal and HTC cowpeas were analysed by Proton Induced X-ray Emission (PIXE) spectrometry. The phytate, tannin and total phenolic contents were analysed together with in vitro mineral bioaccessibility. In HTC cowpeas, Ca and Mg were more concentrated in the cell wall-middle lamella area of the parenchyma cells. This, together with the reduction in phytate content, confirmed 1 the ‗ph ytase-phytate-mineral' hypothesis as a mechanism for development of the HTC defect. Despite the phytate reduction in stored cowpeas, the HTC defect decreased the bioaccessibility of Ca, Fe and Zn in cowpeas.

Starch Granules from Cowpea, Black, and Carioca Beans in Raw and Cooked Forms

“…The presence of starch granules inside the amyloplasts is still observed in the cooked cotyledon image, suggesting that the cooking conditions were suitable for gelatinization, possibly due to a long storage period of the seeds, which characterizes an HTC phenomenon [19,21]. Figure 4 shows the optical microscopy of carioca beans in cross section.…”

“…HTC phenomenon is one of the main obstacles to the consumption of beans grown in countries of Latin America and Africa, where ambient temperatures and relative humidity are high throughout the year, conditions that increase the possibility of occurrence of this phenomenon. At the microstructural level, the visible result of HTC seems to be related to the inability of the middle lamella of cotyledon cells to soften or dissolve and separate the cells [19,21].…”

“…Kruger et al [21] evaluated the role that minerals play in the development of the HTC phenomenon in cowpea and its effect on in vitro bioaccessibility. The mineral distribution in normal and HTC cotyledons of cowpea bean was analyzed by proton-induced X-ray emission spectrometry (PIXE).…”

Cowpea storage under nitrogen‐modified atmosphere at different temperatures: Impact on grain structure, cooking quality, in vitro starch digestibility, and phenolic extractability

“…HTC is a widely discussed phenomenon that occurs in beans stored under the inappropriate temperature, moisture content, and relative humidity conditions. Various complementary studies have been performed in order to explain this phenomenon, reporting HTC occurrence to be mainly associated with: (1) insolubilization of cell wall polymers (Shiga, Lajolo, & Filisetti, ); (2) lignification of the cotyledon cell wall constituents (Nasar‐Abbas et al, ); (3) reduction of pectin solubility as a result of mineral‐pectin interactions (Kruger, Minnis‐ndimba, Mtshali, & Minnaar, ); (4) covalent cross‐linking of pectin as well as increased interactions between starch and proteins, which hinders starch gelatinization (Njoroge et al, ); and (5) increased extent of interactions between the divalent ions and pectic polysaccharides (Yi et al, ).…”

“…(2) lignification of the cotyledon cell wall constituents (Nasar-Abbas et al, 2008b); (3) reduction of pectin solubility as a result of mineral-pectin interactions (Kruger, Minnis-ndimba, Mtshali, & Minnaar, 2014); (4) covalent cross-linking of pectin as well as increased interactions between starch and proteins, which hinders starch gelatinization (Njoroge et al, 2015); and (5) increased extent of interactions between the divalent ions and pectic polysaccharides (Yi et al, 2016).…”