Instability of common beans during storage causes hardening: The role of glass transition phenomena.

Long-term storage of common beans leads to loss of cooking quality and an ill-defined solution, appropriate storage, is recommended. Therefore, the polymer science theory of glasses that hypothesizes stability of a system below its glass transition temperature (Tg) was applied to determine bean stability during storage in relation to cooking behavior. Since composition influences Tg, powders of cotyledons and seed coats in addition to whole beans were equilibrated above different saturated salt solutions in order to generate materials with different moisture contents. A thermal mechanical compression test which measures compressibility changes in a system upon reaching its glass-rubber transition temperature region was conducted to obtain the Tg. A Tg-moisture relation was established, whose relevance was confirmed by storage and cooking experiments which showed development of hard-to-cook in beans stored above Tg but not below it. Therefore, this relation constitutes a stability map for storage of common beans.