Methods of Home Food Preservation by Freeze Drying
Freeze dried food is nothing new, it has been around since the 1940s but lay in the realm of large commercial operations. In recent years the prospect of freeze drying your own food has become feasible. Drying with sun has been done for Centuries and various forms of heat drying have been popular for decades, so why not freeze drying.
Dehydrating and freeze drying are similar, but not the same. Dehydrating uses either light [sunlight] or heat to evaporate moisture out of food and is easily done at home. Freeze drying also uses heat at lower exposures. Freeze drying requires an investment in equipment that will freeze the food as well as expedite evaporation. Food that is freeze dried with rare exceptions will rehydrate for consumption more readily.
Freeze drying starts with freezing. The food is then placed in a vacuum chamber under low heat. The small amount of heat rapidly evaporates the crystalized water, it goes from ice to vapor bypassing the liquid phase, this is known as sublimination. A secondary drying phase follows durring which any moisture that escaped the sublimination process is extracted, this too invloves the use of heat.
The food is then refrozen and nitrogen sealed for storage. Free oxygen leads to food spoilage, it oxidizes when combined with oxygen. Nitrogen in this process replaces the oxygen present and lengthens the foods shelf life.
The end result, the freeze dried food once rehydrated, so long as nothing went wrong in the process is generally high quality so far as taste is concerned. Failure to rehydrate the food leaves you with a flavorfull snack with the texture of styrofoam.
A major drawback to freeze drying at home is the cost. Not only the initial investment in equipment but the energy expenditure as well. Freeze drying is an energy hog, it is one of the most energy intensive methods of preserving food. Commercial operations freeze dry their food in huge quantities, which makes it cost effective. Small batch freeze drying ventures, whether it be for resale or personal use need to factor in the energy expenditure as well as the carbon footprint before diving in. There are some products available retail that are designed to use less electric and claim to be more more energy efficient. Harvest Right is the leading manufacturer that has pioneered freeze driers for the consumer and they lay claim to being reasonably energy efficient.
Freeze drying for personal use will pay for itself in the long run so long as you are able to process enough food. A cupboard full of freeze dried produce, either home grown , purchased ‘in season’ or in bulk or even leftovers can substanially reduce a families grocery bills.
Freeze drying with Dry Ice
There is also a mode of freeze drying which although not nearly as reliable as the use of vacuum chambers and assorted expensive equipment, can be done with appliances and equipment you probably already have. The food should be surrounded with c02 / dry ice which creates as close to a zero humidity environment as possible’ The dry ice forces the moisture out of the foods.
To do this you’ll a box that can hold enough dry ice to completely envelope the food. Plastic storage bins or food storage containers such as tupperware work just swell, but they should be small enough to fit in your freezer.
Place a thin layer of dry ice on the bottom followed by a layer of food followed by a layer of dry ice and so forth till you almost reach the box top. [Do not use freezer bags]. Leave an inch or two of headspace at the top of the box, but not much more, for gases to circulate, and drill ample holes in the lid so gases can escape. If the gases can’t escape you could create a minor explosion and a major mess to clean up. The escaping gases will not only contain the evaporating c02 from the dry ice but the moisture trapped in the food – hence sublimination as described earlier in this article.
Next place the box in the freezer it could take several weeks but the longer it takes the better the end result. The dry ice should solidify and evaporate durring this time, but its still advisable to check on it daily. Sometimes ice will build up and clog the holes, it should be removed. Also make sure the lid is secure as you want the gases to escape slowly. Once the dry ice is gone what remains is freeze dried food.
Dry Ice is hard to come by if you don’t know where to find it. In rural areas some walmarts and costco stores carry it as well as some other independent grocers. If that doesn’t pan out try using dryicedirectory.com which locates sources based on your area code.