Cooking Answers

by Deb on October 7, 2011

Cooking Science

How scientific a cook are you? Close your eyes and click back for the questions.

1.     Meat goes brown because of reactions in the proteins caused by heat or sometimes chemicals. That’s why some marinades will make meat look ‘cooked.’ The main protein in meat is called myoglobin which has a heme ring on it. This is why meat looks red. When it is cooked it denatures and unfolds, letting the heme contact the air. This oxidises the iron in the heme, making it go brown like rust.

2.     Boiling water is a physical change, a change of state from liquid to gas. It has nothing to do with the dissolved gases in the water and is about the water itself changing.

The difference between liquids and gases comes down to the speed of the molecules. When they are moving relatively fast they will slip past each other easily, which is why liquids flow and move, but they are still stuck together. When they get much faster they begin to push apart and escape. As you heat water the heat energy makes the molecules move faster until little pockets of gas form as bubbles.

Have a look at the videos at the bottom for playing with boiling, evaporating and freezing.

3.     Microwaves are part of the electromagnetic spectrum, so they are the same type of radiation as radio waves, x-rays and visible light. These waves are all the same type of energy, but the wavelength and frequency gradually change as you move along the spectrum. The frequency of microwaves just happens to be one that is easily absorbed by water and fats, and when they absorb energy they convert it into heat and cook.

electromagnetic-spectrum

The frequency of microwaves isn’t actually the best one for water – if it were all the microwaves would be absorbed by the edges of the food and nothing would get to the inside. They are a payoff between being absorbed well enough for cooking and being able to penetrate.

And they don’t ‘cook from the inside out’ – they will cook all the places they can reach at the same time. If you have a consistent liquid food then it will all cook, if you have lots of liquid on the outside that will cook well and the inside won’t cook as quickly, but if you have something liquid on the inside and dry on the outside then the inside will cook better.

There are ‘hot spots’ that can form either because of the way the liquid is spread through the food or because of the way the bouncing waves interact. This is one of the reasons they turn, to avoid hot spots. It’s also why you are advised to stir food or let it sit before eating, to let normal old heat conduction even out the temperature. It definitely has nothing to do with microwaves ‘staying in the food’ – they don’t hang around, any more than light hangs around after the switch is turned off.

Microwaves cannot escape from the oven unless the grill on the door is broken. The holes in the grill are smaller than the wavelength of the microwave so it just bounces back into the food. And I can’t imagine anything that would break the grill that hasn’t done far more extensive damage to the rest of the oven!

4.     Mayonnaise is a mixture of things that don’t normally mix – lemon juice or vinegar and oil. If you just added the two together they would always settle out into layers, no matter how much you mix them. The key is to make an emulsion, using the natural emulsifier lecithin found in egg yolks.

Lecithin is similar to a detergent – one end interacts with the watery liquids, one end interacts with the oils. It breaks the oil up into globules and spreads it through the juice, a bit like spreading marbles through water. This is what happens in homogenised milk or cream too.

lecithin

This is a nice basic mayonnaise recipe you can then add your own herbs and spices too. Although given it only lasts for a week with fresh ingredients I’d be reducing the quantities!

5.     The picture above is many different types of chillis, or members of the Capsicum genus. Most species of capsicum contain thecapsicum chemical capsaicin to give them their bite. Like most poisonous or unpleasant chemicals, the role of capsaicin is to deter animals from eating them. Capsicum seeds are spread by birds, which don’t have the receptor for capsaicin and aren’t bothered by it. If animals ate them they would grind up the seeds with their teeth and stop them from being able to germinate.

Capsaicin is not produced by the seeds themselves, but in the white ribs inside the fruit and around the seeds. It binds to the same receptors that register heat and pain so it tricks the brain into thinking it is being burnt. It doesn’t mix with water, so water will not help but oil or milk are the best ways to reduce the pain. While it is poisonous, the amount in chillis is tiny and not a risk. Women are more sensitive to painful stimulus than men and this includes capsaicin – they experience foods with chilli or paprika as much spicier or hotter than men do.

There are medical applications for capsaicin, including as pain relief! It appears to work by saturating the receptors. It is being investigated to help with diabetes because it affects insulin production and also in cancer. And of course it is used as a non-lethal deterrent in riot control and pepper sprays.

According to YouTube, throwing boiling water into air well below freezing is common entertainment in far northern winters.

And here’s some longer and more scientific playing with water, boiling, freezing and vacuums.

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