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Does your cupboard hold a package of unfinished crackers? An old bag of whole grain flour? Some leftover nuts from baking? Or perhaps some vegetable oil you've been slow to finish?

If so, you may be harboring dangerous, rancid foods.

Protecting against rancidity -- which occurs when oils oxidize -- has long been a challenge for home cooks, but a recent perfect stew of factors has made the issue more serious. This situation comes courtesy of the rising popularity of "healthy" polyunsaturated fats, whole grain flours and warehouse stores. They've resulted in American pantries full of food that goes rancid much faster than we're used to.

Add to that Americans' growing acclimation to the taste of rancid foods, and the problem gets bigger.

So what's the problem with eating rancid oils?

"There are at least two," says lipid specialist and University of Massachusetts professor Eric Decker. "One is that they lose their vitamins, but they also can develop potentially toxic compounds" that have been linked to advanced aging, neurological disorders, heart disease and cancer.

"They're carcinogenic, pro-inflammatory and very toxic," says integrative medicine specialist Andrew Weil. "They are also widespread in the food chain."

The growing problem comes as a byproduct of Americans and food manufacturers swapping transfats for polyunsaturates in their products over the past 10 years. For all of their artery-blocking evil, transfats had at least one big benefit: They were very stable, meaning they took forever to go rancid. The same is true of highly refined white flours.

But when these flours and fats were replaced with whole grain flours and polyunsaturates, such as corn and soybean oil, that shelf stability collapsed.

"Manufacturers noticed this and had to change their delivery schedules and formulations," says Kantha Shelke a food scientist and spokeswoman for the Institute of Food Technologists. "And some consumers became irrationally angry that their food was not lasting as long as it had before."

While monounsaturates (like olive or peanut oil) also can go rancid after about a year, they are still 10 times more stable than polyunsaturates, according to Decker.

"People need to minimize their use [of polyunsaturated oils]," Weil says. "And if you do use them, keep them in the refrigerator in the dark, and buy only small amounts that you use up quickly."

And while some consumers can sniff out (and toss out) rancid foods, many don't know the telltale stale, grassy, paintlike odor. Others may not be able to detect them through layers of other flavorings. And still others might feel compelled to consume them out of thriftiness or hopes that a strong sauce will mask the taste.

Because air, light and heat speed up oxidation, it's normally a bad idea to, for example, buy vegetable oil in a clear bottle and place it on the counter in a warm kitchen for several months.

Exotic oils (macadamia, walnut, sesame, fish, flaxseed, etc.), nuts and whole grain flours are also major candidates for fast rancidity, and should all be stored in the refrigerator or freezer. Whole intact grains and nuts in their shells, however, last much longer.

Frequent shopping for small quantities of fresh and freshly processed foods has served much of the world well in avoiding rancid food. Americans, however, favor a different grocery-shopping pattern that involves less-frequent trips for larger quantities of shelf-stable foods.

Though some hope that our sense of smell and taste can help us avoid rancid foods, recent studies raise doubts. Shelke notes that new immigrants to America often think peanut butter smells rancid while American natives think it just smells like peanut butter.