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Mechanistic insight into hard-to-cook development in common beans: Integrating reaction kinetics and state diagrams
Book - Dissertation
In recent years, the shift toward plant-based foods has largely increased the global awareness of the nutritional importance of legumes and their potential role in sustainable food systems. Common beans are and have been a staple source of affordable proteins and micronutrients for many people in developing nations and the increasing demand for healthy and sustainable foods in developed nations has seen a revival of interest in legumes consumption in general. Postharvest storage is a fundamental part of the common beans value chain as it facilitates a steady supply by bridging the gap between different harvesting seasons. However, long term storage of beans at high temperature (>25 °C) and relative humidity (>65%), relevant conditions in the tropical regions where beans form a significant part of the diet, results in development of hard-to-cook (HTC), a cooking quality defect that causes long cooking times of beans. The mechanisms involved in HTC development during storage of beans are rather complex and despite the tremendous efforts devoted to understanding the occurrence of this phenomenon the mechanisms largely remain unresolved. In this doctoral study, an integrated approach combining material science (glass transition temperature) and reaction kinetics was elaborated to gain insight into the mechanism of HTC development during storage of beans. As a first step, the plausibility of the most classical hypothesis of HTC development (pectin-cation-phytate theory) was evaluated by assessing the storage-induced changes in intrinsic compositional factors linked to this hypothesis in seven common bean varieties that have different susceptibilities (low, intermediate and high) to develop HTC. In particular, decrease in phytate content during ageing significantly correlated (r=-0.66, p<0.05) with increase in cooking times of the different bean varieties under study. Given that no significant changes in pectin degree of methylesterification were observed during ageing and increase in cell wall-bound calcium content was mainly significant during a subsequent cooking step following ageing, it was concluded that endogenous phytate hydrolysis is the most significant step of the pectin-cation-phytate theory and cation migration/mobility is the rate limiting process. While it has been established that temperature and relative humidity (influencing the moisture content of the bean) affect the rate of HTC development, the precise interactions between these extrinsic factors and the physical environment of the bean matrix to facilitate (bio)chemical reactions resulting in cooking quality changes of stored beans is not well elaborated. Subsequently, the role of extrinsic factors (storage temperature, moisture content and time) in facilitating the nature and kinetics of (bio)chemical reactions was evaluated in red kidney beans. The rate constants of different reactions (HTC development, phytate hydrolysis and volatile generating (bio)chemical reactions) were linked to storage above Tg which governs the physical state of the bean matrix. Storage temperature, moisture content and time did not influence the texture of beans before cooking but they significantly influenced their cooking behavior. Further modelling of cooking rate constants as a function of storage time resulted in HTC development rate constants that varied between 6.48±3.35 ×10-3 to 44.2±2.96 ×10-3 week-1 for beans stored at respectively 8.7% and 14.5% moisture content and 35 °C. A regression analysis on the influence of storage conditions on HTC development rate constants resulted in a significant interaction between storage temperature and moisture content whereby these two factors showed a synergistic effect. There was a high correlation (r=0.742, p<0.05) between HTC development rate constants and storage above Tg whereby the further above the Tg the beans were stored the higher the rate constants for HTC development were, and vice versa and this relation was dependent on moisture content of the beans. An evaluation of the kinetics of endogenous phytate hydrolysis in beans stored at varying temperature and moisture content relative to their Tg resulted in phytate hydrolysis rate constants that ranged between 0.008±0.002 to 0.058±0.003 week-1 for beans stored at respectively, 25 and 42°C. It was revealed that the rate of phytate hydrolysis is mostly influenced by storage temperature with limited influence of bean moisture content. As such, the overall Tg of the beans did not seem to explain the plasticizing effect of temperature and moisture content expected to influence endogenous phytate hydrolysis and this was attributed to possible existence of local differences in water binding capacity. In spite of that, the rate constants of phytate hydrolysis had a high significant correlation (r>0.98, p<0.05) with HTC development rate constants indicating that beans with a high rate of phytate hydrolysis during storage are more susceptible to HTC development. An evaluation of volatile generating (bio)chemical reactions during storage of beans at varying temperature and moisture content resulted in a high number of volatile compounds being detected as storage temperature and moisture content increased and they ranged between 53 and 120 for beans stored at respectively, 6.9% and 14.5% moisture content at 35 °C. The volatile marker compounds identified were mainly alcohols, ketones and benzaldehydes and are typical for protein degradation and lipid oxidation reactions which are the main (bio)chemical reactions associated with storage-induced ageing of beans. Evaluation of the kinetics revealed that evolutions of particular marker compounds were relevant for specific storage conditions signifying that the nature and rate of (bio)chemical reactions are dependent on storage conditions. The rate constants of evolution of marker compounds highly correlated (r>0.6, p<0.05) with storage above Tg indicating that Tg can be used to largely control the rate of these reactions. This doctoral thesis provides a profound understanding of the mechanisms involved in HTC development through an in-depth elaboration of how extrinsic factors during storage of beans influence the nature and kinetics of (bio)chemical reactions linked to this phenomenon and how they are intricately dependent on the physical state of the beans. The insights presented here are crucial in developing practical solutions to control the occurrence of this phenomenon and highlight useful gaps for further scientific research.