Chemistry and cooking: solvents


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Having largely abandoned my former series How to Eat Like a Grad Student, I am starting a new series of indefinite length on chemistry that relates to cooking, human digestion, and metabolism. This is sometimes called molecular gastronomy. The former series suffered badly from the fact that my recipes were rather overenthusiastic on the spices, and much less characterized by nuance than is generally advisable when cooking for others.

Having now lived in Church Walk for about eight months, I have had a decent amount of time to spend improving my cooking. Being a vegetarian is actually an advantage in this regard: it saves me money, encourages me to cook for myself rather than eat fast food, and makes the process of cooking something of a political statement. As such, I devote more effort to it.

Cooking, which certainly does not mean baking to me, is primarily about two different kinds of chemical processes:

  1. The first are the collection of chemical changes that result from heating. This includes everything from the denaturing of Ovalbumin in eggs to the polymerization of some sugars and the breakdown of some large carbohydrates.
  2. The other major category of chemical processes has to with solvents.

Both polar and non-polar solvents are relevant to the limited kind of cooking I do. Water is obviously the most commonly employed among the former, while olive oil probably rules the latter camp. Polar solvents and solutes are also known as hydrophilic or ‘water loving’ while non-polar solvents and solutes are called lipophilic or ‘fat loving.’

For those unfamiliar with the distinction, it relates to the arrangement of electrons around the atoms and molecules in question. There are two broad kinds of arrangements. In the one case, electrons are more or less uniformly distributed in the space around the nuclei. Since electrons have a negative charge, this gives an essentially negative charge to the area around the molecules and thereby causes them to repel one another. Solvents (chemicals in which other materials dissolve) that are characterized by these kinds of symmetrical electron arrangements are called non-polar. In cooking, these are usually fats.

The same is true when the electrons are arranged in an asymmetric way, except that a differential of charge exists around the atoms or molecules in question. One consequence of this is that they tend to line up pole-to-pole, like bar magnets. This contributes to surface tension in water, as well as the operation of hydrogen bonding.

Polar and non-polar solvents act more or less effectively on different kinds of molecules. Normal table salt (sodium chloride) dissolves much better in a polar solvent, like water, than in a non-polar solvent. Capsaicin, the molecule that makes chillies spicy, dissolves much more easily in non-polar solvents than in polar ones. That is why it is easy to make spices flavourful by heating them in oil. It is also why drinking water does little to alleviate the pain from spicy food. Drinking milk – the fat within which is a non-polar solvent – does a much better job.

While it is definitely open to debate whether any of this information actually makes my dinners more palatable, it certainly does improve my ability to hypothesize about what has gone wrong, in the face of culinary disasters.

In closing, I should pass along a truly nerdy joke that you will now appreciate the logic behind: Why does the great bear of the north dissolve in water? Because it’s polar.

{ 4 comments… read them below or add one }

B October 15, 2006 at 7:37 pm

I am surprised you didn’t mention your biggest solvent-related complaint: how the minerals in Oxford water apparently screw up the production of coffee.

Anonymous October 16, 2006 at 6:37 pm

I think you would be better off with a good cookbook than a bunch of Wikipedia links.

Jessica October 17, 2006 at 12:28 am

Some cookbooks for the science nerd type (none of them are strictly vegetarian, they’re still good for learning techniques):

Cookwise: The Hows and Whys of Cooking Revealed with 235 Great-tasting Recipes by Shirley Corriher

What Einstein Told His Cook: Kitchen Science Explained and its sequel by Robert Wolke

On Food And Cooking: The Science And Lore Of The Kitchen by Harold McGee

I’m Just Here for the Food: Food + Heat = Cooking by Alton Brown. His other book about baking is a really good guide if you would like to get into baking.

Antonia October 17, 2006 at 9:39 am

Or you could look into Heston Blumenthal’s approach to cooking, seeing as he’s Oxfordshire based and a molecular gastronomy pioneer.

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