The Changing Face
of Shelf Life
April 1996 -- Cover Story
By: Scott Hegenbart
Editor*
*April 1991-July 1996
Preserving food for distribution to consumers is the foundation upon which the food industry was born. Although the concept has been established, technologies for extending shelf life and for improving the quality of stability-enhanced foods are far from static. In fact, researchers continually search for ways to enhance shelf life for a wide variety of reasons. These range from the food safety and distribution challenges of centralized food processing and the global marketplace to the heightened demands for quality and variety from time-starved consumers. Fortunately, many advances in ingredients, processing and packaging that had been academic curiosities are now being commercialized.
"Many of the preservation methods we used in the past no longer apply, or they're no longer as acceptable. Consumers don't want all the sugar, they don't want the salt, they don't want the fat, and they are concerned about the vitamin loss of heat treatment," says Susan Brewer, Ph.D., associate professor of food chemistry, University of Illinois, Champaign. "It has become a real challenge to the food industry to provide food that tastes 'like what mom used to make' and is low in fat and sodium, and can be taken from the freezer to table in under three minutes. That's an incredible set of demands."
Ingredients called to action
Although consumers crave the ultimate in quality and safety, they have equally strong concerns about the ingredients product designers use to maintain food quality and safety. Consequently, much research is devoted to finding more "natural" alternatives to ingredients such as antioxidants and antimicrobials.
"Research on natural antimicrobials is at an all-time high," says Robert Hutkins, Ph.D., associate professor, department of food science and technology, University of Nebraska, Lincoln. "Just about every food science department has at least one person working on bacteriosins (or natural antimicrobials). It's just a huge area in the U.S., as well as Europe."
Many foods possess inherent antimicrobial systems. Milk, for example, contains the antimicrobial lactoferrin, while egg whites contain lysozyme, which has been shown to either kill or prevent the growth of Listeria monocytogenes in several foods. In addition, many microorganisms produce metabolites that are bacteriosins.
"It's been known for years that carrots are antagonistic to pathogens," says Eric Johnson, Sc.D., associate professor, University of Wisconsin, Madison. "I think plants are really an untapped source of antimicrobials."
Several methods for including natural antimicrobials in food products have been investigated. The most obvious of these is to isolate the active antimicrobials, purify them and use the resulting substance as a direct additive. According to Johnson, lysozyme is permitted in Europe to help prevent gas formation in cheese. In addition, lactoferrin and lactoferrin derivatives have been developed for use in Japan.
In the United States, two purified bacteriosins obtained from bacterial culture are approved for food-additive use. Natamycin, produced by a strain of Streptomyces chattanoogensis, acts as a mold inhibitor for cheese and wine. Nisin, derived through the pure culture fermentation of Streptococcus lactis, inhibits Clostridium botulinum spores and toxin formation in pasteurized cheese spreads, and canned fruits and vegetables.
Although natamycin and nisin come from "natural" sources, each had to undergo scrutiny by the U.S. Food and Drug Administration because they are isolated, purified substances. Natamycin is approved under 21 CFR 172.155 with a limitation of 200 to 300 ppm applied to cut cheese. Nisin has been given generally recognized as safe (GRAS) status under 21 CFR 184.1538 with a limitation of 250 ppm in finished products.
The FDA review process is one obstacle that discourages suppliers from exploring this method of natural antimicrobial addition. Another possible reason that more natural antimicrobials are not available is the fact that words such as "nisin" and "natamycin" are just as unfamiliar to consumers as more common antimicrobials. Consequently, in consumers' eyes they may appear as just another additive on the label.
Quality with culture
To overcome these challenges, researchers are devising ways to add bacteriosins indirectly. One way to do this requires researchers first to identify food-grade bacterial cultures that produce bacteriosins. These are then grown on a food ingredient substrate such as corn syrup solids, skim milk or whey. The resulting culture is concentrated and spray-dried to yield a bacteriosin-containing food ingredient that may be labeled as simply "cultured corn syrup solids," etc. Such food ingredients also may contain organic acids and other substances that contribute to preservation. In some instances, the culture may even modify proteins in the substrate so the ingredient can offer moisture-binding properties.
