Polyphenol oxidase activity in common beans with different post-harvest time

In Mexico, part of the bean production is stored for months to satisfy the demand during the year. The environmental conditions of the warehouse and the storage time influence the quality of the grain. In light-colored bean varieties, the darkening of the seedcoat occurs, which leads to a decrease in market value, since it is associated with an aged grain. We have previously found that polyphenol oxidase activity in some varieties in accelerated storage tests is associated with seedcoat darkening. The objective of this work was to determine the activity of polyphenol oxidase (PPO) and its possible association with darkening of the seedcoat in light-colored bean genotypes with postharvest times from 1 to 3 years. Seven bean varieties were studied, provided by the CEVAMEX-INIFAP Bean Genetic Improvement Program. Azufradoro, a yellow colored variety was included as slow darkening reference. The PPO activity was determined for these genotypes. The color of the seed coat was measured using a Konika Minolta CM5 reflectance spectrophotometer. At a longer postharvest time, six of the varieties showed visible darkening of the seedcoat. The values of L* tended to decrease, while those of a* and b* increased, which indicated an increase in red and yellow tones. Statistical differences in PPO activity were observed between varieties. The Azufradoro variety showed the highest color stability and the lowest PPO activity. A positive statistical correlation was identified between the PPO activity and the a* b* variables of grain color. These results indicate that the enzyme polyphenol oxidase participates in the formation of pigments that modify the color of the seedcoat.