Exploring the science and history behind freeze-drying food preservation.

One of my favorite breakfasts is a bowl of flakes with desiccated berries—usually strawberries, raspberries and cherries. I love the way that the berries look all shrivelled but are still bursting with flavor when the milk soaks into them. This is the result of a process called lyophilization, more commonly known as “freeze-drying.”

Strictly speaking the process should be called “vacuum freeze drying” because the process involves freezing the produce to a very low temperature then lowering the pressure below the triple point of water so that the ice crystals turn directly to gas and can be drawn out of the cells of the produce, leaving it intact but almost completely dry.

The sublimation of the ice crystals is achieved by placing the produce on a gently heated shelf in the vacuum chamber alongside a low temperature coil or heat exchanger that captures the vapor released from the produce by refreezing it. Typically, the heated shelf is still below freezing (32°F [0°C]) and the cold surface, known as the condenser, could be as low as –58°F (–50°C). By the end of this process the produce will have lost over 95% of its water content and it will last for many months at ambient temperature, for example, in my cereal box, provided it is kept dry. The end result is fruit (or meat, or coffee or pharmaceuticals) that looks dead and shrivelled but retains all of its energy, flavor and nutrients.

The cycling of pressure and temperature required to freeze the produce, lower the pressure, heat the produce and refreeze the liberated gas means that this has to be done as a batch process. The manual handling of batches coupled with the very low temperatures required and the large scale of vacuum equipment make it expensive and energy intensive. It is estimated that lyophilization uses almost twice as much energy as regular freezing, so it tends only to be applied to produce that has its value enhanced by the process, like the berries in my cereal. Although it has been suggested that the Incas used low overnight temperatures and strong sunlight during the day to lyophilize potatoes high in the Andes almost 1,000 years ago, their process didn’t involve the vacuum chamber and so was not so effective.

Vacuum freeze drying was originally developed in 1890 but really took off with the freeze-drying of blood plasma and penicillin during World War II. In the 1960s it was applied to food for astronauts, most famously their ice cream. We all ate “space ice cream” as kids, at cafeterias, summer camp and science museums, however, to quote Walt Cunningham, lunar module pilot on Apollo 7, “We never had any of that!” It seems that astronauts were the only people not eating space ice cream in the 60s and 70s.