Fermentation Primer: Lesson I

Q: What is Fermentation?

The simplest answer is that fermentation is the transformation of sugars into alcohols by yeast or other micro organisms. In reality however, entire industries exist thanks to fermentation and what it is capable of producing in the food, beverage, biofuel and medical fields. Enormous budgets are spent each year worldwide, studying its processes and searching for ways to improve them. The study of fermentation is so important that it even has its own name: zymology. 

Despite having a science dedicated to its study, many questions about fermentation still remain unanswered. Questions such as:

• Why did yeast develop the ability to ferment sugars into alcohol in the first place?

• What is the evolutionary future of yeast?

For us in the distilled spirits industry, fermentation represents a key transformation of raw materials: neither our distilleries nor our products would exist without it!

In order to fully understand the importance and impact of fermentation in rum production, we must start by taking a closer look at the organisms responsible for it: yeast.

Yeast are a species of single-celled organisms from the fungi kingdom. Currently there are about 1,500 species of yeast identified, most of which are in the phylum Ascomycota (also known as “sac fungi” for their sac-like structure), only a few being Basidiomycota. Yeast are found worldwide in soils and on plant surfaces and are especially abundant in sugary mediums such as flower nectar and fruits. There are hundreds of economically important varieties of ascomycete yeast; the types commonly used in the production of bread, beer, and wine are selected strains of Saccharomyces cerevisiae. Some yeast are mild to dangerous pathogens of humans and other animals, especially Candida albicans, Histoplasma and Blastomyces. 

The sac fungi are separated into subgroups based on whether asci (the sexual spore-bearing cell) arise singly or are borne in one of several types of fruiting structures, or ascocarps, and on the method of discharge of the ascospores. Many ascomycetes are plant pathogens, some are animal pathogens, a few are edible mushrooms, and many live on dead organic matter (as saprobes). The largest and most commonly known ascomycetes include the morel and the truffle. Other ascomycetes include important plant pathogens, such as those that cause powdery mildew of grape (Uncinula necator), Dutch elm disease (Ophiostoma ulmi ), chestnut blight (Cryphonectria parasitica), and apple scab (Venturia inequalis). But perhaps the most indispensable fungus of all is an ascomycete, the “common yeast” (Saccharomyces cerevisiae), whose varieties leaven the dough in bread-making and ferment grain or sugars to produce alcoholic beverages.

One of the most prominent features of S. cerevisiae is its ability to rapidly convert sugars to ethanol and carbon dioxide at both anaerobic and aerobic conditions. Under aerobic conditions, respiration is possible with oxygen as the final electron acceptor, but S. cerevisiae exhibits alcoholic fermentation until the sugar reaches a low level, thanks to its glucose repression mechanism. This phenomenon is called the Crabtree effect, and the yeast expressing this trait are said to be “Crabtree-positive”. 

On the other hand, “Crabtree-negative” yeast lack fermentative products and, under aerobic conditions, biomass and carbon dioxide are their only output. It is interesting to point out that science does not yet know if the glucose repression mechanism was the original step to promote evolution of the Crabtree effect, or if it is the result of the evolution of some yeast lineages.

The Crabtree positive yeast, such as Saccharomyces cerevisiae, prefer fermentation to respiration, even under fully aerobic conditions. The selective pressures that drove the evolution of this trait remain controversial because of the low ATP yield of fermentation compared to that of respiration (ATP or Adenosine triphosphate is a molecule that plays a key role in metabolism, particularly in energy transfer within cells). The rate advantage of fermentation over aerobic respiration is insufficient to provide an overall growth advantage. Thus, the rapid consumption of glucose and the utilization of ethanol are essential for the success of the aerobic fermentation strategy for the yeast, suggesting that selection derived from competition with bacteria could have provided the impetus for the evolution of the Crabtree positive trait.

Join us again next month, as we continue our deep-dive into this fascinating world!