In Mexico, as a center of origin of beans, a wide variety of dry 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 tend to darken and this trait affects its market value. The objective of this work was to evaluate the seed coat propensity to darken in light-colored dry bean varieties during the first posharvest days. Five Bayo type, and one Canarian varieties were studied. Azufradoro, a yellow colored variety was included as slow darkening reference. 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 experimental samples 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 after the removal of the grains from the pods and the subsequent ones were done every seven days. Likewise, polyphenol oxidase (PPO) activity in the 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 controls. Bayomex, was the variety that showed the greatest darkening of the seed coat and also the one in which the highest PPO activity was detected. Azufradoro showed little darkening and PPO activity was not detectable. Bayo Inifap and Bayo Azteca were the varieties with lesser grain darkening. ?a* detected after 14 days postharvest was correlated (r=0.75*) with the one observed after 77 days. The results show that it is possible to identify the propensity to darken in the early days after harvest without performing accelerated aging tests.
Audience Take Away:
• Through the use of reflectance spectrometry it is possible to detect changes in color in the early stages of darkening
• The results of this work show that is possible to identify slow darkening genotypes in a simple way