August 03, 2010

A bone to pick

January 6th, 2010

Boredom is, at times, a wonderful thing. Those of you who've seen any of my Facebook Statuses (stati?) over the last few days would have good reason to think I''m going barmy; after all, I do complain of being bored rather a lot. However, I quite like the frame of mind one reaches on being SO bored that your mind starts to wonder about peculiar and random things that usually you just accept without question, such as why teaspoons are teaspoon shaped, how they make paperclips, or if pandas like the taste of bamboo. A lot of my most innovative thoughts have spawned from serious boredom, and, happily, today is no exception.

Although I've been back in Leamington for quite some time now, I've not yet had the chance to go shopping, and I don't have much in the way of food in the house. As most of you will appreciate, rooting through a student freezer is always a risky business; I imagine some of the stuff in there is so old that it would take a team of dedicated forensic scientists to identify it. But anyway, I was hunting around to see what surprises I could find for my tea and I came across a salmon fillet that I'd bought a few weeks ago on my usual prowl around Somerfield for all the reduced delights just before closing time. I love salmon. It's probably one of my favourite foods, so I cooked it up with some rice and peas. Delicious.

There's only one problem with salmon, which I guess you're already anticipating before I say it: those pesky little bones. In today's average-sized fillet, I picked out no fewer than thirteen little bones, which, I must say, rather tainted my enjoyment of the whole thing. I mean, what on earth are they there for?! Maybe somebody put them there just to be annoying. After all, surely they're far too small and bendy to provide any kind of support to such an enormous fish as a salmon. I couldn't help it; I simply had to find out...

As I've already said, salmon are big. HUGE, in fact. Some of the seven species of salmon (namely Chum and Chinook) can grow up to a metre in length and weigh up to 25lbs. THAT'S ENORMOUS!! By comparison, that's about the weight of the average 2 year old, or of about 65 spuds! Yeah, they're pretty hench. Surely those daft little bones can't make a huge amount of difference to such a massive body. So what ARE they for? Indeed, as I found out, their purpose is NOT primarily to confer structural support to the fish; they're actually involved in movement. To make it easier to explain, let me first tell you a little bit about muscles.

When we eat fish or meat, it is muscle tissue that we are eating; a fact that, when I first discovered, temporarily startled me. I don't really know why; it just felt a bit weird. Muscle, as you probably already know, is tissue which brings about movement. I know a lot of biology, but I have to say that the way that muscles work is probably one of the most fascinatingly simple mechanisms that I have ever come across, and one that never fails to amaze me. Indeed, it is finding out things like this that makes me remember why on earth I am still studying this stuff after three years. So yeah, muscles. They're really cool.

The very fundamental unit of muscle in humans is called a sarcomere, and muscle cells are packed full of parallel sarcomeres. A whole muscle contains thousands of interacting sarcomeres which, when all stimulated simultaneously, bring about muscle contraction and movement.

As with most biological systems, sarcomeres are made of protein. Two different types of protein, to be precise, called actin and myosin. Within a sarcomere, actin and myosin fibres interlock with each other, and it is the sliding of these filaments over one another which is the basis of muscle contraction. Let me explain. Essentially, muscle contraction starts when the muscle cell is stimulated by an electrical impulse from a nerve cell. This electrical impulse causes calcium to be released from stores inside the cell. Calcium causes changes in various other associated proteins which allows the myosin and actin to interact (with the help of a bit of energy). The muscle fibres then slide over one another and interact further, which decreases the length of the sarcomere. That's the key point! This sliding DECREASES THE LENGTH of the sarcomere. When this happens in many hundreds of sarcomeres in a muscle, the length of the whole muscle decreases. Muscles in humans are anchored to bones by tendons, so contraction of muscles causes the movement of the bones that they're attached to.

