Freeze dryer and lyophilizer are synonymous names for the same equipment. A freeze dryer executes a water removal process typically used to preserve perishable materials, to extend shelf life or make the material more convenient for transport. Freeze dryers work by freezing the material, then reducing the pressure and adding heat to allow the frozen water in the material to sublimate.
What is a Freeze Drying?
Lyophiization, more colloquially known as “freeze-drying,” is a process by which food, blood plasma, pharmaceuticals, or tissue can be preserved for upwards of twenty years. The method involves freezing the selected material and then placing it in a vacuum, which dehydrates and evaporates the ice without compromising the freeze-dried material’s physical properties. This “ice-drying” process can be referred to as sublimation plus air desorption drying. This method is often used for biomedical and/or biological specimens, but it can also be used to preserve and extend the shelf lives of various food products. Freeze-drying is a dependable way of preserving a number of different types of foods and beverages, to the extent that it is often used by campers, hikers, and backpackers, as well as astronauts and soldiers sustaining on military rations.
Freeze-drying as we know it now is a very modern procedure, but it has its origins in various pre-modern civilizations in Central America, Eastern Asia, and Northern Europe. Archaeologists have revealed that Andean communities utilized their high-altitude environments to “freeze dry” meat and vegetables with alternating strong sunlight during the day and frost at night. The Incans referred to freeze-dried meat products as charqui. Another staple of the Incan died was chuño, a powder ground from freeze-dried potatoes. Evidence indicates that Japanese monks utilized a similar process in the mountains near Osaka around 1200 A.D. They packed bean curd into the snow-laden mountainsides and let low temperatures and atmospheric pressure sublimate the ice. The resulting product was known as koyadofu, a type of dried tofu. Finally, Vikings in Northern Scandinavia would utilize wooden racks to freeze-dry codfish, effectively preserving the foodstuffs for up to two years.
The process of laboratory lyophilization as we understand it today was developed in 1906 by French physician Jacques-Arsène d’Arsonval at the College de France in Paris. The process was popularized during World War II, when massive quantities of blood serum needed to be preserved in transit from Europe to the US to treat the wounded. Soon thereafter, freeze-drying became a dependable method by which physicians would preserve penicillin and other pharmaceuticals, as well as various other heat-sensitive biological materials. It wasn’t until the 1950s that commercial food processing organizations began using lyophilization in the preservation of food products. Since the mid-20th century, the freeze-dried food market has grown consistently and is anticipated to grow at a rate of approximately 7.4% per year.
How freeze-drying works?
Simply put, freeze-drying is the removal of water from a frozen product using a process called sublimation. Sublimation occurs when a frozen liquid transforms directly to a gaseous state without passing back through the liquid phase. The process of freeze-drying consists of three phases: prefreezing, primary drying, and secondary drying.
Food items that are freeze-dried must first be prefrozen below its eutectic temperature, or simply put, freezing the materials (solute) that make up the food. Although a product may appear to be frozen because of all the ice that is present, in actuality it is not completely frozen until all of the solute is frozen as well.
After prefreezing, ice must be removed from the product through sublimation. This requires careful control of two parameters; temperature and pressure. The rate of sublimation depends on the difference in vapor pressure of the product compared to the vapor pressure of the ice collector. Molecules move from the higher pressure sample to the lower pressure sample. Since vapor pressure is related to temperature, it is also necessary for the product temperature to be warmer than the ice collector temperature.
After primary drying, all ice has sublimated but some liquid is still present in the product. Continued drying is necessary to remove the remaining water. The process for removing this excess water is called isothermal desorption. The excess water is desorbed from the product by making the product temperature higher than the ambient temperature.
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.