Lesson 6 - Beer Flavour and Stability
In this lesson we will discuss beer flavour and beer flavour stability. This will include the effects raw material has on beer flavour, followed by a discussion on common taints, their flavours and consumer reaction. We will examine the flavour changes that occur during beer storage: focussing on oxidation reactions, the species of oxygen involved, and how we can utilise antioxidants to protect against aged beer flavour change. Finally, we will summarise the selection of raw materials and process optimisations available to improve beer flavour stability.
Microbial Contamination
Flavour changes from the growth of microorganisms are covered in the Hygiene unit (lesson 4). We will, however, briefly summarise some of the key impacts on beer flavour, which are often diverse because multiple flavour compounds are produced. The levels occurring in beer depend on the severity of the contamination as well as the style and baseline flavour that are characteristics to the beer.
Lactic acid bacteria
They produce lactic and other organic acids, leading to sourness, as well as diacetyl, which gives the beer a buttery slick taste and aroma.
Acetic acid bacteria
The chief product formed is acetic acid, better known as vinegar. In most beers, this will be considered as an off-flavour and provoke consumer rejection at levels exceeding taste thresholds (around 90 mg/L depending on the beer style and pH).
Zymomonas
Zymomonas products include acetaldehyde and hydrogen sulphide, which together contribute an unpleasant ‘rotten apple’ off-flavour.
Enterobacteriaceae
Enterobacteriaceae is a diverse group of bacteria produces DMS, diacetyl, acetaldehyde and lactic acid, giving flavours of parsnip and unpleasant sulphur to beer.
Pectinatus
Pectinatus bacteria produce unpleasant sulphurs such as hydrogen sulphide, DMS, methyl mercaptan, and various fatty acids. Flavours are described as being like rotten eggs and sour.
Megasphaera
Megasphaera produces hydrogen sulphide with its characteristic rotten egg aroma, as well as butyric acid with its vomit like aroma. It contaminates low alcohol beers with a flavour described as putrid.
Wild yeasts
Wild yeast produces smoky and clove like compounds such as 4-vinyl-guaiacol, as well as 4-ethylguaiacol and 4-ethylphenol, which are described as being like a barnyard, horse sweat and adhesive plasters.