Tasty T-Day Science: why does vinegar keep poached eggs together?

Yum. Poached eggs. The stuff of foodie dreams, with the runny yolk that makes a sauce for whatever lies beneath it. And for those of you who get your Thanksgiving on early in the day (and there are lots of you, don’t lie), your day of thanks may very well begin with one of these delicacies on toast. Or a Toad in the Hole, for you Brits.

If you’ve noticed, most people who make poached eggs with some regularly tend to use a few drops of vinegar in the water. Why is this? The answer has nothing whatsoever to do with flavour. It has to do with physics and specifically, with a concept known as molecular polarity.

But let’s back up a step. Poaching is about cooking food in hot water. Boiling, essentially. The thing with an egg is: you face the obvious problem of trying to poach something which is itself liquid. Dropping the egg into a pot of boiling water should, we would expect, cause the egg to spread out evenly in the pot. But that is not what we want when we poach an egg. We want a nice, fluffy cloud of egg that can be taken out whole and dropped onto whatever foods we wish to bathe in unctuous goodness. This requires that the white or albumen of the egg poach quickly and more or less in one place.

Water’s molecular structure. Note the polarity of the two elements. Photo:

The trick, though, is that water has a chemical structure that is built to be magnetic. With its negatively-charged oxygen ion on one side and its positively-charged hydrogen ion on the other, water forms a natural magnet. That magnetism is called molecular polarity, and allows it to do two things: create a meniscus at the top of a column of water and more importantly for our discussion, naturally adhere to other surfaces.

An example of water’s molecular polarity in action. Note the beads of water formed by surface tension.
Notice. Them.
Photo: Blue Waikiki

That ability to adhere to other surfaces is the problem, because it’s what draws the egg out of its nice shape and into nastiness. But vinegar, while it still has some molecular polarity, is nowhere near as magnetic. By introducing a few drops of vinegar into the water, you can change the overall ability of the cooking liquid to leech albumen out of shape.

So, yet another reason that vinegar is a must-have for any kitchen, even if you don’t particularly like the taste. What else is vinegar good for? Well, it is a natural counterbalance to heat. If you’ve made that chili a wee bit too hot for the little ‘uns, add a couple of dashes of vinegar to the pot. You’ll never taste the acid of the vinegar, but the heat will be magically cut. Hmm… Maybe I just came up with another Tasty T-Day Science article…

And Spotify users, don’t forget to check out Jillian and I on Spotify, where we’ve created a Turkey Day set list of truly awful “turkeys.” Great, cheesy fun!