Friendly bacteria might not be an easy notion to wrap your brain around in the context of food these days.

Monica Corrado says bring 'em on. And she's not the only one.

Pack raw food into a jar, then seal it to keep out air, says the Takoma Park teacher of lactofermentation. Leave at room temperature and let feisty, naturally occurring microbes go to town for several days or even weeks. Open. Taste. Feast.

To see Corrado lick her lips after lapping up some of her "live" homemade ketchup, to watch her eyes dance as she opens a jar of her bubbling salsa and, yes, to taste her hissing peach chutney, redolent with crushed red pepper, is to concede that she might be on to something.

Part science, part art, lactofermentation is an ancient method of food preservation using live bacterial cultures. Anathema though it may seem to a generation of antibacterial hand-gel obsessives, the technique is increasingly being embraced by DIY aficionados and whole-food advocates who like the idea of low-tech preservation and also believe that unpasteurized foods aid digestion and boost immunity.

As Corrado puts it, "We're live people. We're not meant to eat only dead food!"

A former caterer of organic cuisine, Corrado became intrigued with lactofermentation a decade ago after meeting Sally Fallon, author of "Nourishing Traditions" (NewTrends, 1999). The nearly-700-page tome on nutrition is considered essential background reading by fermentation fetishists, but Corrado found it lacking in practical advice. Five years ago, she began teaching classes at farmers markets, food co-ops and private residences in the Washington area. The first thing she instructs students? Get over the fear of bacteria.

For every salmonella or E. coli - "the bad guys," as Corrado calls them - there are many more "good guys," such as lactobacilli. Healthful bacteria outnumber the body's other cells 10-1; trillions of them populate the intestinal tract alone.

Lactobacilli in particular flourish when food is submerged in liquid in an anaerobic (or oxygen-free) environment. The bacteria feed on the sugars and multiply in such great numbers that they overwhelm any contaminants trying to muscle their way in. "The lactobacilli can number a billion per gram of tissue at the height of fermentation; it's amazing," says Roger McFeeters, a USDA scientist who oversees the agency's fermentation laboratory in Raleigh, N.C. "The bad bacteria can't compete."

According to McFeeters, lactofermentation has caused no known food-borne illness. The Food and Drug Administration places no specific regulations on vendors of live ferments. "As far as we know, it's been going on for thousands of years," says the scientist. "It's perfectly safe."

Anthropologists say the practice of alcoholic fermentation, involving yeast, is more than 8,000 years old, while lactofermentation is thought to have developed later, alongside agriculture and long before refrigeration, as farmers sought ways to prevent food from spoiling after harvest. Historians credit lactoferments with at least one achievement: British sea captain James Cook's completion of an around-the-world sail without losing a single man to scurvy, thanks to the 60 barrels of sauerkraut he packed for the 18th-century adventure.

The practice has endured in many countries but fell out of favor in American kitchens with the advent of the processed-food industry. Although many edibles and beverages are subject to fermentation at some stage of industrial production, the live cultures usually are killed through cooking or pasteurization before the products go to market. (Most yogurts advertised as containing live cultures are pasteurized before the cultures are added.)