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Microscopic Food Analysis

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

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To investigate the connection between ingredients, processing and texture, the main types of instrument used are scanning electron microscopes (cryo-EM and element analysis). They supply high-resolution 3D images. Food analysts can obtain quantitative information from microscope images. In-situ microscopy makes dynamic studies possible.

Microbiology: Bacteria, Moulds, and Viruses

Moulds, bacteria, and viruses are all types of microorganism. Certain microorganisms are essential for production of, amongst other things, cheese, yoghurt, or alcoholic drinks such as beer. Conversely, microorganisms can result in food perishing, meaning that such foods need to be conserved, or they can trigger illnesses that are transmitted via foods. Common examples of this are species of salmonella and moulds. Microorganisms that are relevant in food accordingly range from pathogenic or toxinogenic microorganisms to spoilage microorganisms to organisms used by food technologists in fermentation and maturation.


Fluorescence microscopy using dyes is available to demonstrate the presence of microorganisms such as bacteria. Gram staining is typically used to investigate and observe bacteria such as Staphylococcus aureus, E. coli, Salmonella, Campylobacter, and Shigella. Blue staining of starch using iodine, and Fast Green FCF or Acid fuchsin are valuable methods, particularly for localization of proteins.

An Example of a Practical Application: The Brewing Process

As part of the brewing process, for example, light microscopes are important monitoring tools. The aim behind microscopic examination in the brewery is optimal and comprehensive evidence of the raw materials, of the yeast growth during the process, and of potential foreign bodies. This examination is also fast and efficient, and it is used to accompany production. The darkfield process has proven effective thanks to its high contrast and high sensitivity. Contaminants are spotted visually (see Fig. 2), while high throughputs are achievable.

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So how is beer actually made?

A diagram showing this explanation can be found in the photo gallery (see Fig. 3). At the start of the brewing process, barley malt is milled using a grist mill. This produces various fractions, and the spelt of the grain is preserved (1). During mashing, the malt grist is mixed with water in the mash tun to make mash. The enzymes contained in the malt grain (amylases) become active at different temperatures and split the insoluble starch into soluble sugar. As they do so, substances vital for brewing are released from the malt in solution (2). The final temperature stage disables the enzymes.

In what is known as a lauter tun, the solid components of the mash are separated from the liquid components. The solids, referred to as draff, remain in the lauter tun at the end of the process and are sold as animal feed. The liquid, the wort, containing all the soluble substances from the malt grain, is pumped into the wort kettle (3) and the hops are added. Next, the wort is boiled for around an hour (4). The more hops that are added, the bitterer the resulting beer. Aromatic or bitter hops are used depending on the type of beer. In the whirlpool (5), any solids remaining after boiling are separated off before the wort is lowered to the pitching temperature in the wort cooler (6) and the yeast is added. Fermentation can now begin.

The yeast starts fermentation into alcohol in the fermentation tank, converting the dissolved malt sugar into carbon dioxide and alcohol. At the end of the fermentation process, the bottom yeast settles at the bottom of the fermentation tank and can be drawn off. The brewer refers to the liquid as green beer (“Jungbier”) at this stage (7). It is cooled to a temperature just above freezing point and racked into the conditioning tanks. Depending on the beer type, it can stay here for up to three months. During storage, the flavor of the beer becomes more rounded, undesirable by-products from fermentation are dissipated, the residual sugar breaks down, the carbon dioxide is bound in the beer and the beer clarifies through settling (8). After conditioning, the beer generally undergoes filtration. This is where any cloudiness still present in the beer is removed. The result is a clear, bright product that is called bright by the brewer (9). The beer is now decanted into bottles, barrels or cans (10).

When checking the wide range of samples from the entire brewing process, the origin of the samples is critically important. This makes it possible to look for discrepancies in the microscopic image in a targeted fashion. If discrepancies are found, further analyses can be carried out for identification. These might include, for example, evidence of live/dead condition, by staining microorganisms using fluorescent dyes.

* K. Unger, A. Koenen: Carl Zeiss Microscopy GmbH, 07745 Jena, Germany