Biochemische mechanismen die verantwoordelijk zijn voor na-oogstveroudering en kookkinetiek in verschillende accessies van gewone bonen

Dry common beans (Phaseolus vulgaris L.) are an essential component in sustainable and affordable human diets worldwide. However, their wider utilisation is constrained by long cooking times, particularly attributed to the development of the hard-to-cook (HTC) defect that emerges during postharvest storage at typical (sub)tropical temperature and relative humidity conditions. This is especially an important challenge in the Global South where beans are a staple food and urbanisation in those regions has led to the increasing demand for convenient and energy-efficient foods. Despite extensive prior research, bean cooking behaviour and HTC development remain partially understood, likely due to most of the previous studies focusing on a limited number of accessions, often on a single accession. There are inconclusive results regarding the influence of the significant inter- and intra-accession variability on softening and HTC development as well as analytical challenges in evaluating the associated structural and biochemical changes. This doctoral thesis therefore aimed to provide a holistic understanding of bean cooking behaviour: cooking/softening kinetics, time to cook, and underlying softening mechanisms and biochemical changes underpinning the pectin-cation-phytate mechanism of HTC development across a diverse range of common bean accessions. In the first part of this research, a kinetic approach was applied to study bean softening and related biochemical changes during cooking at ≈95 °C in diverse fresh and aged bean accessions. Objective compression texture analysis of single beans was used for hardness measurement. Texture evolution curves were modelled using conventional fractional conversion and modified three-parameter models incorporating a lag time. The results demonstrated that bean cooking kinetics differ markedly among a diverse panel of twenty-four bean accessions and whether they are fresh or aged. Ageing had little to no effect on the hardness of soaked beans (initial hardness) within an accession, which was rather strongly correlated with bean size. The postharvest ageing-induced HTC defect, observed across all accessions, manifested by reduced softening/cooking rate constants, prominent lag phases, and longer times to cook. Aged beans of an accession reached similar final (relative) hardness to that of their corresponding fresh beans when sufficiently cooked for long time. In fresh beans, the variation in time to cook among accessions was mainly determined by initial hardness and the highly variable softening rate constant, while in aged beans, initial hardness and lag time were the dominant parameters and to a lesser extent the softening rate constant, which showed limited variation among accessions. To elucidate the biochemical changes underlying bean softening, five bean accessions with different cooking behaviour were selected from a pool of the twenty-four accessions. A texture-based classification approach was used to narrow the texture range, and analyses were conducted on split beans (half cotyledons). During cooking, half cotyledons of fresh beans softened faster compared to their corresponding whole beans, whereas half cotyledons and whole aged beans had comparable softening rates. This quantitative kinetic study revealed that pectin solubilisation in the cotyledon is the rate limiting mechanism of bean softening during cooking across all accessions, fresh and aged. In contrast, protein denaturation and starch gelatinisation play little to no influence on overall softening. The extent and rate of pectin solubilisation strongly correlated with hardness decay and softening rate constants, while residual starch gelatinisation and residual protein denaturation were completed far in advance of the palatable texture and showed no meaningful association with cooking kinetics. Ageing reduced the rate of pectin solubilisation, consistent with the delayed softening observed in aged beans. The second part of this dissertation focused on elucidating the pectin-cation-phytate interactions underlying HTC development that affects the cooking behaviour of bean accessions. Ageing significantly reduced the phytate (inositol hexakisphosphate, InsP6) content among diverse beans in a panel of twenty-four accessions. InsP6 dephosphorylation resulted in some accumulation of InsP5, while lower inositol phosphate isomers (InsP2-4) showed minimal net accumulation, indicating rapid turnover of dephosphorylation intermediates. The conversion of InsP6 to InsP5 was identified as the rate limiting step in the phytate hydrolysis cascade, and InsP6 quantification appeared to be the most relevant indicator of phytate hydrolysis. Thermal pretreatment inactivating phytase markedly suppressed ageing-induced InsP6 dephosphorylation, confirming phytase-mediated hydrolysis. The dephosphorylation of InsP6 releases Ca2+, which can subsequently migrate to and crosslink with pectin in the cotyledon cell wall. Analysis of carefully isolated cell wall material (CWM) from texture-classified cotyledons of fresh and aged bean showed a significant ageing-induced increase in cell wall-bound calcium across the five selected accessions, which strongly correlated with InsP6 content and confirmed phytate hydrolysis as the primary source of Ca2+. Cell wall-bound calcium strongly correlated with the softening rate constant and time to cook, indicating that cell wall integrity reinforcement during ageing delays softening during cooking (resulting in longer cooking times). In addition, significant, but slight reductions in pectin degree of methylesterification (DM) were observed during ageing in more than half of the bean accessions. These findings provided strong evidence supporting the pectin-cation-phytate mechanism of HTC development across bean accessions. Overall, this doctoral research provides an integrated kinetic and biochemical framework that advances fundamental understanding of bean cooking behaviour and HTC development. These insights are critical for the design and optimization of the bean cooking process and for developing practical strategies to mitigate the HTC defect. Building on the findings and the methodological aspects including compression texture analysis on a single bean, modelling of bean cooking kinetics, texture-based classification, cotyledon CWM isolation, and the isomer-specific profiling of inositol phosphates in this thesis, key directions for further scientific investigations are highlighted. These include plausibility of the lignification hypothesis, quantification of InsP1, precooked dry beans and breading solutions to prevent the HTC defect.