"Fermented foods evolved because of their shelf-life properties," says Sandra Curtis, technical manager for cultures, Quest International, Hoffman Estates, IL. "This approach takes the functional metabolites and transfers them into foods that are non-cultured."
By selecting the proper culture, substrate and process conditions, many such shelf-life-extending ingredients have been created. They already have been used in processed meats, salad dressings, certain types of cooking sauces, and prepared salads. In dairy products, cultured natural preservatives have successfully extended the refrigerator life of cottage cheese when added to the dressing, and have controlled yeast and mold in yogurt.
"Using these ingredients depends a great deal on the product's formulation, so suppliers must provide quite a bit of customer service," says Curtis. "You have to look at the formula and know what other inhibitors -- such as salt level, water activity, whether the product will be hot-packed, and whether it will be packaged with a certain gas environment -- then determine which type of ingredient to use."
Although this technology has been commercialized to a certain extent, university researchers continue to refine the technique and expand the range of usable substrates. Through work supported by the Minnesota Dairy Foods Research Center, researchers at the University of Minnesota, St. Paul, have developed a cultured shelf-life extender using whey protein.
"The bacteriosin-containing whey protein (BCWP) is very effective as an inhibitor and inhibits gram-positive spoilage organisms such as Lactobacillus," says Edmund Zottola, Ph.D., professor of food microbiology, department of food science and nutrition, University of Minnesota. "We have used it in some meat-containing dishes, traditional cheese, and in pasteurized-processed and cold-pack cheese."
In addition to spoilage organisms, the BCWP inhibits the growth of Clostridium botulinum, Listeria and Staphylococcus aureus. According to Zottola, experiments demonstrated that the bacteriosin could destroy 103 of Listeria in ice cream base.
Besides dairy-based research, the University of Minnesota has been looking for natural inhibitors to Escherichia coli 0157:H7 with support from the Minnesota Beef Council. Unlike other strains of E. coli, 0157:H7 can survive for extended periods in a frozen state. In fact, several researchers have shown that it can remain viable in frozen hamburger patties for over a year.
"We theorize that there might be something in hamburger that is inhibitory to 0157:H7," says Zottola. "We're in the process of screening those (substances) that are the most effective so that we can create a bacteriosin-containing substance that can be put into a frozen hamburger patty to control the organism."
So far, Zottola's group has screened over 1,000 non-pathogenic organisms and narrowed the field down to 20 potential candidates. Then they will select the most effective one or two for detailed testing.
Active inhibition
As promising as bacteriosin-containing ingredients are, some researchers have concentrated on eliminating the ingredient processing step by directly adding the bacteriosin-producing organism to the food itself. One version of this method -- called "in situ preservation" or "competitive exclusion," among other terms -- was developed and patented by the University of Nebraska's Hutkins.
The technique involves adding food-grade lactic acid bacteria, such as Lactobacillus or Pediococcus, to a food. The food system would be designed so that these organisms don't grow or carry out fermentation in the food, but release the bacteriocin "pediocin." The pediocin would either kill or inhibit pathogens and spoilage organisms such as Listeria, Clostridium and other lactic acid bacteria.
"Ordinarily, these organisms will ferment if the temperature is right and if there is a fermentable substrate," says Hutkins. "If these conditions are restricted, the organism won't ferment but it will still be metabolically active and produce the antimicrobial substance."
Designing the product to prevent the lactic acid bacteria from fermenting relies on common techniques for microbial control: high salt levels, low water activity, and reduced-temperature storage and handling. According to Hutkins, determining the correct temperature and water activity are the most effective controls. Controlling these conditions, however, can be challenging, and each food system must be evaluated individually with shelf-life testing and distribution/temperature abuse studies to establish the effectiveness of the system.
