The red juice that often collects in a package of red meat is not blood, as many assume. Most of the blood is removed during processing and any that remains is usually contained within the muscle tissue.
The red liquid, instead, is a mixture of water and a protein called myoglobin, whose purpose is to help ship oxygen to muscle cells. Myoglobin is deeply pigmented, which is why the more myoglobin a meat contains, the darker (or redder) the meat will be.
Red meat is comprised of muscles that are used for extensive activity. Remember, myoglobin’s role is to help bring oxygen to the muscles, and oxygen is required to give muscles energy.
So the more the muscles are used, the more myoglobin they’ll contain (and the redder in color they’ll be). This is why when you prepare “white” meat such as poultry or fish, you won’t find any “blood” in the package – the white meat contains hardly any myoglobin.
Myoglobin is what makes meat ‘white,’ ‘dark,’ or ‘red’
The level of myoglobin in meat is what ultimately dictates whether it will be “red,” “dark,” or “white.” The muscles in red meat are used for standing, walking, and other frequent activity, and they’re made up of slow-twitch muscle fibers. Red meats’ high levels of myoglobin make it red or dark in color.
White meat, on the other hand, is made up of fast-twitch muscle fibers and is comprised of muscles used for quick bursts of activity only. They get energy from glycogen and contain little myoglobin.
Some animals, like chickens, contain both white and dark meat, with the dark meat found primarily in their leg muscles. If you’ve ever wondered why wild poultry contains mostly dark meat, it’s because they fly frequently, and the increased muscle usage means the meat contains more myoglobin.
Pigs are often referred to as the “other white meat,” and that’s because, while they contain myoglobin in their muscles, the levels are not as concentrated as they are in cattle (likely because pigs are not as active). Fish, too, are typically considered white meat because most of them are able to float in the water without requiring much muscle use.
Certain types of migratory fish, however, which swim briskly for extended periods, have dark meat, and that is again because of the increased myoglobin (examples would be tuna and shark).
Myoglobin also tells you when your meat is overcooked
The color changes that occur as meat is cooked are also due to myoglobin. In white meat, which will be translucent when it’s raw, proteins coagulate as it is cooked, resulting in the whitish-opaque appearance. 
In red meat, myoglobin changes from red to tan and grayish brown as it is heated. As reported by the New York Times, this color change also has to do with moisture, which is why well-done meat that’s turned gray-brown is often dry: 
“Oxygenated myoglobin is red, but when its structure is changed by heat or by other molecules, it changes color. That’s why redness in cooked meat signifies juiciness: As meat cooks, the heat causes the other meat proteins to coagulate and squeeze out their moisture. Myoglobin stays unchanged and red as the meat juices flow, then turns from red to gray-brown as the release of moisture ends and the meat becomes dry.”
Carbon monoxide used so your meat appears fresher than it actually is
When myoglobin is exposed to air, it eventually turns brown. This is why color can be a good indicator of the freshness of your meat, with red meat being fresher than meat that’s turned brown.
In the US, however, the US Food and Drug Administration (FDA) allows the use of carbon monoxide as a “preservative.” It works by attaching to myoglobin’s iron atom, turning it bright red and preventing it from interacting with oxygen.
This practice is banned in Europe and Japan because, even though it might prevent color changes, it certainly doesn’t prevent bacterial growth.  Carbon-monoxide-treated meat may appear fresh for weeks even though it’s already gone bad (it’s also used to preserve color in fish).
Are there health differences between white and dark meat?
Dark meat has more fat than white meat, which is also why it’s often juicier. In poultry, the dark meat also tends to have more nutrients than white meat, including B vitamins, iron, zinc, and selenium.
As far as red meat is concerned, it has gotten a bad reputation because of its saturated fat content, but this is a myth. It’s widely stated that eating red meat causes heart disease, an association that is often blamed (incorrectly) on its impact on cholesterol levels. Yet, research has repeatedly shown that the dietary cholesterol-heart disease connection is incorrect.
