Lesson 8 - Fermentation Unit Final SAQ
You have now completed the Fermentation unit. This lesson provides the opportunity to test your knowledge with a last set of self-assessment questions.
Suggested Response - Q3
**Some brewing fermentations are spontaneous, using an inoculum made of mixtures of different yeast strains and bacteria. How do the metabolic potentials of the individual members of these mixed populations explain the sequence of events that are often seen?
Response
In the wild, all micro-organisms occur in mixed populations.
A source of nutrients and a particular set of physical conditions will allow some, but not all, micro-organisms to grow. Where these combinations occur regularly – as would be the case for a brewery where a particular fermentation medium, such as a cereal-based wort, is used – the local microflora in the environment would naturally become enriched in those organisms capable of growth.
In a spontaneous fermentation, the medium is naturally contaminated with a mixture of different microorganisms. Wort cooling is arranged to make sure this happens, typically in shallow vessels open to the atmosphere. Usually great care is taken to make sure that the environment surrounding the wort cooler is not disturbed so that the microflora is likewise unchanged.
The growth of a sub-set of micro-organisms on the wort will change the conditions by removing some nutrients, producing metabolic by-products, and changing the environmental conditions. These changes may allow the growth of another sub-set of the population which was not able to grow at first.
Continued changes may enable the growth of further sub-populations and in this way successive blooms of growth may be seen.
Aerophilic and alcohol-intolerant yeasts such as Hansenula, Candida, and Torulopsis grow rapidly at first but are rapidly overtaken by fermentative yeast.
Saccharomyces yeasts tend to quickly dominate the early stages of natural fermentations because of their ability to grow rapidly and change the environment in ways that prevent the growth of many potential competitors. Examples include acidification of the medium, rapid depletion of fermentable sugars, exhaustion of dissolved oxygen, and formation of alcohol. This combination deters many potential competitors.
In the later stages, when conditions have become more acid, lactic acid bacteria become dominant. The actions of these further acidifies the medium and prevents the growth of many other organisms.
In the late stages of the fermentation, Brettanomyces, which can tolerate the combination of low pH, anaerobiosis, and high alcohol, often predominates.