Water Physics 101

Water Physics 101

We recently announced the progress of The Anova Precision Oven, a connected, countertop steam oven that will ship in 2020. Steam will fundamentally change the way people think about cooking, so we've launched a series of blog posts to prepare the #anovafoodnerd community for the impending revolution.
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Water plays a critical role in almost all cooking. Sometimes it’s obvious, like when boiling or steaming. But water’s role is just as vital even when it’s invisible to the naked eye, like when roasting a cut of meat or baking a batch of cookies. Unfortunately, your sunday afternoon cooking show doesn’t explain how water plays a role in cooking. And trying to cook without understanding water is like trying to launch a rocket without understanding gravity.

We can do better. In this post, I’m going to introduce you to three new terms that will prepare you for the world of combi cooking, and may forever change the way you think about cooking in general. Your new words-of-the-day are relative humidity, evaporative cooling, and wet bulb temperature. Let’s go on an adventure.

Water Rules(!)

We begin with a surprising revelation: most foods are mostly water.

Ingredient % Water
Broccoli 91%
Egg White 88%
Salmon 62%
Steak 57%
White Bread 37%
White Bread Toast 30%

And don’t get me started on the water content of avocado toast…

Even foods that we don’t think of as “wet” still contain lots of water. And, as we recall from middle school science class, water behaves according to a very specific set of rules.

In particular, water has a boiling point. Liquid water can’t get hotter than its boiling point, which (here at sea level) is 212°F / 100°C. Anything hotter than that is no longer water, it’s steam.

Here’s an experiment that you can try at home. Take a pan of water and place it in your oven. Now turn your oven on to 300°F and let it heat up. Stick a thermometer in the water: it won’t be hotter than 212°F. Crank your oven up to 400°F. The water still won’t top 212°F. Set the oven to broil. Still two-twelve on-the-dot.

Big whoop, right? Well, recall that most foods are mostly water. Instead of your pan of water, imagine a rib roast, or a head of cauliflower, or bundt pan full of whole eggs. They behave like the water does. This means that no matter what temperature you set the oven, any food that’s still wet will experience a different temperature than the temperature of the oven. And we, the cooks, care a lot more about the temperature of the food than the temperature of the oven. Put a pin in this. We’ll be back in a moment.

Relative Humidity

Humidity is a measure of the amount of water in air. But as the temperature of air changes, the amount of water it can hold changes. Hotter air is capable of holding more water than cold air. This is why summer months feel muggy, but in winter months we break out the chapstick.

So we use the term relative humidity as a shorthand way to talk about the fraction of actual vs. possible humidity in air. And a difference in relative humidity in your oven has a dramatic impact on how your food cooks.

Let’s look at a real world example of the impact of humidity, not on food, but on spa-goers.

Sauna v. Sauna

Imagine a dry sauna set to 200°F. It’s hot - nearly at the boiling point of water - but is a realistic sauna temperature. When you first go inside, you notice that the air is very warm, but you’re comfortable. Your body starts to sweat, and as the sweat evaporates, it cools you off. After twenty minutes, you’re ready for a glass of water (with lemon if you’re civilized), but you’re otherwise relaxed and happy.

Now let’s change one tiny variable…

Picture the same 200°F sauna, but now with very high humidity. Upon entering, you begin literally cooking immediately, straight-up steaming like a human dumpling. Sweating does nothing to cool you down, and so you exit the sauna as quickly as possible, seeking medical attention. This was a terrible idea.

So what’s the difference?

Evaporative Cooling

In the dry sauna, your sweat evaporates into the dry air. As it does, it carries tremendous amounts of energy away from your body. As a result, your body cools down. This is called evaporative cooling.

But recall from above that air can only hold so much water. If the air is already fully saturated, there’s nowhere for new water to evaporate to. Sweating doesn’t cool you off because your sweat can’t evaporate. The air is already at 100% relative humidity. This is why the humid sauna is such a death trap: your body is being raised to the temperature of the environment without any counteracting force to cool it down.

Know what else reaches 100% relative humidity? The inside of a sealed sous vide bag. As the temperature inside the bag rises, water from your food evaporates. But, because there’s only a tiny amount of air left in the bag, that air quickly becomes saturated and there’s no room for additional water to evaporate. In effect, all sous vide cooking happens at 100% relative humidity. Remember this factoid for the next step in our journey.

Wet bulb temperature

Now we come to our third and final bit of new terminology for the day. Somewhere in your kitchen you probably have a thermometer. Hold it out in front of you, turn it on, and take a reading. You’ve just measured dry bulb temperature. This is the temperature we’re used to in everyday life: it’s the temperature you read on the weather report, the temperature you set your home thermostat, and the temperature you set on your oven dial. This is, broadly speaking, the temperature of a thermometer in air.

But most foods are full of water, not air. Try this instead: dip a paper towel in room temperature water, then drape it over the probe end of your thermometer. You’ll get a different reading - a lower reading. That’s because, even at room temperature, some of the water from that paper towel is evaporating into the air. And, as it does, it’s cooling everything left behind. You’ve just made a wet bulb thermometer and you’ve taken your first measurement of wet bulb temperature: the dry bulb temperature, minus the effect of evaporative cooling!

Again, what’s the big deal?

As long as your food is wet, it behaves according to the wet bulb temperature. This is the most important part of cooking that cookbooks never teach you about. The temperature you set on your traditional oven only knows about dry bulb, but your food heats according to wet bulb. Not cool.

We know from the Sous Vide Enlightenment that doneness is all about temperature. If you want your chicken breast or salmon fillet cooked to the perfect doneness, you just set the water bath to the correct temperature.

But remember what we just said about the inside of sous vide bags? They rise to 100% relative humidity, which is a very special number. You see, at 100% relative humidity, dry bulb temperature and wet bulb temperature are the same. Your sous vide bath is really controlling wet bulb temperature. But if we replace the sealed bag with a steam-filled oven, we get the same result: a steam oven running at 100% relative humidity operates just like… sous vide!

Feel free to take a few minutes to unblow your own mind.

Believe it or not, things get even better. If you know the wet bulb temperature of your cooking environment, you can achieve sous-vide-like results even at lower humidity. And this is a powerful advantage.

For instance, suppose you’re cooking a skin-on turkey leg. Inside a sous vide bag, everything is humidified, including the skin which soaks up water and becomes rubbery. Getting that skin crispy after cooking becomes a major challenge. But if you were to cook the same turkey leg at the same wet bulb temperature but in a dry oven, the meat will achieve the same perfect doneness while the skin stays dry and ready to crackle. This, my friends, is the future.

But this would require some kind of sophisticated, cutting-edge, cleverly designed, effortlessly charming piece of steam oven technology. Oh, wait. 2019 Anova Precision Oven
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