Biochemical mechanisms responsible for postharvest ageing and cooking kinetics in common beans across diverse accessions

Promovendus/a: Henry Tafiire

Promotor(en): Prof. dr. ir. Tara Grauwet, Prof. dr. ir. Marc Hendrickx, Dr. Patrick Ogwok

The Dry beans are one of the world’s most important staple foods. They are nutritious, affordable, and vital for sustainable diets. However, in many parts of the Global South, where beans are widely grown and consumed, people face a problem of beans taking longer times to cook. When dry beans are stored in the warm, humid conditions typical of these regions, they develop what is known as the hard‑to‑cook (HTC) defect. With the increasing urbanisation in these regions, most people are shifting away from the consumption of dry beans due to the inconvenience and high energy requirements for HTC beans. Although researchers have studied the HTC defect for decades, a full understanding of bean softening and HTC development is lacking, making it difficult to optimize the bean cooking process and to develop practical solutions for this defect. This is largely because previous studies were conducted on a limited number of bean accessions - often on a single accession -, yet variations exist within and between bean accessions (of different genetics and growth conditions) that may affect their behaviour.

This doctoral research set out to fill that gap by studying the bean cooking behaviour across a large and diverse set of bean types. The aim was to uncover not only how fast beans soften during cooking, but why – what biochemical processes drive this softening, and what changes during postharvest storage cause the HTC defect?

How beans soften

The first part of the research examined how both fresh (non-aged) and aged (stored) beans from twenty-four accessions soften during cooking. Using a precise method that measures the hardness of individual seeds, the study showed that different bean accessions, fresh or aged, differ considerably in their cooking behaviour. Ageing did not generally change how hard beans were before cooking, but it consistently made them to soften more slowly, meaning they needed longer times to reach edible softness compared to fresh beans. Aged beans developed long “lag times” – periods at the beginning of cooking when almost no softening happened. Despite this delay, aged beans could eventually reach the same softness as fresh beans if cooked long enough.

What really controls bean softening?

To understand the biochemical reasons behind bean softening, the research zoomed in on five selected accessions. The results showed that the changes (or solubilisation) of pectin, the glue-like substance in bean cell walls, is the key step determining how fast beans soften in both fresh and aged accessions. Other biochemical changes such as protein denaturation or starch melting were completed in the early phases of cooking and had little to no effect on overall bean softening. Ageing consistently slowed down the pectin changes across accessions, explaining why aged beans need more time to reach edible softness.

How hard‑to‑cook occurs?

In this part of the thesis, it was investigated why ageing slows pectin changes and softening. The research linked this to a cascade of chemical events involving phytate, calcium and pectin. During storage, phytate in bean cotyledons is broken down by the enzyme phytase, releasing calcium. This calcium then migrates into the cotyledon cell walls and binds tightly to pectin, effectively adding extra crosslinks that reinforce the cell wall, like adding extra rivets to strengthen a bridge. This calcium-reinforced pectin becomes more resistant to solubilisation during cooking, leading to the slow softening characteristic of beans with the HTC defect. The study confirmed this mechanism across all bean accessions and demonstrated a strong link between increased cell wall-bound calcium, greater phytate breakdown, and longer times to cook.

These findings provide a comprehensive picture of how and why beans soften during cooking and how the HTC defect develops. This knowledge can guide food processors, plant breeders, and researchers in designing or optimising bean cooking processes and developing practical strategies to prevent or reduce the development of the HTC defect.