Freeze-drying of food
Freeze-drying, or lyophilization, is a preservation method that removes moisture from foods while maintaining their nutrients and flavor. This process involves freezing the food and then placing it in a vacuum chamber where heat is applied carefully, allowing the frozen water to evaporate directly from ice to vapor—a process known as sublimation. This results in a lightweight, porous product that retains its original shape and can be easily reconstituted with water.
The technique gained prominence during World War I and II, when the need for preserved foods for soldiers led to advances in the method. Modern freeze-drying began in the late 1960s, and it is commonly used for a variety of foods, including fruits, vegetables, and instant beverages, as well as in pharmaceuticals and biological specimen preservation. While freeze-dried foods are ideal for long-term storage due to their resistance to microbial growth, the process can be costly and is not widely utilized for all food types. It is especially popular among outdoor enthusiasts for its convenience and portability.
On this Page
Subject Terms
Freeze-drying of food
The first modern quick-freezing process was developed by Clarence Birdseye in 1925; he used refrigerated moving metal belts to quick-freeze fish.
Definition
Freeze-drying, also called lyophilization, is a method of preserving substances for future use by removing water from them. Freeze-dried foods retain their nutrients almost intact. Their flavor characteristics are almost undiminished, and the process prevents the growth of microbes.
Overview
Food was not dried in great volume in the United States until World War I (1914-1918), when dried food became important for feeding soldiers. During World War II (1939-1945), the need for dried foods for soldiers led to the development of such items as instant coffee and dried milk. Modern freeze-drying techniques began in the late 1960’s.
Freeze-drying differs from other drying methods because the substance is frozen into a solid state (at a temperature of about -29° Celsius) before being dried. The substance is then placed on trays in a refrigerated vacuum chamber, and heat is carefully applied until the frozen moisture content is evaporated without melting. A technician controls the rate of heating so that the pressure inside the vacuum chamber never becomes great enough to melt the ice in the substance. The process of changing the ice directly from a solid to a vapor without its first becoming a liquid is known as sublimation.
As the ice vaporizes, the food maintains its shape but becomes a porous (full of tiny holes), spongelike, lightweight dry solid. Drying takes from four to twelve hours, depending on the type of substance, the particle size, and the drying system used, with more than 90 percent of the water being removed. Freeze-dried foods are usually packed in an inert gas, such as nitrogen, and then packaged in moisture-proof containers. Since freeze-drying prevents microbial growth and freeze-dried foods can regain a close approximation of their original shape, texture, and flavor when reconstituted with the addition of water, freeze-drying is an ideal method for storing food supplies.
Among the foods most commonly preserved by freeze-drying are soup mixes, strawberries, mushrooms, bamboo sprouts, shrimp, a variety of vegetables, and beverages, especially instant coffee, tea, and dried milk. Many other substances are also freeze-dried. Drug companies use the process to prepare many medicines, including medicines derived from plants, since the low temperature at which the process takes place allows serums and other drug solutions to retain their original characteristics. Biologists use the freeze-drying process to prepare animal specimens for displays in museums, or to prepare parts of organisms for microscopic studies. The process is also used to restore valuable papers damaged by water, and military personnel, hikers, and campers often carry freeze-dried foods because the products are light and compact. Although it has many diverse, practical applications, freeze-drying is not used extensively for food because the difficulties in freeze-drying animal and plant cells make it relatively uneconomical.
Abla, Kawthar K., et al. "Freeze-Drying: A Flourishing Strategy to Fabricate Stable Pharmaceutical and Biological Products." International Journal of Pharmaceutics, 25 Nov. 2022, doi.org/10.1016/j.ijpharm.2022.122233. Accessed 27 Dec. 2024.
Lang, Ariane. "How Does Freeze-Drying Work and Are Freeze-Dried Foods Healthy?" Healthline, 30 Jan. 2024, www.healthline.com/nutrition/freeze-drying. Accessed 27 Dec. 2024.
McHugh, Tara. "Freeze-Drying Fundamentals." IFT, 1 Feb. 2018, www.ift.org/news-and-publications/food-technology-magazine/issues/2018/february/columns/processing-freeze-drying-foods. Accessed 27 Dec. 2024.