Making Progress in
Food Preservation
March 1998 -- New Technologies

By: Elaine Knehr
Contributing Editor

  Tomatoes picked ripe for cross-country shipment. Dairy products possessing double their previous shelf lives. Pathogen-free meat and poultry. These are the goals of the food product designer, and industry is working to make them a reality.

  A number of available technologies help the food processor deliver safe and quality products. Some processes are new; others are familiar technologies applied in novel ways.

The heat is off

  Thermal food processes -- such as canning and pasteurization -- deliver a safe product, but often at the expense of the food's freshness.

  " 'Nonthermal' generally means lower impact on organoleptic and nutritional qualities," says Don Quass, director, food technology alliance, Electric Power Research Institute, Palo Alto, CA. Two "nonthermal pasteurization" technologies are irradiation and ultra-high pressure.

  Irradiation. In 1963, FDA approved irradiation to treat food. Initially, wheat and wheat flour were irradiated to eliminate insects. Over the years, irradiation has been approved in the United States for pork, spices, fruits, vegetables and poultry. In December 1997, red meat joined the list, and a petition to irradiate seafood is pending.

  In irradiation, the food is exposed to a source of ionizing radiation. Contrary to one popular misconception, the process doesn't make food radioactive, since the food never comes in contact with radioactive material. Irradiation can be likened to a person being X-rayed for medical reasons.

  Irradiation reduces or eliminates pathogenic and spoilage microorganisms in food. Specific doses of radiation can kill the rapidly growing cells of insects, pathogens and spoilage organisms. These include E. coli O157:H7, Salmonella, Campylobacter and Trichinella spiralis. Since the cells of the food itself are not multiplying, the process has little effect on the food. The nutritional value and organoleptic qualities of food remain almost unchanged. Irradiation can minimally affect some very sensitive vitamins, such as thiamin. It is estimated that if the entire U.S. pork supply were irradiated, only 2.3% of B1 in American diets would be lost, according to an April 1996 paper, "Radiation Pasteurization of Food," published by the Council for Agricultural Science and Technology. In addition, a small fraction of ascorbic acid in fruits is converted to another equally usable form.

  FDA-approved sources of ionizing radiation are gamma rays produced by the radioisotopes cobalt-60 or cesium-137, and machine-generated X-rays and electrons. The amount of ionizing energy absorbed is measured in units of grays or kilograys (kGy).

  "The best point to irradiate a food is after it has been packaged," says Frank Fraser, vice president, market development, MDS Nordion, Kanata, Ontario. "This eliminates recontamination during processing. Cobalt-60 can be used to treat items like chicken pieces or cases or pallet loads of product. Unstable cobalt-60 gives off gamma radiation as it decays to a stable form. This energy is absorbed by the product. Exposure time is varied to get the desired dose of radiation."

  Radiation produced by an electron beam accelerator penetrates less than gamma rays. "An e-beam can penetrate about 3 in. of material," Quass explains. "The food should be thin or small pieces, like strawberries. Hamburger patties can be treated with a two-sided approach, penetrating 1.5 in. from each side. Penetration of the accelerated electron beam is improved by passing the beam through a metal film. This produces X-rays, which have penetrating power similar to gamma rays."

  In addition to producing a safe product, irradiation extends shelf life and increases product yield. "The process is cost-effective," Fraser says. "Strawberries have been picked at 80% to 90% maturity and irradiated. With mold growth inhibited, there is less spoilage. It is possible to get a 23-day, refrigerated shelf life. And the strawberries taste better because they are vine-ripened."

  FDA requires food treated with irradiation to be labeled with a green radura symbol and the phrase "treated by irradiation" or "treated with radiation." As Fraser points out, these statements "can then be followed by a statement of the benefits, such as 'to control foodborne diseases.' "

  "Consumers are becoming more accepting of irradiated foods," Fraser claims. "Irradiation has a big role to play in assuring the safety of the food supply."

  Ultra-high pressure. This process involves packaging the food product in a flexible pouch or container, then placing it in a chamber under ultra-high pressure (80,000 to 100,000 psi) for a few minutes. Since the process is not dependent on package geometry, size can range from individual pouches to large bulk containers.

  Food products containing pieces (such as fruit dices) benefit from this process. A high-pressure-pasteurized guacamole is now commercially available in the United States.

  "There are opportunities for new products when ultra-high pressure is used," states Quass. "For example, the process modifies dairy proteins. Egg whites undergo a mild gelation effect, resulting in a flan-type product. High-pressure-treated milk has increased cheese yield."

  Pumpable high pressure is being investigated for juice and possibly milk. "The goals are to extend shelf life and to produce a fresh-like product without thermal pasteurization," Quass explains. "A current use for high-pressure pumps is water-cutting. For this application, the pumps generate about 40,000 lb./sq. in. of pressure. The problem with applying this technology to juice is that it takes about 80,000 lbs. of pressure to pasteurize juice. It is not a simple task to generate this amount of pressure."

The heat is on

  An advancement in thermal processing is ohmic heating. In principle, electric energy is transformed into thermal energy uniformly throughout the product. Rapid heating results, and better nutritional and organoleptic qualities are possible when compared with conventional in-can sterilization.

  "Ohmic heating employs electrodes immersed on a pipe," Quass says. "Product is pumped through the pipe as current flows between the electrodes." Depth of penetration is not limited. The extent of heating is determined by the electrical conductivity through the product, plus residence time in the electric field.

  "Ohmic heating is useful for foods that burn-on or have particulates that plug up plate heat exchangers," continues Quass. "Instead of using a scraped surface heat exchanger for stew, for example, ohmic heating can reduce the come-up time, and improve product quality."

