Lesson 4 - Types of Microorganisms
In this lesson we cover organisms that occur in beer besides pitching yeast. Firstly, we look at speciality organisms involved in making traditional Belgian and wild beer styles. We then cover the spoilage bacteria, classified as Gram-positive and Gram-negative. Finally, we cover the wild yeasts as contaminants of beer processing and finished product.
Classification Of Beer Spoilage Organisms
When we discuss spoilage microorganisms in the brewing process, we are talking about a small sub-set of bacteria and yeasts and does not include moulds. We will discuss the various Gram-positive and Gram-negative beer spoiling bacteria and then later we will discuss the beer spoiling yeasts in detail.
Moulds
Moulds are not considered beer spoilers due to their inability to proliferate in the absence of oxygen but they can contaminate raw materials, especially if these are not stored dry. If the raw material does become contaminated, the products of mould infestation on brewing grains can cause gushing in beer or introduce the presence of carcinogenic mycotoxins.
Bacteria
Beer spoilage bacteria are characterised as being able to grow and flourish in beer, creating spoilage of flavour and clarity. The brewing environment presents a difficult environment for bacteria to proliferate. The presence of hop acids and low pH in the brewhouse are inhibitory. After boiling, wort is effectively sterile. During fermentation, oxygen is quickly depleted, pH is lowered further, and alcohol, carbon dioxide, and sulphur dioxide are formed. At the end of fermentation, the availability of nutrients is reduced and temperature lowered. All of these factors create hostile conditions, under which only a select few species can contaminate beer. The most dangerous for brewers are the Gram-positive lactic acid bacteria.
A review of bacteria in brewing can be downloaded from this site: