The moisture plasticizing effect on enzyme-catalyzed reactions in model and real systems in view of legume ageing and their hard to cook development

Abstract

This study aims to understand the role of storage conditions (temperature, T and moisture content) in the reactions involved in the cation-pectin-phytate hypothesis, postulated to cause the Hard-to-cook (HTC) defect in beans as influenced by the plasticizing effect of moisture (glass transition ( T g ) concept). The enzyme catalysed reactions, demethylesterification of pectin and hydrolysis of phytic acid, were monitored respectively in simplified pectin and phytic acid/protein model systems. In addition, both reactions were studied in a real system, red kidney bean cotyledon during storage at 35 °C and different relative humidity conditions (6%, 54%, 66%, 72% and 82%) for 15 weeks. The T g -moisture relation of the model systems and the bean cotyledon were established by Dynamic Mechanical Thermal Analysis (DMTA). The T g -line, fitted using the Gordon-Taylor equation, was combined with the moisture sorption isotherm to create a stability diagram. The stability diagram obtained can be used to ascertain the biochemical stability during post-harvest ageing of beans. Results displayed that there was significant hydrolysis of phytic acid in the phytic acid/protein model system and the real system when stored above their glass transition values. On the other hand, in the model system the pectin degree of methylesterifcation (DM) did not significantly change during storage. In the real system, significant, although small, changes of the DM were noted when T - T g > 20 °C. An examination of the glass transition lines revealed that the reactions were influenced by the plasticizing effect of moisture. • The reactions in the cation-pectin-phytate hypothesis were studied in model and real systems. • Hydrolysis of phytic acid is controlled by glass transition. • Glass transition lines of red kidney bean cotyledon and its' cell wall polymers were created. • DMTA based T g lines are better suited for explaining (bio)chemical stability rather than TMCT. • Stability map supports in establishing suitable post-harvest storage conditions.