Lesson 8 - Hygiene End of Unit SAQ
This lesson allows you to test yourself on the material covered in the Hygiene unit, and determine the level to which you have attained the unit’s outcomes.
Suggested Response - Q4
*** Your brewery has just experienced a bad quality month. You have received microbiological trade complaints relating to three different batches. The complaints were related to beer that displayed the following characteristics:
Complaint 1 - A bottled lager, exhibiting turbidity and ropiness, with a strong butter aroma and sour taste.
Complaint 2 – A bottled ale, exhibiting turbidity and giving an unpleasant sulphuric and vegetal/parsnip aroma.
Complaint 3 – A traditional, barrel-aged strong ale with a sour taste and vinegary aroma.
Explain the likely cause of each contaminant and describe from where in the brewing process the contamination could have originated. In addition, describe how these contaminants exploit the brewing conditions in order to grow and spoil the beer.
Response:
Complaint 1: Ropiness in beer is formed by the Gram-positive lactic acid bacteria (LAB), particularly Pediococcus species. These bacteria also produce high levels of diacetyl, which has a distinctive buttery flavour and aroma. Pediococcus damnosus can be found in wort as they can exist in microaerophilic conditions. However, they are more often detected from late fermentation through to package.
They are able to grow under low temperatures and low pH (< 4.2) conditions. They produce lactic acid from carbohydrates, giving beer a sour taste. They do not occur in brewing raw materials. However, they can colonise the brewery environment, making them a serious threat to beer and its quality.
Complaint 2: These symptoms are typical of Gram-negative Enterobacteriaceae spoilage; a probable culprit is Obesumbacterium proteus. This bacterium is found in ale yeast, which it inhibits during fermentation, allowing the yeast to increase in number over successive yeast generations to reach a level where the beer is spoiled.
Obesumbacteria are facultative anaerobes, meaning they use a fermentative metabolic pathway, but can also grow in aerobic conditions. They are catalase positive, which enables then to break down the reactive oxygen species – hydrogen peroxide. As they are Gram-negative, they are insensitive to hop antimicrobials and flourish in the high sugar environment, producing DMS, organic acids, and 2, 3- butanediol, which gives the beer its vegetal character. As they are alcohol and pH sensitive, they are not found in finished beer.
Complaint 3: Barrel-aged beer is currently experiencing a renaissance. However, the inherent barrel process brings an increased risk of oxygen in the headspace as well as in the wood during the filling stage.
The acetic acid bacteria (AAB), Acetobacter and Gluconobacter, are Gram-negative, catalase positive, hop resistant obligate aerobes. This means that they will only grow in oxygen rich environments. They are very resistant to ethanol (up to 10% survival) and can also tolerate high levels of bitterness.
As the beer has a distinct vinegar aroma, this suggests that the primary contaminant is Gluconobacter oxydans rather than the Acetobacter species. Gluconobacter form a pellicle on the beer surface, producing acetic acid from the oxidation of ethanol. Acetobacter do the same but can also further oxidise the acetic acid to water and CO2.