More than a chill pill
The first freeze-dried meals came out of a NASA lab in the 1960s, but the first foods to ever be freeze-dried were potatoes, 700 years ago. Chuno, as the preparation was called, fuelled the Inca army as it marched along the Andes, annexing new territories for a growing empire. The low temperatures and low air-pressure levels at those altitudes were ideal for this preservation technique.
Ages later, in 1909, biologist LF Shackell would replicate this process in a laboratory, refining the procedure so that it could be used in a more widespread manner. By the 1940s, freeze-drying was helping preserve blood plasma and penicillin in the battlefield, during World War 2.
Then came the space race of the 1960s. America’s National Aeronautics and Space Administration (NASA) poured millions into finding the best freeze-drying methods for food. As with so much else — the internet (which originated in a DARPA project), aviator sunglasses (designed for the US Air Corps), GPS (developed by the DOD), even microwaves — the advances made in pursuit of a nationalistic goal yielded a popular new consumer product. In this case, freezedried and indefinitely shelf-stable foods ranging from coffee and herbs to vegetables, meat and fish. (Foods that do not freeze dry well, incidentally, include those high in sugar or fats, such as honey, jams, butter and mayonnaise. The natural sugars hold on to water even at low pressure levels, and fats simply don’t reconstitute well when mixed with water.)
So, how does it work? Freeze-drying combines the best aspects of freezing and drying, while eliminating their negative effects. Frozen foods, for instance, need a constant power source to stay frozen. Drying alters texture, often making foods chewy and less flavourful. Freeze-dried foods remain shelfstable at room temperature and retain much of the original textures and flavours when rehydrated.
The procedure works by sublimating the water out of foods. Substances such as naphthalene balls and camphor sublimate at room temperature, going straight from the solid to the gaseous state.
Water can sublimate too, if air pressure and ambient temperature are very low. How low? In this preservation method, temperatures are dropped to -50 degrees Celsius, and a vacuum is used to reduce air pressure to .06 atm, a 94% drop from normal atmospheric pressure at sea level.
There are three stages to freeze-drying. First, the freezing turns the water in the food into ice crystals. Next, in the primary drying stage, air pressure is dropped and the food is heated slightly so that the water sublimates, leaving pores where it had been. This process is slow and heavily controlled in order to preserve the texture and nutrients, and can take up to two days. The final process is secondary drying. Here, the food is brought back to room temperature and dried further. The ingredient has by now typically lost about 99% of its water content, giving it a shelf life of up to 25 years.
(It’s important to note here that it may not be very tasty, a few years in. The fats in the food may go rancid, the packaging may absorb moisture from the air. It is best to consume such products within a year or two of manufacture.)
A significant advantage is that the extremely low temperatures lock nutrients in. Studies have shown that freeze-drying preserves fibres, minerals and nutrients such as carotenoids and Vitamin C to a far greater degree than does hot-air drying. It preserves flavour better too.
Most delicate aroma compounds perish at high temperatures (which is why precooked meals sealed in at high temperatures often taste bland).
In foods where aroma plays a key role, such as coffee and herbs, the eventual flavour of the freeze-dried version is far superior to that of the air-dried. Since the procedure causes no change in volume (only the weight is altered), so the food, when reconstituted with water, is visually and texturally very close to the original. This is why freeze-dried strawberries are usually added to cornflakes rather than dehydrated ones.
In the scientific community, this procedure is known by another name: Lyophilisation. “Lyo” is Ancient Greek for “solvent”, “phil” is “to love”. Freeze-dried items love to dissolve. This makes the process a convenient one for the manufacture of pharmaceutical drugs.
In a full-circle development, freeze-dried ready meals are now becoming popular too. Once, instant noodles were the only such option. They lasted months on the shelf because they were deep-fried. Freeze-dried meals are more wholesome and more flavourful. And they’re not just for astronauts anymore.