A polished approach to food safety
Kitchen hygiene should be an important part of the recipe for any meal. It involves not only frequent hand washing and cleansing of food preparation surfaces with sanitizing rinses (see To disinfect your salad), but also making sure that foods spend as little time as possible at room temperature.
In most cases, the goal of these practices is to limit the growth and transfer of germs that hitched a ride into the kitchen along with the food. But where do these germs come from?
Most of those in chicken and eggs appear to trace to infections passed around the chicken coop (SN: 5/26/96, p. 326). In other cases, they can come from a single "bad apple," so to speak -- one contaminated carcass or vegetable that sheds a few of its germs on equipment in the food-processing plant, which then goes on to taint subsequent foods running through that equipment.
In hopes of stemming this cross-contamination of materials in food-processing plants, researchers have been studying what types of surfaces offer the least receptive homes for germs. Judy W. Arnold believes she has stumbled onto a particularly inhospitable one in electropolished steel.
|A pair of stainless steel samples 4 to 6 hours after being exposed to bacteria rinsed off of poultry carcasses. The standard stainless steel (left) shows liberal settlement by the microbes, whereas the electropolished steel (right) contains almost no bacterial squatters. Photos: Sandra Silvers/ USDA-ARS|
Shiny is better
A microbiologist with the Department of Agricultureís 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. Thatís 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 doesnít 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."
processors clean their equipment frequently with high-pressure sprays and
liberal dowsings of disinfectants, but they canít do this between each
animal processed. The evisceration machines in some poultry plants, for
instance, process 90 to 140 birds a minute.
Whatís 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, thatís what electropolished steel seems to offer, Arnold says.
In follow-up studies, she hopes to identify ways of optimizing this metalís resistance to bacteria.
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Judy W. Arnold
Poultry Processing and Meat Quality Research Unit
Russell Agricultural Research Center
950 College Station Rd.
Athens, GA 30604
This week's Food for Thought has been prepared by Janet Raloff, senior editor of Science News.