Wednesday, January 20, 2010

The Science Behind Poaching an Egg

(via Tastespotting)

Poached eggs are delicious – at least I think so. I suspect that a number of other people do, as well, yet they remain relatively unpopular when compared to other methods of egg preparation. This may be because oftentimes the result of boiling an egg without its shell resembles Egg-drop soup more than Eggs Benedict.

Science to the rescue! Actually, poaching an egg is not that difficult, especially if you think a little bit about what’s going on inside your pot. With a little bit of know-how behind you, preparing perfect poached eggs, without space-hogging and expensive unitaskers, is no problem.

What’s going on when you cook an egg? The answer is all about proteins. Egg whites are made of proteins and water. Proteins themselves are large (macro) molecules that consist of long chains of smaller molecules called amino acids folded up into complex structures. When proteins are subjected to heat, they denature, that is, the chains unfold and lose their structure. This often causes the proteins to lose solubility (they can no longer be dissolved in liquid). In the case of ovalbumin, the main protein component of egg whites, this causes the egg white to solidify and look white and cooked. The key to perfect poached eggs is to have this happen as fast as possible when cooking the egg, before the egg white has a chance dissipate into the water in the pot.

How to do this? Well, it has been shown that ovalbumin is more easily denatured at acidic pH1. This is the science behind why vinegar is often added to egg poaching water. This recipe outlines the basic approach I take. A simple experiment shows that adding 1 tsp. (4.9 mL) of white vinegar (5% acetic acid v/v) in 2 quarts of NYC tap water (1.9L) changes the pH from 6.75 to 3.43 at room temperature (25C) (Remember that pH 7 is neutral, lower pH is acidic, and higher pH is basic or alkaline). At a simmer (95C), the pH of tap water is changed from 6.84 to 3.87. This more acidic pH, combined with the heat of simmering water, rapidly denatures the ovalbumin in the egg whites, creating a coating of cooked egg that the interior of the egg is trapped in while cooking. This prevents the egg from falling apart and the result is a perfectly poached egg.

Poached Egg Protocol:

Materials: 1-4 fresh eggs (egg white proteins break down over time, resulting in a runnier white that spreads out more in the pot), 2qt or larger pot, small bowl, white vinegar, slotted spoon, paper towel

  • Bring ~2qt. water to a low simmer (NOT a vigorous boil) and add 1 tsp. white vinegar.

  • Crack one egg into a small bowl before sliding it into the simmering water so as not to break up the egg on it’s way into the water.

  • Repeat for subsequent eggs, spacing eggs out from each other so that they heat quickly and evenly and don’t stick together.

  • Cook eggs 2-3 minutes (or until desired done-ness level). Eggs will continue to cook as long as they are hot, so expect them to cook a little more after they have been removed from the pot.

  • Evacuate eggs to a paper towel on a plate using a slotted spoon.

  • Bon Appetit!

1Koseki et al. Conformational Changes in Ovalbumin at Acid pH. J Biochem.1988; 103: 425-430


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