Researchers are examining ways to reduce the risk of bacterial contamination in lettuce by applying a protein known to increase resistance to disease.

Their work also shows the protein, harpin, prolongs the shelf life of processed lettuce used in bagged salads, said Jorge Fonseca, the lead researcher an assistant specialist in the Department of Plant Sciences at the University of Arizona's Yuma Agricultural Center.

Research shows protein can prolong lettuce shelf life
           Courtesy Jorge Fonseca, University of Arizona

Harpin, a protein that can boost a plant's protection against bacteria, was applied to these cored heads of lettuce used during a research project by the University of Arizona, Rutgers Cooperative Extension, and University of California-Davis.

"I thought why not test it against actual bacteria that grows naturally and could potentially harm also the consumers," Fonseca said.

The findings were published in January in the Journal of the American Society for Horticultural Science.

Fonseca worked with researchers from Rutgers Cooperative Extension and the University of California-Davis, testing two-acres of sniper, desert queen and sahara varieties of lettuce in California, New Jersey and Arizona between 2004 and 2005.

Fonseca said lettuce in the California growing regions around Salinas and Watsonville responded best in terms of quality to the lowest doses of harpin and showed the smallest bacteria populations. Harpin causes the lettuce to produce phenolic compounds that can improve a plant's chemical defenses against microbes, according to the study.

The team's research, though not completely published, has generated interest from salad processors, Fonseca said, who have contacted him for more information about harpin's possible commercial applications.

Samantha Cabaluna, director of communications for Natural Selection Foods, San Juan Bautista, Calif., which owns the organic Earthbound Farm brand, said the company isn't familiar with the study but "we are closely following and supporting a variety of research projects."

Harpin was applied through a spray at different strengths to test areas five days before harvest, and a control group at each site was treated only with tap water. Immediately after harvest, according to the study, six lettuce heads from each group were stored in coolers for three hours before being cut into pieces and stored in sealed plastic bags for 20 days.

Fonseca said the research team wasn't looking directly for quality improvements but found harpin's greatest effect was prolonged shelf life when larger doses of the protein were applied. Fonseca said more research needs to be done in this area as well as harpin's effect on specific types of bacteria. The team also didn't look at the protein's effect on specific strains of bacteria like E.coli O157:H7, instead focusing on the general background of aerobic bacteria in fresh-cut lettuce.

According to the study, overall results in New Jersey showed no major differences among treatments, but in Arizona, microbial population was lower and visual quality was higher in lettuce treated higher levels of harpin.

Fonseca said overall there was very little quality improvement seen during the research in New Jersey, some improvement in Arizona, and the best quality seen in the California lettuce. Fonseca said the difference in quality is due to how long harpin was applied before harvest and how much was used.