Techniques for Evaluating
Packaging Materials
February 1997 -- QA/QC

By: Ray Marsili
Contributing Editor

  Container shape and style, label graphics and colors are all unquestionably important elements in food packaging design. If the appearance and visual appeal are right, packaging can be a potent marketing tool. It tells consumers what the product is and why it's superior to competitor products. It can communicate value, help create brand image, and most importantly, inspire purchase.  But before you turn loose the creativity of your graphic design team, you'll need to carefully evaluate new packaging for its most basic function of all -- how well it protects and preserves the food product's taste and appearance. If you do a poor job with your evaluation, the consequences could prove disastrous, leading to a massive product failure or an expensive product recall.  Considering the highly competitive nature of today's food business and the money it takes to launch a new product, your packaging's capability to provide an appropriate, safe level of protection at a reasonable cost may be as important to your product's success as what you actually put in the package.  But a proper evaluation won't come easily. It will require some good hard science. It may not be as glamorous as designing attractive packaging graphics, but it's undoubtedly more important. Following are a few suggestions of what to watch for when developing packaging for new products, and techniques you can use to monitor potential quality control problems related to packaging materials.Packaging-related off-flavors  There are several ways that packaging can contribute to flavor and shelf life problems. Numerous problems can occur when packaging provides insufficient barrier protection for a particular product. Oxygen transmission through the carton and into the food product can result in oxidation reactions and the formation of off-flavors. For some types of food products, transmission of water vapor through the packaging and moisture absorption by the product can result in microbial growth and unacceptably short shelf life. External chemical contaminants can also pass through the packaging material and into the food product, imparting highly objectionable flavors and odors.  Aluminum foil affords excellent barrier properties, but it is expensive and difficult to machine. While numerous types of plastic materials have been used as packaging films, the problem with these materials is that they offer varying degrees of barrier protection for oxygen, water vapor, and aroma compounds. EVOH, for example, provides excellent barrier properties for oxygen transmission but it isn't as effective at stopping water vapor. On the other hand, a film like EVAL-F provides superior protection for water vapor transmission but performs poorly as a barrier for oxygen permeation.  As a result, packaging suppliers have created multi-layered films that combine the optimum barrier properties of different types of packaging films. For example, polypropylene and high-density polyethylene (HDPE) are relatively inexpensive materials for cartons and packages, but they don't provide much barrier protection from oxygen transmission. Using a packaging material consisting of a lamination of polypropylene and EVOH offers significantly better barrier protection; 1 mil (0.001 in.) of EVOH provides the same barrier properties as 4.5 in. of polypropylene or HDPE -- and it produces much less solid waste to dispose of in a landfill.  To properly evaluate the barrier properties of packaging films, it is important to determine the transmission rates of various desirable flavor chemicals from the food through the film. This type of information can be used in many important ways. For example, it can reveal how much "scalping" of important flavor-contributing chemicals will occur (e.g., how much decanal, ethyl butyrate, acetaldehyde, and other flavor chemicals are lost by orange juice to the inner lining of the carton over time), a factor that affects shelf life.  The Aromatran™ instrument from Mocon/Modern Controls Inc., Minneapolis, is a useful tool for screening packaging materials for their barrier properties and for determining the potential for scalping of flavors. It can also be used to assess the migration of malodorous packaging components into the food product.  Mocon markets two versions of the Aromatran. Model 1A is a fully modular and automated system designed to test single permeants under dry conditions or at a specified precise relative humidity. Model 2 is an effective, semi-automated means of testing multiple permeants and has a built-in cryotrap that provides increased sensitivity for high-barrier materials.  When you consider all the possibilities, choosing the most appropriate packaging material can be confusing. Your prime goal is to pay for only as much barrier protection as you need; you can spend more for superior protection, but your customers probably wouldn't want to pay for it.  Give careful consideration to the entire package during the evaluation process. That is the advice that Kenneth Marsh, Ph.D., president and research director of Kenneth S. Marsh & Associates, a Woodstock, IL-based food and pharmaceutical packaging consultancy, gives his clients. Failing to do so can be a very expensive mistake. Marsh cites the following example:  A manufacturer of a high-fat sandwich spread wanted to test the barrier properties of two lid stocks for its new product. The researcher responsible for the project chose to compare polyester -- a medium barrier lid, with foil -- an absolute, but significantly more expensive barrier. Shelf life studies and organoleptic paneling indicated that there was no significant difference between the polyester and the foil lid stock. However, during testing with both types of lid stocks, the product rapidly turned rancid. The entire new product concept was scrapped as a result of these tests.  