Find records of fraud such as those discussed in this column and more in the Food Fraud Database. Image credit: Susanne Kuehne
Honey is an easy target for food fraud and adulteration with sucrose, high fructose corn syrup, molasses and other sugars are not uncommon. To quickly identify adulterants, a method using Raman spectroscopy and pattern recognition analysis was developed. To verify the method, 97 samples were tested with the new method, and the tests confirmed with HPLC, with the result that 17% of the commercial honey samples showed fraud from added sugars.
Raman spectroscopy (RS) can be used to identify layers in polymer food packaging films to better understand the laminated plastic’s chemical composition. A Raman spectrum is obtained by illuminating a sample with a laser and collecting and measuring scattered light with a spectrometer. Coupling the spectrometer to a microscope with a mapping stage allows an accurate way to create a chemical map of a film’s composition and structure. The map provides valuable information to better understand the packaging’s barrier properties, structural integrity and layers.
The RS method can be useful for conducting failure analysis (why did a food package fail to meet standards), supply chain validation (is the plastic what the supplier claims), decision making (which plastic should be used), and evaluating package appearance (why is there discoloring, haze or particle inclusions in the film). It provides important information for design, purchasing, product success and other decisions that food manufacturers and packagers regularly face.
Take for example the packaging used for frozen hamburger patties. The film used must be transparent to display the hamburger patties, but it also needs to provide an oxygen barrier in order to prevent the ground beef from turning brown. As such, a polymer layer with low oxygen permeability must be incorporated into the laminated film, along with other components like nylon for strength and polyethylene for heat sealing and water barrier. The most common polymer used as an oxygen barrier is ethylene-vinyl alcohol copolymer (EVOH).
It is important that the film used to package these hamburger patties includes a good heat seal as well as a proper oxygen barrier layer. The possible absence of either of these could result in the undesired effect of ground beef turning brown. Manufacturers may want to test packaging for an EVOH layer to make a purchasing decision or verify a supplier’s claims. Additionally, if the packaging fails, an analysis can determine if the failure was due to having no EVOH barrier layer in the product or if there is a need to investigate other potential issues with the packaging. Regardless of the reason, RS provides a preferable method for rapidly evaluating the plastic for an EVOH oxygen barrier layer.
The RS method can be used to determine the construction of the laminated film and confirm that it meets specifications. Using the combination of RS with microscopy and mapping allows both identification of the polymers and the evaluator to correlate the composition to the layer structure of the laminated film. This method provides a map showing the composition of each layer in the film. In some cases, the Raman map will show layers that are not resolved in the visible micrograph image. Thus, with RS, one test provides both the structure and composition of each layer of the laminated film.
This sample table illustrates composition and thickness of each layer of a laminated film. Table courtesy of Intertek.
To start, a small section of the film (5 x 10 mm) is cut and mounted with a photocuring resin. A cross section of the mounted film is then cut to expose the layers for analysis. This cross-section is placed on the mapping microscope stage of the Raman instrument. A micrograph image with a 100X objective is obtained and a Raman map of the cross-section with 1 µm2 pixel resolution collected.
A map image is obtained by classical least squares (CLS) fitting example spectra to each of the spectra collected from the cross-section. The example spectra for the CLS fits are averages (mean) of the spectra in the center of each layer with a unique composition as determined by the data (see Figure 1). The final result is a color-coded map that can be superimposed on the micrograph image to show the composition and thickness of each layer in the laminated film. For example, a film with six layers composed of Nylon 6, polyethylene or EVOH would have varying thickness and placement of each layer to achieve the desired result for the product.
Figure 1. Example spectra used to create the CLS model for map image.
The composition map can confirm the presence of an oxygen barrier layer of EVOH, as well as the overall construction of the laminated film. Knowing the thickness of the barrier layer is important since the gas permeability is a function of the film thickness. Determination of the overall film structure allows the end-user to confirm the film meets the specifications from the supplier. In turn, this can be used to make important purchasing decisions or insights into what caused a packaging failure.
While good, successful results will confirm the presence of an EVOH layer, the RS map may also show only polymers that don’t have the required oxygen barrier properties (see Figure 2). The manufacturer would need to check it against a supplier spec sheet. It may ultimately show that the lack of an EVOH layer is what caused the issue with the packaging. If the test is being used for decision-making purposes, the manufacturer would know not to use the product. If a supply chain validation is being run, after checking the spec sheet, the manufacturer may need to correct the situation.
Figure 2. Raman map overlaid with image of film cross section. Green = nylon; Red = polyethylene; Yellow = ethylene vinyl alcohol copolymer (EVOH).
What if the analysis confirmed that an EVOH layer was present, but the test was done for a failure analysis, meaning the packaging did fail at some point? If the EVOH later is present but the meat is still turning brown and/or spoiling, other potential problems would need to be evaluated. In this case, the issues would most likely be with the heat seal and additional testing of the heat seal would be necessary. Thanks to the RS analysis, the investigation into the packaging failure can proceed, and the issue with the heat seal identified.
