HOW THE COMMON BEAN FOOD CHAIN AFFECTS MINERAL QUALITY

In order to prevent and overcome mineral deficiencies, healthy diets with a balanced nutritional profile are key. Common beans and other pulses contain significant amounts of minerals and are increasingly being suggested as important components of healthy diets. However, the mineral quality of a food product is not only determined by its mineral content but also by the bioaccessibility of the minerals. While the total mineral content comprises all minerals present in the food product, the bioaccessible mineral fraction is the amount of minerals that is readily available for absorption in the human body. In this PhD work, magnesium (Mg) and calcium (Ca)  were chosen as representatives of major minerals (>100 mg/day), while iron (Fe) and zinc (Zn) were chosen to represent trace minerals (<100 mg/day). Moreover, all of the aforementioned minerals are reported to be linked to widespread mineral deficiencies.

Within the common bean food matrix, nutrients, including minerals, are encapsulated by a cell wall that is indigestible for humans. In addition, common beans contain indigestible compounds which can chelate minerals, i.e. mineral antinutrients. Since minerals can only be absorbed in the human body when they are able to diffuse to the absorption site and  are in a free, non-chelated state (in this work denoted as soluble minerals), we hypothesized that those physical (i.e. encapsulation by a cell wall) and/or chemical (i.e. chelation by indigestible molecules such as phytic acid and pectin) barriers reduce the amount of minerals available for absorption. To investigate to which extent these barriers impede the accessibility of minerals, the soluble mineral content was determined in cooked bean cotyledons before and after cell wall disruption as well as before and after selective phytic acid and pectin degradation. It could be concluded that the cell wall does not form a natural barrier for mineral diffusion in cooked common bean cotyledons, whereas the presence of antinutrients does decrease the soluble mineral content and as a result, potentially reduces the mineral bioaccessibility. Consequently, although beans are a valuable source of minerals, estimating their mineral quality purely based on mineral content may lead to a considerable overestimation.

Along the common bean food chain, several stages are hypothesized to influence the mineral concentration as well as the level of mineral chelation and therefore the mineral quality. In this research, the chain from postharvest storage and cooking up to digestion was taken into consideration during evaluation of the mineral quality of common beans. Postharvest storage at increased temperature and R.H. (35°C and 80% R.H.) already induced development of the hard-to-cook defect after 8 weeks of storage. A known consequence of this effect is the need for prolonged cooking times in order to obtain a palatable solid food product. Moreover, the interaction between minerals and antinutrients is hypothesized to play an important role in the occurrence of this defect. The effect of postharvest storage on mineral bioaccessibility in common beans, however, had  not yet been investigated. Upon prolonged storage, associated with development of the hard-to-cook defect, we observed the cell wall-chelated mineral fraction to increase and consequently the mineral bioaccessibility to reduce. Additionally, mineral bioaccessibility was found to decrease with cooking time, which was hypothesized to be caused by the chelation of free minerals by compounds which gain accessibility as a result of the cooking process (e.g. more extensive pectin solubilisation). Since the composition of seed coat and cotyledons in common beans is different, the effect of dehulling on mineral bioaccessibility was evaluated as well, taking into account different storage and cooking times. Overall, in contrast to Mg and Ca, the bioaccessible amount of Fe and Zn increased when beans were dehulled.

From these results, we can conclude that, if a maximal mineral bioaccessibility in cooked common beans is desired, it is advised to select fresh beans (avoiding storage at increased temperature and R.H.), keep the cooking time as short as possible and dehull the beans in case of specific need for Fe and/or Zn. Despite these recommendations, a certain mineral fraction seems to remain chelated by mineral antinutrients. A possible strategy to maximally exploit the total mineral delivery potential of common beans was explored in this research as well. This strategy consisted of degrading mineral antinutrients before or during digestion in order to reduce their mineral binding capacity and in turn release more minerals. For example, when a healthy man (18-65 years) would consume 100 g of cooked common bean cotyledons, this would contribute to approximately 20% of his required daily Fe intake. When he would consume 100 g of cooked common bean cotyledons along with antinutrient degrading enzymes (e.g. using a stomach resistant capsule), this contribution would increase to approximately 30%. 

Through a more fundamental approach, i.e. using pectin-mineral model systems, mineral competition between Zn and Ca for binding pectin was observed. This effect results in  increasing Zn bioaccessibility when the Ca concentration exceeds the Zn concentration, which plateaus at a fourfold excess of Ca over Zn. In common beans for example, the Ca concentration is already over 10 times the Zn concentration. Therefore, in meals that mainly consist of common beans, applying this strategy (i.e. addition of Ca) is not believed to further increase Zn bioaccessibility. 

Overall, it can be concluded that common beans contain interesting levels of minerals, but the presence of mineral antinutrients hampers their bioaccessibility. To increase the mineral delivery potential, the solution lays within reducing the mineral binding capacity of mineral antinutrients. Unfortunately, this cannot be achieved by conventional thermal processing (cooking) but dehulling before cooking and selecting appropriate cooking times can avoid a further reduction of the mineral bioaccessibility. Moreover, enzymatic degradation of these antinutrients (before or during digestion) proved to have significant potential in increasing the mineral bioaccessibility of common beans.