High temperature and humidity storage alter starch properties of faba (Vicia faba) and adzuki beans (Vigna angularis) associated with hard-to-cook quality

Hard-to-cook (HTC) beans are characterised by extended cooking times. Although the changes in cell walls limiting hydration in HTC beans are widely investigated, the role of macro-molecules (starch and protein, which constitute >80 % of beans) are almost overlooked. This study investigates the structural changes in starch associated with the HTC quality in faba and adzuki beans stored at contrasting temperature and humidity regimes. Beans were stored at 4 °C (control) and 40 °C with relative humidity (RH) levels of 60 % and 80 %. Significant changes in starch properties were observed, particularly in beans stored at 40 °C and 80 % RH, with swelling power decreasing by 7 % and 12 % for faba and adzuki beans, respectively. Additionally, gelatinisation behaviour was negatively affected, with peak temperatures increasing for adzuki beans (from 66.64 °C to 68.46 °C) and enthalpy rising for faba beans (from 9.25 J/g to 10.64 J/g) along with an increase in relative crystallinity. Overall findings indicate that storage at elevated temperature (40 °C) under both moderate and high humidity conditions (60 % and 80 %) primarily or partially leads to developing HTC beans due to molecular rearrangement of starch at helical and crystalline levels.