Fermentation Primer Lesson VII
From Sugar to Alcohol
Fermentation Primer - Lesson VII
In last month’s lesson we discussed how scientists classify yeast and how they identify new varieties. In this month’s issue we shift our attention to some of the most common yeast varieties used by rum producers.
Key to fermentation
Q: Why did nature evolve more than one type of alcohol-producing yeast?
In a simple world, if you wanted to produce alcohol through fermentation, you would pick “the yeast” and add it to “the sugar source” and voila! We are, however, not living in a simple world, so our choices for yeast species and varieties are plentiful. But why is this the case?
Plants, like yeast, have evolved over millions of years. Modern fruit producing plants evolved at the end of the Cretaceous age, more than 125 million years ago. Their arrival brought to microbial communities a new, larger and increasingly-abundant source of food, based on simple sugars. But ancient yeasts could hardly produce the same amount of new biomass as their contemporary bacteria and could therefore be out competed. Scientists speculate that the slower growth rate of yeast was counteracted by their production of compounds that could inhibit the growth rate of bacteria, such as ethanol and acetate. Because the sources of sugars were varied (many types of fruiting trees and plants), yeast species and varieties adapted and evolved to be more efficient at living on or around specific plants or fruits.
The Saccharomycetaceae family covers over 200 million years of the yeast evolutionary history and includes six post Whole-Genome Duplication (post-WGD ) genera:
Saccharomyces,
Kazachstania,
Naumovia,
Nakaseomyces,
Tetrapisispora, and
Vanderwaltozyma.
It also includes six non-WGD genera:
Zygosaccharomyces,
Zygotorulaspora,
Torulaspora, Lachancea,
Kluyveromyces, and
Eremothecium.
chart of strands of yeast
The chart on the right shows a rough phylogenetic relationship among these genera.
Two significant events are known to have happened during its evolution: 1) WGD , which took place approximately 100 million years ago, and 2) the loss of Respiratory Complex I, which took place approximately 150 million years ago. The figure above was adopted from Hagman et al. (2013) by Dashko, Zhou, Compagno and Piskur (2014).
Concurrent with the evolutionary events described above (and perhaps as one of the driving forces behind them) fruit-producing plants adapted themselves to different climates, nutrients and soil conditions, resulting in fruits (sugar sources) with varied chemical compositions that further shaped their surrounding fauna. Yeast adaptation was and continues to be the “key” to its survival.
Many yeasts have been used by distilleries around the world. In this series we will focus on four of the most common:
• Saccharomyces cerevisiae
• Pichia kluyveri
• Lachancea thermotolerans
• Torulaspora delbrueckii
Each of the above species has many different varieties, each one offering traits that make it more specialized at handling unique environments or sugar sources. Of the four, Saccharomyces cerevisiae is the most studied and scientists have already identified over 1,000 genetic variants, highlighting its adaptability and versatility.
Join us again next month as we continue to explore this fascinating topic.