(Bio)chemical stability and Physical Collapse in Pulses and Pulse-based products: A Case study on Red Kidney Beans

Promovendus/a: Shruti Aravindakshan

Promotor(en): Prof. dr. ir. Marc Hendrickx

To feed the rising world population sustainably, great emphasis is placed on the increase in the consumption of plant-based foods. In this context, pulses specifically beans have been receiving much attention due to their nutritional quality and sustainability aspects. However, one of the major factors that limit its utilization is the development of a textural defect when subjected to adverse post-harvest storage conditions which increase their cooking time and reduce the nutritional quality and the absence of convenient bean-based products. Similarly, processing and storage conditions during the preparation of quick-cooking dehydrated beans can affect consumer attributes. The choice of suitable processing and storage conditions depends on the intrinsic composition and structural characteristics and its interaction with the external environment. Utilizing a scientific basis to evaluate the biochemical and physical changes induced by processing and storage has ushered in the use of the concept of the glass transition concept of stability. Thus far, this approach has found little use in the context of the stability of dry (precooked) beans.

In this context, the aim of this PhD was twofold. Firstly, the role of storage conditions on the enzyme-catalyzed reactions in the view of hard-to-cook development during post-harvest storage was investigated. The reactions involved in the cation-pectin-phytate hypothesis (hydrolysis of phytic acid and demethylesterification of pectin) and lignification hypothesis (change in phenolic acids) were investigated in red kidney bean at controlled storage conditions. Results revealed that all the aforementioned (bio)chemical changes coincided with the presence of hardening except for the demethylesterification of pectin, which is thereby ruled out as a requirement for the development of the Hard-to-cook defect. Specifically, diminished levels of the overall phytic acid content and an increase in the free phenolic acids were noticed upon storage duration. The former was shown to be plasticized by moisture while both moisture and temperature played a role in the evolution of phenolic acids. These reactions are dependent on the state of the individual local substructures and not on the entire matrix (cotyledon).

The second part centred on the physical stability of quick-cooking dehydrated beans, whereby the impact of processing and storage on the rehydration attributes and quality characteristics were investigated. Pre-cooked dehydrated beans characterized as being convenient, shelf-stable, value-added and quick-cooking are a suitable alternative to fresh, dry beans. The effect of processing conditions (drying technique) and storage conditions (time and temperature) on the rehydration properties and quality characteristics were investigated. To do so, quick-cooking beans were first prepared by air, vacuum and freeze-drying and subsequently, the air-dried beans were subjected to the storage study. Freeze-dried beans resulted in 9-10 times faster rehydration time compared to air and/or vacuum drying. The rapid rehydration is attributed to capillarity resulting from the porous, uniform, honeycomb-like microstructure and minimal shrinkage caused by freeze-drying as opposed to the other techniques. However, irrespective of the drying technique, the rehydration time was about 50-60 % shorter than the cooking time of fresh whole beans. Irrespective of the drying choice, other characteristics including rehydration ability, colour and texture were comparable. Storage resulted in a decrease in the rehydration rate constants with increasing storage temperature and duration. The rehydration ability also significantly decreased with increased storage duration (>28 °C) suggesting a strong inverse link with hardness. Although there was no overall colour change with storage, the formation of new volatiles associated with non-enzymatic chemical reactions occurred at elevated temperatures (28-42 °C).

Overall, the findings of this doctoral thesis demonstrate the link between the state of the matrix and (bio)chemical deterioration in fresh beans. On the other hand, it is sufficiently effective in minimizing but not entirely preventing the changes occurring upon storage of quick-cooking dehydrated beans as it is likely dominated by the effects induced by the processing. Therefore, Tg-moisture relation and stability maps are a fundamental scientific basis in post-harvest and post-processing storage considerations. The current study inspires additional research on determining the Tg of the substructures of the bean tissue, which could eventually result in an increased understanding of the local mobility-based changes. Furthermore, the demonstrated link between processing/storage, structure and functionality is indispensable in the design of quick-cooking dehydrated beans. Thereby unlocking the opportunity and need for targeted processing to tailor application-based specific functionality of bean-based convenience products.