The color of fresh meat is considered one of the most influential factors related to fresh meat purchasing decisions. To many consumers, it can be a troubling thing, to go to the self-serve retail meat case and see one steak that is a bright, cherry-red color (packaged on a tray and wrapped in film) and right beside it is a dull, purple appearing steak (packaged in vacuum). Why the color difference? Even if those two steaks were cut from the same loin, they can appear very differently.
The reason for this apparent difference is probably due to how the meat was packaged. In order for meat to “bloom” (meat industry jargon for turning from purple to red), exposure of the primary pigment in meat (myoglobin) to oxygen is needed (*meat color is a super-complicated thing; for now, let’s presume oxygen is the only substance that can cause meat to bloom; I’ll delve into others in later entries). Thus, if fresh meat (“fresh meat” meaning steaks, chops, ground beef, etc. — not salami, bacon, ham…) is packaged in a way that lets it contact oxygen (this is how most meat in self-serve meat cases are packaged), or displayed fresh at the meat counter, it should look red. Problematically, once the steak is cut and exposed to air, oxidation (going rancid or “off”) may begin. To mitigate oxidative deterioration and essentially keep meat fresher longer, there is vacuum packaging (some folks use the blanket term “Cryovac” in lieu of vacuum), in which meat is packaged without oxygen, and thus the fresh meat would appear a dull, purplish color. Vacuum packaging is pretty handy – take the air away, and meat will keep (frozen or refrigerated) longer.
Below is an illustration of the relationships among different states of myoglobin in fresh meat:
There is a lot happening in this diagram! (1) Let’s start with DEOXYMYOGLOBIN in the upper left, which appears purplish. This is the color of meat when myoglobin is in its native state, or immediately after cutting and before blooming. For example, purple is the color of meat in the middle of a steak (i.e., When you cut across a raw, fresh steak that’s red on the surface, it should be purple in the middle. If you let the steak sit for a bit exposed to air, that color will change, or bloom, to cherry red.) (2) In the presence of oxygen (better referred to as oxygenation), fresh meat blooms and turns its characteristic red color. This form of myoglobin is called OXYMYOGLOBIN. After prolonged exposure to oxygen, (3) we then have METMYOGLOBIN, which appears brown. If you’ve ever been to the grocery and see brown spots on the “Reduced for Quick Sale” fresh meats, those superficial blemishes are METMYOGLOBIN. (Those little brown spots may not look appealing, but may not mean the meat is not safe to eat after cooking. However, if you’ve any reason to believe it’s not safe – such as smells spoiled – don’t eat it!) After the meat oxygenates and turns red, it will eventually oxidize and turn brown.
Getting into the chemistry of the matter, the state of the iron in myoglobin (the heme pigment – this is the iron than makes red meat “high in iron”) is a determining factor to fresh meat color. DEOXYMYOGLOBIN and OXYMYOGLOBIN contains iron in the ferrous (Fe 2+) state and METMYOGLOBIN contains iron in the ferric (Fe 3+) state. Let’s dig deeper into this ferrous/ferric business…
Electron management is the key to meat color management. As outlined above, the difference between desirable, red fresh meat and undesirable, brown meat is one electron. Yep, one. Follow the arrows in the diagram, and you can see how the different color forms relate to each other. A classic example of these color dynamics in action that you may have observed yourself are the different colors of beef present in one ground beef vacuum chub. Meat may look red or purple on the outside, but have a brown, muddy appearance in the middle. That’s totally okay — look above at the color cycles. The red (bloomed) ground beef was put into a vacuum package, and before it turns purple, it turns brown. Since the beef has gone through this natural color cycle a few times (from purple to red to brown to purple…), the enzymes in the meat that allow for this cycle to continue are worn out (those guys tucker out pretty quickly and easily). Thus, the meat may stop at brown and stay there. That’s just how the color dynamics work — it does not necessarily mean the beef has gone bad.
I’m working an entry as to why cooked beef color is not a good indicator of doneness, and why a meat thermometer should be used to ensure that any ground meat is cooked to 16o°F. (UPDATED: cooked ground beef color post here) There’s another thing happening in the upper right of the myoglobin color forms diagram — CARBOXYMYOGLOBIN. I’ve left that out of the color dynamics explanation for now, but will address it soon. (UPDATED: Carboxymyoglobin post here)