Attack of the killer tomatoes?
You know that saying "Do as I say, not as I do"? I've been thinking about that lately, as I've delved into issues of food safety and tomatoes. Particularly one of my obsessions: 12-Hour Tomatoes.
They're spectacular: Roasted all day (or overnight) in a low oven, they don't get dried exactly, but a little crisp around the edges and then jammy and concentrated inside. After I first made them for my sister's wedding in Maine almost a decade ago, for a few years I would prepare a few dozen a few times every summer, using them to enliven pickle and cheese trays and throwing them on salads and into pasta dishes, but keeping them for just a couple of weeks at a time. Then, thanks to such easy access to farmers markets, I started buying a case or two of tomatoes every August and packing them in olive oil in jars, refrigerating them for months. Call it faux-canning.
When I experimented with pizza toppings and techniques recently, of course I had to include them. But eagle-eyed readers noticed that in the original recipe I referred to weeks of storage and made no mention of oil, while in the pizza piece I bragged about using them all winter long. As I prepared to update the original recipe, a little voice inside my head asked: Are you sure these are safe?
I thought they were, for various reasons, including the fact that I knew they weren't being kept in the "temperature danger zone" of 40 to 140 degrees that my food-safety instructor in culinary school drummed into our heads. But I enlisted some expert advice just in case.
I turned to Sam Beattie, consumer food safety extension specialist at Iowa State University, and he had some immediate concerns. He started with this caveat about himself and colleague Catherine Strohbehn: "We will always come down as overly cautious," he wrote in an e-mail. "We can think of all the worst-case scenarios and unfortunately have seen some of them."
The problem, he said, is that tomatoes used to be considered a high-acid food when it comes to preserving (giving them extra insurance against bacterial growth). However, newer cultivars and overripe tomatoes have less acidity and therefore require special treatment. If I were fully drying them (as in using a dehydrator), they'd be safer because there would be so little water -- and moisture is what the "spoilage microorganisms" need to grow, he said.
Canning, which uses a vacuum seal, or covering a foodstuff with oil is a great way to protect flavor and prevent oxidation. But the lack of oxygen in those methods can allow the dreaded bacterium Clostridium botulinum to grow within the small amount of moisture in the tomatoes and potentially cause botulism.
I thought that was a problem only when trying to hold something at room temperature, in that danger zone I referred to earlier: Doesn't my refrigerator prevent the growth of botulinum? Yes, but only to an extent, Beattie wrote. Most strains won't grow at 40 degrees Fahrenheit or less. But a couple of strains can grow and produce toxin at 38 degrees, and because so many home refrigerators aren't kept that cool (according to one study, many approach 50 degrees!), there is cause for worry. He wrote:
So, worst-case scenario: Low-acidity tomatoes are harvested. Drying process is carried out incompletely, with residual water left in the tomato. Semi-dry tomato is covered with oil, resulting in an anaerobic environment for the botulism spores in the water droplets in the tomatoes. Jar is sealed and put into 48-degree F refrigerator. Two months later, we have a jar of normal-looking dried tomatoes in oil that have ... botulinal toxin.
What to do?
I don't really want to fully can them, making them shelf-stable, because I think that would compromise their texture, color and possibly even flavor. (Experts such as Beattie recommend that before water-bath canning tomatoes, you acidify them with lemon juice or citric acid.) What if I am using heirloom tomatoes, surely higher in acidity than the newer varieties? Short of testing their pH, Beattie is reluctant to say that would save me. He suggests freezing them instead, first removing as much of the air as possible from the container, and preferably storing them in a chest freezer that would better maintain quality than a "frost-free" one with a continuous freeze/thaw cycle. Or, he suggested, packing them into food-safe plastic bags and covering them with oil before freezing also should help protect the quality.
I like the way he thinks, but my freezer doesn't have room for one more thing in it, at least not until I eat out of it for a while without putting anything back in.
And besides, theory and planning aside, what about the jars of these I have left in the refrigerator from last summer? I have suffered no effects of botulism, so maybe they're fine. (Beattie says that any symptoms of botulinum poisoning would be commensurate with the strain and the amount. "Sure, a little might not kill you -- just make you wish you were dead," he wrote.)
I bought a refrigerator/freezer thermometer, got out my trusty Thermapen and took some readings. First, the refrigerator itself was hovering at around 37 degrees. I turned it down a smidge just to be extra-safe. Then I pulled out one of the jars, popped it open, and stuck the instant-read pen in through the layer of congealed oil and into the tomatoes.
It read 35 degrees -- and that was after a few minutes. Phew. I made a pizza to celebrate.
For readers, however, consider me officially reluctant to officially declare my 12-Hour Tomatoes are officially safe to simply cover with oil and refrigerate, unless you know your refrigerator's temperature. We'll update the recipe to suggest a thermometer. Or a freezer.
-- Joe Yonan
March 12, 2010; 10:00 AM ET
| Tags: Joe Yonan, pizza, tomatoes
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