By giving a chemical image of the packaging, RS analysis provides a wealth of information about a film that can be vital to a food manufacturer or processor. Knowing why certain films may not be working, either due to faults in chemical makeup or the need to look elsewhere, such as the heat seal, RS quickly and efficiently provides information and answers to help get products to market and meet consumer demand.
Laminated plastics are common and popular food packaging options. They are strong and flexible, making them ideal for both packing and presentation, and can be used for cooking, frozen foods, drink pouches, snack products and even pet food. Yet, unreliable plastics can create a problem for food packaging and the safety of a product.
If a grade of plastic is not what was promised or needed, there can be issues that lead to spoilage, spills and messes, crystallization, mold or other risks. Additionally, there may be concerns about how laminated films will interact with the product itself, as it could impact food safety or lifecycle. For these reasons, it is critical to have accurate information when evaluating the plastics films used in food packaging.
Raman Spectroscopy
Raman spectroscopy (RS) is a powerful method of identifying and characterizing chemical compounds based on light scattering by a sample. It can be used to identify layers in food packaging films to accurately understand the chemical makeup of the laminated plastic. The effect is named for its inventor, C.V. Raman, who was awarded the Nobel prize in physics for its discovery in 1930. It is a non-destructive method that uses an induced-dipole mechanism to probe the vibrations of the chemical bonds in a molecule. The Raman spectrum shows a pattern of molecular vibrations that represents a detailed chemical fingerprint of a material, providing insights into the product composition.
A Raman spectrum is obtained by illuminating the sample with a laser and collecting and measuring the scattered light with a spectrometer. The molecular vibrational modes vary depending on the geometry and electronic structure of the chemical compound present in the sample. By controlling the position of the laser focus point on a sample, a map of the composition can be created. This provides valuable information on the plastic film related to its composition, such as number of layers, thickness of each layer and overall make-up.
In the food packaging and safety industry, this technique can be used to evaluate laminated plastic films by examining polymers, minerals, and/or inorganic fillers and pigments present in the film. Specific food packaging products that can benefit from RS assessments include heat seals, containers, lids, films and wrappers both for durability and performance and for diffusion, permeation or other concerns.
Benefits and Limitations
There are numerous benefits to using the RS method. A major advantage is that there is virtually no sample preparation necessary; spectra can be obtained without direct contact, such as through the sides of glass vials or through windows in reaction cells. As a non-destructive technique, it allows an easy, highly accurate way to take a sample, create a chemical composition map and better understand films’ barrier properties, structural integrity and layers. It has broad applicability and works using conventional microscope optics.
There are, of course, limitations to the approach, as well. Fluorescent components or impurities in a sample can emit a photoluminescent background that overwhelms the Raman scattering. Samples can also be damaged by the laser if too much power is used, or the sample absorbs light at the laser wavelength. Samples that do fluoresce and samples that are photolabile act as common interferences for the RS method. In many cases, these interferences can be overcome with the proper choice of laser and sampling techniques. Additionally, while RS provides an accurate analysis of laminated films, the technique cannot be used on metals or metallic compounds (which can be assessed using scanning electron microscopy or light optical microscopy) or organic pigments or ink layers (which can be assessed with other infrared techniques).
Using RS for Food Packaging
RS can offer a variety of insights for food packaging films:
Failure analysis. If a plastic used for a heat seal in a fruit or yogurt cup fails, it could result in a mess for manufacturers, stores or the consumer. Exposure to air or elements could also lead to spoilage, particularly for refrigerated foods. Inconsistent plastic packaging could result in weak points that break, crack or puncture, which could also result in mold, mess or other spoilage concerns. If a manufacturer experiences a failure in a heat seal or packaging leading to leakage or spoilage, RS analysis can help determine why the failure occurred (was in the plastic film or something else) to help prevent future issues.
Supply chain validation. It is extremely important that the plastic films coming from suppliers are what they are promising and what the manufacturer needs. RS analysis can be used to determine the chemical make-up and morphology of packaging to confirm a supplier’s claims before proceeding with use of the film in food packaging and products.
Simple decision making. If a manufacturer is trying to decide which material to use, RS can provide answers. For example, if there is a need for moisture non-permeating films and there are multiple options available, an RS chemical map can illustrate what to expect with each option, aiding in the decision-making process when combined with other known factors such as cost or timing. If there is an additive in the food product that may diffuse into the film, RS can determine which material might best resist the potential problem.
Packaging appearance. If there is a swirl or haze in the packaging, RS can compare the area with the issue to a clear section to determine if the defect in the film is a foreign polymer or an inorganic pigment or filler, identifying the source of the problem.
RS analysis provides a wealth of information in a manner that is non-destructive. Giving a chemical fingerprint to identify composition with extremely good spatial resolution gives manufacturers valuable information that can be used to mitigate issues, correct problems or make important decisions. These actions in turn can help ensure food safety, which builds brand image and manufacturer equity. Ultimately, RS analysis can play an important role in the success of a product, a brand or a company.
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