Keeping Food Fresh Longer
Southwest Research Institute's experience in the area of polymer film research over the last 10 years has allowed engineers in the Department of Applied Chemistry and Chemical Engineering to develop a number of specialty polymers with novel applications tailored to the needs of particular clients. Projects have included the design of polymer- ceramic-nanocomposites used for example as dental restoratives, biodegradable films for packaging, photocurable polymer systems for lithography, and electrochromic polymers.
One recent project, originally sponsored by a national packaging company and now by Bernard Technologies, formerly Cambridge Chemical Company of Chicago, Illinois, has resulted in a series of biocidal polymer films and coatings that release controlled amounts of the biocide, chlorine dioxide, in response to temperature and humidity in the atmosphere. The technology shows particular promise for the food packaging industry and also has many potential applications in the health care field. Dosages can be adjusted to kill mold spores with an initial release at concentrations of 0.5-1 ppm or higher, followed by a longer period of sustained release at a lower level for further biocidal action. Recently, these polymer films were successfully used to destroy E. coli in meat products.
Patents have been granted to SwRI, and some are pending, on the technology, which is licensed exclusively to Bernard Technologies but allows royalties to be collected by the Institute.
The sponsors came to us with a problem to be solved, says SwRI Project Manager Dr. Stephen T. Wellinghoff. By the initial and timely use of divisional overhead money, we were able to carry out some of the critical fundamental research on controlled release in advance of the contract award this allowed us to provide the clients with a creative solution.
The packaging company was looking for new methods to improve the longevity of paperboard boxes and paper containers in particular, to protect them against mold. An example of this problem can be seen in the packaging of some bar soaps that may already be mildewed before purchase.
Product storage is particularly critical in warm regions, such as the American South and tropical areas, where there are serious shelf-life limitations and subsequent warehouse losses for many products.
Rapid spoilage is a major concern for the food industry, especially in the case of small soft fruits such as strawberries, raspberries, and blueberries. Berries stored and packaged in warehouses often carry mold spores on their surfaces from the fields in which they were picked. In high temperatures and humidity, the spores can start to mature quickly on the sugary surfaces of the berries. It is critical to package such products in a way that prevents the mold from bursting into growth before the fruit is eaten.
There were a number of factors to be considered in the design of this particular project. The choice of chemical materials to be used in the polymer formulation were necessarily confined to the GRAS (Generally Recognized As Safe) index. These are materials recognized as safe by the federal Food and Drug Administration (FDA) for people working in the food industry; they are assumed to be safe for human ingestion. Also, the design of an effective and flexible time release mechanism for the biocide had to be developed, and aesthetic considerations such as the optical clarity and color of the material, important in the food industry, had to be addressed. Finally, large-scale packaging costs needed to be restricted to less than a penny per box or container for many industrial clients to consider their use.
Early in 1995, the FDA approved chlorine dioxide (ClO2) for additional food purposes. The chemical has been widely recognized since the 1960s as a potentially useful substance for the control of bacteria and fungi and is already used for skin disinfection, contact lens care and cleaning, swimming pool maintenance, and the treatment of drinking water.
Chlorine dioxide is a broad spectrum biocide with the advantage that bacteria, fungi, and viruses do not build up a natural resistance to it, as they do to many biocides. Because of the action mechanism in ClO2, which attacks membranes and fundamental cellular processes, mutant strains are not selected. A group of industries, interested in the manufacture of chlorine dioxide in large quantities for use in controlling biological contaminants for the food industry, for example, is supporting an effort for FDA fast- track approval in an even wider range of products.
The ClO2 releasing films are both versatile and safe, and available ClO2 can be as large as 10-5 mole per cc of thick film, a fairly significant amount. The most complex part of the design of the new polymer was development of a controlled release system for the biocidal films and coatings to produce an initial delayed release of the gas at concentrations of 0.51 ppm or higher, followed by a longer period of sustained release at a lower level when further biocidal action is needed. Proven effectiveness for long-term controlled release is critical to the packaging industry, where products may be stored for many months.
There are two components in the controlled release design. One is an acid releasing component that can be released quickly, in a matter of days or hours, or more slowly, with release occurring over a period of months. The second is a chlorite containing component.
Atmospheric moisture and temperature affects the polymer film, causing hydrolysis of an acid anhydride and releasing the acid, which then diffuses internally in the film to release the ClO2 from the chlorite. In this way, temperature and humidity levels control the rate of acid release, which in turn controls release of the ClO2. As temperature and humidity rise, so does the rate of release.
Mold and bacteria growth is highly correlated to temperature and humidity, and data recorded in the laboratory at SwRI have clearly demonstrated the impact of a weather front passing through San Antonio on the biocidal release rate and even the effects of a laboratory door being opened or closed.
Different methods of film application are being explored. The polymer can either be sprayed or melt-coated onto a container. An alternative approach is to punch a stamp- sized coupon on the bottom of a box that would permit the ClO2 to diffuse up through the fruits or vegetables. Chlorine dioxide easily dissolves in the water on the surface of berries.
Chlorine dioxide is effective at extremely low levels on the order of several ppm, and it can kill molds, bacteria, and viruses at levels below the point at which an odor can be detected (this is widely considered to be 10 ppm). In addition, the release system developed by SwRI scientists is both optically clear and aesthetically pleasing. This was achieved through the development of a one-phase solution formed between the sodium chlorite and the polymer, resulting in no small particles to scatter light. This means that if clear plastic packaging is needed for a product, the polymer can be extruded between two other barrier films to make a clear, multilayered sandwich.
Other potential applications for ClO2 include clear plastic packaging for meats, hospital floor coatings, and clear plastic bandaging for surgical applications. It is increasingly common for surgeons to put a clear plastic bandage on a wound surface after an operation, so the patient can monitor the healing process and inform the doctor if anything unexpected occurs. Use of this clear plastic bandage for surgical applications also allows the release of ClO2 at a controlled rate. Chlorine dioxide is a deep-penetrating wound disinfectant that can improve the chances of keeping incision wounds relatively free of bacteria.
Institute scientists have identified a number of potential release systems that are effective for moisture-induced controlled release of ClO2 to kill growing molds, bacteria, and viruses. A theoretical model has also been developed that provides guidelines on the design and cost of the release system for each container type, in addition to a computer controlled ClO2 measurement system suitable for long-term studies.
The author would like to acknowledge colleagues in the Institute's Chemistry and Chemical Engineering Division who contributed to this work: Joel Kampa, Craig M. Wall, and Charles K. Baker.
Published in the Summer 1995 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.