Still, minimally processed, refrigerated food products -- which aren't sterile -- can benefit greatly from the technique. This is especially true for ready-to-eat foods in this category such as hot dogs, prepared sandwiches, luncheon meats, pasta salads, dressings and prepared entrees. Sausage, cheese and other fermented products also are candidates. In these, the technique would be more easily applied because controlling fermentation isn't as critical.
"Many of the other applications for this sort of in situ preservation are for fermented foods because of the difficulty in preventing fermentation," says Hutkins. "In fermented foods, the antimicrobial-producing organism might not be the primary fermenting organism, but if it ferments, it's not necessarily going to affect the product quality."
In non-fermented foods, in situ preservation offers the added benefit of potentially acting as a safety fail-safe for consumers. If the product were not kept at the proper temperature, the lactic acid bacteria would ferment and spoil the product. This protects consumers in two ways. First, the sour taste of the acid would be an obvious indicator that the product is spoiled and the consumer would be less likely to eat the product. Second, even if a certain amount of the product were consumed, the consumer would have ingested non-pathogenic lactic acid bacteria instead of something like Clostridium botulinum because it would have been inhibited.
Reducing oxidation
Controlling microorganisms is as much an issue of food safety as food quality. Controlling oxidation may not be as critical to safety, but it is crucial for maintaining product quality. Fats and oils, in particular, are very sensitive to oxidation and can generate off-flavors at very low oxidation levels.
"The longer foods or food ingredients have been around, the more likely they are to suffer form oxidation," says Brewer. "Oxidation changes odors, flavors, colors, and destroys the vitamins so they're no longer in their active form."
Commercialized natural antioxidants have a head start on natural antimicrobials. Various spice extracts and many different forms of vitamin E are readily available to food designers for formulation use. Thanks to the perceived health benefits of antioxidants, new sources of natural antioxidants are being discovered as researchers search for phytochemicals.
"We're evaluating new antioxidants derived from different combinations of plant extracts," says Alegria Caragay, director in technology and product development at Arthur D. Little, Cambridge, MA. "We're trying to kill two birds with one stone by delivering disease-prevention qualities while reducing the qualitative effects of oxidation."
Besides adding antioxidants, food components also can be protected from oxidation by encapsulating them. Although not a new technology, newer encapsulation methods are being commercialized for use on food ingredients.
Spray-drying a flavor onto a carrier has long been used to stabilize flavors. The heat of spray-drying, however, can volatilize some of the flavor's desirable aromatics. In addition, many spray-drying techniques leave flavor-bearing oil on the surface of the particle where it is exposed to oxygen. Newer techniques using microencapsulation completely envelope the flavor and reduce the incidence of oxidized flavor oils.
Consumer demand for "natural" ingredients also has expanded the need for encapsulation. Many product designers are formulating with non-certified colors such as beet extract instead of the FD&C colors. These "natural" colors are more sensitive to light, oxidation and changes in pH. Encapsulation systems can be designed to protect against all three.
Advancing processing technology
Processing a food product is a virtual requirement to obtain extended shelf life. Unfortunately, such processing usually involves heat treatment that can reduce organoleptic quality. Over the years, researchers have optimized time/temperature ratios to minimize the exposure of the food to heat. Now, newer process technologies promise to reduce or, in some cases, eliminate heat exposure. Some of these processes are not so new, but have recently made significant advances toward commercialization.
Ohmic heating reduces heat exposure by dramatically reducing the time it takes to bring product up to sterilization temperature (the "come-up" time). The technique works by passing an electric current through the food product. The dissipation of the electrical energy in the product mass generates heat.
In addition to heating rapidly, ohmic heating heats particulate matter as quickly as fluids. In some cases, particulates heat even more rapidly. With conventional heating, the larger the particle, the more time is required to heat its center to the sterilization temperature. Consequently, ohmic heating allows more even heating of the entire system and the opportunity for product designers to formulate products with larger "chunks."
