Lesson 8 - Hygiene End of Unit SAQ

*** Due to poor planning, a standard lager brew with 15 IBU was cooled and aerated into a fermenter that was only rinsed before filling. The next yeast will only be available in 48 hours’ time. The previous brew in the vessel was a Weiss beer.

Discuss the types of microorganisms that could contaminate the wort and describe the ways in which they could affect the finished beer.

Response:

We must first note that the wort contains medium levels of hop iso-α-acids, a high relative pH (5.2-5.5), abundant simple and complex sugars, a diverse amino acid spectrum, and oxygen content.

Although oxygen levels may be 9 mg/L, this is not conducive to the survival of anaerobes, which typically grow at dissolved oxygen (DO) levels of less than 1.0 mg/L.

The hop resistant strains of the Gram-positive lactic acid bacteria can contaminate wort. Lactobacillus species are aerotolerant anaerobes and Pediococcus are anaerobic. However, any growth is likely to be very slow due to IBU levels of > 10 creating a degree of inhibition.

The most probable contaminants in the wort are the water borne Gram-negative, hop resistant Enterobacteriaceae. These are facultative anaerobes, meaning they can grow with or without oxygen. They metabolise wort sugars and survive at a high pH. The genera involved in wort spoilage are Enterobacter, Obesumbacterium, Escherichia, Klebsiella, and Citrobacter.

This group can grow very rapidly, with an optimal cell division time of 20 minutes, resulting in a million-fold increase in 7 hours.

Enterobacteriaceae inhibit the growth of brewing yeast and their own growth is only inhibited by a drop in pH and ethanol production from fermentation.

These bacteria produce DMS, diacetyl, acetaldehyde, esters, and lactic acid, which in turn produces turbidity and parsnip or vegetal and unpleasant sulphur flavours. They can also reduce nitrate to nitrite, which can react with wort amines to form nitrosamines (which are classified as carcinogens).

In a well-designed plant with correct CIP processes before wort cooling, Enterobacteriaceae are mainly likely to occur as a result of poorly maintained wort plate heat exchangers (PHE), where water is leaking from the cooling side into the wort. Modern PHE are designed to be operated with a lower water pressure to prevent this occurring.

The fermenter was only rinsed, and CIP was not used. This risks the growth of any remaining yeast cells from the previous fermentation. As this was a Weiss beer, a POF1 (phenolic off-flavour) positive yeast would have been used. These yeasts can produce the phenol 4-vinylguiacol or 4VG, from ferulic acid originating in malt. Other flavour active compounds such as 4-vinylphenol and styrene are also formed.