July 2006's heat wave may have been a factor in the extensive spinach… (Al Seib / Los Angeles Times )
Heating spinach remains a tried-and-true method of killing nasty pathogens like E. coli, but how long do you need to heat it? Can you warm it up, or do you need to cook it to a crisp?
That's the burning question addressed by research presented at this week's Interscience Conference on Antimicrobial Agents and Chemotherapy in Boston. Charles Pavia, a microbiologist at the New York Institute of Technology, infected spinach with bacteria from the September 2006 E. coli outbreak that struck several regions of the United States, then microwaved or boiled the greens for different lengths of time to see how many bacteria survived.
While most studies call it a day when around 90% of a pathogen is destroyed, Pavia said in a telephone interview that he wanted to find what it took to totally decontaminate the spinach. Boiling for 30 seconds destroyed 91% to 93% of bacteria, a minute raised the kill rate to 96% to 98%, and two minutes of heating eradicated 99%. Microwaving showed similar numbers. Finally, the spinach was heated for four minutes, at which point no signs of live bacteria showed up in the Petri dishes.
But wait -- four minutes for 10 grams of well-chopped spinach? That seems like overkill. Sure, I don't want any nasty bugs in my vegetables, but I'm guessing that four minutes of zapping would render the spinach pretty unappetizing.
Lest we forget, boiling spinach for too long leaches valuable vitamins from those leafy greens. And even though microwaving preserves nutrients far better than boiling, how well those nutrients are preserved is inversely related to how much water is in the food.
It's true that's something to consider, Pavia said, but he pointed out that the three-minute mark hadn't been tested (so it might be just as good as four), and the frozen spinach we buy in the supermarkets has already been cooked too.
The study could also assuage those overcookers who boil their greens to a pulp, Pavia added. So do we need to cook all spinach for four minutes to ensure that we won't get sick from it? The problem with trying to answer that question, Pavia said, is that it's not clear exactly how many microbes it would take to infect a person. Some require an army a million strong, while others need to invade only in groups of 100 or so.
Many infection thresholds for other bugs were determined several decades ago, back when ethical guidelines were less restrictive. Volunteers would consume a certain amount of pathogen and researchers would see whether or not they got sick at those exposure levels, Pavia said.
So without that key bit of information, it's hard to say exactly at what point you can switch off the stove or stop zapping the greens, he explained. Cooking for two minutes appears to kill off enough bacteria to be a worthwhile practice.
As for those who can't give up their fresh spinach salads?
"I think the take-home message really is, if you don't want to cook your spinach, don't give it to a child, based on what we know about who gets sickest the most," Pavia said.
-- Amina Khan / Los Angeles Times
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