A Watched Pot – Food Chemistry No. 1

Food is chemistry. I think that’s one of the reasons that it holds such mystique: talented cooks are not only preparers of nutrition, they are practical chemists. There is a lot to learn about the natural world just by making food – and if you truly understand food and food preparation, you have also gained an intuitive understanding of chemistry, the modification of molecules to create a desirable result.

Food and chemistry have always gone hand-in-hand. One of the earliest and most popular alchemical processes was the manufacture of aqua vitae – the “water of life”. Today we call the process “distillation”, and transliteration of “aqua vitae” gives rise to the words “whisky”, “eau de vie”, and “vodka”.

The Alchymist, In Search of the Philosophers' Stone by Joseph Wright of Derby, 1771.

When the aqua vitae starts glowing, it’s time to call a cab.

While historically very popular (indeed!), this is an experiment that we don’t encourage anyone at home to repeat, because it’s highly illegal in many countries, including the United States. The process, however, is chemistry at its simplest: use the difference in the boiling points of alcohol and water to separate them. One adds heat to a pot of a lighter-strength alcoholic beverage and condenses the vapor that evaporates first – single malt whiskey is essentially distilled beer, brandy is distilled wine, vodka is pretty much any fermented liquid distilled until the distiller passes out from the fumes.

What is really interesting about this is the process of boiling: the transition of a liquid substance into a gaseous substance. It is precisely because boiling is such a common process to perform in the kitchen, yet full of interesting chemistry under the surface, that we have chosen to cover it here in our first blog entry about Food Chemistry.

What’s so great about boiling?

To understand boiling, it is first necessary to understand evaporation: the process by which a liquid slowly dissipates into the atmosphere. If you set a cup of water out on your counter, you should find that the water disappears over time. What’s happening here is that the molecules of water are constantly jostling around one another like the crowd at a parade that hasn’t started yet.

“I understood there would be an inflatable Scooby Doo balloon here?”

Over time, people come and go into the crowd, getting coffee or taking a restroom break. When the inflatable cartoon characters pass by, more people are attracted to the scene: we call that condensation. When nothing’s going on, more people wander away, and that’s evaporation. Since there are no molecule-sized Scooby Doo parade balloons, molecules make do with temperature: they tend to group together when it is cold, and wander away when it is warm. Also, like aimless crowds, molecules in the liquid tend to stick together when they have nothing better to do: some wander away, some come back, and most hang out on the edge, thinking about wandering away, but not really getting up the gumption. That’s why liquids are liquid at room temperature: they’re more active than solids, but too lazy to be a gas.

When molecules leap into the air as vapor, they increase the pressure very slightly in their surroundings: where there was nothing, there is now something. When it comes to liquids, the measurement of the pressure of this vapor is called, well, “vapor pressure“. When people do it, I tend to think of them as “those annoying crowds that amble down the sidewalk in no hurry and take up all the space just to pester me”, which increases my blood pressure, but that’s neither here nor there.

When we turn up the heat on our liquid, we encourage more and more molecules to break away from the group, increasing the vapor pressure. Eventually, we get to the point where the vapor pressure of the liquid equals the surrounding pressure, which means that the molecules wander off freely into the environment. This is the technical definition of a boiling point: the state at which the vapor pressure of a liquid equals the surrounding pressure. The molecules in the liquid are so hyper that they stop being a liquid and wander off into the atmosphere.

Just like my attention. Boooo-ring…

OK, this is pretty basic stuff. Pot of water plus fire equals steam. Why go through all the effort to define boiling as “the point at which the vapor pressure of a liquid equals the surrounding pressure”? What about we just call it “roasting water it until it bubbles”.

The whole vapor pressure thing is important because it highlights some unexpected features of boiling. Here’s one you may know as a trivia question, but may not have thought about how weird it is: it is impossible to heat a liquid beyond its boiling temperature, as long as you keep its pressure constant. Water boils at 212℉ at sea-level atmospheric pressure, no matter how much heat you add to it.

If you put a pizza in a 500 degree oven, eventually that pizza will become 500 degrees, potentially bursting into flame or dribbling molten cheese all over the place. That’s sensible, if unfortunate. However, if you put a pot of water in the same oven, the liquid water will never get hotter than 212 degrees (although its steam can be). What gives?

When you have different pressures in an open space, like the atmosphere, everything will rebalance by having stuff move from the high pressure to the low pressure area – the same thing that causes wind, but on a smaller scale. Increasing the heat you add to the liquid will make it boil faster and more vigorously, but not hotter. When you put more energy into the water, it would increase its temperature, and therefore its vapor pressure, but it can’t, because the extra pressure equalizes into the air. As soon as the heat gets high enough in the molecules, the pressure draws the molecule away from the liquid, taking the energy with it. It reaches equilibrium, and balances right at the one temperature.

