Thursday, January 14, 2010

The Science of Cooking: Part 1

If you’re just getting started in the kitchen – or even if you’ve been cooking for years or even decades – an understanding of the science behind cooking can make your adventures in the kitchen more interesting and can be very useful when creating your own culinary masterpieces. This is part 1 of a 3-part series on the science of cooking. The next two parts will appear over the next few weeks in our blog and will each investigate the science behind four culinary phenomena. This week we look at the browning of apples after they've been cut, emulsion, why onions make us cry, and the effects of adding salt when boiling water, and also offer some tips to enhance your experience in the kitchen.

Why do apples turn brown after they’ve been cut?

Apples and other produce items including bananas, potatoes, and avocadoes contain an enzyme that causes them to oxidize when exposed to air. This oxidation results in the exposed fruit turning brown, similar to the rusting of iron. Although the browned fruit is totally fine to ingest, it is not very appetizing! There are a number of ways to inactivate this enzyme, thereby preventing the unsightly color change. These include cooking the fruit, lowering the pH of the exposed fruit’s surface with lemon juice or another acid, reducing the fruit’s exposure to oxygen by vacuum sealing it or keeping it under water, or by using certain preservatives such as sulfur dioxide.

Eggs yolks as emulsifier

Anyone who has been to fifth grade knows that oil and water don’t mix. But what about recipes that call for both ingredients? Interestingly, egg yolks act as an emulsifier, or a substance that allows unblendable (or “immiscible”) liquids to stay mixed. Egg yolks contain proteins that have amino acids that attract water and amino acids that repel water, and also contain a protein known as lecithin, a fat emulsifier. Lecithin prevents tiny oil droplets from bonding with one another, preventing the oil from separating from the water. This is why eggs are an important ingredient in many recipes, including hollandaise and mayonnaise. Honey, gelatin, and mustard are also examples of emulsifiers, and soy lecithin is often used as a food additive in such items as candy bars and baked goods to aid in emulsion.

Why do onions make me cry?

Cutting an onion releases enzymes contained within the cells of the onion, which then react with sulfenic acids that form when the onion is cut, resulting in a volatile sulfur compound. This compound reacts with water from your tears to form sulfuric acid, which burns your eyes (and triggers the release of more tears). It’s possible to inactivate the enzyme by cooking the onion. You can also decrease discomfort by cutting onions underwater, chewing gum or bread while cutting the onion, refrigerating the onion prior to use (this slows the reaction), or by not cutting the root off, as the enzyme is most concentrated in the root. Using a sharp knife will also reduce the amount of the sulfur compound that is released.

Boiling water with salt

Adding salt to water before you boil it increases the temperature at which it boils. This allows food to cook at a higher temperature than 212°F, thereby decreasing the overall cooking time. Of course, the amount of salt you add will affect how drastic the change in cooking time is, and many scientists argue that the effect of adding salt to water before boiling it is negligible and that salt should be used solely as a flavor enhancer. Because adding salt increases the amount of time it takes for water to start boiling, the overall time required for your meal may be unchanged.

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