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Science News Online - Food for Thought - 2/14/98

February [1] 14, 1998

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.

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).

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.