Hindustan Times ST (Mumbai) - Live
More than a chill pill
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.