Title : Evaluation of the seed coat darkening in dry bean genotypes from the first post-harvest days
In Mexico, as a center of origin of beans, a wide variety of common bean genotypes (Phaseolus vulgaris L.) are cultivated. Depending on the areas, consumers may prefer light or dark colored grains. Most of the beans are grown under rainfed conditions during the spring-summer season and are marketed throughout the year. Light colored seeds are prone to darkening during storage, which decreases its commercial value because it is associated with an aged grain. In this work the seed coat propensity to darkening of six light-colored beans was evaluated: four from the Bayo variety, one Canarian variety and one yellow colored variety. The varieties were grown under rainfed conditions. Once ripe, four plants of each variety were cut. The grains of two plants were mixed to form a repetition. Two repetitions and one control of each genotype were analyzed. The controls were kept refrigerated, while the two repetitions were left exposed to natural light in the laboratory for 77 days. The color of the seed coat was measured using a Konika Minolta CM5 reflectance spectrophotometer. The first measurement was done during the removal of the grains from the pods and the subsequent ones were done every seven days. Likewise, polyphenol oxidase (PPO) activity in genotypes was measured. The propensity to darken was different between genotypes. The color change of the seed coat was mainly detected as a decrease in the value L* (lightness) and an increase in red tones a*. There were also differences in PPO activity. Consistently, samples exposed to natural light showed greater PPO activity than their corresponding controls. In the Bayo type, greater PPO activity was detected in Bayomex, a variety that showed greater darkening of the seed coat and less activity in Bayo Inifap, which was the variety with less grain darkening. Darkening of the seed coat was detected after 14 days postharvest, and correlated with the one observed after 77 days. Azufradoro showed scarce darkening and no association was observed with the activity of the PPO. The results show that it is possible to identify the propensity to darkening in the early days after harvest without performing artificial aging tests at high temperature and relative humidity and without the need for a UV lamp.