Freeze-drying, or lyophilization, is like "suspended animation" for food. Freeze-dried meals stored in Mylar bags lasts for decades. When you're finally ready to eat it, revitalize it with a little water. Even after all those years, the taste and texture will be pretty much the same. That's some amazing chemistry. What better way to learn about solids, liquids, gas, atmospheric pressure, and molecular levels?
Freeze-drying or lyophilization is a dehydration technique based on the sublimation of water in a product. This means that the product’s water content transitions from a solid to a gaseous state — or from ice to vapor — without going through the liquid state. Just like evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it.
Water will sublime from a solid (ice) to a gas (vapor) when the molecules have enough energy to break free but the conditions aren't right for a liquid to form. There are two major factors that determine what phase (solid, liquid or gas) a substance will take: heat and atmospheric pressure. When food is placed in a freeze dryer chamber, it is frozen solid, which separates the water from everything around it, on a molecular level, even though the water is still present.
The vacuum pump forces air out of the chamber, lowering the atmospheric pressure below .06 ATM. The heating units apply a small amount of heat to the shelves, causing the ice to change phase. Since the pressure is so low, the ice turns directly into water vapor. The water vapor flows out of the freeze-drying chamber, past the freezing coil. The water vapor condenses onto the freezing coil in solid ice form, in the same way water condenses as frost on a cold day. This continues for many hours (even days) while the material gradually dries out.
The process takes so long because overheating the foods can significantly change the composition and structure. Additionally, accelerating the sublimation process could produce more water vapor in a period of time then the pumping system can remove from the chamber. This could rehydrate the foods slightly and degrade their quality.
Raw foods contain a large amount of water — 80 to 95 percent. The water activity of foods varies. During the process of freeze-drying, both water molecules that are bound to other molecules (sugars, salts, proteins) and water molecules that are free must be removed.
The significant reduction of the water activity of the final food product contributes to its shelf-life and food safety. The water activity for most properly freeze-dried food products ranges from 0.08 to 0.33. Once the food is dried sufficiently, it's sealed in a moisture-free mylar package with an oxygen absorber. As long as the package is sealed properly and not punctured, the food can still be restored to its original form with a bit of water even after 25+ years. However, if even a very small amount of moisture remains in the packaging, the food will eventually spoil.
Discover how fun and delicious freeze-dried desserts and snacks can be! From freeze-dried ice cream to gummy bears and the very favorite – Skittles! There is a high demand for freeze-dried candy. The amazing texture of freeze dried candies such as saltwater taffy is worth letting your taste buds explore.
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.
Bring highschool chemistry back to life. You may know that P1V1/T1 = P2V2/T2 is the equation for the predictable three-way relationship between pressure, volume, and temperature in any fluid. Pressure and volume are inversely proportional – volume tends to contract when pressure increases and vice versa. Think about a clown making balloon animals. As a balloon twists, it contracts its volume. This increases the pressure, causing the balloon to feel 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.
Semisolids like honey and maple syrup form a “glass,” a very viscous liquid that tends to resist dissolving in water when freeze-drying them. Thus, powdered honey is usually combined with something else. Experiment with 1 teaspoon of cane sugar per 2/3 cup honey to freeze dry and make a powdered honey.
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 might explode if you forget it overnight. 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.
Freeze-drying removes water from food, but not oils. Oil doesn't gasify during sublimation the same way that water does. The result is that these candies do not change much. This can be experienced through freeze-drying candies with peanut butter or lots of nuts. The Big Hunk Bar is a perfect way to demonstrate how freeze drying will not affect the nuts, however, the nougat surrounding the peanuts is very much impacted. A freeze-dried Big Hunk offers an exciting contrast of textures with a crunchy nougart texture and realitvely chewy nuts.
Oreos are so low in moisture that they do better if left out for about a week to reach a stale state to obtain more moisture before freeze-drying. This is a great science lesson for kiddos. One would typically hypothesize the opposite. Most foods dry out when they go stale. However, Oreos are actually drier that the surrounding air. Thus, they absorb atmosperic moisture. This gives the Oreos a softer texture and doing the opposite of a freeze dryer.
After you freeze-dry taffy, it melts as soon as it comes in contact with your tongue. It’s like the Forth of July for your mouth!
Sharing tips with the community and learning from each other's experiences is such a blessing.