Shiny is better

A microbiologist with the Department of Agriculture’s Poultry Processing and Meat Quality Research Unit in Athens, Ga., Arnold has been testing the attractiveness of various materials used in poultry-processing equipment to the Salmonella, Campylobacter, and other food-poisoning bacteria that commonly infect chickens.

In general, she notes, the more porous the surface, the better bacteria like it. That’s why conveyor belts made from chains of polyethylene links are particularly vulnerable. Her findings suggest that the more old-fashioned, solid rubber belts might be preferable.

While ordinary stainless steel, a standby of food processors, is not porous, it does provide a safe haven for microbes. Its surface appears smooth to the naked eye, but nooks and crannies show up under scanning electron microscopes. These microcrevices can give at least some bacterial squatters shelter from the chemical and mechanical cleansers designed to evict them.

Electropolished stainless steel undergoes an acid bath, followed by a finishing step that sends an electric current through the metal. What emerges is a far shinier steel with an almost chromelike luster.

More important, fewer bacteria took up housekeeping on the electropolished steel.

Arnold is at work investigating why. She suspects that the finishing step may impart a negative charge to the surface that repels bacteria, which tend to carry a negative charge themselves. However, she points out, "charge is probably only one of several factors that play an important role." After all, she notes, the surface charge on the metal probably doesn’t last long in an environment subjected to all type of charged materials, including water, manure, blood, and tissue. "So the smoothness of this steel probably makes a great deal of difference too."

Between cleansings

Food processors clean their equipment frequently with high-pressure sprays and liberal dowsings of disinfectants, but they can’t do this between each animal processed. The evisceration machines in some poultry plants, for instance, process 90 to 140 birds a minute. What’s more, bacteria are a very sociable lot. A pioneering bug will quickly set up colonies of hundreds or thousands of progeny. Each of the initial homesteaders lays down filamentous fibrils that not only help anchor it to the metal but also tend to link it to adjacent bacteria (see photo 1). Successive waves of unrelated bacteria then join them to create a multicultural community of microbes. Often, some of the later emigrants exude a gluey film that eventually covers the entire community. Studies have shown that such biofilms are particularly resistant to removal (SN: 7/20/85, p. 42).

Bacteria on a metal surface exude fibrils that bind cells into a strong, raftlike structure.   Photo: Sandra Silvers/ USDA-ARS

Manufacturers are looking for ways to cut down on the buildup of bacteria and to evict them before the biofilm producers arrive. If food processors can limit the initial adhesion of these microbes so that cleansers can do a more thorough job -- or better yet, so that smaller quantities of disinfectants are needed -- they stand to improve not only the safety of their products, but also their bottom line.

Though more expensive than ordinary stainless steel, that’s what electropolished steel seems to offer, Arnold says.

In follow-up studies, she hopes to identify ways of optimizing this metal’s resistance to bacteria.

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This week's Food for Thought has been prepared by Janet Raloff, senior editor of Science News.

copyright 1998 Science Service