"You can adjust the heating rate so you don't have to overheat to sterilize particulates," says Sudhir Sastry, Ph.D., professor, department of food, agricultural and biological engineering, Ohio State University, Columbus. "The net effect is that the heat treatment is reduced."
Proper ohmic treatment begins with the formulation. For the technique to work, the product must be pumpable and both the solid phase and liquid phase must be electrically conductive. Because many food products contain salt, this base is usually covered. However, designers may choose to adjust the conductivity of the phases so that one heats more rapidly, or design both to heat at the same rate.
"Another thing we've found is you can make a mixture heat faster by increasing the viscosity of the carrier," says Sastry. "This runs contrary to what we know about conventional heating. There, you need viscosity to maintain homogeneity, but it inhibits heat transfer. In ohmic heating, the increase in viscosity helps the system heat more quickly while suspending particulates."
Once the conductivity and viscosity of the formula are balanced, the product designer must fine-tune the ohmic heating by determining the correct processing voltage and the optimum distance between electrodes. Currently, several food companies are using the technology. One of these companies is Papetti's Hygrade Egg Products Inc., Elizabeth, NJ.
Papetti's used to pasteurize its liquid eggs using plate heat exchangers. The company heated the eggs to 140°F for 3-1/2 minutes to kill Salmonella and used higher temperatures to produce extended-shelf-life liquid eggs. In this temperature range, the eggs would coagulate and accumulate on the exchanger plates, reducing their efficiency. This necessitated shutting the line down for cleaning every four to six hours.
Papetti's researchers created a process in-house in which the liquid egg passes through a series of five electrodes. The electrical current passing through the conductive egg ohmically heats them to the required 140°F. Because the electrodes themselves remain cool, coagulated egg build-up is no longer a problem and the line runs 15 to 18 hours between cleanings.
Pulsed electric fields form another electrical process that can reduce microbial loads. Like ohmic heating, the technique involves applying electrical energy to a conductive fluid food that is passed between two electrodes. The two processes differ in that the pulsed electric field method applies short, high-voltage bursts that rupture the cell walls of microorganisms to inactivate them without generating significant amounts of heat.
"The effect is strictly on the bacteria because of their structure," says Wayne Clark, Ph.D., president and chief operating officer, PurePulse Technologies Inc., San Diego. "There is no effect on the product. You don't change the taste, or the physical and chemical properties."
Experiments using the process have killed as many as six logs of Listeria in milk without exceeding 55°C. While pulsed electric fields do kill vegetative microorganisms, they don't kill spores. Consequently, it is a pasteurization technique rather than one for sterilization.
As with ohmic heating, products processed with pulsed electric fields must meet certain conductivity requirements. According to Clark, most foods fall within the range of what the technology can process. Extremely salty products may require special equipment designs. In the fall of 1995, the U.S. Food and Drug Administration determined that the process can be used broadly to pasteurize foods and beverages. According to industry sources, the first commercial application of the technology should begin installation this summer.
Hydrostatic pressure is a non-thermal process that was first explored in the 19th century. This technique involves packaging the food in a watertight container and placing it into a water-filled chamber. Once the container is sealed, the pressure in it is increased to anywhere from 1,000 atm to 10,000 atm, which inactivates microorganisms. Many products outside the United States -- most notably, in Japan -- are processed using this method. Within the United States, commercial equipment for hydrostatic pressure processing is available and industry sources indicate that commercial-scale installations are planned for the very near future.
Pulsed light uses an intense flash of sunlight-like light to extend the shelf life of foods and kill microorganisms on food and packaging surfaces. For transparent foods, the effects penetrate as deeply as the light. Already tested by researchers on bakery foods, seafood, meats, fruits and vegetables, and many other foods, results indicate the process significantly reduces microbial loads and enhances shelf life with no change in nutritional properties. A petition for the approval of pulsed-light treatment was submitted to the FDA in February 1994.