'Zoning in

  Ozone (O3) has been used for many years in municipal water treatment, aquarium environments and other nonfood applications. In May 1997, ozone received GRAS (generally recognized as safe) status as a sanitizer or disinfectant for foods. GRAS affirmation was attained when an expert panel submitted a report to FDA documenting the usefulness and safety of ozone for food.

  An unstable gas formed from oxygen, ozone functions in the food industry as a surface sanitizer or disinfectant. As it decomposes into oxygen, "its high oxidizing potential leads to its disinfection potential," explains Robert Kim, director of marketing and sales, Novazone, Wexford, PA. "Bacteria is quickly destroyed, leaving behind oxygen. Ozone is a broad spectrum biocide. It is not specific to any microorganism. Ozone will kill bacteria, viruses and molds.

  "Ozone decomposes quickly," Kim says. "There is no residual for consumers to ingest. And, unlike chlorine, there is no buildup of residual compounds."

  OSHA regulates the use of ozone at 0.1 ppm concentration in ambient air over an eight-hour period. At high levels, ozone causes breathing and eye irritation. "Ozone is a self-policing gas," Kim points out. "A person can smell it at very low levels."

  Ozone's high reactivity means it must be generated on-site, as needed. While this requires equipment investment, advantages include no storage requirements, product handling or spills during transport.

  Food applications utilize ozone in water or as a gas. "Fruits and vegetables can be sprayed with ozonated water prior to packaging," Kim says. "Ozone is beneficial in the chiller water used for chickens. Carcasses can be sprayed before cutting. Following treatment, products are packaged and handled by normal distribution methods. Ozonated water also is useful in sanitizing processing equipment, pipe systems and floors. When cleaning is finished, all that remains is water."

  Gaseous ozone has the potential to sanitize storage rooms, trucks and refrigerators. Storage rooms might contain bins of fruits and vegetables, sausages or meat carcasses. Ozone gas keeps both the air and surfaces clean.

  The possibility exists of "over-ozonating" a food. Fat in meat might oxidize and go rancid. Too much ozone changes the color of some fruits. "We are in the developing stages of food applications for ozone," Kim stresses. "There are still questions to be answered about contact times, usage levels and effect on the product."

It's a gas

  The food industry utilizes industrial gases in processes ranging from cryogenic cooling and freezing to modified atmosphere packaging. New industrial gas applications extend the shelf life and improve the safety of some food products.

  Cornell University, with funding from Dairy Management Inc., Rosemont, IL, and Praxair, Inc., Burr Ridge, IL, has developed a process for directly adding carbon dioxide to cottage cheese to increase shelf life. A known antimicrobial agent, CO2 naturally occurs in milk. Unfortunately, much of it is lost during processing. The Cornell researchers developed a method of dissolving CO2 into the cream dressing before mixing it with the cheese curd. With moderate barrier packaging, shelf life might be 60 days or more.

  "Now that our technology is fully commercialized for cottage cheese," says Armand Paradis, senior development associate, Praxair, "we are looking at applying it to whole fluid milk, yogurt, soft-serve ice cream, yogurt mix and ricotta cheese. Shelf life can be doubled, and in some cases, tripled. This technology is geared toward modified atmosphere processing, with the gas being injected on the processing line. Food processors might be more familiar with modified atmosphere packaging, where the gas is introduced at the last point in their process."

  Cryogenic cooling is being investigated to ensure the safety of table eggs. "Lowering the temperature of eggs to 40° to 45°F, and then maintaining that temperature during distribution, reduces the threat of salmonella," says Debbie Benjamin, senior development associate, Praxair. "It is a challenge for the egg processor to reduce the temperature of the volume of eggs handled. And once the eggs are palletized, the insulation effect makes it difficult to reduce the temperature in a reasonable amount of time.

  "Praxair's technology to reduce the temperature of the eggs before packaging is still in development," she says. "One concern is the limited amount of plant floor space which might be available for additional equipment."

  Meat and poultry carcasses can be cooled cryogenically to bring the temperature down quickly. This limits bacteria growth. "A Canadian processor is cryogenically chilling poultry," Benjamin notes. "Since the birds are cooled cryogenically instead of in a chill bath, the possibility of cross-contamination from the chill water is eliminated."

  "The opportunity exists to use industrial gas applications in conjunction with other novel preservation technologies," Paradis says. "Testing suggests, for example, that there are advantages to combining irradiation with modified atmosphere packaging. The combinations can be left to your imagination."

Lengthening the life

  With worries about food safety, consumers are more careful about checking use-by dates and looking for signs of spoilage. Shelf-life extenders are useful in lengthening the life of some meat products, including hot dogs, turkey products and hamburger patties.

  "Ingredients commonly used to extend the shelf life of meats are citric acid, ascorbic acid, sodium diacetate and sodium acetate," says Barbara Schmitt, national manager of technical sales, SpiceTec, Ltd., Carol Stream, IL. "These acidic ingredients deter the growth of bacteria. They are volatile, however, and break down the texture of raw meat."

  The volatile components can be protected with a modified, low-heat extrusion process, whereby active ingredients are entrapped within a protective matrix. The matrix prevents the release of the ingredients until cooking. Hot dog shelf life has been extended from 50 to up to 90 days.

  "Currently under experimentation is utilizing this matrix with antioxidants," Schmitt says. "Shelf life is extended by limiting rancidity. The meat also keeps its red color for a longer time."

  The shelf-life extenders are included in the seasoning blend to aid dispersion. Proper dispersion is critical in attaining maximum effect of the protected ingredients.

  This technology -- among other methods -- promises to meet food processor needs, taking ideas from the "what if" stage to "what's next?" in the progress toward better product preservation.

  Free-lance technical writer Elaine Knehr holds a bachelor's degree in food technology and a master's degree in business. Her 10 years of experience in product development covers a wide range of food products.

Back to top