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why does freeze-dried candy expand?

Why Does Freeze Dried Candy Expand?

Oct 12, 2022

Janette dArgy

When candy is freeze dried, its texture changes into an airy, crunchy, melt-in-your-mouth marvel, but what makes freeze dried candy expand? This is one of the great mysteries of the universe, and luckily, we have the answer for you.

Freeze dried candy expands because of a process called sublimation. Warming the candy from a very low temperature while under very low pressure changes the moisture in the candy from ice directly to vapor. As the water expands, so does the candy.

But how does freeze drying create the conditions for sublimation? Why does this process make candy expand but not other foods? Let’s dig a little deeper into the details.

How Does a Freeze-Dryer Cause Sublimation?

A freeze dryer sets the stage for sublimation by reducing the temperature to a frigid -50 F. This is done quickly to keep the ice crystals from growing too big.

Then a vacuum pump sucks the air almost entirely out of the chamber. At this point, not much has changed about the frozen food; it’s just really cold. 

Slowly, the temperature begins to rise. As it does, because of the extremely low pressure, the ice crystals begin to vaporize, similar to how dry ice does at room temperature. The gas expands rapidly into the chamber and refreezes on its walls. Sublimation has occurred.

A freeze drying program might repeat this cycle up to three times to remove 98 to 99% of the moisture.

Why Does Candy Expand in a Freeze Dryer?

Most candy is not exactly solid, but not liquid, either. It’s somewhere in between because we like chewy things. We call matter in this state semisolid, amorphous solid, or glass. It acts like a solid in the short term, but at a consistent moisture level, it would flow very slowly at room temperature. There’s no structure in these parts of the candy to prevent it from doing so.

When the moisture in the candy begins to sublime, expanding and flying outward, the force creates bubbles in the semisolid candy, and it expands along with the water vapor. 

The sugar content seems to give the candy elasticity and keep it from cracking apart during sublimation. When the moisture is gone, the freeze dried candy is completely solid, so it maintains the rigid foam-like structure indefinitely.

Some candies that expand the most during freeze drying are Skittles, taffy, caramel, and even Jolly Ranchers.

Why Don’t Some Candies Expand?

Sublimation doesn’t cause all candy to expand. Several types of freeze dried candy don’t expand or even really freeze dry. They mostly fit into one of two categories:

Low moisture content – Because sublimation works on the moisture in food to make it expand if there’s no moisture, but there won’t be much change in structure. Candy corns are in this category. They get crunchy, but they don’t expand much at all.

High oil content – Candies that include a lot of oil typically don’t expand much or even change texture. Since the oil doesn’t evaporate, the candy stays in its original state. Candy with peanut butter is a good example of this.

A few other candy-like things are rumored to not do well in a freeze-dryer. Honey and syrup are examples of this. It’s because after a typical run in the freeze dryer, they get thick and chewy, but they still retain much of their moisture. They take what is described as an amorphous solid state. Hmm, sounds familiar. 

Our theory is that since honey and syrup have so much moisture, it just might take more than one run in the freeze dryer to remove all the water.

Why Don’t Other Foods Expand When Freeze Dried?

It’s true; most foods don’t expand when they’re freeze dried. They retain their original shape. So what’s the difference? Most foods, including fruits, vegetables, and meats have rigid cellular structures. These structures hold their shapes even when put through the sublimation process in a freeze dryer.

You’ll notice that when cooked or dehydrated, most foods will shrink. This is because the water in food helps prop up the structure of the cells. You can think of a cell as a water balloon. The balloon is round because it’s filled with water. If you let the water out, the balloon will shrink. But the balloon can only hold so much water before it breaks. This is why it won’t expand.

Then why don’t these foods shrink in a freeze-dryer? The only point it might be malleable enough to deflate is when the vapor is forming, but the moisture is expanding so quickly it continues to create pressure until it’s gone. It’s like crystalizing the balloon before you let the water out, allowing it to hold its shape even when empty.

Are There Other Foods that Expand?

Candies aren’t the only amorphous solids that like to expand when freeze dried. They may not all do it the same way as candy because they don’t have the sugar content to maintain cohesiveness, but if they have some other way to hold together, they will expand. Here are some examples:

Egg whites – They don’t always expand, but we’ve seen them do it. We think it’s because they have such a high protein content that’s great for building foam. This is why meringue exists. And just like in meringue, it doesn’t work if you mix the yolk in – too much fat for the proteins to bind correctly.

Bread dough – While it doesn’t create the same foam texture as candy or egg whites, it does expand in a freeze dryer even when no yeast is present. We credit gluten for this – the hero of spongy bread and paper mache. 

Mashed potatoes – Well, mashed potatoes don’t expand, but how could they? Even though they are semisolid, they don’t have anything to bind them together as the moisture sublimes. They’re not particularly high in protein; they don’t have gluten. They have starch but very little sugar. So instead, they end up cracked, porous, and brittle.

Visit Candy Jan for Sweet, Puffy Goodness

We hope this exploration of the mysteries of Freeze dried candy was enlightening for you. If you’re like us, it made you hungry. And we’ve got you covered. We have all sorts of billowing, foamy candy to choose from. Can’t decide which to choose? We also offer sample packs. Visit our shop to start your order today.