FOOD PRESERVATION : FREEZE-DRYING, IRRADIATION AND VACUUM PACKING

Humans have used processes associated with freeze-drying for centuries by placing foods at cooler high altitudes with low atmospheric pressure where water content is naturally vaporized. Also called lyophilization, freeze-drying involves moisture being removed from objects through sublimation. Modern freeze-drying techniques dehydrate frozen foods in vacuum chambers, which apply low pressure and cause vaporization.

Freeze-drying reduces foods’ weight for storage and transportation. Freeze-dried foods do not require refrigeration but do need dry storage spaces. Adding water to freeze-dried foods reconstitutes them. Unlike dehydration, freeze-drying does not remove components that give foods flavors. The nutritional qualities of foods are also retained. The process inhibits microorganisms and chemical reactions causing food to spoil because the water pathogens and enzymes need to thrive is absent.

Engineers developed freeze-dryers for specific tasks that use vacuum pumps and chambers to pull moisture out of food. Commercially, freeze-dried foods are popular because they require less storage space than other packaged food. Freeze-drying extends the shelf life of products. Freeze-drying technology has been applied to consumer food products since the 1930s. Coffee, first freeze-dried in 1938, is the most familiar commercial freezedried food product. The Nestle Company was the first to freeze-dry coffee because Brazil requested assistance to deal with coffee bean surpluses. The successful freeze-drying of coffee resulted in the creation of powdered drinks such as Tang, which contrary to popular belief was invented by General Foods not the National Aeronautics and Space Administration (NASA). Many types of soups and noodles are freeze-dried. Researchers develop and patent new freeze-drying processes to improve and vary foods for consumers.

Freeze-drying process

Freeze-drying was appropriated as the best method to preserve food for astronauts in space flight. During his pioneering February 1962 orbit, John Glenn was the first human to eat food in space. When he expressed his dissatisfaction, food engineers attempted to improve the taste of space food while maintaining its nutritional qualities and minimizing its weight. As space technology advanced, more elaborate freeze-dried meals provided variety for longer duration space missions, which required compact, lightweight food  sufficient to feed crews at the international space station.

As early as 1963, radiation was used to control mold in wheat flour. The next year, white potatoes were irradiated to inhibit sprouting. By 1983, the Institute of Food Technologists released a study about the potential of radiation to sterilize and preserve food. Those experts declared that food processors might be reluctant to accept expensive irradiation technology unless they were confident that consumers would purchase irradiated goods. The Institute of Food Technologists warned that prices of irradiated food had to be affordably competitive with nonradiated foodstuffs and fulfill people’s demands.

Irradiation process

Irradiation is less successful than freeze-drying. Prior to irradiation, millions of people worldwide became ill annually due to contaminated foods with several thousand being hospitalized or dying due to food-borne pathogens. By exposing food to an electron beam, irradiation enhances food safety. Irradiated human and animal feed, especially grain, can be transported over distances and stored for a long duration without spoiling or posing contamination hazards. The radura is the international food packaging symbol for irradiation.

Small doses of radiation alters microbe DNA and kills approximately 99.9 percent of bacteria and parasites in meats. This exposure does not alter nutrients. Irradiation permits people to consume slightly cooked meats, including rare steaks and hamburgers. Most food irradiation uses cobalt-60 isotopes, but researchers developed alternative techniques such as gamma rays from cesium-137 and linear accelerators that transform electrons aimed at food into x-rays, which have greater penetration than electron beams. Those beams consist of high-energy electrons expelled from an electron gun and can only penetrate several centimeters compared to gamma and xrays reaching depths of several feet. Irradiation sources are kept in water tanks that absorb the radiation until they are used to sterilize food in a thick concrete chamber.

Countries in North America, Europe, Asia, Africa, and the Middle East accepted irradiation. The World Health Organization endorsed irradiated food as safe for consumption. The U.S. Food and Drug Administration (FDA) approved irradiation of pork, fruit, vegetables, spices, and herbs in 1986 and poultry in 1990. Eight years later, the FDA approved the process of irradiating red meat to kill dangerous microorganisms and pests and slow meat spoilage. Despite federal approval, some states banned irradiation. During the 1990s, scientists improved irradiation methods to destroy toxins and bacteria, especially Escherichia coli, Salmonella, Shigella, and Campylobacter, in foods. The Institute of Food Technologists published a document that noted that the FDA’s endorsement of irradiation legitimized that food preservation technology. Astronauts routinely eat irradiated food to prevent food-related sicknesses in space.

Despite irradiation’s benefits, some consumers boycott purchasing or consuming irradiated foods that they consider dangerous and insist instead that facilities where agricultural goods are processed should be sanitized to reduce threats of contaminating toxins. Irradiation supporters assert that public opinion parallels how people initially reacted to milk pasteurization before accepting that process. Studies concerning how people and animals react to correctly irradiated food indicate that the foods are safe and not radioactive. Irradiation facilities are regulated by government licensing, and workers undergo rigorous training. Fatal accidents have occurred only when workers ignored rules regarding exposure to radioactive materials.

Irradiation has several significant limitations. Viruses are too small for irradiation dosages appropriate for safe food handling. Prions linked to bovine spongiform encephalopathy lack nucleic acid thus making irradiation ineffective. Vacuum-packing food technologies involve a process that removes empty spaces around foods being packaged. Vacuum technology uses environments artificially modified to have atmospheric pressures that are lower than natural conditions.

Vacuum packing extends the shelf life of food. The U.K. Advisory Committee on the Microbiological Safety of Foods warned that anaerobic pathogens such as C. botulinum can grow in vacuum-packed foods. Because vacuum packing often results in rubbery sliced cheese, some manufacturers use the modified atmosphere packaging (MAP) system, which utilizes gases to fill spaces so that cheese can mature to become tastier inside packaging.

By Elizabeth D. Schafer in "Encyclopedia of 20th-Century Technology", Colin A. Hempstead, editor; William E. Worthington, associate editor, published in 2005 by Routledge New York, excerpts volume one p.365-367. Adapted and illustrated to be posted by Leopoldo Costa. 

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