Freeze-drying has been around more than 100 years. But how does freeze drying work? And why is it better than simply dehydrating?
People have developed many food preservation methods to extend life and nutrition of their edibles during seasonal changes or travel. Anthropologists have identified some of the first methods of preserving food as curing and fermentation. These included drying meat and plant products with heat and airflow, smoke, or salt, to remove moisture. Fermentation includes making cheeses and yogurt, vinegars, and alcoholic beverages. Scientists found evidence of curing as early as 12,000 BC and cheesemaking at 6,000 BC.
Many preservation techniques developed by location: Civilizations in colder climates, such as northern Europe and the homesteads of the Old West, used cooling methods such as freezing, root cellars, and burying food within clay jugs. Warmer locations learned, early on, how to ferment; anthropologists found strong evidence of fermentation within Babylon, ancient Egypt, Sudan, and Mexico.
Then came the modern methods: Nicolas Appert invented home canning in 1806, Louis Pasteur developed pasteurization in 1862. Now we have irradiation, chemical preservatives, and hurdle technology: combining several modern approaches to control pathogens.
Most of these modern food preservation methods cannot be practiced outside a commercial facility. Homesteaders may employ water bath or pressure canning, dehydration, and freezing to extend harvests into lean times. New products such as the Harvest Right freeze dryer now allow individuals to freeze dry their bounty in small batches.
How Does Freeze Drying Work?
Invented in 1906 but developed within World War II, freeze drying involves freezing the material then reducing air pressure so water evaporates rather than melts.
Pharmaceutical companies may freeze-dry products which break down quickly when exposed to air and water. Samples or crime scene evidence may be stored with this method so certain properties remain when scientists need them. But freeze-drying isn’t just for consumables. Because water evaporates without heat, the method has successfully restored rare, water-damaged manuscripts.
Middle school teachers explain evaporation in science class as the heating of water until it turns to vapor and rises from an object, but how does freeze drying work without heat? Sublimation is the transition of a solid directly into a gas. This occurs when temperature and atmospheric pressure don’t allow the liquid form to occur. If it’s not the right temperature or pressure for water to exist, it can only be ice or vapor.
The method uses heat, but just enough to bring material out of the frozen state. Low atmospheric pressure means water immediately becomes vapor. Air then sweeps the water vapor past a freezing coil, which turns it back into ice so it can be removed. This process may occur several times and take hours or days, for thick items, or to avoid overheating. Once freeze-drying is complete, products enter moisture-free packaging, often vacuum-sealed with oxygen-absorbing materials inside.
How Does Freeze Drying Work for Food Storage?
Removing water preserves food because:
- Microorganisms, like bacteria, cannot live without water. If they cannot survive, they cannot feed on food to decompose it or cause disease.
- Enzymes also cannot react without water. This keeps food from spoiling, ripening, or turning bitter due to enzymatic action.
- Removing water removes up to 90% of food’s total weight.
Dehydration also removes water but has drawbacks regarding food quality. Some nutrients perish when introduced to heat, and most dehydration methods involve heat in one way or another. Heat can also change food’s flavor and texture.
Freeze-dried food hydrates quicker and better, whereas dehydrated food may need to be soaked or simmered for hours. It also weighs less and lasts longer because up to 99% of water evaporates; dehydrated food may retain some moisture, especially if people want their apple slices to still be tender, not tooth-breaking hard.
Modern equipment, which allows freeze-drying at home, also allows people to preserve almost everything, from fruit to meal leftovers and even frozen confections. The Harvest Right device can sit on a countertop. Controlled by a computer, it freezes food to minus 40 degrees. A vacuum pump kicks in. It then gradually warms the food. Water sublimates then a fan blows it out of the machine. The process takes about 24 hours, for food that is ½-inch or thinner.
Food prepared with home freeze drying equipment doesn’t take much preparation; apples should be soaked in lemon water or a citric acid solution to prevent browning and some food should be cut or compressed to less than ½-inch thick. Ice cream can be processed alongside meats and produce. Once the process is complete, food is the same color and shape but considerably lighter in weight.
If freeze-drying equipment is not attainable, many people choose to purchase products from food preservation companies. Lightweight #10 cans of potato pearls, dried bacon, and even powdered butter can last decades on shelves. Some people even prepare entire meals by scooping dry ingredients into mason jars then labeling with hydration and cooking instructions.
Always store freeze-dried food in airtight containers. Both enzymes and microbes need oxygen as well as water, and our breathable air always contains at least a little humidity. That oxygen and moisture can ruin your food preservation efforts. Home vacuum-sealers like Food Savers are inexpensive, and moisture absorbers can be ordered in bulk. If storing in mason jars, ensure containers are completely dry before adding contents. Store in cool locations, if possible, to avoid heat as the third factor, which may shorten food’s lifetime.
Food storage experts who have tried both dehydrated and freeze-dried food usually prefer the latter. Sweet corn remains sweet and can be eaten as a snack, crunched between teeth. Lean meats sit within cans at room temperature, ready to be shaken into soups. Backpackers stash nuts and freeze-dried berries within pockets, washing them down with bottled water. And those hoping to avoid waste can preserve their leftovers to hydrate another day.
How does freeze drying work for you? Have you tried commercially prepared food storage? Or have you tried freeze-drying your own bounty?