Brewery bacterial contaminants

Brewery bacterial contaminants
By Moritz Kallmeyer
Master Brewer of Drayman’s Brewery & Distillery, Silverton Pretoria, January 2004

Bacterial cells are the oldest inhabitants of planet earth. Rocks which were found by geologists in the Baberton area, timedated to be 380 million years old, showed the familiar rod shaped bacterial cells clearly when examined under a craftscope. Bacterial cells were here first and the fact that they still thrive today is evidence of their incredible ability to adapt. Some are found in the ice on the poles, others in the hot geysers that spew 100°C water, where nothing seemingly can survive.

French scientist Louis Pasteur is the father of modern brewing. His work on yeast allowed brewers to understand, for the first time, exactly what happened during fermentation. Previously beer frequently went off and became undrinkable and the brewers had no idea why. Most brewing companies in those days (and some home-brewers nowadays!) expected losses of 20% or more through waste and regularly had to destroy whole batches of sour beer.

The old saying that cleanliness is next to Godliness is especially applicable in the running of a brewery. Some brewers have dubbed themselves as glorified janitors! The brewer himself, with the degree of craftbiological control he practices during the entire process, controls the nature and magnitude of bacterial contamination in a brewery fermentation. It is a small consolation for a brewer to know that the number of bacterial genera that could survive in a brewing process are relatively small. Then again, brewing processes, as practiced by some brewers seriously lack sufficient craftbiological checks. Several factors account for the limited growth of bacteria during a brewery fermentation. Such spoilage organisms may be sensitive to hop resins (the higher the hop rate, the lower the risk of bacterial contamination), a high ethanol concentration, low pH, lack of oxygen during most of the process, limited sugar after fermentation, early heating steps and low temperature maintained during fermentation and processing.

Just on a pronunciation note. It doesn’t matter if the word is written in its original Latin form in italic or not. All “c’s” and “ k’s” for that matter are pronounced as “k’s”. Lactobacillus Bacteria (pronounced Lak-to-ba-kil-lus Bak-te-ria), craftcococcus (pronounced Mikrokokkus) Acetobacter (pronounced Aketobakter). Nowadays in normal writing the bacteria name is usually not written in italic and the pronunciation thereof, by the uninformed, has been concocted into a mix of c’s (as in sea) and k’s (as in cookie). Hopefully after studying this article you will be able to name and correctly pronounce the specific bacterial contaminant in someone’s beer at the next meeting!

In order to identify different bacteria under a craftscope, various dyes are used like methylene blue. There are some procedures (like differential staining) that use more than one stain in order to distinguish differences in the chemical composition of the bacterial cell. One of the most important differential staining techniques is the Gram stain or gram-reaction. Gram –positive bacteria that are of significance to brewing include the members of the lactic acid bacteria especially the genera Lactobacillus and Pediococcus. The important gram-negative bacteria family that have been responsible for beer spoilage are Acetic acid bacteria, Zymomonas, and a few members of the family Enterobacteriacea, Pectinatus cerevisiiphilus and Megasphaera. Certain endospore- forming bacteria belonging to the genus Bacillus and certain craftcoccus species (craftcoccus kristinae) have, on occasion caused problems in breweries.

Gram-Positive Bacteria

Lactobacillus
These are catalase-negative, nonsporulating, rod-shaped organisms that are either homofermentative or heterofermentative, based on temperature tolerance and the mode of fermentation. There are several species of lactobacilli isolated from beer, and these constitute the predominant spoilage organisms in the beer industry. They are resistant to hop bittering compounds , and some of these lactobacilli have been implicated in the production of diacetyl that is responsible for the “buttery” flavour. Although all lactobacilli produce lactic acid, the level of the acid accumulated does not reach the high threshold levels needed to make a significant flavour impact on the final beer. However, the spoilage can be observed as “silky” turbidity.

