Microbiological Examination of Beer

General Considerations

• All beers should be examined for the state of primary yeast (see topic 16) and the presence of bacteria and wild yeast after forcing.
• Clarity and aroma will indicate the soundness or otherwise of the beer.
• Plant cleaning and sterilising procedures should be considered in the light of the results of forcing tests.

Range of Values

• Beer should be visibly bright and of sound aroma after forcing.
• Primary yeast cells may be distorted, granulated, partly autolysed. or healthy.
• Ideally bacteria and wild yeast should be absent. The upper limit tolerated by each beer should be defined either as the numbet of organisms per field or per millilitre.

Operational Protocols

• A sample for forcing should be taken from every tank or fermentation vessel and should be forced at 27° C (81° F) for 7 – 14 days.
• Where infection is found, plant cleaning and sterilising methods should be carefully examined.
• Detergent concentrations should be checked after each cleaning cycle.

Measurement Protocols

• A microscopic examination of beer at rack will indicate cases of substantial contamination.
• Contaminating organisms at lower levels can be detected and enumerated by membrane filtration of beer and incubation of filters on either a general purpose medium or on a range of detection media (Institute of Brewing Recommended Methods and EBC Analytica Microbiologica, see Appendix 1).
• Forcing tests indicate the development of contaminating organisms while the beer is in trade. The beer is decanted and checked for gravity, clarity and aroma. The sediment is examined microscopically for primary yeast, bacteria and wild yeast.