This CHOW Tip turns out a perfect sous-vide egg with ease. But not all readers feel easy about chef Janine Falvo’s technique, which involves tying up a raw egg in plastic wrap and poaching it in simmering water. As one reader commented, “Is anyone else scared of the plastic wrap decomposing and releasing toxins?” Our reply: Sure.

To find out if this practice is safe, we consulted Daniel Schmidt, an associate professor in the Department of Plastics Engineering at the University of Massachusetts Lowell, who warned us that while he’s neither a toxicologist nor an MD, he does know a lot about plastic.

Schmidt says the main thing to worry about when cooking in plastic is what’s known as a “plasticizer,” the substance that makes plastic soft and supple. Plasticizers are made of molecules smaller and more mobile than the macromolecules in the plastics they condition. “When molecules are small and mobile, they can migrate much more easily,” he explains, “so the exposure potential is automatically higher.”

What do these molecules do once they migrate into your body? For men, it can mean reduced sperm counts or testicular atrophy. That’s the bad news. Here’s the good news: Most plastic wraps, zip-top bags, and other plastic things you buy in the grocery store are plasticizer-free.

Older readers may recall cling wrap getting less clingy some years back. That’s when most wraps made the switch from plasticized polyvinylidene chloride (PVDC) to polyethylene. “It’s only some of the food service companies and smaller firms that supply the older type,” says Schmidt, adding that the PVDC kind has a plastic-y smell and sometimes a blue or purple tint on the roll.

Polyethelene is much more stable than plasticized plastics. It’s fine at the low temperatures typical of sous-vide cooking (which generally don’t creep above 140 degrees Fahrenheit). They do, however, start to soften at temperatures above 176 degrees Fahrenheit; that’s probably why some of the responders on the sous-vide egg post experienced melted wrap.

Schmidt says he’d feed food cooked in plastic wrap to his toddler daughter, no problem. He worries more about the potential for bacteria in food that isn’t fully cooked.

Still, he understands the level of concern that’s caused some cooks to seek out materials like silicone bags. And while many plastics do, under stress, emit estrogenic compounds, Schmidt suggests that’s not such a big deal, compared to risks—like inhaling car exhaust and exposure to UV rays—we take for granted.

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