The Sweet Smell of Food . . . Via Odor-Scavenging Packaging
Active packaging, about which much has been written and discussed since its admission into graduate school too recently, has been thrust into the realm of trying to control unwanted odors within closed packages.
Some food scientists and technologists are cognizant of the generation, concentration, and other mechanisms that propel odors when consumers open food packages. Some flavor scientists have recognized the phenomenon by adding desirable aromas to package interiors. And many are attempting to enhance the perception received by consumers through incorporation of flavors into package materials and adjuncts. At the other end of the spectrum, removal of undesirable odors within closed packages has become one objective in the rapidly emerging discipline of food and beverage packaging science.
Often referred to in the past as “confinement odors,” these packaged food attributes may be the result of microbiological growth, biochemical activity, or physical gas dynamics. The malodors may diminish the characteristic flavors of the base food by reducing the aromatics or by altering the balance required for flavor. More frequently, the presence of adverse odors in the package headspace masks the real thing and produces a negative response from consumers expecting fresh or characteristic aromas of the contained product.
Within closed poultry packages, trivial quantities of sulfide compounds from harmless minor microbiological activity may be detected as spoilage by consumers sniffing the space immediately upon opening packages. Analogous headspace odors may arise in packages of fresh meat, seafood, and even fresh produce. Warmed-over-flavors of chilled prepared foods are other representatives of this activity. The presence of acetaldehyde arising from breakdown of polyester during carbonated beverage bottle fabrication and of polyethylene degradation products adjacent to packaging films and coatings is hardly uncommon. Perhaps the most pervading, however, is the low level of oxidative rancidity appearing in headspaces of snack and bakery goods, nut products, and even confectionery.
Although almost always of little or no nutritional or safety consequence, these odors may be viewed by consumers as signals that the product is less than perfect or even spoiled. Removal or reduction of these headspace odors might be regarded as a mechanism to render the otherwise fine packaged products as acceptable. Conversely, reduction of adverse odors might be considered hiding something from consumers. Regardless of the ethics of the action, odor removal from headspace is an activity within active packaging today.
During the 1990s, two programs from DuPont were directed toward obviation of undesirable odors. One involved the incorporation of molecular sieves into polyethylene to remove oxidation odors from processing the plastic resin. Analogous programs from other sources incorporated tocopherols as antioxidants into the resin prior to extrusion.
The second program was aimed more at the low-level odors generated by lipid oxidation of dry foods. In this research, the target was snack, cereal, and bakery goods subject to oxidative rancidity and thus generating aldehyde and ketone odors characteristic of peroxide splitting of unsaturated lipid bonds. Polyethylene imine could react with the volatile short-chain odorous compounds and effectively remove them from the package headspace. However effective these plastic additives are technically in removing the adverse odors, they have not become a commercial success.
Studies for Military Rations
Some of the paucity of application might be attributable to the relatively short shelf life of the contained food products in domestic distribution. But, for military rations, the reverse represents a problem. Food for soldiers and sailors should contain fat for flavor and caloric value. Furthermore, such food must be in distribution channels for prolonged periods—years—to be available instantly for operations. Distribution at above-ambient temperature conditions can be the norm. Thus, all conditions are or approach optimum for development of oxidative odors within packages.
Those professionals responsible for protecting food for the troops have recognized that quality is an important attribute, and that headspace odor should be, if not pleasant, at least not unacceptable. Oxidative odors are not desirable in packages of bakery goods, prepared foods, and high-energy confections.
Government-sponsored research to retard formation of the undesirable end products through controlled delivery of the antioxidant tocopherol (vitamin E) from package structures is underway under the direction of Kit Yam and Karen Schaich at the Rutgers University Dept. of Food Science. Simultaneously, research is being conducted at the University of Georgia to remove the undesirable odors from the headspaces of hermetically sealed barrier packages using active packaging.
• Tocopherol Approach at Rutgers. Yam and Schaich have begun their Natick Soldier Systems contract (“Use of Natural Antioxidants in Packaging Materials for Shelf Life Extension of Combat Rations”) by incorporating tocopherol into polyolefinic plastic film in much the same manner as this compound class is added to retard oxidation of the plastic during thermal processing for extrusion. The ultimate objective is for a controlled migratory release of tocopherol from the plastic in contact with the contained food product to provide a continuous antioxidant effect to the product. If the tocopherol were a food ingredient, its antioxidant effect would be a one-time event and oxidation could persist after it is totally consumed. With controlled release, the antioxidant effect is prolonged to enhance the shelf life.
Special laboratory devices have been constructed to ensure contact of the tocopherol-containing film with the oxidation-prone product while product evaluation is performed. Meanwhile, alternative tocopherol incorporation technologies to effect total control over the migration process are being developed.
• Odor Scavenger Approach at Georgia. The University of Georgia work is in the form of master’s degree research (“Oxidative Odor Control Through Active Packaging Odor Scavengers”) conducted by Heather Oliver directed by us folks at Packaging/Brody, Inc.
