The Science Behind Freeze-Dried Candy That Makes It so Wonderful
No one can deny that trying a freeze-dried Skittle is a eureka-worthy experience, and the science behind freeze-dried candy is equally as fascinating. The flavor and texture of freeze-dried candy result from a unique process that occurs in the freeze dryer. Here’s a quick rundown.
What does a freeze dryer do?
The freeze-drying process begins by cooling food way down to -50 degrees Fahrenheit. For reference, that’s just as close to the freezing point of water as a balmy 114-degree summer day in the Sahara Desert – just the opposite direction.
Once the food is nice and cold, the air in the chamber is removed by a vacuum pump, leaving the food under very low pressure. You may know that lowering pressure also lowers the temperature at which water and other chemicals change state. In this case, the ice is under such low pressure that it will evaporate before it’s warm enough to melt. This process is called sublimation.
The temperature of the freeze-dryer chamber is slowly warmed to a comfortable -10 degrees. As the ice crystals in the food sublime, they gasify and disperse. This process is typically repeated a couple of times.
Finally, the temperature of the chamber is slowly raised to 120 degrees. This ensures that most of the remaining moisture is evaporated. This way, 98 to 99% of the moisture is removed in freeze-drying.
Why does candy puff up so much?
If you’ve eaten a lot of freeze-dried food, you know it doesn’t shrink like dehydrated food. Most foods maintain the same shape they were in when you put them in. So why does candy expand so much? This is an interesting bit of science behind freeze-dried candy.
Pressure, volume, and temperature
If you’ve studied chemistry, you may know there’s a predictable three-way relationship between pressure, volume, and temperature in any fluid. This equation represents that relationship:
P1V1/T1 = P2V2/T2
What this means in English is:
Pressure and volume are inversely proportional – volume tends to contract when pressure increases and vice versa. Think about a clown making balloon animals. As he twists the balloon, it contracts its volume. This increases the pressure, making it feel more firm.
Pressure and temperature are directly proportional – temperature also wants to increase when pressure increases. Blow a can of compressed air onto your hand. As the air decompresses, it will feel colder than the room temperature.
Temperature and volume are directly proportional – volume naturally increases proportionally when temperature increases. This is precisely how mercury thermometers work. As the mercury warms up, it expands, and the level rises to show higher temperatures.
Wouldn’t we expect candy to contract in such low temperatures?
In a freeze-dryer, the temperature is brought so low, according to the relationship between temperature and volume (sometimes called Charles’s Law), wouldn’t we expect the volume of the candy to decrease?
There are a couple of things going on here.
Water doesn’t always follow the rules of thermodynamics like other substances. Most liquids do contract when frozen, but water expands. That’s why if you put a can of soda in the freezer, it will have exploded when you remember it the next day. But this isn’t the main reason candy expands so much.
When you cool candy and the water inside to -50 degrees, the surrounding pressure will decrease a bit, but the volume doesn’t change that much. On the other hand, when you reduce the pressure to almost nothing and start to raise the system's temperature, that’s when Charles’s Law takes effect. The newly gasified water expands quickly, and so does the candy it’s tied up in.
Why doesn’t candy maintain its shape like other freeze-dried foods?
It’s true that most foods don’t look much different after being put in the freeze dryer. Some freeze-dried candy balloons up to several times their original size. Why is this? With most candy, we’re dealing with semisolid substances (aka quasi-solid or amorphous solid). This means it’s somewhere between a solid and a liquid. It maintains its shape pretty well but will give if compelled to. Think of play-doh.
A food like an apple slice has a cellular structure that basically maintains its shape through changes in temperature and pressure. But the semisolid nature of candy – whether it be a gummy bear or a Jolly Rancher, will rearrange itself readily. This means when sublimation occurs and the moisture gasifies, there’s nothing to keep it from blowing up like a balloon as the vapor vacates.
Isn’t it difficult to freeze-dry sugary foods?
You may have read elsewhere that high sugar substances don’t freeze-dry well. Typical examples of this are honey and maple syrup. When freeze-dried, these substances form a “glass,” a very viscous liquid that tends to resist dissolving in water. And while you can find powdered honey fairly easily, it’s usually combined with something else.
The theory is that since sugars bind so tightly to water, it’s complicated to pull the water out in the freeze-drying process. While this seems logical, experience trumps theory. And in our experience, candies with high sugar content freeze-dry very nicely.
So why does it work for some sugary substances and not others?
Well, as we said, the result of freeze-drying honey is often a viscous liquid called a glass – which is really another name for a semisolid. As you’ll remember, that’s what most candy is before it’s freeze-dried. It won't work very well if you try to dissolve saltwater taffy in water. But after you freeze-dry taffy, it melts as soon as it comes in contact with your tongue. It’s a wonderful experience.
So maybe honey simply has too much water in it for one cycle in a freeze dryer. It would be interesting to see how it would respond to a second round. Sounds like a fun experiment to us.
If the science is making you hungry, come to Candy Jan
While the science behind freeze-dried candy is fascinating, you don’t need to understand it all to know it’s delicious.
Candy Jan has a wide variety of unique freeze-dried confections to experiment with yourself. Contact us today, browse through our shop, and have your order delivered right to your door.