A related technology involves the use of ultraviolet light. Like pulsed light, the ultraviolet light is only effective on the exposed surface unless the product is transparent. At least one manufacturer offers a turnkey system that uses UV light to sterilize water. The use of such a system offers the opportunity to reduce microorganisms added to food products through the formula water.
Microwaves for sterilization and shelf-life enhancement have been commercialized for years, but they haven't caught the attention of the U.S. food industry. Charles Buffler, vice president, Microwave Research Center, Marlborough, NH, says that P&T Foods in Belgium has a line of prepared meals sterilized using microwave energy. Europe, however, has much more enthusiastically embraced microwave treatment as a way to extend the shelf life of bread. "Ever since the European Union came into existence, the regulations have prevented companies from adding preservatives to foods shipped across country borders," says Buffler. "In order to ship bread, many bakeries use microwave pasteurization -- particularly in Germany."
The procedure simply involves running pallets of packaged bread loaves through a low-energy microwave tunnel. The treatment can extend the shelf life from around a week to 10 days up to two, three or even four weeks.
Irradiation inhibits microorganisms because exposure to ionizing radiation disrupts the cellular processes associated with sprouting, ripening, or growth. The process was first recognized with the discovery of X-rays in 1895. Currently, irradiation is approved for some limited uses in the United States: to control bacteria, parasites and insects in spices; to destroy trichinae in pork; to inhibit growth and maturation of fresh fruit; and to disinfest insect-infested foods. In addition to the limited nature of irradiation's approval, consumers' unfounded fears of food becoming radioactive preclude many processors from taking advantage of the technology. Still, industry sources believe that consumers may yet see the safety advantages of the technology.
A package deal
Of course, the most innovative ingredients and most advanced processing techniques are useless without the proper package to protect the product. As is the case with ingredients and processing, developments in packaging also are helping to enhance stability and quality in shelf-life-enhanced foods. According to Aaron Brody, managing director of RubbrightBrody, Eagan, MN, glass-coated plastic films have been the buzz of packaging professionals for several years.
"So far we haven't produced any commercially in the United States," says Brody. "The one product out on the market was made in Europe, but was withdrawn because it didn't see much success. This lack of success, however, had nothing to do with the technology itself."
Brody, who is also a visiting professor in the department of food science and technology of the University of Georgia, Athens, believes that the more innovative packaging advances won't necessarily depend on new, high-tech materials, but on unique applications of existing ones. One example is hot-filling beverages into polyester bottles.
"It was first used about five or six years ago, and processors said, 'It's too expensive,' and 'The bottle looks funny,' " says Brody. "Now we have enough polyester-bottle capacity so that many juice processors are converting from glass or even metal to polyester bottles."
Although polyester has only modest gas- and vapor-barrier properties, it blends well with other barrier materials that can enhance its performance. Polyesters also are unique in that they can withstand the 180° to 190°F temperatures commonly used to process isotonic beverages and juices.
"Co-injections, co-extrusions and all that are really making it possible for us to consider that polyester will be one of the few packaging materials to survive into the future," says Brody. "Polyester with some nylon will be the barrier of the future, along with many others that use polyester as the backbone."
Among the advances in materials, Brody believes one of the more promising ones will be liquid crystal, but it won't be widely available for some time. "Liquid crystals are remarkable materials with extraordinary barrier properties," says Brody. "Their cost, however, is prohibitive so manufacturers are trying to blend them with polyester to take advantage of each: the moderate price of the polyester, and the barrier properties of the liquid crystal."
Other combinations of existing materials also are showing promise. In Japan and Europe, packaging suppliers are laminating polyester film to either steel or aluminum and forming it into cans. This is said to provide a superior way to keep the food away from the metal.
