Chapter 21

Beer Spoilage Organisms

Monitoring Microbes

The objective of a microbiological quality assurance program is to detect and prevent contamination of in-process and finished beer by beer spoilage organisms. A robust cleaning and sanitization program as well a thorough sampling program is essential in maintaining high standards. In a typical quality assurance program, samples are collected and analyzed for spoilage microbes from several quality control points across the beer brewing process. Typical sample points are wort, pitching yeast, fermenter, lager tank (if used), bright tanks, rinse water after cleaning tanks, hoses, bottling lines, and packaged beer. Monitoring microbes effectively and efficiently during the brewing process requires applying several different techniques including microscopic investigation, microbiological culture plating, adenosine triphosphate (ATP) bioluminescence testing, or polymerase chain reaction (PCR) identification.

Microscopic Observation

The simplest way to monitor microbes is by making direct observations of the beer using a microscope capable of 1000x magnification. While a magnification of 400 is sufficient for studying yeasts and molds, a magnification of 800 to1000 is usually needed for reliable examination of the morphology of bacteria. The microscope is a very powerful aid to the beermaker because, with a good microscope and a little experience, it is possible to instantaneously detect any substantial population of microbes in the beer.

Microbial Culture Plating

Contrary to direct microscopic observation, where the cells can actually be observed, viable culture plating relies on the ability of cells to reproduce, resulting in a visible colony on the surface of the nutrient media. Enumerating microorganisms in beer requires the use of the appropriate sterile media, as well as knowledge of aseptic techniques (laboratory procedures that ensure the technician does not contaminate the media with anything not present in the beer)..

Spread Plate Technique

Spread plating is useful if there is a high microbial population. Plating is a procedure that is usually well within the capabilities of most small breweries and should be for all larger ones (Figure 21.5). Several types of culture media exist for the detection of the organism of interest. Some media are considered general all-purpose media and support growth of a large variety of organisms. Whereas there are a large range of culture media specifically for microbiological analyses of beers.

Membrane Filtration Technique

Membrane filtration is useful when dealing with beer that has a very low level of viable microorganisms. This method uses a vacuum to pull a large volume of beer through a small membrane filter that retains the cells, thus concentrating the bacteria or yeast.

Media Types

While there are very many media available, the following represent a good spectrum for the detection of most brewery microorganisms. The media presented are; Wallerstein Laboratories Nutrient Medium, Universal Beer Agar, MacConkey Agar, Lee’s Multi-Differential Agar (a.k.a. Schwarz Differential Agar), Hsu’s Lactobacillus/Pediococcus Medium and Carr’s Bromocresol Green Medium.

Adenosine Triphosphate (ATP) Bioluminescence

In recent years, many large and mid-size breweries have adopted a technique referred to ATP bioluminescence. Adenosine triphosphate (ATP) is found in all living or once living biological cells and is used as a proxy of microbial contamination. Adenosine triphosphate bioluminescence does not identify specific microorganisms that could be contaminants, but it does provide a rapid analytical indication of the effectiveness of cleaning and sanitizing practices at specific locations.

How to Test for ATP

Adenosine triphosphate testing systems contain 3 main parts—sample collection swabs, a luminometer, and software to analyze the data output. Sample collection swabs are used to swab down surfaces or collect water samples and inserting the swab into a luminometer that quickly provides a reading (Figures 21.6 and 21.7). These pen sized swabs contain bioluminescent reagents that react with the ATP in the sample to emit light.

ATP Test Results

There are generally two results given, a number in RLU and whether this quantification indicates a PASS or a FAIL to the sanitation process. This PASS or FAIL determination (a specific number of RLUs) is somewhat arbitrary. The RLU measurement can be analyzed and archived to help establish acceptable baseline ATP readings, pass/fail thresholds, and to provide insights into cleaning and sanitation processes.

Scope of the Bioluminescence Method

Although ATP swabbing gives us a good indication at the total quantity of cells left behind after sanitation, this type of assay only signals either the presence or absence of cells. Adenosine triphosphate tests do NOT distinguish between microbial and organic matter cell type.

Polymerase Chain Reaction (PCR)

A new emerging technology is real-time quantitative polymerase chain reaction (PCR) analysis. PCR is essentially an exponential DNA synthesis reaction in a test tube. Results can be expressed on the same day, and it is possible to perform a simultaneous quantification of more than one pathogen in a single assay. Overall, it allows for a quicker analysis that is just as sensitive as traditional PCR.

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