Going back to the crowd: imagine the atmosphere as riot police with those shield things keeping the crowd in one place. I’m not sure what the liquid did to raise the ire of the government, but it’s in a constant state of suppression. When you add heat to the liquid, it’s like firing up the crowd, and since they can’t go anywhere, they just get madder and madder. Eventually, if you get them really mad, they burst out entirely, and the police can’t hold them back. At this point riling them up doesn’t make them more mad: they’re already as hot as they can get, and yelling at them more just makes them run faster. The ones on the edge that are breaking through just carry riot energy away from the main crowd, who are waiting their turn to get boiling mad.

It seems counter-intuitive that a simmering pot of water is the same temperature as, say, a cup of water over a flame thrower, but it’s true. Want proof? Let’s adjourn to the stove, er, chemistry lab!

To prove it, I set up the experiment below. I have two Erlenmeyer flasks with 400ml of water straight out of my tap, and a digital thermometer with two probes I built specifically for this experiment. It is chemistry, after all. We must at least make an effort to look scientific.

I put one flask over medium low heat (on the left) and one flask over medium high heat (right), then took some time lapse photos and stitched them into a movie. Yes, I’m making you watch a pot until it boils.

And here’s that as a graph, since the characters on the LCD are obliterated by the compression used on the movie. Besides, the shape of the graph is what’s important:

© Knife & Fork Project

Hot, steamy data.

So you see, the hot (red line) flask boiled faster than the medium (yellow line) flask, but both flasks flattened out when they hit the boiling point. Therefore, it doesn’t matter how much heat the burner produces, that boiling point will stop the water from getting any hotter. Furthermore, when you compare the first and second derivative values of the curves, you see the increase in temperature is linearly proporti–

Yeah, OK, so what does this have to do with cooking?

Lots.

As we already mentioned, distillation is the process of separating a mixture based on boiling points. Pure ethanol boils at 172.4℉, while water boils at 212℉. This is what makes distillation so effective: no matter how much heat you add, you will always boil off the ethanol first at a set lower temperature before moving on to boiling off the water. This precise temperature depends on the mixture of ethanol to water, but it is always less than 212℉.

Let’s talk about reducing. Often times, when making a syrup, stew, or soup, you boil the liquid to remove water and concentrate flavor. It’s essentially the opposite of distillation: you’re boiling off the water first, leaving behind things that boil at a much higher temperature.

Like chicken, which doesn’t boil nearly as easily.

Also, have you ever noticed how many recipes have steps like this: “Bring to a boil, cover and reduce to a simmer”? The creator of the recipe knows that whether you gently simmer or you boil the knickers off of your food, it will still be sitting at around 212℉, so you might as well preserve the moisture and save energy with the simmer.

This also explains why pasta makers just tell you to “boil for 7-10 minutes”: they know that water’s all about the same temperature when it boils. Also, it explains that fine print about water boiling at different temperatures based on your altitude: the higher you are, the thinner the air, and a lower target pressure to hit with that vapor pressure balancing act. In fact, if you had a glass of water in space, it would boil right away with no heating at all because its vapor pressure is always at or above the pressure of the surrounding nothingness. Space is not a good place to cook pasta.

Lastly, recall that sneaky thing I said before about “it is impossible to heat a liquid beyond its boiling temperature, as long as you keep its pressure constant“? What if we didn’t keep its pressure constant? What if we put it in a sealed pot and then boiled it? The pressure in the pot as a whole increases as we increase the vapor pressure of the water, because there’s nowhere for that pressure to go. As we increase the surrounding pressure, the water boils at a higher temperature, but otherwise behaves exactly the same as in an open pot. Add one of those jiggly things on top to regulate the pressure, and we’ve got ourselves a pressure cooker, which cooks just like boiling on the stove but hotter and therefore faster. Profit!

Indeed, fortunes have been made on this very feature of boiling liquids. Colonel Harland Sanders took the long process of pan frying chicken and quickened it by using a pressure fryer: a pressure cooker filled with oil. This raises the temperature of the water inside the chicken as it fries, which raises the pressure, cooking the chicken faster and leaving it juicier than it would be in a regular deep fryer.

High pressure? High heat? This just doesn’t end well for me, does it?

So you see, even if you knew that water boiled at 212℉ before, now you know why it does, and why it’s possible to distill alcohol, and how pressure cookers work. You know why pasta makers just put times and not cooking temperatures on their instructions. You know the difference in temperature between a boil and a simmer. You know why it’s hard to drink in the vacuum of space. You watched a pot, and know it does boil – after about 13 minutes on medium-high heat, in fact.

All this because you know the chemistry behind boiling, just one reason why cooking is awesome.

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