Her recently completed research focused on a typical product, a peanut butter–flavored, nutrient-dense confectionery bar for use within the Meals Ready-to-eat (MRE) military ration. The peanut butter bar was selected from among the several versions of the high-energy density bars because the peanut butter version, liked by troops when fresh, exhibited not-atypical rancidity and hence unacceptability after aging.
Oliver identified a variety of potential means to remove lipid oxidation odors applying principles of physical adsorption. Viable alternatives were, among others, molecular sieves, cyclodextrins, and activated carbon.
Molecular sieves are essentially highly porous solids of the zeolite class with high surface areas derived from three-dimensional silicon/aluminum tetrahedrons bound to each other. Cation-exchange properties of molecular sieves allow the material to reversibly sorb and desorb guest molecules small enough to enter the pores or channels, such as odorous gases. Molecular sieves are selective, in that they permit small molecules such as odors to enter and be captured, while having no effect on larger molecules. The sodium form of molecular sieves was evaluated during this research.
Cyclodextrins are homogeneous cyclic oligosaccharides with six or more glucopyranose links. The molecule takes the form of a torus with a hollow interior of a specific volume due to the specific coupling of the glucose monomers. Hydrogen atoms and glucosidic oxygen-bridge atoms form the interior lining of the cavity, creating an electron-rich, nonpolar environment. The internal cavity, which is lipophilic and hydrophobic, is attractive to hydrocarbon molecules such as the odorous gases. Differences in cyclodextrin cavity size provide a basis for selectivity of adsorption.
Activated carbon is the granddaddy of odor adorbers because of its exceptionally high internal surface areas containing micropores or mostly slit-shaped spaces between cross-linked flat aromatic sheets. Although some molecular selectivity is possible, the general rule is that activated carbon adsorbs, i.e., removes a broad range of odor molecules.
Oliver initially incorporated the active granular materials into sachets, much like those employed for oxygen and moisture scavengers by Tom Powers and his colleagues at Multisorb Technologies. Each sachet is a twin-web pouch, with one face being a gas barrier and the other a porous spun-bonded DuPont Tyvek® polyolefin membrane. During later testing, sachets were replaced with labels containing activated carbon, also supplied by Multisorb Technologies. Labels are flat versions of the sachet containing much less adsorbent material spread out over a larger, flatter area, to be applied to the package interior as a coupon or label.
Confectionery bars supplied through Natick Soldier Systems program manager Laurie Kline were tested. The bars were inserted into closed glass containers with no odor scavengers and with sachets containing the odor scavengers. Containers were stored at temperatures representing cold or control, ambient temperature, and elevated temperature to accelerate the odor generation and removal effects.
Evaluations were conducted by trained odor panelists sniffing the headspace of the glass containers after equilibration. Gas chromatographic analyses of headspace gases were performed. Lipid oxidation testing was conducted on the bars themselves to measure the actual biochemical changes. Subsequent sensory testing was conducted with surrogate “consumers” (students with the general demographics as troops) to determine product acceptability.
The results indicated that the odor-scavenging properties within sachets of the molecular sieves were too low to warrant further study. The cyclodextrins exhibited too variable a result to be studied in this context, but the military sponsors are continuing the work. The old standby activated carbon worked very well in removing adverse odors from the confectionery bar package headspace while the bar continued to biochemically change through oxidation. Despite the biochemistry and the removal of headspace odors, the bars themselves remained acceptable to surrogate consumers in flavor-by-mouth evaluations. Objective chemical and instrumental analyses supported the sensory test results.
Results from testing of labels suggested that the reduced quantities of active materials available to react with the odors were equally unsatisfactory except for the activated carbon.
Oliver’s research continues to attempt to incorporate the activated carbon into direct package contact material, polyethylene. In conjunction with CSP Technologies, activated carbon was incorporated into the company’s unique three-phase polymer with microscopic interconnected transmitting channels to allow transport of molecules. To date, incorporation of odor adsorbers into polyethylene film has proven to be a challenge, but Oliver continues her research in the hope of finding a means to connect the odor scavengers with the undesirable odors.
Alteration of package headspace by odor scavenging imposes some challenges. Optimally, the change mechanism should target the specific contributors to either malodors or flavor aromatics. Any vehicle that explicitly or inadvertently causes change in other areas might be viewed as an attempt to deceive the consumer. Furthermore, activity behind the plastic, i.e., within the solid mass of plastic structure, is difficult to control with most present thermoplastic technology. Although the odorous compounds may be removed from the headspace by odor scavengers, the product continues to oxidize, producing malodorous compounds that should be countered; hence, the tocopherol migration approach.
What a fascinating path we tread when first we enter into the perilous road of active packaging—but to what benefits for food scientists and technologists might this adolescent discipline lead?
by AARON L. BRODY
President and CEO, Packaging/Brody, Inc.