The company learned later that the product's unacceptable shelf life had nothing to do with the lid stock or the product's formula. The food designer responsible for the project decided early in the product's development stages to use a polypropylene tub. There was infinitely more oxygen transmission through the tub than through the lid stock. So it didn't matter which type of lid was used; rapid development of rancidity was inevitable. There was nothing wrong with the product's concept or formulation. The only problem: It was put in the wrong type of package. This simple oversight cost the company some $1.5 million in wasted research and development expenses. The product concept was killed.Light exposureEven light transmission through packaging material can be a problem. Light radiation can initiate free radical chain reactions in food components and cause several different types of off-flavor problems. A prime example of this problem is butterfat oxidation of milkfat by light. The increasing use of HDPE milk jugs has promoted the occurrence of light-induced off-flavors in milk. Exposure of milk in blow-mold plastic containers to the fluorescent light of supermarket dairy cases is responsible for the development of light-induced off-flavors in some 80% of store samples, according to some studies.Packaging films and inks  Although plastic packaging material consists primarily of non-volatile high-molecular weight polymers, volatile low-molecular weight compounds are often added to improve functional properties of the materials: plasticizers to improve flexibility, antioxidants to prevent oxidation of the plastic polymers or the food inside the packaging, and UV blockers to prevent "yellowing" of polymeric material when it is exposed to light. Additional additives include polymerization accelerators, cross-linking agents, anti-static chemicals and lubricants. While all these additives serve a valuable function, they can also be potential contributors to off-flavors.  Solvents associated with packaging inks and with resins used in bonding the various layers of laminated packaging films can be absorbed by the product and contribute off-flavors. The presence of monomers (single units of polymers) can contribute significant off-flavors. Styrene monomers at 0.5% in polystyrene are responsible for the characteristic plastic odor of many packaged foods. Fats and oils in food products can help mask off-flavors from monomers and odorous residues from plastic processing. However, as consumers shift toward lower fat products, off-flavors contributed directly by packaging materials may become more of a problem.  One example of a food off-flavor from solvent packaging material occurred with half-and-half packaged in 13 ml polystyrene cups. Normal-tasting control samples and samples with a "chemical, solvent-like" off-flavor were analyzed by purge-and-trap gas chromatography with mass spectrometry (P&T/GC-MS) detection. The technique involves bubbling helium gas through the sample, sweeping the volatiles off and onto a trapping material, and then desorbing the trapped volatiles (which should include the chemical causing the off-flavor) into a GC column.  The GC column separates all volatile chemicals into individual peaks, and the mass spectrometry detector performs positive identification of each peak. In addition, some chemists split the effluent from the column's exit, with a portion going to the MS detector and the remainder directed to a "sniff port." With this approach, researchers use the MS detector to identify the chemicals that are present and the sniff port (also known as an olfactory detector) to evaluate the odor contribution of each component.  As shown on the following page, the most significant differences between the chromatograms of the control and the tainted samples were an increase in the acetone levels and the presence of a significant propylacetate peak in the tainted samples. The odor of the propylacetate peak was characterized with the sniff port and judged to be similar to the sample's odor defect.  The packaging lid stock consisted of three layers: an outer paper layer impregnated with water-based inks, a middle metal-foil layer, and an inner plastic film used for heat sealing. The inner heat-seal film, which is actuated by heat and pressure to seal the lid to the cup, is applied to the foil as a slurry, and then dried. The solvent used by the packaging supplier to form the slurry was propylacetate. After application of the plastic film to the foil, sheets of the lid stock material pass through drying ovens to remove all residual propylacetate solvent. According to the packaging supplier's specifications, the lid stock should contain less than 1,000 mg propylacetate per 3,000 sq. ft. of material. After the problem was reported to the packaging supplier, quality control specifications were more closely monitored and no further off-flavors occurred.  