Meat
I have been reading McGee on Food and Cooking, which is 800 solid pages on How Food Works. In the interests of increasing my recall, I'm going to write about some of the things I'm learning.
Today's topic: what's the difference between white meat and red meat?
I admit, hitherto I hadn't thought of this as an interesting question to ask. Chickens have feathers, cows have milk - different coloured meat just seemed like another thing to add to the list. But amazingly, it turns out that not only is there a simple answer, but I already knew it!
First, we have to ask: what is meat, anyway? There's basically two kinds: meat is either muscle or offal. If you're eating offal, it's usually named (kidney, liver, etc.), so everything else is muscle. And if you're anything like me (i.e. you have a passing interest in sports science), you may know that there are two basic types of muscle: fast twitch and slow twitch.
Fast twitch fibres are used for short bursts of intense activity. They operate by burning a carbohydrate called glycogen. They will use oxygen to burn this, but if insufficient oxygen is available, they operate anaerobically and produce lactic acid as a by-product. As such, they quickly fatigue, until the body can flush out the lactic acid and replace the glycogen.
On the other hand, slow twitch fibres only operate aerobically. They operate by burning fat - and "burning", of course, is another word for "oxidizing". Slow twitch fibres are much thinner than fast twitch (to allow for better blood supply), and have many small fat droplets to provide energy. But fat's no good on its own. To burn the fat, the muscles have a protein called myoglobin, which takes oxygen from the blood and holds onto it until its needed, and other proteins called cytochromes that use the protein to oxidise the fat.
The name "myoglobin" should look a little familiar; it is, of course, similar to the haemoglobin in your blood, and contains an iron atom. Cytochromes, likewise, contain iron.
So this, then, explains meat colour. Fast twitch fibres are white, and slow twitch are red. The chicken doesn't fly much, but when it does fly, it's in short bursts, so chicken breast is almost entirely white. Chicken legs are a little more pink, because chickens do walk around a little (well, some of them). Cows spend their lives walking around to find grass, so their meat is redder. Pigs don't move much, so their meat is quite white, but wild pigs have darker meat.
The heme group in myoglobin changes colour, depending on the presence of oxygen. If the iron atom is holding an oxygen molecule, it is red. If the iron atom loses its oxygen, it is purple. And if the iron has been oxidised itself (i.e. lost an electron, and become unable to hold oxygen at all), then it is brown. The similarity with blood should be obvious. So fresh steak will be red on the surface, where oxygen is present, but probably purple on the inside. If the enzymes are still active when you cut it open, the cut edge will redden as oxygen is taken from the air and attached to myoglobin. But even if it doesn't - the iron is still there.
And the fat cells that provide fuel for red meat contribute to its taste (which is why, I guess, chicken breast seems so dry).
[you may also remember this news story from a little while ago. Meat packers pump extra oxygen into meat packs before sealing them, to keep them looking red. Do I care? The only risk of old meat is that bacteria may be growing on the surface. If the experts trust it to be safe, then I don't see this as a problem.]
Today's topic: what's the difference between white meat and red meat?
I admit, hitherto I hadn't thought of this as an interesting question to ask. Chickens have feathers, cows have milk - different coloured meat just seemed like another thing to add to the list. But amazingly, it turns out that not only is there a simple answer, but I already knew it!
First, we have to ask: what is meat, anyway? There's basically two kinds: meat is either muscle or offal. If you're eating offal, it's usually named (kidney, liver, etc.), so everything else is muscle. And if you're anything like me (i.e. you have a passing interest in sports science), you may know that there are two basic types of muscle: fast twitch and slow twitch.
Fast twitch fibres are used for short bursts of intense activity. They operate by burning a carbohydrate called glycogen. They will use oxygen to burn this, but if insufficient oxygen is available, they operate anaerobically and produce lactic acid as a by-product. As such, they quickly fatigue, until the body can flush out the lactic acid and replace the glycogen.
On the other hand, slow twitch fibres only operate aerobically. They operate by burning fat - and "burning", of course, is another word for "oxidizing". Slow twitch fibres are much thinner than fast twitch (to allow for better blood supply), and have many small fat droplets to provide energy. But fat's no good on its own. To burn the fat, the muscles have a protein called myoglobin, which takes oxygen from the blood and holds onto it until its needed, and other proteins called cytochromes that use the protein to oxidise the fat.
The name "myoglobin" should look a little familiar; it is, of course, similar to the haemoglobin in your blood, and contains an iron atom. Cytochromes, likewise, contain iron.
So this, then, explains meat colour. Fast twitch fibres are white, and slow twitch are red. The chicken doesn't fly much, but when it does fly, it's in short bursts, so chicken breast is almost entirely white. Chicken legs are a little more pink, because chickens do walk around a little (well, some of them). Cows spend their lives walking around to find grass, so their meat is redder. Pigs don't move much, so their meat is quite white, but wild pigs have darker meat.
The heme group in myoglobin changes colour, depending on the presence of oxygen. If the iron atom is holding an oxygen molecule, it is red. If the iron atom loses its oxygen, it is purple. And if the iron has been oxidised itself (i.e. lost an electron, and become unable to hold oxygen at all), then it is brown. The similarity with blood should be obvious. So fresh steak will be red on the surface, where oxygen is present, but probably purple on the inside. If the enzymes are still active when you cut it open, the cut edge will redden as oxygen is taken from the air and attached to myoglobin. But even if it doesn't - the iron is still there.
And the fat cells that provide fuel for red meat contribute to its taste (which is why, I guess, chicken breast seems so dry).
[you may also remember this news story from a little while ago. Meat packers pump extra oxygen into meat packs before sealing them, to keep them looking red. Do I care? The only risk of old meat is that bacteria may be growing on the surface. If the experts trust it to be safe, then I don't see this as a problem.]