我也找到一篇文章，也是這樣說的。

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.

https://www.myrecipes.com/ingredients/beef-recipes/is-my-ground-beef-safe-if-it-has-changed-color