Most People who don't like rare or medium-rare steak cite the reasoning as the color of the meat being off-putting.
These people will also mistakenly say that the "red juice" is blood and that they don't want to eat blood.
However, this isn't the case at all. The "red juice" is a combination of a water-soluble protein called myoglobin and water, not blood.
To Preface: This article is fairly nuanced but the short of it is, the protein myoglobin is purple in color and will change to hues of red or brown based on iron/oxygen interaction.
Blood is removed from the animal during slaughter; Blood gets its color from hemoglobin. Meaning, the "red juice" in your steak or hamburgers is actually just water and myoglobin, not blood.
A more optimal word for "meat" is muscle tissue. Skeletal muscle tissue contains approximately 60-70% water, 10-20% protein, 2-22% fat, and 1% ash, depending on the species of animal.
Meaning, muscle tissue is primarily water/moisture content.
I won't really go into the intricacies of animal slaughter but when an animal is properly slaughtered, the blood is effectively removed and blood circulation stops.
The beef is then hung by its hind legs and eviscerated; Meaning, the offal or innards (hence viscera) are removed. The beef is also skinned.
The remaining carcass consists of the meat (outlined above), fat, and bones. The hung carcass is then halved along the backbone; The symmetrical halves are called "sides" of beef.
After time in the cooler, each side is split between the 12th and 13th rib to create the forequarter and hindquarter.
To put it simply: Blood is removed from the animal during the slaughter process.
This is a quick mini-biology lesson:
Most people know and understand that blood is red in color. However, this color is due to hemoglobin. Red blood cells (RBC) contain hemoglobin, which is a protein that carries oxygen through the body.
Cows use aerobic respiration which requires oxygen to break down glucose to generate adenosine triphosphate or ATP for muscle contractions.
When a cow respires, RBCs pick up oxygen (specifically 4 oxygen molecules) in the lungs and carries it to the muscle cells.
At the cell wall, if the oxygen levels are low, the oxygen molecules pass through the cell membrane. Within the cell, the protein myoglobin holds onto the oxygen (one molecule) until it's required by the cell.
An iron compound "heme" is what allows myoglobin to hold onto oxygen. As we know, iron can "rust" and turn shades of brown in color.
Myoglobin is purple-ish in color and will change color based on the iron's interaction with oxygen.
This is why meat is purple when it's vacuum sealed. A vacuum creates an anaerobic environment (a lack of free oxygen). Meaning there is nothing to "rust" the color of the meat.
Here's a steak in Cryovac, vacuum packaging:
Since our atmosphere is made up of oxygen, the color of meat is then considered categorically unstable.
Once the vacuum seal is broken, the color of the meat changes or "blooms" from purple to red. The longer the meat comes into contact with oxygen it will deteriorate and brown.
Specifically, these stages are referred to as:
This is also why certain parts of animals - like chickens - are called "dark meat." These muscles require a consistent energy source - like oxygen - to function. Requiring more myoglobin to store oxygen in cells which results in them being "dark."
Typically people who don't like "blood" - or as we now know, red oxymyoglobin - in their steak will ask for it to be cooked to medium or well-done.
This is typically at a measured internal temperature of 140F for medium or 160F+ for well-done.
The reason for this is because at temperatures above 140F, the protein myoglobin loses its stability (denatures) and forms a new molecule called hemichrome (denatured globin and oxidized heme iron).
The color that forms is brown/tan in color.
We also get browning via the maillard reaction of amino acids and reducing sugars which creates a pleasurable exterior crust.
To start, I'm not you and what I may like you may not. I like my steak medium-rare, my Dad likes it rare, my Mom likes it well-done.
A lot of people in the "Steak World" can act somewhat elitist in terms of finishing temperatures (even I used to be guilty of this).
Often you'll hear phrases like these in the food/steak industry:
You get the gist.
Often there's this bravado that's associated with rare/medium-rare steak.
However, if you don't like your steak red - regardless if it's blood or not - that's your prerogative. At the end of the day, you're paying for the steak, not someone else - eat it how you like it.
Scientifically though, the reason Rare/Medium rare steaks are considered "better" has everything to do with juiciness. As we learned above, meat (muscles) are primarily water content.
As steak is cooked, it will lose free moisture - which we call juiciness.
A steak that's cooked to rare/medium rare will lose 2-3% of its moisture, where-as a steak cooked to well-done will lose 18% of it's moisture (The Food Lab: Better Home Cooking Through Science, pg. 693, J. Kenji Lopez-Alt).
Below 140F the meat is tenderizing more than it's contracting, meaning less moisture loss. After 140F, the contraction speeds up and as a result, more moisture is lost.
My only qualm with well-done steak lovers is when they want the steak to be both well-done and juicy - which as we just learned - isn't possible.
Fatty steaks usually help compensate for this fact (like a chuck eye or ribeye steak) where-as lean steaks suffer (filet mignon and tenderloin).