Most of the time I adapt questions that come across my desk (aka prep table) into these Ask-A-Chef articles. Then, sometimes I ask myself a question. I am fortunate enough to have a kitchen with some of the most cutting edge gadgets in the food service industry. We have pacojets, anti-griddles, and control freaks. But one of the coolest things we have is a freeze dryer. We’ve packed this thing full of all sorts of food. Some results are better than others and it’s always an experience when opening the chamber after 24-36 hours of anticipation. But how does this contraption actually work? Is it magic?….. It seems like magic. So I figured I would do some research to figure out what is actually happening in this magic machine
“How does freeze drying work?”
Those two words seen above are actual words. I swear I didn’t make them up to sound smarter than I am! Cryodessication is easier to figure out than the latter. Cryo meaning “frozen” and Dessicate means “to completely dry up” so in short it’s just a cool way to say freeze dry. Lyophilization is a bit more difficult to decipher. Lyo is adapted from latin and it means “to dissolve” and philization uses phile which means “love”. So Lyophilization is not only fun to say it has a fun meaning; A love for dissolving! Now that we all have our vocabulary lesson out of the way we can talk about how the magic actually works. The freeze part of freeze drying happens first. The food placed in a freeze dryer will need to be frozen to a very low temperature, between -40 and -50F. Now the freezing can be done in two ways, freezing quickly will make the ice crystals very small. The smaller thy ice crystals the less damage they do to the cell structure of the food. The slower you freeze something the larger the ice crystals and the more damage they do. The reason this matters is that larger ice crystals are easier for the drying process to happen in the next phase. I recommend a faster freezing process and a longer drying time to make for the best outcome. Sooo how does this food that is packed with frozen Ice crystals magically dry out? Once the food has reached the correct temperature a vacuum will seal the chamber and increase the pressure. At this point the trays will begin to gently be heated. So as the ice crystals begin to melt the go through a process called sublimation. Buckle up because this is where things get crazy. Sublimation occurs when the temperature and pressure of a product cause all three phases of the substance to line up. This is called the “triple point” in thermodynamics. What it means is each substance has a point where it turns into a solid, liquid, or a gas. At normal pressure and room temperature we know how water acts. It freezes at 32F and boils at 212Fand inbetween it is liquid. But at lower pressure the frozen solid food can slowly begin to be heated and the ice crystals bypass the liquid phase (sublimate) and turn into water vapor. The process happens slowly to allow the ice crystals to be removed without returning to the liquid state. This is why freeze dried foods look exactly like they did when they entered the chamber. It’s a truly amazing culinary accomplishment and hope one day they become accessible and affordable for everyone.
If you happen to have access to a freeze dryer one of our favorite things to do is freeze dry gummy bears. The pressure causes the sugars and gelatin to puff into an amazingly light and crispy puffed candy.