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.
Why Meat Stays Red: Myoglobin And Nitrites By DON ULIN Posted May 10, 2011 Listen Now: Why Meat Stays Red: Myoglobin And Nitrites -2:01
Check any package of bacon, hotdogs, or cold cuts and chances are you'll find nitrites listed among the ingredients.
Although nitrites have caused cancer in laboratory animals and may well do the same in people they are among the oldest and most common food preservatives and serve a variety of functions.
What Are Nitrites?
Nitrites and their close relatives, nitrates are simple compounds made of oxygen and nitrogen attached to some other element such as sodium or potassium. Potassium nitrate, or saltpeter, as it was called, was first used in the middle ages.
Today, sodium nitrite is the most common food preservative.
Red Meat
Fresh meat in the supermarket is red because of the pigment called "myoglobin," which stores oxygen in muscle cells. But myoglobin is only red when it is bonded to oxygen molecules.
In live animals, the blood carries oxygen to the myoglobin; in freshly cut meat the oxygen comes directly from the air. But the red color of freshly cut meat is temporary since aging, cooking, and bacteria, all separate the oxygen from the myoglobin, turning the meat a brownish-gray color.
What Nitrites Do
Nitrites keep meat red by bonding to the myoglobin and acting as a substitute for the oxygen. Oxygen and sodium nitrate both turn myoglobin red, but nitrate attaches with a more stable bond and so the color lasts longer.
Although for several hundred years, nitrites and nitrates have been used to preserve the color of meat, more recent evidence shows that these chemicals also inhibit the growth of bacteria, including the bacteria that cause the deadly disease, botulism.
常常是外面一层很红很新鲜的样子，但里面靠中心的却颜色发灰/暗，是因为冷气传不到内部还是什么别的原因？
链接
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
肉（任何肉）的色彩化学有点复杂，而我们所谓的「红肉」能让变色龙甘拜下风。不过简而言之，肉的颜色绝大部分取决于三种有色蛋白质的相对分量而定，就如同艺术家将三种颜料在调色盘上混合以达到想要的色调。这三种蛋白质分别为现切牛肉时深紫红色的去氧肌红蛋白（deoxymyoglobin）、鲜艳粉红色的氧合肌红蛋白（oxymyoglobin），以及灰棕色的变性肌红蛋白（metmyoglobin）。还有其他形式的肌红蛋白，不过以上述的三种肌红蛋白为主。
这三种有色化学物质彼此呈动态平衡，也就是都能互相转换。相对量是根据当时可使用的氧气、酵素与抗氧化剂而定。不过这三种有色化合物并不会对口味或健康造成任何影响。它们的作用纯粹就是化妆品而已。
无论是任何形式的肌红蛋白，在猪肉与小牛肉的含量都很少，不过牛肉的肌红蛋白含量则多很多。肉类市场努力地要保持肉色为受消费者喜爱的鲜红色──氧合肌红蛋白状态。不过为什么呢？
首先，必须要有氧气存在，才能将新鲜肉类的紫色去氧肌红蛋白，变为红色的氧合肌红蛋白或棕色的变性肌红蛋白。根据肉中可使用的氧气量，可决定这两种肌红蛋白何者占优势。若在几乎无氧的环境下，例如真空包装里，肉将保持其去氧肌红蛋白的紫色。你可能以为这样可以解决棕肉的问题，可惜的是，消费者不喜欢紫色；他们要的是红色。因此真空包装甚少应用于新鲜（非腌制）肉类的零售贩卖上。
当肉只能接触到少量氧气时，棕色的变性肌红蛋白会成为最容易产生的化学反应形式。因此曝露于空气的牛肉表面，可能显示出鲜红的氧合肌红蛋白，而底下被剥夺氧气的肉，则缓慢地变为棕色。大多数的零售市场会以坚硬的塑胶盘包装肉，并覆盖上透氧塑胶膜（通常是聚氯乙烯，PVC）。它的表面富含氧气，因此肉的颜色鲜红。不过氧气能向下扩散的距离有限，越往下肉色就越呈现棕色。
链接
https://indianapublicmedia.org/amomentofscience/meat-stays-red-myoglobin-nitrites.php
Why Meat Stays Red: Myoglobin And Nitrites By DON ULIN Posted May 10, 2011 Listen Now: Why Meat Stays Red: Myoglobin And Nitrites -2:01Check any package of bacon, hotdogs, or cold cuts and chances are you'll find nitrites listed among the ingredients.
Although nitrites have caused cancer in laboratory animals and may well do the same in people they are among the oldest and most common food preservatives and serve a variety of functions.
What Are Nitrites?Nitrites and their close relatives, nitrates are simple compounds made of oxygen and nitrogen attached to some other element such as sodium or potassium. Potassium nitrate, or saltpeter, as it was called, was first used in the middle ages.
Today, sodium nitrite is the most common food preservative.
Red MeatFresh meat in the supermarket is red because of the pigment called "myoglobin," which stores oxygen in muscle cells. But myoglobin is only red when it is bonded to oxygen molecules.
In live animals, the blood carries oxygen to the myoglobin; in freshly cut meat the oxygen comes directly from the air. But the red color of freshly cut meat is temporary since aging, cooking, and bacteria, all separate the oxygen from the myoglobin, turning the meat a brownish-gray color.
What Nitrites DoNitrites keep meat red by bonding to the myoglobin and acting as a substitute for the oxygen. Oxygen and sodium nitrate both turn myoglobin red, but nitrate attaches with a more stable bond and so the color lasts longer.
Although for several hundred years, nitrites and nitrates have been used to preserve the color of meat, more recent evidence shows that these chemicals also inhibit the growth of bacteria, including the bacteria that cause the deadly disease, botulism.