This example illustrates a couple of important points. First, by having in place good QC testing techniques to monitor packaging-related off-flavors, problems can be resolved quickly, before they generate complaints and potentially lost customers. Also, it is noteworthy that the packaging supplier was eager to cooperate with the food company's QC chemist to resolve the problem. For example, the food company's chemist wasn't aware that propylacetate was used as a solvent in the manufacturing process of the film or that specifications were established for its control.  This type of manufacturer/supplier cooperation is critical to efficient problem solving. Another point: If packaging suppliers know that sensitive, reliable testing techniques are in place, they will probably be more diligent in monitoring the quality of the packaging materials they are supplying.New tools  P&T/GC-MS with olfactory detection is a valuable technique for resolving packaging related off-flavors. But it does have some disadvantages. For one, it is time consuming and requires a highly-trained, experienced chemist. In addition, samples with complex chromatographic profiles can be difficult to interpret. It is not unusual for real-world food samples to contain a hundred or more chromatographic peaks.  One alternate approach is solid-phase microextraction (SPME). SPME is really a sample preparation technique for GC analysis (a replacement for the purging and trapping step in the P&T/GC-MS method). SPME is a relatively new technique that promises to offer numerous advantages over other sample preparation methods for food aroma analysis:It can be easily automated for increased sample throughput.It is fast and solvent free.It is inexpensive.It is sensitive (with ppt detection limits for some compounds).It is ideal for quick screening.  Sample preparation with SPME involves a few simple steps. It uses a fused silica fiber coated with an adsorbent that is mounted on a modified GC syringe to extract analytes from samples, and then passes these analytes directly into a heated GC injector. The coated fiber acts like a sponge, concentrating the organic analytes on its surface so they can be transferred to the GC.  A portion of the packaging material (usually 0.1 to 2.0 grams) is placed in a GC vial and capped. The vial is then punctured with the SPME device and the fiber is exposed to the air in the vial, which is heated to approximately 120°C. After an equilibration time of approximately 30 to 45 minutes, the SPME device is removed from the sample vial and inserted into the GC injector. While in the injector, analytes are thermally desorbed from the fiber to the GC column, where individual chemicals are separated and transferred to the GC detector for quantification, and in the case of a mass spectrometry detector, for identification.  Fibers coated with non-polar polydimethylsiloxane and the more polar polyacrylate are commercially available at the present time. For most analyses, such as volatile flavor compounds, a fiber having a 100 µm coating of polydimethylsiloxane is preferred. For analysis of high boiling point components, such as polyaromatic hydrocarbons, fibers with a 7 µm thickness of polydimethylsiloxane usually work best.  The technique has been used successfully to monitor ppb levels of acrylonitrile, styrene, alpha-methyl styrene, and butylated hydroxytoluene in a packaging polymer made with acrylonitrile, polybutadiene, styrene, alpha-methyl styrene, and styrene butadiene rubber. For more details, consult Varian Application Note Number 7, "Determination of Residual Solvents in Polymers with Solid Phase Microextraction (SPME) and GC/MS." (Varian Analytical Systems, San Fernando, CA.)Electronic nose  Another promising tool for evaluating packaging materials is the electronic nose. Unlike P&T/GC-MS and SPME, the electronic nose is a non-chromatographic technique, and it doesn't attempt to separate or resolve all individual volatile components. Instead, it uses an array of sensors that responds to each volatile chemical in a slightly different way -- much in the way the human nose functions.  The human nose has approximately 10,000 odor sensors that are non-specific but can be very sensitive to certain odors. The human nose neither tries to identify nor quantify the different constituents. Signals from human olfactory sensors are transmitted to the brain for processing. The brain then interprets what the sum of all these signals is describing in terms of odor.  Today's electronic nose instruments attempt to do the same thing with much fewer sensors and a simulated brain consisting of a computer and sophisticated software. Electronic nose instruments are designed to mimic the human olfactory system. The human nose is used to smell or detect an aroma, which is actually one or a mixture of many organic volatile chemicals emitted by a particular source. In essence, there are really three major parts to the human system: sensors; the conversion of the sensor outputs; and the analysis of the data. Electronic nose instruments, regardless of the manufacturer, contain these same three components.  The heart of the electronic nose is the sensor. Like the human nose, the sensors used in these instruments are highly sensitive, but not specific. This characteristic is essential for any aroma sensor. Different vapors may contain hundreds of different compounds, so it is neither possible nor desirable to monitor each of these chemicals individually. Instead, sensors must respond to a large number of different chemicals.  