Population explosion in a can
Q: Why does a can of food ever go bad if there are
no bacteria in it? And if there are bacteria, why
doesn't it go bad immediately, given the geometric
progression of reproduction?
A: You're right--a can of food with no bacteria in it
when canned, and canned correctly, can last a long
time.
(Photo: Sandy Anderson-Miller, Complete Guide to Canning) Home canning.
Dale Blumenthal of the U.S. Food and Drug
Administration tells this true story: In 1865, the
steamboat, Bertrand, started down the Mississippi River heavily laden with provisions for gold
mining camps. She snagged, swamped, and sunk to the bottom of the river. A century later,
salvagers found the steamboat under 30 feet of silt with her cargo largely undamaged. They
recovered brandied peaches, oysters, plum tomatoes, honey, and mixed vegetables.
Later, chemists at the National Food Processors Association examined the canned food and
detected no bacterial growth. The 100-year-old food is edible.
Why does a can of food ever go bad? "With time, the normal process of chemical breakdown
occurs," says Dane Bernard, Vice President, Food Safety Programs, National Food Processors
Association. "This can affect the flavor and nutrients of the food but as long as the container is
intact, the food will be safe to eat."
With enough time, chemical breakdown may produce gas inside a can even though no
microorganisms are present. The food may not be spoiled, says Bernard, but always throw away
swollen cans.
To can food, canners place uncontaminated food in airtight clean containers and heat them to 250
degrees Fahrenheit, which destroys microorganisms. If a can gets punctured, bacteria will enter
and the food goes bad in hours, says Bernard.
In about 12 hours and in only a half teaspoon of space, a single bacterium can produce about five
billion offspring (the number of stars in the Milky Way), under conditions of unhindered growth,
says Ronald Hurlbert, professor of microbiology at the Washington State University.
"Such rampant fermentation, says Bernard, "is normally accompanied by production of lots of
gas". You've seen the results: a can badly bulging at both ends.
Further Surfing:
Dale Blumenthal's The canning process: Old preservation technique goes modern, U.S. Food
and Drug Administration, September 1990
Sandy Anderson-Miller's Canning Tips
Chapter 4: Bacterial Nutrition Requirements, Microbiology 101/102 by Dr. Ronald E. Hurlbert,
Internet Text, Washington State University
Food Science, National Food Processors Association, the food safety people
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