In fish, the basic unit of muscle is the same as in mammals, and it contracts in the same way. However, in fish the muscles are not anchored to bones via tendons. Instead, blocks of muscle run all the way along the fish's body separated by tough sheets of protein, known as myosepta. These sheets run right from the core of the fish (where they are attached to the skeleton) to the outside (where they are attached to the overlying skin). The fish moves by passing a wave of contraction from the head to the tail along one side of the fish, then along the other side, which causes the fish's body to move from side to side, which produces force which propels the fish through the water. Make sense?

Effectively, this wave of contraction is like a mexican wave in a football stadium.
Imagine that the blocks of seats in a stadium are the muscle blocks, and that the staircases between them are the myosepta. A mexican wave might start at one block and move all the way across all of the blocks until it reaches the other side of the stadium. However,if the people at the edges of the blocks are a bit dopey, it might take them a while to realise that the previous block is mexican-waving, and they woudn't notice that a wave is going on until it's right at the edge of the previous block. This means that the next block won't start waving until the previous one is finished. This is the same in the different muscle blocks of the fish: The myosepta serve to separate the blocks of muscle so that the side of a fish is not just one continuous lump of muscle from one end to the other. If the myosepta didn't exist, then the fish wouldn't be able to create this sequential wave of contraction. Instead, all of the muscle along
the side of the fish would contract simultaneously, and the fish wouldn't be able to move elegantly through the water; it would just be flailing around looking stupid while being laughed at by all his speedy fishy friends.

Ok, so now I have digressed unashamedly from my original point, I feel it's high time to return. The pin-bones in fish are also known as inter-muscular bones, and, as the name suggests, are found in between the blocks of muscle, within the myosepta. They are what are known as "floating bones", which means that they are not attached to the main skeleton. (This explains why they are able to elude capture even after the fish is filleted). As I said before, They are not involved in structural support; certainly not to the fish's structural integrity, anyway. It is thought that their purpose is to further stiffen the myocommata, which unifies the rate of contraction between the individual sarcomeres in each muscle block, and helps direct the muscular forces among them. What I mean by this, is that by stiffening up these dividing sheets, it means that the forces that are generated by the contracting muscle are hugely amplified, so the fish can generate forces with up to a huge magnitude if it needs to swim really quickly. Clever, innit.

On a relatively minor point, the bones are not as soft in a living fish as they are when you unsuspectingly bite into them; If they were, the only purpose they would effectively serve would be to ruin my dinner! They certainly wouldn't help the fish in any way! I can't find any direct information as to WHY they become so soft, but I speculate that it's just because the cooking process causes a loss of all the minerals in bone that are there to strengthen them. Bones are made of primarily two components; protein and calcium phosphate, along with a few cells and other components thrown in for good measure. The protein is there to make bones resistant to snapping, stretching, or otherwise being distorted or broken, and the calcium phosphate is there to harden the bones and make them really really strong. Without the protein, bones become really brittle, and without the calcium phosphate, bones become bendy and soft.

So these little pin bones are highly variable amongst different species of fish, and salmon are estimated to have around forty. Sometimes it seems that some fillets have more than others, although this is almost always down to the filleting process, not anatomical differences between the individual fishes. Call me a snob, but I don't think it's very classy to serve bones in food (whaddya know!) so many high class fishmongers will remove the pin-bones before the fish is sold. However, cheaper fillets and mass-produced supermarket fish often have them left in. You would probably have good grounds to shoot the cook if you found pin-bones in your £50 fillet in a swanky London restaurant (actually, maybe shooting him might be a bit of an overkill), but I think the presence of pin-bones in a run-of-the-mill, £2.99 jobbie from Tesco's is probably as certain as me tripping up the steps on graduation day.

Anyway, to end, let me just say this. Pin-bones are just a fact of life that must be accepted. If they really put you off your succulent salmon fillet so bad that they simply HAVE to be removed before cooking, or if Gordon Ramsay's coming for dinner, you could always use your toenail tweezers to get rid of them... now there's a thought to put you off your dinner!

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