Underexplored role of cell wall modifications in the hard-to-cook (HTC) phenomenon

Prolonged storage of legumes under high temperature (>40 °C) and relative humidity (RH) (>60 %) conditions triggers the hard-to-cook (HTC) defect, negatively impacting cooking performance and nutritional value. Although previous studies have focused on starch, protein, and polyphenol changes, the role of cell wall modifications remains underexplored. Cotyledon cell walls act as a key barrier to water uptake and thermal softening by enclosing starch and protein matrices. We hypothesised that storage under HTC-inducing conditions would cause structural and compositional changes in cotyledon cell walls, reducing pectin solubility and increasing cross-linking, thereby limiting water absorption and contributing to the HTC defect. This study examined structural and compositional changes in the cell walls of faba and adzuki beans stored under HTC-inducing conditions (40 °C and > 60 % RH) compared to controlled storage (4 °C, 25 % RH). The changes in pectin solubility, monosaccharide profile and microscopy images provided visual evidence of a more compact cotyledon cell arrangement, with fewer intercellular spaces and distinct tri-cellular junctions. Over nine months, phytic acid declined, and free phenolic acids became bound and migrated to the seed coat, contributing to seed darkening. These changes increased seed hydrophobicity and limited water absorption. Overall, cell wall alterations, along with phytate and polyphenol transformations, are key contributors to the HTC phenomenon. This new knowledge provides mechanistic insights that can support storage and processing strategies aimed at improving legume cooking quality, energy efficiency, and nutritional value.