All About Yeast Contamination

Contamination affects the quality of the beer. Most contaminants will produce off-flavors, acids and non-desirable aromas. They can also produce hazy beers and films. They may compete with the production strain for essential nutrients; they can also induce stuck fermentation or over-attenuated beers. There are two major groups of microorganisms responsible for contaminating wort and beer: 1) Wild Yeast and 2) Bacteria.

1) Wild yeasts – Not all yeast are desirable in a brewing fermentation ‘wild yeast’ are usually defined as: ‘any yeast which is not deliberately used and under full control’. Wild yeast contamination can occur if the pitching/cropping yeast is in contact with air and anything added in the fermenter is a potential source of contamination. Hygiene is key. Wild yeasts are not killed by acid washing and therefore production yeast contaminated with wild yeast needs to be discarded to avoid product defect. Wild yeast infection can result in hazy beers, off-flavors production, decrease production of ethanol and rate of attenuation, superattenuation. They can also cause aberrant flocculation; they tend to stay in suspension and do not interact well with finings. Wild yeast can either be of the Saccharomyces or non-Saccharomyces genus.

Saccharomyces wild yeast
Cross-contamination with another S. cerevisiae strain can cause flavor defects as well as unusual fermentation performance. The most troublesome Saccharomyces wild yeast is Saccharomyces diastaticus. This particular yeast has the ability to break down the dextrins which are not being used by S. cerevisiae strains resulting in overattenuated beers. Saccharomyces wild yeasts are facultative anaerobes.

Non-Saccharomyces wild yeast
There are many different genus and species of non-Saccharomyces yeast which could cause problems in the beer. They tend to be aerobic organisms. The most detected ones are Brettanomyces spp which produce acid and strong mousey flavors; Pichia spp which form films, haze and various unusual esters; Candida and Hansenula spp grow fast and form films.

Note: Non-Saccharomyces yeast can sometimes be used deliberately for specific products. They can create different flavor/aromas to increase the unique character of a particular beer. As examples, Torulaspora delbrueckii is often used for Wheat Beer production to generate the clove-like flavor typical of these beers and Brettanomyces bruxellensis gives the characteristic sour taste to Belgian Lambic Beers. Some of them are available in dry form which is very practical as there is no need to propagate and the risk of cross-contamination is diminished.

2) Bacteria – Bacteria can grow rapidly in wort and beer, any fresh wort must be pitched as soon as possible with active yeast to prevent bacterial growth. Bacteria form a very diverse group; when viewed under the microscope they appear very small when compared to brewing yeast (see section on detection of contamination). They appear in two basic shapes; round (cocci) and rod (bacilli). They can further be identified by a staining procedure known as ‘Gram stain’. Bacteria will usually be either Gram positive (purple) or negative (pink) depending on how they react to this stain. This property is related to the structure of their cell walls. Many bacteria are sensitive to the adverse conditions prevailing in wort and beer, including low pH, alcohol content, hop extracts and low oxygen concentration. However some groups are able to thrive under these conditions and are therefore particularly dangerous as they cause off-flavors and compete with yeast for essential nutrients. Listed below are some of the main groups involved in brewing infections.

Lactic acid bacteria – Potentially the most troublesome spoilage organisms due to their ability to grow in low oxygen conditions and their tolerance of low pH, high alcohol and hop extracts. They produce diacetyl. They can either be rod shaped (Lactobacilli) or cocci (Pediococci) and are both Gram positive.

Obesumbacterium proteus (Hafnia protea) – Also known as common brewers bacteria, they favour wort at pH levels of 5·0 – 5·5 but is inhibited in beer at pH levels of 3·8 – 4·2. CBB is not likely to grow in beer unless its pH is unusually high. Still, they cannot be ignored as potential beer spoilage contaminates because, if they are present in sufficiently high levels in the pitching yeast, they will impart off flavors in the beer. When the wort pH becomes inhibitory during fermentation, these bacteria tend to group with pitching yeast cells, as they flocculate. If the start of the fermentation is slow due to decreased yeast cell viability or lack of oxygen, then the bacteria have a prolonged opportunity for growth before the pH becomes inhibitory. It is therefore important to get a vigorous start to fermentation to avoid this situation.

Acetic acid bacteria – These are Gram negative short rods named Acetobacter and Gluconobacter. As the name implies, they produce vinegar (acetic acid) from ethanol. Some members of the group will produce more extensive gassing of an infected beer, often leading to strong head retention. They are obligate aerobes and as such are commonly known as ‘wort bacteria’. Prevention of access of air to the beer at all stages of processing after fermentation offers the readiest means of control.

Pectinatus and Megasphaera – These organisms are particularly troublesome as they favour strictly anaerobic conditions. Hygiene in the bottling/canning line is the best way to avoid contamination. They cause turbidity, produce acetic acid (Pectinatus), short chain fatty acids (Megasphaera) and H2S. Pectinatus is rod shaped whereas Megasphaera is cocci, they are both Gram negative.

Zymomonas – This is a Gram negative short rod. It is also a potent spoilage organism since it survives in low oxygen conditions where the pH is low and the alcohol concentration relatively high. Fortunately it is not often isolated in breweries and seems to be mostly present in ale breweries.

Potential Sources of Contamination – Contamination can infiltrate from a number of sources: starting materials (raw materials and additives), the brewing plant (vessels, piping, etc.) and the brewing environment.

Starting Materials – It is important that the pitching yeast be free from contamination. Everything which comes into contact with the yeast must be sterilized. The wort leaving the kettle has been through the boiling process and therefore should be relatively sterile. But if the wort cooling system is not clean, contamination can be introduced before it enters the fermentation vessel. Any other additions made at this stage (i.e. yeast foods, or unsterilized water) is risky, they should be preferably added before the end of the boiling period to ensure sterilization.

Brewing Plant – Any surface that comes into contact with wort, beer, or yeast should be thoroughly cleaned and sterilized. (i.e. vessels, piping, and implements.) Soiled surfaces can support a microbiological growth which can be introduced into the beer. Any recurrent contamination may indicate the presence of a biofilm. Biofilms are particularly difficult to clean as they can bind strongly to the vessel or pipe. Localization of the biofilm may be necessary before adequate cleaning can be performed.

The Brewing Environment – Microorganisms are ever-present in the air, often in association with dust particles or airborne moisture droplets. They can also be introduced to the environment by insects and other pests. Every effort must be made to keep the brewing environment as clean as possible and to minimize the ingress of outside contamination. Wherever possible all vessels should be covered to reduce the risk of aerial contamination.

Detection of contamination – The easiest way to check for contamination, bacteria or wild yeast (see sections above), in production yeast is to observe a drop of slurry under a microscope. 400X is the standard magnification to observe both yeast and bacteria together. Any tiny dots (1? in size) moving around is likely to indicate the presence of bacteria. Brewing yeast cells should be round or oval shaped and around 10? (Familiarize yourself with the shape of your production yeast), any suspiciously-shaped yeast could indicate a contamination of wild yeast. However, microscopy does not ensure that you will detect contaminants; if you can see them it means that they are present in high number. A more secure and sensitive option is to use selective media for wild yeast and bacteria. These media are available either already made from or there are recommended methods such as those from the MOA (Methods of Analysis – ASBC) which describe how to make your own media.

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