So I was cleaning out the freezer today, and came across a number of plastic freezer bags that were fully inflated, like pillows. Has this ever happened to you? You’re pretty sure you squeezed as much air as you could out of the bags before sealing and freezing, and some weeks later, you find them mysteriously puffed up?
As luck would have it, I was reading up the other day about lyophilization, or freeze drying, because I’m really interested in the crisp, airy texture that lyophilized fruit and vegetables possess and was trying to figure out if I could do it at home without buying a lyophilizer. I’m especially enamoured of the Crispy Green products, which are light and crunchy and taste of pure fruit. I also love the freeze dried shiitake mushrooms, green beans, and okra I occasionally find at Whole Foods for grinding into dust for garniture, or even just snacking. Anyway, the way lyophilization works is by flash freezing the product, so the water content forms the smallest possible ice crystals and avoids damaging the cell walls. Then the product is subjected to low pressure (vacuum), so the solid water content sublimates to gas without passing through the liquid phase. The product remaining is nearly devoid of any water content, and its structure remains intact.
Interesting, no? Freezer burn is similar, but unintended and undesirable; because it is uncontrolled and unintentional sublimation of water vapor from frozen product, it causes irrevocable damage. But it got me thinking – if freezer burn is essentially incomplete or imperfect lyophilization, then it stands to reason that freezer burn taking place in sealed plastic bags would cause them to puff, as the water vapor has nowhere to go. And there’s the answer to the mystery – the gas inside the bags isn’t air. It’s water vapor sublimating off the frozen food.
So what use is this information? Well, two things. First, the more slowly food is frozen, the larger the ice crystals that will form, so food should be chilled down properly before freezing. Second, the more surface ice forms, the more is available to sublimate and cause freezer burn. If you find a bag in the freezer, and it’s starting to puff, but the food doesn’t appear damaged yet, consider using it immediately to avoid waste.