Eliminating microbes from leafy greens

Strong chlorine washes used by processors to eliminate dangerous pathogens in fresh produce are ineffective but bombarding them with intense radiation kills nearly all the germs, according to a study by U.S. Department of Agriculture researchers presented to American Chemical Society at its annual conference Thursday.

The scientists said irradiation could be the best means to eliminate pathogens inside fruits and vegetables and on difficult-to-reach spots. Irradiation is a process of blasting food with charged beams of gamma rays, x-rays or electrons to kill insects and pathogens. The process does not warm the food, unlike microwave radiation or cooking.

Commercial processes usually wash leafy greens three times, using high concentrations of chlorine dissolved in aqueous solutions. But questions have long been raised about the adequacy of these procedures against E. coli, Salmonella, and other dangerous microbes.

Such microbes are able to evade chlorinated water by penetrating into leaves. They also coalesce into tight-knit colonies, known as biofilms, sticking to the surfaces. These measures make them difficult to kill.

The team evaluated three different procedures for eliminating microbes, using romaine lettuce and baby spinach leaves. The leaves were cut up and soaked in a bath filled with E. coli. They used a vacuum process to enable the bugs to penetrate into the leaves.

The leaves were then cleaned for three minutes in a plain water wash and in highly chlorinated water. The third procedure was irradiation. Some of the findings were:

* The plain water wash did not do anything much to lower E. coli levels.

* The chemical treatment was ineffective in spinach and eliminated less than 90 percent of the E. coli on the surface of lettuce leaves but none of the bacteria inside leaves.

* Radiation achieved significant reductions of E. coli in both vegetables. At the highest level of radiation, 99.99 percent of E. coli in lettuce was killed, and just a shade lower for spinach at 99.90 percent.

* Biofilms consisting of salmonella colonies were more easily killed by irradiation than E. coli (this was in another comparative test).

* Killing bacteria inside leaves required two times as much radiation as for killing bacteria on the surface.

For all its apparent effectiveness, irradiation is controversial. Consumer groups believe there are safety issues that need to be studied further.  Some experts fear the radiation could damage cells in the leaves and shorten shelf life. Processing times might also become longer, and there may be unknown effects on the environment.

Safety Tip:

* Strengthen your immune system. Bodies need some exposure to pathogens to stimulate antibody production. Find ways to strengthen your immune system through exercise, good sleep, and proper diet.