Margarita Barros and Mikal Saltveit, scientists from 'University of California - Davis Campus' have published on International Journal Postharvest Biology and Technology, a work on the relation between microbial growth and the wound-induced phenolic accumulation in fresh-cut lettuce.Lettuce wounding during cutting and fresh-cut processing causes several physiological responses, among which is the synthesis and accumulation of phenolic compounds.
The chlorogenic acid is the most abundant among the phenolic compounds induced by wounding and it accumulates in cut leaves held at 10°C for 2 days.
The scientists isolated and cultivated on agar TPS (tryptic soy agar) the most important 5 bacteria responsible of lettuce decay: Erwinia carotovora, Erwinia chrisanthemi, Pseudomonas putida, Pseudomonas fluorescens and Pseudomonas syringae.
Then, the scientists treated the bacteria with chlorogenic acid at the same concentration that was found in the wounded lettuce; they found that the phenolic compound did not affect the bacteria. Thus, the scientists observed that the growth of E. carotovora and P. fluorescens increased when bacteria were grown on medium containing homogenized non wounded lettuce and decreased when grown on medium containing homogenized wounded lettuce leaves that were held at 10°C for 2 days.
These results showed the antimicrobial activity of phenolic compounds. However, the wound-induced phenolic accumulation is significantly correlated to the tissue browning that is a quality decay factor in fresh-cut lettuce, in fact the phenolic accumulation is generally inhibited by using 1-hexanol or heating shock treatments. The scientists observed that treating the lettuce to stop the phenolic synthesis favors the microbial growth, especially E. carotovora growth.
This study has highlighted an important critical issue: if all treatments that reduce tissue browning by suppressing the wound-induced phenolic accumulation also make the vegetable more susceptible to the microbial growth. Probably it should be better that the research be focused on preventing the brown pigments formation rather than studying treatments to reduce the wound-induced phenolic accumulation.
Source: Barros M., Saltveit M.E., ‘Microbial growth in fresh-cut lettuce increases when wound-induced phenolic accumulation is suppressed’, 2013, Postharvest Biology and Technology, Issue No, 83, pagg. 34-39. http://www.sciencedirect.com/science/article/pii/S0925521413000793