"You can now can more aggressive products," says Brody. "Also, the cans may be produced without all the volatile and organic emissions that occur when you're laying down the typical solvent-coated can."
Another unique packaging combination was the subject of a petition submitted to the FDA in late February. "The petition described the incorporation of active sanitizers into packaging film," says Richard Whelan, senior consultant at Arthur D. Little. "The result is a film with bactericidal properties."
Less is more
Although many packaging advances seem to be focused on creating more impervious barriers, at least one food trend requires the opposite. Consumers are more frequently relying on fresh-cut produce and ready-made salad "kits." When vegetables are harvested, they continue to respire and require a certain amount of oxygen to maintain optimum freshness. A high-barrier package would actually be a detriment.
To accommodate this, many packages have been designed with small holes in them to let air pass through. New materials, however, have been designed to allow specific oxygen transmission rates. The exact oxygen transmission rate can even be customized to meet the different oxygen requirements of various fruits and vegetables.
The environmental concerns of consumers have created another packaging area in which less is more: degradable packaging. Because many degradable packaging materials are made from food materials, research into this area has significantly advanced the area of edible films. Using edible films to extend shelf life may not be a new technology, but some applications of edible film are solving new problems. For example, applying a film to a pie crust can minimize moisture transfer from the filling. In fruit-containing breakfast cereals, an edible barrier could prevent the cereal from picking up moisture from the fruit pieces and becoming stale while the fruit pieces dried out and became undesirably firm.
Most edible films consist of proteins, polysaccharides, and waxes and lipids. As with ordinary packaging materials, each of the components provides certain benefits. Consequently, recent edible-film research has focused on combining the components.
"You cannot achieve every packaging objective using a single component," says Aris Gennadios, Ph.D., postdoctoral research associate, University of Nebraska, Industrial Agricultural Product Center, Lincoln. "For example, protein films and polysaccharide films are very potent oxygen barriers -- even better than many polyolefin packaging materials. This is, however, limited to relatively dry conditions. As environmental moisture increases, these films pick up water, plasticize and lose their oxygen barrier ability."
On the other hand, lipid materials are excellent water-vapor barriers. But they have problems of their own. They have poor structural integrity and must be melted prior to application.
"What is happening now is the development of multi-component films that combine proteins and/or polysaccharide with the lipids," says Gennadios. "A protein or a polysaccharide will give good structural support and good gas barrier properties while the lipid provides water barrier properties... These films can either be prepared as emulsions in which all the materials are mixed together, or prepared as a multilayer film in which you take the polysaccharide or protein film then deposit the lipid material on top."
Although edible films are promising, Gennadios points out several challenges that must be overcome before they can obtain significant commercial use. First, even multi-component edible films are somewhat fragile. Packaging machinery often puts tension on packaging film, which may cause pinholes in the finished package. Another problem is that the emulsion forms of edible films are commonly applied by dipping. If the product is a piece of chicken, the possibility of cross-contamination exists.
"Cost also would be a factor," says Gennadios. "Some of the proteins and polysaccharides are much more expensive compared with conventional polymer packaging material."
Another thing to keep in mind with protein films is that many proteins are known allergens.
Nevertheless, edible films have great potential and already have some commercial applications. Sausage casings made through extrusion are one example. Another is corn zein coating, which when applied to nuts can act as an oxygen barrier and delay the development of rancid off-flavors.
Although preservation and shelf-life extension are often thought of as mature technologies, they are very active areas of food research. Because consumer demands for nutritionally altered foods, greater convenience and reduced additives continually present food-preservation challenges, the need for new shelf-life technologies is only going to increase. Fortunately, many researchers are busy looking for creative new solutions. These research projects will, no doubt, enjoy a long shelf life of their own.
Back to top
.png?width=850&auto=webp&quality=95&format=jpg&disable=upscale "The Changing Face")
.png?width=800&auto=webp&quality=80&disable=upscale)