German China

Microscopic Food Analysis

Looking at the Detail of Foods: What Are We Eating and Drinking?

| Author / Editor: Kristin Unger und Anke Koenen* / Dr. Ilka Ottleben

Packaging and Production — Foreign Bodies & Raw Materials

In the practice of official food monitoring, foreign bodies of all kinds in foods represent a commonly-recurring challenge. One possible cause of contamination is interventions in the manufacturing process, such as repairs. Natural constituents in the raw materials such as stones, pips, stalks, bones and the like call for agreements with the suppliers in the form of specifications. Poor personal hygiene can also result in foreign bodies such as jewellery, make-up, nail polish, and hair being introduced into foods.

Which foreign bodies were found in a food and where they originate from is critical. Clarification may also be needed for a legal classification and evaluation. Unfortunately, identification is not always as simple as for glass or metal particles (from packaging material or plant parts), and in some cases it may call for considerable analytical effort and a lot of experience.

Initially, most foreign bodies are identified via macroscopic examination. This can also clarify whether the body exhibits an animal, plant-based or artificial structure. Brightfield microscopy and polarization microscopy provide more accurate results if the structure is artificial and contains elements of textile fibres for instance. For plant-based components, particularly where an element of starch is suspected (e.g. bread or potato parts), it is possible to detect the presence of starch using Lugol’s solution. Histological stains are not always necessary, but they can be very helpful – particularly if the foreign body is suspected to be of animal or parasitic origin or the investigation material is so pale that cell structures are barely recognizable. For example, various types of tissue and cell nuclei can be shown very clearly using a haematoxylin-eosin (HE) stain. This stain is also highly suitable for showing fungal hyphae – particularly as it is a stain that is easy to apply and very commonly found.

Hairs and fibres make up a relatively high proportion of the foreign bodies found in foods. Examining whether it is a fibre or a hair is a relatively straightforward task. It becomes more difficult, though, when attempting to distinguish between human and animal hair. However, this will not necessarily require an electron microscope to display the cuticle scales (scales on the surface of the hair). A conventional brightfield microscope with at least 400 times magnification can work well here. Human and animal hairs can be differentiated not only by using the scale shape, size and spacing from one another. The medullar index, too, provides valuable indications as well. The medulla is the air-filled innermost layer of the hair, which in central Europeans is generally fragmented or entirely absent.

Structure and Function — Composition, Crystals

The microscopic structure of foods determines their function. Food technologists are developing complex new food products. Numerous thickening agents are used in sauces and desserts. These include gelatine, starch, pectins and various polysaccharides such as Xanthan gum or locust bean gum. The requirements for a commercial food product are generally different from those for a home-made equivalent, since it needs to withstand the cooking process, the cooling or freezing operation, transport, long-term storage and a second cooking process prior to consumption.

Microscopes help make the structures in various test products visible. That way, the most suitable stabilizing or thickening agent can be determined for a specific application. Moreover, microscopic analyses can also differentiate between wheat starch added in a particular production stage and a modified maize starch introduced later in the process. The difference between gelatinized and ungelatinized starch granules explains, for example, the loss of viscosity in a sauce product.

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