Wet Bulb, Dry Bulb, and the Science of Cooking with Water

thermodynamics wet bulb dry bulb

Cooking, in all its forms, involves controlling water. That’s a basic fact. Heat’s interaction with internal moisture causes changes to food as it cooks. After all, food is mostly water. Notably, some foods are more than 90 percent water. So, it makes sense that food behaves like water when heated. Let’s delve into wet bulb, dry bulb, and the science of cooking with water.

wet bulb dry bulb thermometer

Dry Bulb

Dry-bulb temperature is measured by a bare thermometer with no radiant heat affecting it (sunlight, radiators, Chippendales dancers, etc.). It’s what we think of as the air temperature. Wet-bulb temperature is a little trickier. Simply put, wet bulb is the lowest temperature possible under ambient conditions by the evaporation of water only. Sounds complicated, but it’s not.

Wet Bulb

Wet-bulb is measured with a thermometer covered by a water-soaked wick or other porous fabric. Remember, evaporation is a cooling process. Therefore, the drier the air, the faster moisture evaporates, and the lower the web-bulb temperature. Wet-bulb is always lower than dry-bulb. However, there is an exception. At 100% relative humidity, wet-bulb and dry-bulb temperatures are equal. Certainly, it’s physically impossible for the wet bulb to be higher than the dry bulb.

Author Nathan Myhrvold eloquently described the role that water plays in cooking in his book series, Modernist Cuisine:

Water is also the medium in which most cooking is done. Sometimes we use it directly, as when boiling, steaming, extracting, or cooking sous vide. Water plays a role in ostensibly “dry” processes such as roasting and baking as well, yet many chefs fail to account for its effects. Whether it’s a liquid boiling or simmering in a pot, a vapor rising from a steamer, the humidity in an oven’s air, the liquid circulating in a cooking bath, or the crushed ice in a blender, the unique properties of water come into play in all manner of culinary operations. Faced with such powerful and ubiquitous phenomena, cooks must learn how to manipulate water or risk being foiled by it.

food is water

Long before Myhrvold, our founder Winston Shelton discovered water’s role in cooking. He deduced that to control food’s temperature, you must control food’s internal moisture. This led to the invention of CVap® (Controlled Vapor Technology). Shelton developed it as a hot food holding technology. But he realized it could also revolutionize cooking.

New Technology, New Words

Never limited by existing language, Winston fondly coined new terms. A significant Winstonism was thermoisturization. He defined it this way:

Thermoisturize: Taken from the words thermalize and moisturize…to mean the process of thermalizing while moisturizing. Also, ‘thermoisturization.’ The scientific world uses ‘thermalizing’ to represent the heat transfer phenomena in each of the processes. ‘Thermoisturization’ is coined to represent the simultaneous thermalization and moisturization of foods.

Our Corporate Research Chef, Samantha Brown, explains thermoisturization in more layman’s terms here.

The Laws of Thermoisturization

Shelton wrote The Laws of Thermoisturization. These explain the mechanics of controlling wet bulb and dry bulb temperatures inside an oven utilizing two heat sources. Ultimately, the reason wet bulb temperature control is more important is as follows:

  1. The large amount of moisture in all fresh foods.
  2. The enormous heat energy in water vapor.
  3. The ability to accurately control the amount of water vapor in the food’s atmosphere.

Certainly, that’s a lot of scientific jargon. A much simpler explanation of CVap can be found in this quirky video.

CVap generates heated vapor from a water reservoir at the unit’s base. We refer to it as the evaporator. If food is cooler than the evaporator, the heated vapor condenses on food, heating it. On the other hand, food that’s hotter than the vapor temperature evaporates moisture, cooling it. Since evaporation and condensation are highly effective heat transfer phenomena, food temperature must equalize with the vapor temperature. Ultimately, food temperature is affected more by the vapor temperature than the air temperature. Food simply can’t drop below the evaporator temperature. This differs from other so-called “humidified” equipment. Only CVap directly controls food temperature via controlled-heat vapor.

thermodynamics wet bulb dry bulb

Conventional Cooking vs. CVap Cooking

In a conventional oven, evaporation increases exponentially with the food’s temperature. Evaporation slows as food’s interior moisture decreases. As a result, the food dries out.
Indeed, evaporation is counterproductive for several reasons:

  1. Evaporative cooling hampers the heating process.
  2. Energy is wasted evaporating moisture instead of heating food.
  3. Food quality decreases as moisture evaporates.

In order to serve at peak quality, food must be removed from a conventional oven as soon as it reaches the desired endpoint temperature. If not, food overcooks.

CVap Cooking

In a CVap oven, food moisture cannot evaporate until food reaches vapor temperature. Critically, food heats more rapidly in the absence of evaporative cooling. As food reaches the selected endpoint, its temperature stops rising. For that reason, food can remain in a CVap oven for extended periods without overcooking.

Relative Humidity is Bull$h*t

Some manufacturers throw the term “relative humidity” around. They claim better control over food quality via relative humidity. However, when it comes to food, relative humidity is BS.

Relative humidity is simply a measurement of how much moisture air can hold before becoming saturated. But it changes, depending on the temperature.

“Controlling” Relative Humidity?

“Controlling” relative humidity is a bit of a misnomer. Relative humidity can be the same percentage at a wide range of temperatures. You can have an RH of 50% with a dry-bulb temp of 62°F or 86°F or 150°F (and so on), depending on the wet-bulb temperature. Consequently, relative humidity’s effect on food temperature and quality is indirect, at best.

CVap is Different

That’s why CVap technology is so different. The other guys attempt to affect food by manipulating the oven’s air. But they have it backward. Only CVap directly controls vapor temperature, which dictates food temperature.  

After all, do you want to heat air, or do you want to heat food?