The rehydration attributes and quality characteristics of 'Quick-cooking' dehydrated beans: Implications of glass transition on storage stability.

Storage stability is an essential consideration for minimizing the deteriorative quality changes in foods post-processing. This study, for the first, time aimed to gain insight into the storage stability of quick-cooking 'convenient' dehydrated beans (Phaseolus vulgaris L.) using the glass transition (Tg) concept. Quick-cooking dehydrated beans were prepared by hydrothermal treatment of fresh beans followed by air-drying and are rehydrated prior to use. The impact of storage temperatures (25, 28, 35 and 42 °C) on the rehydration indices (rate constant and extent) and quality characteristics (colour, texture and volatile profile) of the beans were studied. The results indicate a decrease in the rehydration rate constants with increasing storage temperatures 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). Identification of the critical water contents based on the Tg-moisture relation and the moisture sorption isotherm revealed that dehydrated beans of 10 % moisture content stored below 28 °C are in a glassy state. Overall, the quality characteristics are significantly influenced by storage and the utilization of the glass transition concept allows for identifying suitable storage conditions.

The Impact of Drying and Rehydration on the Structural Properties and Quality Attributes of Pre-Cooked Dried Beans.

Fresh common beans can be made 'instant' to produce fast-cooking beans by first soaking and cooking the beans before drying to create a shelf-stable product that can be rehydrated at the time of use. This study investigated the interplay between the drying process (air, vacuum and freeze drying), the microstructure and functional attributes of rehydrated pre-cooked beans. The microscopic study revealed that the three different drying techniques resulted in distinctly different microstructures, with the freeze drying process resulting in highly porous materials, while the air- and vacuum-dried samples underwent shrinkage. Additionally, the rehydration behavior (modeled using empirical and diffusion models) demonstrates that the high rehydration rate of freeze-dried beans is due to capillarity, while rehydration, in the case of air- and vacuum-dried beans, is primarily diffusion-controlled. Irrespective of the drying technique, the high rehydration capacity supports little to no structural collapse or damage to the cell walls. The color and texture of the rehydrated beans did not differ greatly from those of freshly cooked beans. The total peak area of the volatiles of rehydrated beans was significantly reduced by the drying process, but volatiles characteristic of the cooked bean aroma were retained. This new understanding is beneficial in tailoring the functional properties of pre-cooked dry convenient beans requiring short preparation times.

Evaluation of storage stability of low moisture whole common beans and their fractions through the use of state diagrams.

A material science approach was explored towards understanding storage stability of common dry bean seeds. State diagrams of powders from distinct bean varieties were generated through determination of their glass transition temperatures (Tgs) using differential scanning calorimetry. Confronting the state diagrams with dry matter-temperature combinations during storage facilitated establishing the link between the relative position of the bean storage conditions along the Tg line and extent of hard-to-cook (HTC) development. Generally, Tg increases with dry matter content of the bean powders implying stability at increasingly higher temperatures attributed to the reduced plasticizing effect of water. Whereas Tg lines of powders of the different bean varieties were very similar, distinct differences were observed for the powders of bean substructures. At a given moisture content, the Tg of the cotyledon material was lower than that of the seed coat material and the Tg values of the whole bean powders were dominated by the cotyledon material. Cooking time analysis showed that whole beans stored above their Tg developed the HTC defect, this extent being correlated with the difference between storage temperature and Tg value. Considering the HTC development rate, (R-value, rate of change in cooking time with storage time over a period of 0-4 months or at 0 months of storage) the higher the difference between the storage temperature and the Tg value, the faster the change in cooking time during storage. Exploring the role of the major polymer components of bean cotyledon revealed that at a given moisture content, the cell wall material showed the lowest Tg values compared to the protein and starch isolates (Tg cell wall < Tg protein < Tg starch isolate). Confronting these values with the HTC development rates (change of cooking time with storage time) supports involvement of the cell wall material and probably protein changes in the development of this defect.