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I have a question on a situation that everyone runs into:
If you remove the mold that forms atop an opened can of tomato paste, and carefully remove any visible moldy portions so that the next spoonful coming out doesn't touch any mold -- is it safe to use that paste? If you tasted it, and it tasted OK, I would assume it's still a bad idea to push your luck and say "Well it tastes OK, so I'll use it anyway?"
And yes, I'm asking this because I just tossed my umpteenth opened can of moldy paste, and am wondering if I could've used the OK-looking (and tasting) paste on the bottom of the can. But no, I haven't spread it on a slide and looked for microscopic remnants of mold on the can surface or in the paste itself.
Your question is one that we have all probably asked at one time or another. The USDA and FDA use a test known as the "Howard Mold Count" to determine the amount of mold in products that you purchase. There are regulations about the acceptable levels of mold in certain foods. The minute you open that can of tomato paste, any mold that is already in the can can grow with the exposure to oxygen. The longer it is in the fridge, the more chance of mold growing.
Most of the molds are benign and not likely to hurt you, but they will taint the flavor of the tomato paste. While the molds might not hurt you, I myself would not risk it. In the comments below several readers have posted ways to safely store the unused tomato paste for later use. Rather than keeping a jar of paste in the refrigerator and taking the health risk, I would use one of their methods to store the unused paste (and have done so).
On the other hand, there's an even more convenient solution than freezing the unused, canned tomato paste. There are many manufacturers that sell tomato paste in a tube. Just squeeze out what you need and reclose the tube. This is widely available in many grocery stores, and while it's a little higher priced than a can of traditional tomato paste, none gets wasted. I use Amore Tomato Paste.
Timothy S. Harlan, MD, FACP