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Wed May 08, 2002 8:37 amFood.com Groupie
By Robert L. Wolke
Wednesday, February 20, 2002
Recently I wrote about what happens during the process of simmering meat or fish bones to make a stock. Several readers wrote to tell me they had a bone to pick with what I had written. Rather than treating the issue as a bone of contention or accusing me of being boneheaded, they politely informed me that I had pulled a boner by not going far enough and omitting certain information. I readily admit that, and make no bones about it. But because the issue has to do with bacterial growth, I have had to bone up on bacteriology to correct the omission.
In the earlier column I tended to agree with one of my readers that the conventional warning about cooling the stock quickly so that it passes rapidly through the temperature "danger zone" for bacterial growth (40 to 140 degrees), was perhaps overly cautious, inasmuch as the stock had just been simmered for hours and all bacteria should therefore have been killed. I did point out, however, that there is still a danger of new bacteria entering the picture as the bones are removed with a nonsterile utensil, following which the stock is strained into a nonsterile container through nonsterile cheesecloth in a nonsterile holder, and that there are few more attractive growth media for bacteria than a protein-rich meat stock.
All true, but incomplete according to my correspondents, among them a food safety consultant and a biochemist at the National Institutes of Health. It seems that all bacteria are not necessarily killed at 212 degrees Fahrenheit. Some of them can survive by forming highly invulnerable spores.
Most species of bacteria reproduce by binary fission, each organism splitting into two whole new organisms. That's why they can grow at exponential rates. Once they get started, bacteria can increase their numbers from, say, 1,000 to 2,000 to 4,000 to 8,000 to 16,000 to 32,000, etc., as often as every 10 minutes, until they possibly reach as many as 10 billion in every milliliter (one-thirtieth of an ounce) of your meat stock.
But when conditions aren't conducive to growth, or are even out-and-out hostile, some species of bacteria, fungi and green plants can ride it out in dormant forms called spores. Armored within tough, horny shells, the spores are capable of surviving boiling water, nutritional deprivation, dryness, freezing, ultraviolet light and other radiations, corrosive chemicals and detergents. When conditions improve, such as when your stock cools to a comfortable growth temperature, the spores will grow into whole new individuals that can then continue to reproduce in the normal way.
A common pathogenic -- disease-causing -- genus of spore-forming bacteria found in soil, water and the intestinal tracts of humans and animals is Clostridium, especially the species C. perfringens, which is a major cause of food poisoning, and the much rarer C. botulinum, which produces botulin toxin, one of the most potent poisons known. Clostridium bacteria don't need oxygen to live; in fact they can't survive in air, so the interior of a pot of stock is a perfect growth medium.
To kill these spores, temperatures higher than 212 degrees are needed. That's why medical and surgical equipment is sterilized in an autoclave, a sort of pressure cooker. Under higher pressures, water boils at higher temperatures. Pressure cookers and autoclaves are closed containers in which the steam pressure of boiling water builds up enough to raise its boiling temperature to about 250 degrees, high enough to kill most bacterial spores.
The insides of food cans are excellent oxygen-free breeding places for Clostridium spores. That's why, after being filled and sealed, canned foods are sterilized by heating them in high-pressure steam kettles or cookers at temperatures of 240-250 degrees.
Let me emphasize, then, that it is indeed essential for safety's sake to cool your stock as quickly as possible to prevent the growth of bacteria.
In this column I have been able to cover only the bare bones of the subject of bacterial growth, in spite of having become bone-weary from hefting reference tomes in my quest for bacteriological veracity. Knowing my readers, I can guess that this won't be the end of this discussion. I can feel it in my bones.
Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Barber: More Scientific Answers to Everyday Questions" (Dell Publications, $12.95). His next book, "What Einstein Told His Cook: Kitchen Science Explained" will be published by W.W. Norton in May. Send your kitchen questions to email@example.com
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