Pediococcus
These are catalase-negative , homofermentative cocci that form characteristic pairs or tetrads due to their cell division in two planes. Pediococcus damnosus is the most common spoilage organism of the genus found in breweries that produce lager beer. The organisms are seldom found in the pitching yeast, but are found at times during the late fermentation or in the final beer. The spoilage by pediococci is somewhat similar to that caused by lactobacilli. Pediococci are responsible for the sarcina sickness and give rise to high acidity and buttery aroma due to the production of diacetyl.

Other Gram-positive craftorganisms
Several gram-positive craftorganisms, although less important as spoilage organisms, have been detected in brewery fermentations. Although heterofermentative cocci like Leuconostoc mesenteroides have been detected in breweries, they are not known to produce any beer spoilage. On the contrary, Streptococcus lactis and craftcoccus kristinae, which are relatively acid-tolerant and hop-resistant, have been responsible for beer spoilage. Likewise, thermophillic endospore-forming bacilli like B.coagulans and B. searothermophilus have been isolated from breweries. These organisms are known to produce high levels of lactic acid when sweet wort is left for an extended period of time at elevated temperatures.

Gram-Negative Bacteria

Acetic Acid Bacteria
These are gram-negative, rod shaped bacteria capable of producing acetic acid from ethanol. Acetobacter and Gluconnobacter are two important genera under the group acetic acid bacteria that are traditionally associated with brewery fermentations. Acetobacter is known to oxidize ethanol to CO² and water via the hexose monophosphate pathway and TCA cycle. In the case of Gluconobacter, the hexose monophosphate shunt constitutes the most important route for sugar metabolism. The entire glycolytic and TCA cycles are not functional in gluconobacter. Beer is not expected to contain oxygen in the final form, and these organisms cannot thrive in beer under highly anaerobic conditions. Problems with Acetobacter and Gluconobacter can only be seen in beers that have low oxygen tension due to process defects.

Obesumbacterium proteus
The best- known brewery contaminant in the family Enterobacteriaceae is Obesumbacterium proteus. I have a strong suspicion that most homebrewers have encountered this bacterium some or other time. It is a gram-negative, nonacid-fast straight rod that can be found multiplying in the pitching yeast. It can grow in unhopped wort and is able to tolerate pH values ranging from 4.4 to 9.0. This organism is known to suppress the fermentation process and is also responsible for the increased level of dimethyl sulfide, dimethyl disulfide, diacetyl and fusel oils. Beer contaminated with Obesumbacterium proteus has a characteristic parsnip-like or fruity ordour.

Zymomonas
These are gram-negative rods that occur mostly as single cells, in pairs, chains, or filaments, and most strains are non-motile. Motile strains have one to four flagella. Zymomonas mobilis is the most common brewery contaminant, and its most distinctive characteristic is the ability to convert glucose or fructose to ethanol and CO² via the Entner-Doudoroff pathway. Its growth is only inhibited by around 8% ethanol. It can also be completely killed by a few minutes exposure to 60°C. Zymomonas mobilis is known to produce unacceptable levels of acetaldehyde and hydrogen sulfide in lager beer.

Other Gram-Negative craftorganisms
Several other less important gram-negative organisms have been reported in brewery fermentations. One such gram-negative contaminant is Pectinatus cervisiiphilus, which is known to produce acetic acid, propionic acid, acetoin and hydrogen sulphide, either in fermenting wort or packaged beer. Beers contaminated with this organism are generally turbid and have a rotten egg odour. The other organism of minor significance is the gram-negative coccus, Megasphaera, which imparts cloudiness and an unpleasant puke-like odour to the beer. These characteristics are primarily due to the production of butyric acid during fermentation.

What is important to remember here for us brewers is that even a bacteria described as of “minor significance” by some white-coat researcher can be emotionally devastating for a brewer trying in vain solve the mystery of his butyric smelling witbier!

References:

1. Reed, G., Nagodawithana T.W.,1991. Yeast Technology 2nd edition , p.124-126.