Lesson 7 - Microorganism Detection and Control SAQ

*** Discuss the factors affecting the ability of bacteria to grow in wort or beer. How do different species exploit the changing processing conditions to survive?

Response:

Wort and beer contain the essential nutrients that microorganisms need to grow. Brewing processing materials and conditions can make this difficult. However, the diversity of bacteria and yeast allows certain groups to exploit these conditions.

The factors promoting or restricting growth are:

• Carbon source
• Nitrogen source
• Presence of oxygen
• Absence of oxygen
• Temperature
• pH
• Alcohol

In the production process, we consider the following stages as growth opportunities for bacteria:

• Water addition
• Mashing
• Cooled wort / early fermentation
• End of fermentation/maturation
• Packaging

Brewing water opens the product up to contamination by coliforms. Intestinal coliforms such as E. coli should be absent and would indicate faecal presence. More commonly, free-living coliforms such as Klebsiella, Enterobacter and Hafnia species can develop in wort.

Other bacteria linked to contaminated brewing water (chilled liquor/dilution water), such as Pseudomonas, typically do not grow in wort or beer.  However, this genus produces biofilms that can harbour other beer spoilers and spore formers (Bacillus and wild yeast), increasing hygiene risks within the brewery plant.

During mashing, the microbial concentration decreases due to adverse heat and PH conditions. However, this does allow for the growth of niche organisms such as Lactobacillus, which produce lactic acid, are thermotolerant (heat tolerant), and grow at mash pH. This process is exploited for mash pH adjustment in Reinheitsgebot brewing, where acid addition is not permitted.

Other bacteria can grow in the mashing process, for example Bacillus, which can produce excess acid and also form nitrites from nitrate reduction. Clostridium species can grow in mash, forming butyric acid, which gives beer an unpleasant, ‘baby vomit’ smell.

Excessive bacterial growth can retard wort separation through viscous polysaccharide production. Any fungal growth can have the opposite effect, through β-glucanase production.

The wort boiling step effectively sterilises the wort, killing all microorganisms. The addition of hops inhibits Gram-positive bacteria, although some have developed resistance to hop acids through the horA gene.

In order to minimise the chance of spoilage, the nutrient rich wort is cooled as quickly as possible and pitched with yeast. If pitching is delayed, this supports opportunistic contamination by the Gram-negative Enterobacteriaceae, such as Klebsiella, Citrobacter, Enterobacter, Obesumbacterium, and Escherichia, together with that of the Gram-positive bacteria Lactobacillus and Pediococcus.

Enterobacteria are Gram-negative, hop-resistant, facultative anaerobes. They can thus grow in early fermentation but are inhibited by ethanol and the lower pH of later fermentation. O. Proteus and Enterobacter survive by associating with top fermenting brewing yeast; they reach levels of spoilage over successive repitchings.

The Gram-positive, hop-resistant LAB are anaerobic but are aerotolerant and ethanol tolerant. They can grow in both fermenting beer and packaged beer. They pose a major threat, producing sourness and diacetyl in beer.

The obligate anaerobes, Pectinatus and Megasphaera, can grow in unpasteurised, packaged beer with low dissolved oxygen (DO) content, producing unpleasant and sulphuric tastes, turbidity, and fatty acids.

Zymomonas exploits a traditional British ale practice, whereby sugar primings are added to the finished beer. This bacterium can withstand low pH and high alcohol; it uses the priming sucrose as an energy source to grow.