Instrument manufacturers assemble a group of six to 32 different sensors into an array to ensure that every compound in a vapor is detected by at least one sensor and every different vapor gives a unique fingerprint from the array of sensors. This ensures that slight changes in the composition of the vapor -- perhaps, for example, because of a taint -- will result in a different fingerprint.  Analyzing signals generated by the sensor array output is a critical step in the operation of the electronic nose. Data presentation techniques employing scaled polar plots, difference plots, difference rings and other methods allow for qualitative sample-to-sample differentiation. However, quantitative results for specific compounds cannot be as easily obtained as with P&T/GC-MS, SPME/GC-MS and other chromatographic techniques.  The most powerful type of data processing technique being employed in electronic nose instruments is the Artificial Neural Network (ANN). ANNs are self-learning; the more data presented, the more discriminating the instrument becomes. By running numerous standard samples and storing results in computer memory, the application of ANN enables the electronic nose to better "understand" the significance of the sensor array outputs and use this information for future analysis.  The advantage of the electronic nose for monitoring malodors contributed by packaging materials is that the technique is very rapid, and once the nose is properly calibrated, it does not require a highly trained technician to operate it. The instruments have a broad price range, depending on the number of sensors ordered and on other features. Costs can climb as high as a GC/MS system.  Electronic nose manufacturers Neotronics Scientific Inc., Flowery Branch, GA, and Alpha M.O.S. America, St. Louis, have demonstrated that their instruments can effectively monitor low-level concentrations of malodorous chemicals in paper- and cardboard-based packaging materials. In addition, both Neotronics' and Alpha M.O.S.'s instruments are being used by packaged goods companies to monitor levels of additives in packaging film to ensure that off-flavors don't develop when these packaging materials are used. Reliable determination can be made in less than five minutes, and cost per analysis is low.To withstand physical abuse  Once you have designed a packaging system that offers the barrier protection you require at a reasonable cost, you might think that your packaging design work is completed. It's not. Your next challenge is to make sure your packaging materials provide sufficient protection so that your product isn't damaged by the physical abuse encountered during its normal distribution cycle.  Millions of dollars are wasted each year on transit damage and over-packing. Merely devising heavier and sturdier packages drives up shipping costs and generates enormous waste disposal problems. When properly designed, packaging can be less bulky, lighter, easier to handle, and less costly without risking damage to products. But before you can decide on which packaging materials to use, you will have to know how much physical abuse your product actually encounters during its distribution cycle, and how much abuse it can withstand before it becomes unmarketable.  Lansmont Corp., Monterey, CA, and Dallas Instruments, a subsidiary of Lansmont, sell instruments that help food processors use a scientific approach in their packaging development efforts. "Our instruments monitor and analyze product/package abuse due to shock, vibration, dropping, compression, temperature, and humidity that occurs during distribution," says Paul Dinh, marketing manager at Dallas Instruments. Bare product testing can quantify a product's ability to withstand this environment on its own. The product's integrity can then be used to select materials appropriate for the design of an optimum package system. Package testing completes the design cycle and simulates the real world abuse conditions.  Dallas Instruments has recently introduced the SAVER -- Shock and Vibration Environment Recorder -- which they claim is one of the most powerful, yet easiest-to-use field data recorders ever made. This electronic device measures the dynamic and climatic environment experienced by vehicles, products, and packages in transit and distribution. Using a Windows(tm)-based computer, you set up SAVER with a simple dialog, and then send it out to measure the "real world." The resultant data is analyzed by SAVER's software program, SaverWare, to provide decision-supporting answers regarding drop, shock, vibration, temperature, humidity, and other variables.  SAVER's small size and reasonable price tag (in the $10,000 range) makes it suitable for use in a wide variety of situations. For example, it can be hidden in a package to measure the small parcel distribution environment. It can be tucked away in a vehicle to measure ride quality, or in a railcar to measure coupling shocks and in-transit vibrations. It can measure atmospheric pressure changes during transit, compression of cans and other packages, and other variables. It can be placed in remote locations for unattended recording, and used where conventional data systems cannot be used. In short, if you ever wondered exactly what happens to your product during shipment, SAVER will tell you -- bump by bump.  And if you don't have the staff, experience, or time to go through the packaging development steps, you can always hire consultants to do the job for you.Back to top