Techniques for Identifying Food Adulteration and Contamination

Adulteration and contamination are ongoing concerns in the food and beverage industry, as they can result in unsafe products and damage brand integrity. Small particulate or light filth contamination may not be identifiable based on visual inspection, and in some cases may be misidentified. Brown or black particulate might simply be charred or burnt product, but other types of manufacturing debris may be found. Dark particulate can be rubber or metal debris, or mold or microbiological growth. Larger colorless or light-colored particulate may look like glass, but may in fact be plastic or compressed product material. Microbiological testing is used to confirm and identify certain types of organisms, but a negative result does not identify the contamination. A number of analytical techniques for material identification have been adapted for use on barely visible or subvisible particulates. Identification of foreign material contamination can alleviate safety concerns and identify the root cause of the problem.

The first step in particle evaluation is microscopic examination using a low-magnification stereomicroscope. Particles that can be seen without magnification are usually in the 100–300 μm range, but in some instances, smaller particles can be observed. Many types of particulate can be classified and identified based on appearance, color and other characteristics; for example, particulates such as glass and metal have distinctive morphology that allows for tentative identification. Stereomicroscopic examination can be used to select the appropriate analytical technique.

When glass is not glass

Colorless and light-colored particles are often assumed to be glass. However, when examined under a stereomicroscope some particles have a pliable polymeric texture that is not consistent with glass but, rather, a plastic. Other particles may have a compressed, powdery consistency and are clumps of starch or sugar that have been compressed or not thoroughly mixed into the product.

Glass particles have a distinctive morphology when viewed with a stereomicroscope. They are hard, transparent and have conchoidal fracture surfaces. Identification made via a stereomicroscope can be confirmed using other analytical techniques such as higher-magnification polarized light microscopy (PLM); glass particles are isotropic when viewed using crossed polarized light. Another analysis technique, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), provides an elemental profile of the particle, which can be used to confirm the identification of glass. This profile can also be compared to suspected sources of the glass.

If stereomicroscopic examination indicates that the particle is not glass, more specific identification can be made using infrared (IR) spectroscopy.

IR spectroscopy is widely used in many industries to identify organic materials, and can distinguish polymers and product-related materials such as starch and sugar. Infrared spectrometers interfaced to microscopes permit the collection of spectra from very small samples.

Black or brown particles may not be charred product

Figure 1 – Black particle as seen under a stereomicroscope.

Black particles are often observed in products that have been subjected to heat. Products containing carbohydrate    material are susceptible to browning, and more severe charring often results in particles that appear dark brown or black and have a very brittle consistency. Charred organic material has a distinctive appearance when viewed under the microscope. The charred material is very brittle and friable, and appears translucent and amber colored when pressed thin. IR spectroscopy can be used to identify charred organic material, but severely charred material yields a broad, nonspecific spectrum that cannot be identified further. SEM/EDS can be used to confirm that the particle is primarily carbonaceous in composition.

However, black particles are not necessarily charred product. Stereoscopic examination is vital to characterizing the particle and selecting the proper analytical technique. Small black rubber particles may be generated from washers and bushings in production equipment. If the consistency of the particles is indicative of rubber, IR spectroscopy is typically used to identify the type of rubber. The presence of black particles may also result from small metal particles that are generated during manufacture. These particles frequently look black when seen with the naked eye, but are shiny and reflective when viewed under a microscope. Metallic-appearing particles can be prepared for SEM/EDS to identify the type of metal alloy.

Figure 2 – This higher-magnification view of a portion of the particle in Figure 1 shows fungal hyphae and yeast, partially stained for contrast, viewed under a polarized light microscope.

Figure 1 shows a black particle found in a polysaccharide raw material. While it appeared to be a charred organic material, the particle had a soft, somewhat pliable consistency, which was not expected for thermally degraded polysaccharide material. The IR spectrum of the particle suggested carbohydrate and protein material, which is consistent with biological materials, but also could be due to food-based sources. The particle was stained for contrast and examined using PLM; structures consistent with fungal hyphae and budding yeast were observed (Figure 2).

Amorphous residues are not easily identified using light microscopy alone, but are often analyzed using IR spectroscopy. A sample of black residue swabbed from the surface of manufacturing equipment yielded an IR spectrum characteristic of protein and possibly polysaccharide indicative of biological materials, but specific identification could not be made using IR spectroscopy alone. The swab containing the black particle was examined in situ using the SEM/EDS in low vacuum mode. The black residue had stringy morphology consistent with mold.

Guiding the way

Stereomicroscopic identification of small particulate contamination provides a preliminary identification and guides the choice for the most appropriate techniques and instruments to use for further analysis. It is able to determine the source of the particulate, i.e., manufacturing debris, production-related (charred or compressed product) or biological contamination. The identification of biological contamination may indicate the need for further testing for specific identification or viability testing.

Gretchen Shearer is a senior research chemist at McCrone Associates, Inc., 850 Pasquinelli Dr., Westmont, Ill. 60559, U.S.A.; tel.: 630-887-7100, and teaches Pharmaceutical Materials and Contaminants and Infrared Microscopy at Hooke College of Applied Sciences, LLC; e-mail: [email protected]; www.mccrone.com

Comments