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.
Point of Reflection - Reconvene
Welcome back. So what were your thoughts?
There are a number of Gram-negative bacteria that can spoil or potentially spoil beer. However, with improved dissolved oxygen and hygiene control the aerobic acetic acid bacteria, such as Acetobacter and Gluconobacter, have become less of a risk in many modern breweries. The risk does remain high, however, in situations where beer is in greater contact with oxygen, such as cask beer, badly managed draught dispense, and those operations that have poor beer hygiene standards.
Zymomonas contamination is still found in traditional primed cask conditioned ales, which provide a sugar source post package for growth.
The obligate anaerobes Pectinatus and Megasphaera have become more prevalent spoilage organisms as dissolved oxygen control has improved and unpasteurised beer and the case of Megasphaera low alcohol beer have become more popular. They can survive modern cleaning protocols in biofilms and gain access through the filling process, producing intense off flavours, and as such should be considered the most dangerous of Gram-negative spoilers in modern brewing operations focussed on dissolved oxygen reduction for shelf life flavour stability.
Obesumbacterium proteus is found in traditional ale production systems where they associate with the top fermenting yeast.
The Gram-positive hop resistant lactic acid bacteria provide a bigger risk for breweries that do not pasteurise their beer such as traditional smaller and craft breweries. Nevertheless they can spoil beer in process in any site where strict hygiene protocols are not in place, and are the main cause for beer spoilage globally. Increases in diacetyl levels in maturation are often caused by Pediococcus damnosus, which exploits its ability to grow at low temperatures.