Most carnivores have experienced the odd sensation of seeing a butcher scoop out lovely bright-red ground beef for them only to reveal gray meat beneath. Has the gray meat gone bad? We reached out to a butcher and double-checked the USDA to find out.

First things first: The USDA is the best first stop for food safety. It has a whole page devoted to ground beef, how to store it safely, and how to cook it. But we also wanted to talk to someone with hands-on meats-pertise, so we called Sunny Sanchez, a whole-animal butcher at The Meat Hook in Brooklyn, New York, who honed her craft at Lindy & Grundy in Los Angeles. Here’s everything fit to print about knowing whether your ground beef is safe or not.

Why does beef turn gray?

As the USDA explains, “Oxygen from the air reacts with meat pigments to form a bright red color which is usually seen on the surface of meat purchased in the supermarket.” That pigment is called oxymyoglobin, a substance found in all warm-blooded animals, and it explains why the outside of that pile of meaty goodness is red, with gray lurking underneath; it just hasn’t been exposed to the air. Super-fresh meat can look purple, in fact.

What if all of my beef is gray?

Buyer beware: Per the USDA, “the interior of the meat may be grayish brown due to lack of oxygen; however, if all the meat in the package has turned gray or brown, it may be beginning to spoil.” Sanchez is on the fence about whether she’d cook meat that was gray throughout: “I’m a butcher, I eat a lot of meat … if it smells fine and feels fine it’s still OK to eat. [But] if it’s gray throughout and you don’t see natural muscle colors, it might be time to toss it.” She parries that with meat that was gray, but still smelled and felt right, she might toss into a chili “and cook the heck out of it,” but wouldn’t serve it in a medium-rare hamburger.

Is there a better way than color to tell if it’s still good?

“I always tell customers, your nose knows,” says Sanchez. “If it starts to smell funky, just toss it.” She only butchers grassfed meat, which sometimes has a strong grassy scent, and says “normal ground beef should smell light, fresh, bright; if it starts to smell sour, musty or fishy” it’s time for it to go. Same deal if it starts to feel slimy or tacky to the touch: If in doubt, toss it.

How long will ground beef stay fresh?

The USDA suggests you eat fresh ground beef that’s been safely refrigerated within 1-2 days, and that “ground beef is safe indefinitely if kept frozen, but will lose quality over time. It is best if used within 4 months.” Sanchez concurs, and suggests that if you’re buying from a supermarket as opposed to a butcher, ask them to grind it to order, which can extend its life a tiny bit. At her shop, she often offers to vacuum-seal meat, which “will stabilize it for four to five days longer.”

Any other tips?

If you can buy your meat at a small shop, specifically a whole animal shop, says Sanchez, there will be less of a chance of cross-contamination, since one animal is butchered at a time, and “your ground beef will contain the meat of one animal, not several.” It’s food for thought.

Alex Van Buren is a food and travel writer living in Brooklyn, New York whose work has appeared in, Bon Appétit, Martha Stewart Living, Travel & Leisure, New York Magazine, and Epicurious. Follow her on Twitter and Instagram @alexvanburen