For example, a 2010 study from Harvard found no evidence that eating red meats leads to heart disease.  What you need to be concerned about when eating red meat isn’t its impact on your cholesterol levels… it’s whether the animal was fed grains or raised on traditional pasture.
Choose grass-fed (pastured), NOT grain-fed, meat
I’ve often said that the differences between organic, pastured beef and that from animals raised in confined animal feeding operations (CAFOs) are so great that you’re really talking about two completely different animals (and the same applies to other animal meats and animal products such as dairy and eggs).
In the grand scheme of all that is wrong with modern agriculture, the unnatural transition that turned cattle, which naturally eat only grass, into grain-eating ruminants is definitely toward the top of the list. CAFO cows are fattened for slaughter in massive feedlots as quickly as possible (on average between 14 and 18 months) with the help of grains and growth promoting drugs, including antibiotics.
Remember 80% of the antibiotics in the US are fed to farm animals. The antibiotics and grains radically alter the bacterial balance and composition in the animal’s gut. The natural diet for ruminant animals, such as cattle, is plain grass.
When left to their own devices, cattle will not graze on corn or soybeans. Just as in humans, poor gut health in animals promotes disease. This radically altered diet also affects the nutritional composition of the meat.
For example, when raised on a grass-only diet, levels of conjugated linoleic acid (CLA) are three to five times higher in meat compared to CAFO beef. CLA has been found to have a wide array of important health benefits, from fighting cancer to decreasing insulin resistance and improving body composition.
Grass-fed beef also tends to be leaner and have higher levels of vitamins and minerals such as calcium, magnesium, and potassium. It also has a healthier ratio of omega-6 to omega-3 fats.
Unless labeled as grass-fed, virtually all the meat you buy in the grocery store is CAFO beef, and tests have revealed that nearly half of the meat sold in US stores is contaminated with pathogenic bacteria—including antibiotic-resistant strains. Grass-fed beef is not associated with this high frequency of contamination, and their living conditions have everything to do with this improved safety.
This doesn’t only apply to beef, of course. It also applies to poultry, which should be organic and pasture-raised (or free-range certified), as well as fish, which should be wild-caught, not farm-raised.
Where to find naturally raised healthy meat
Currently, meat in supermarkets will be labeled 100% grass-fed if it came from pasture, but if it contains no label, it’s probably CAFO-raised. An alliance of organic and natural health consumers, animal welfare advocates, and anti-GMO activists are working together to tackle the next big food labeling battle: meat, eggs, and dairy products from animals raised on factory farms, or CAFOs.
This campaign, which aims to have CAFO foods labeled, includes a massive program to educate consumers about the negative impacts of factory farming on the environment, on human health, and on animal welfare. It hopes to organize and mobilize millions of consumers to demand labels on beef, pork, poultry, and dairy products derived from these unhealthy and unsustainable so-called “farming” practices.
In the meantime, you can boycott food products from CAFOs and choose to support farmers who produce healthy pastured grass-fed meat, eggs, and dairy products using humane, environmentally-friendly methods. You can do this not only by visiting the farm directly, if you have one nearby, but also by taking part in farmer’s markets and community-supported agriculture programs, many of which offer grass-fed meats. The following organizations can also help you locate grass-fed beef and other farm-fresh foods in your local area, raised in a humane, sustainable manner.
 “Science Of Meat: What Gives Meat Its Color?”. The Accidental Scientist. Web. https://www.exploratorium.edu/cooking/meat/INT-what-meat-color.html
 McGee, Harold. “The Red-Meat Miracle, And Other Tales From The Butcher Case”. The New York Times. N.p., 2007. Web. http://www.nytimes.com/2007/04/04/dining/04curi.html?_r=0
 Micha, R., S. K. Wallace, and D. Mozaffarian. “Red And Processed Meat Consumption And Risk Of Incident Coronary Heart Disease, Stroke, And Diabetes Mellitus: A Systematic Review And Meta-Analysis”. Circulation. N.p., 2010. Print. http://circ.ahajournals.org/content/121/21/2271