Hi I’m Dr. Sabrina Stierwalt, the Everyday Einstein, bringing you Quick and Dirty Tips to help you make sense of science.
Science is a part of everything that we do - and cooking is no exception. Today we explore the scientific principles behind 3 common kitchen shortcuts to help us boil water faster, discern a cooked egg from a raw egg, and keep our guacamole from turning brown.
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Does Water Boil faster if You Add Salt?
You may have heard that putting a pinch of salt into a pot of water as it heats up will help the water to boil faster. However, the opposite is actually true, and to understand why, we need to look back at a previous Everyday Einstein episode discussing the specific heat capacity of water.
See also: Super Powered Water: Specific Heat and Heat of Vaporization
The heat capacity of a substance is the amount of heat that needs to be added (or subtracted) to change the temperature of that substance by a certain number of degrees. So the heat capacity is expressed as a ratio of energy per unit temperature (i.e. degree). The specific heat capacity, often just called the “specific heat,” is the energy difference needed to change the temperature per unit mass or amount of material that you have.
Water has a relatively high specific heat compared to other substances, which means it requires a lot of energy to heat up or cool down. When you add salt to water, you lower the specific heat so that less energy is required to heat the water to the same temperature. Thus, cooks think they are helping their water to boil faster by tossing in the salt.
However, salt water has a higher boiling point, or temperature, at which it changes from a liquid to a gas, than unsalted water. So, even though less energy is needed to heat a pot of salt water, that salt water needs to reach a higher temperature before it will start to boil.
The salt actually makes the water boil slower! Once the salt water is boiling, it boils at a higher temperature.
Does this mean adding the salt will make your pasta cook faster?
Unfortunately, the answer is still "no." Salt water does have a higher boiling point than fresh water, but we are talking about briny sea water here and not the lightly salted water you would want to cook your pasta in.
See also: Why Leftover Pasta Is Good for You
You would need 115 grams of salt just to raise the boiling temperature of roughly 4 cups of water by 1o Celsius. That’s a lot of salt! Your pinch of salt is likely less than one gram.
So keep adding salt to that pasta water – but do it because it makes your pasta taste better, not to speed up your time in the kitchen.
How Can I Tell if an Egg Is Hard-Boiled?
At my house, we often hard-boil a bunch of eggs on the weekend to eat for breakfast throughout the week. I don’t always remember to label which ones are the cooked eggs before putting them back in the fridge. Luckily, a simple physics trick can tell me whether or not my egg is raw before I crack it open to find out.
To test an egg, place it on the counter and try to spin it. Once you hard-boil an egg, it becomes a rigid or solid body instead of a solid shell around a liquid core. So when your fingers exert a force on the outside of the egg, the momentum from that force is carried cleanly through to the center of the egg and the entire egg starts spinning together.
For the raw egg, however, the force from your fingers sets the shell to spinning, but the liquid inside takes a while to catch up - the liquid yolk’s momentum initially keeps it stationary relative to the shell. So the resulting spinning motion is not uniform and the egg wobbles.
If after setting your egg to spin, you’re still not sure whether or not it’s raw, you can use the same momentum but in a different way to crack the case. Now that your egg is spinning, tap the top with your finger to stop the spin. Since the hard-boiled egg is a solid body, when you stop the outer shell, you stop the entire egg from spinning. The momentum is lost and the egg stops.
When you give the raw egg a quick tap, the shell begins to stop spinning, but the momentum of the liquid inside still carries the liquid around in its spinning motion. Once you lift your finger, the egg keeps spinning until eventually the liquid slows down too.
There is similar physics at work here as in the case of an accelerating car that we discussed in an earlier episode about the Earth's rotation. When your car accelerates, you feel pushed back in your seat because your body’s momentum initially keeps you moving at your (and the car’s) original speed.
You can easily do this test as an experiment at home, just don’t spin your eggs too close to the edge of the counter!
Why Do Water and Lemon Keep Guacamole from Turning Brown?
We’ve used math to tell us that our pinch of salt isn’t enough to boil our water faster and we’ve used physics to differentiate between raw and hard-boiled eggs. Now let’s turn to chemistry to help us save our guacamole!
I firmly believe that you can never have too much guacamole, but even I sometimes have leftover dip that I would like to save for later. Unfortunately, even though the bowl of guacamole looks green and delicious going into the fridge, it comes out looking like brown sludge. The guacamole, or specifically the avocados that make up the guacamole, turn brown for the same reason apples do: oxidation.
See also: How to Store Half an Avocado
The same chemical called "catechol" that is found in apples and other fruits is also found in avocados. When catechol reacts with oxygen, other chemicals are formed that set off the fruit’s natural defense mechanisms against exposure to the elements (in this case the air). Some of these chemicals, like o-benzoquinone, like to bond with themselves, which can have the side effect of turning the fruit brown.
See also: Why Do Apples Turn Brown?
The most effective way to fight the oxidation process is to protect your guacamole from oxygen. Before putting your dip back in the fridge, pour a layer of water on top, about 1-2 centimeters thick, depending on the size of your bowl. This water acts as a barrier and effectively seals the guacamole from any contact with the air.
For some added protection, you can also add some lemon juice to lower the pH of your guacamole. The enzymes at work to turn the dip brown work best at certain pH levels, and so the process is slowed when you make the dip more acidic.
That ends our show for today. In the meantime, you can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.
Until next time, this is Dr. Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science.
Adding salt to pot image courtesy of Shutterstock.
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