Photo: Alexi Kostibas

China’s pidan, or preserved eggs, go by many names: preserved egg, hundred-year egg, century egg, thousand-year egg, thousand-year-old egg, and millennium egg. You get the idea—these eggs look like they’ve been sitting around for years and years.

While their putrid-looking greenish-grey yolk and transparent, brown egg white may appear to be the furthest thing from appetizing to Western palettes, for the Chinese, these things are a common delicacy. But now, even Chinese consumers have a reason to avoid 1,000-year-old eggs. Thirty preserved egg companies are being shut down for using industrial copper sulphate, a toxic chemical, to expedite the egg-festering process. South China Morning Post reports:

Industrial copper sulphate usually contains high levels of toxic heavy metals, including arsenic, lead and cadmium, so is banned for use as a food additive.

The eggs are usually preserved with baking soda, salt, and quicklime for about two months. The process turns yolks dark green and the egg white into a stiff, dark jelly. Using copper sulphate could significantly reduce the processing time while achieving the same effect.

For now, the companies—one of which produces 300,000 tons of preserved eggs per year—are on hiatus as investigations continue. One official remarked that nearly all the preserved egg companies used this chemical, and he doesn’t consider it such a big deal. ”There won’t be a problem if you don’t eat too many of them,” he told South China Morning Post.

In other Chinese cuisine news, Quartz reports, watch out for chewing on suspect pork knuckles and chicken legs in the country. Some of those chewy treats were sold more than a year past their expiration date after being washed with detergent to cover up their foulness.

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Unless You Like Toxic Chemicals, Skip This Chinese Delicacy