De rol van pectine polysachariden bij het hard-to-cook probleem van gewone bonen

The importance of common beans (Phaseolus vulgaris L.) in addressing food insecurity and protein-energy malnutrition cannot be underestimated. However, their utilization is hampered by the occurrence of the hard-to-cook (HTC) phenomenon, a condition in which seeds do not imbibe sufficient water during soaking (hard-shell effect) and do not become tender after a reasonable cooking time (HTC defect). Various hypotheses have been postulated to explain the occurrence of the HTC defect, including a well-supported hypothesis involving pectin i.e. the classical pectin hypothesis. Nevertheless, the mechanisms through which pectic polysaccharides are involved in the HTC defect have not been fully elucidated. Therefore, the aim of this research work was to gain detailed mechanistic insights into the role of pectic polysaccharides in the development and manifestation of the HTC defect in common beans by evaluating the classical pectin hypothesis. In preliminary work, selected common beans were classified into either easy-to-cook (ETC) or HTC based on their cooking quality. Furthermore, factors affecting the cooking quality of common beans such as soaking in various brine solutions were explored. From the selected common bean samples, Rose coco, Red haricot, Zebra beans were classified as ETC, while Canadian wonder, Pinto and Soya fupi beans were classified as HTC. In addition, the influence of salts solutions and solutions of different pH on the texture of beans during soaking and thermal treatment pointed at the possible involvement of pectin in the cooking quality of common beans. Subsequently the classical pectin hypothesis was evaluated by examining pectic polysaccharide properties such as sugar composition, degree of methyl-esterification (DM), extractability and molar mass (MM). Differences in pectic polysaccharide properties between ETC and HTC beans were investigated as well as the effect of storage conditions and (pre)treatments on the pectic polysaccharide properties in relation to the development and manifestation of the HTC defect. It was revealed that common bean pectic polysaccharides have relatively higher amounts of neutral sugars, especially arabinose and galactose, compared to the amounts of galacturonic acid and therefore rather highly branched. Moreover, HTC bean pectic polysaccharides are more branched than the ETC beans, especially for the Na2CO3 extractable pectin (NEP). Development of the HTC defect during storage of beans at high temperature (35 °C) and/or relative humidity (83%) was accompanied by a decrease in hot water pectin extractability paralleled by an increase in the alkaline extractable fraction. On the other hand, the amounts of chelator extractable pectin (CEP) for ETC and HTC beans were similar. However, upon soaking and thermal treatment in a solution of Na2CO3, extractability profile of the pectic polysaccharides shifted, the water extractable pectin (WEP) increased while NEP decreased, for both ETC and HTC beans. This observation can be attributed to thermal solubilisation, mainly for the strongly bound NEP, and hence conversion of part of NEP into WEP. This was reflected in enhanced cell separation during cooking due to weak cell-cell adhesion. In contrast, such changes were not observed in samples soaked and thermally treated in a CaCl2 solution, the cell walls disintegrated on grinding due to strong cell-cell adhesive forces. The observed changes in pectic polysaccharides extractability during storage and (pre)treatments were in line with the cooking behaviour of the beans. ETC beans and Na2CO3 treated beans showed minimum cooking time, while HTC beans and CaCl2 treated beans took a longer time to cook. Other pectin characteristics such as DM of the AIR and MM distribution of water extractable polymers did not differ considerably between the ETC beans and HTC beans. However, soaking and thermal treatment in a Na2CO3 solution considerably reduced the DM of the AIR. In conclusion, contribution of pectic polysaccharides to the development of the HTC defect is majorly due to reduced pectin extractability. However, the decrease cannot be explained solely by the classical pectin hypothesis. In contrast to the hypothesis, the DM and the amounts of CEP did not differ considerably between ETC and HTC common beans. Therefore, the decrease in pectin extractability is probably not as a result of PME catalysed pectin demethoxylation but could be due to the release of Ca++ that subsequently cross-links with already lowly methylesterified pectin. However, the occurrence of non-Ca++ cross-links cannot be ruled out. Therefore, in this PhD work an alternative hypothesis is proposed. The decrease in extractability of pectin is probably also due to the possible occurrence of strongly covalent ester links, since, breakage of ester links is crucial for solubilisation of pectin during cooking. With the revealed structure of common bean pectin, especially high level branching of the strongly bound pectins (NEP) in form of arabinans, the following possible ester cross-links can occur: (i) ferulic acid bridges esterified to neutral sugar side chains, (ii) rhamnogalacturonan-II borate diester, and (iii) uronyl ester linkages between carboxyl group of galacturonic acid and hydroxyl group of another glycosyl residue.

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