Textural and (bio) chemical changes during conventional ageing and chemical pre-treatments of common beans: towards understanding the development of the hard-to-cook defect

Soaking followed by cooking is a common method to process raw common bean seeds towards consumption. The hard-to-cook (HTC) defect, that can develop in beans during post-harvest storage at adverse conditions (conventionally aged beans) is an important factor decreasing the cookability of beans, therefore hampering their utilisation. In addition, the texture evolution of beans during cooking could be affected by soaking/cooking media. The studies carried out so far on the HTC defect suggest the pectin-cation-phytate hypothesis facilitated by membrane damages to be the most plausible. This hypothesis states that pectin in the cell wall/middle lamella crosslinks with minerals released from phytate hydrolysis resulting in insoluble pectates, prolonging cooking times required for bean softening. As for texture changes induced by soaking/cooking media (e.g. CaCl2 solutions, sodium acetate buffer and sodium citrate buffer), limited researches explored the mechanisms in detail. Today, the mechanisms of HTC defect/texture changes in beans after storage or treated by specific chemicals have not been fully elucidated. Therefore, the aim of this research work is to investigate the cooking behaviour in relation to compositional changes in conventionally aged beans and beans treated by chemicals to further understand the (bio) chemical reactions involved in texture changes of beans during cooking.

In the first study, starting from the pectin-cation-phytate theory, the texture evolution during cooking and associated composition changes in red kidney beans treated by (0.01 M, 0.05 M and 0.1 M) CaCl2 solutions or sodium acetate buffer (0.1 M, pH 4.4, 41 °C with soaking times: 2, 4, 8 and 12 h) were investigated to understand the relation between (bio) chemical reactions and the cooking behaviour. For beans soaked and cooked in CaCl2 solutions, higher Ca2+ concentrations resulted in longer cooking times and higher final bean hardness values. The phytate content in these beans remained constant during cooking while exogenous Ca2+ migrated into the cotyledons and seed coats during soaking and cooking. For beans soaked in sodium acetate buffer for different times, delayed softening associated with increased phytate hydrolysis was observed in beans with longer soaking times in the buffer. This suggests that endogenous Ca2+ released from phytate hydrolysis contributed to the delayed cooking of these beans. It indicates that both exogenous Ca2+ from soaking/cooking media and endogenous Ca2+ from phytate hydrolysis contributed to the texture changes of beans during cooking. These results show that divalent cations (e.g. Ca2+ and Mg2+) are important factors influencing the cookability of beans.

To further explore the role of Ca2+ migration in texture changes in detail, in situ cation analysis of beans with different pre-treatments (fresh beans, conventionally aged beans, beans treated in CaCl2 solutions or sodium acetate buffer) was carried out using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). In addition, their degree of pectin methyl esterification (DM) was determined. Texture-based bean classification was exploited in selecting samples to relate the (bio) chemical changes to texture at single bean level. In conventionally aged beans, a negative correlation was shown between pectin DM of beans after soaking and cooked bean texture as well as a positive correlation of the cell wall-bound Ca2+ and texture of cooked beans. This indicates that the pectin DM after storage and pectin Ca2+ crosslinking in the cell wall/middle lamella (promoted by heating) largely contributed to the texture changes observed. In beans treated by CaCl2, crosslinking of pectin with exogenous Ca2+ occurred during soaking but even more so during cooking, delaying bean softening. For beans treated by acetate buffer, the pectin crosslinking with endogenous Ca2+ released from phytate hydrolysis at the cell wall/middle lamella can be the crucial factor delaying bean softening during cooking.

Next, the cooking behaviour and phytate changes in ten bean accessions with different pre-treatments (fresh beans soaked in deionised water, conventionally aged beans, beans treated in CaCl2 solution and beans soaked a sodium acetate buffer) were investigated and compared. The potential of using such chemical pre-treatments as a tool to predict the sensitivity of different accessions to HTC development during conventional ageing was explored. Based on the results, it can be concluded that soaking and cooking beans in 0.01 M CaCl2 cannot be used to predict the susceptibility of these bean accessions to develop the HTC. Beans soaked in acetate buffer showed a similar or higher extent of IP6 hydrolysis compared to conventionally aged beans. The significantly positive correlations of IP6 changes in beans undergoing these treatments suggest that soaking in acetate buffer (pH 4.4, 41 °C, soaking time: 12 h) is a good tool to predict IP6 hydrolysis in conventionally aged beans (35 °C, 83% relative humidity, 3 months). In addition, soaking beans in acetate buffer can also be exploited to predicted the cooking time changes in different bean accessions (except for yellow bean-KATB1) after conventional ageing. However, although the HTC to some extents can be predicted in this way, the mechanism of HTC development during conventional ageing might not be identical to the mechanisms involved in the accelerated tests proposed.

In the last chapter the potential of soaking in a sodium citrate buffer (0.1 M, pH 4.4, 41 °C, different soaking times) to increase the cookability of beans was explored. The cooking kinetics of beans soaked in citrate buffer for different times suggest that longer soaking times in the buffer promote bean softening during cooking. An evaluation of (bio) chemical changes indicates that the citrate buffer complexes the minerals released from phytate hydrolysis and also limits phytate hydrolysis, therefore inhibiting the cation mediated cross-linking of pectin at the cell wall/middle lamella. On the other hand, the citrate in the soaking medium might also binds inherently bound Ca2+ e.g. initially bound by pectin. Consequently, pectin solubilisation during cooking in beans treated in a citrate buffer was faster compared to fresh beans. Starch gelatinisation and protein denaturation were not responsible for changes in bean softening.

This PhD work explored the mechanisms of HTC development reflected in the texture changes of beans during cooking after conventional ageing or chemical pre-treatments (with CaCl2, sodium acetate buffer or sodium citrate buffer). It reveals that pectin-Ca2+ crosslinking (Ca2+ from soaking/cooking media or released from phytate hydrolysis) is the main factor influencing the texture evolution of beans (with different pre-treatments) during cooking. In addition, the utility of chemical pre-treatments as an accelerated test to evaluate the sensitivity towards HTC development was evaluated. Soaking beans in sodium acetate buffer is suggested to predict HTC development in conventionally aged beans. Within addition, soaking in a sodium citrate buffer represents a convenient method to accelerate bean softening during cooking.