Fermentation Primer- Lesson VIII
From Sugar to Alcohol
Fermentation Primer - Lesson VIII
In last month’s lesson we discussed how the evolution of fruit-producing plants may have been one of the driving forces behind the proliferation of new yeast types, each tailored to take advantage of specific climate and food source combinations.
Q: If all yeast evolved the ability to produce alcohol to inhibit the growth of competing bacteria, do all yeast species and varieties get along with each other?
While it is possible for some yeast combinations to co-exist in seemingly harmonious ways, some of them have also evolved the ability to produce killer toxins aimed exclusively at killing other yeast species and varieties.
These killer toxins (generally proteins or glycoproteins) are polypeptides that kill sensitive cells of the same or related species, often functioning by creating pores in target cell membranes. Just like with the production of ethanol, the yeast cells that secrete the toxins also have a built-in immunity to them, a fact that is being researched by scientists interested in synthesizing the toxins as effective inoculants for fermentations.
During the last two decades, secreted killer toxins and toxin-producing killer yeasts have found several applications. For instance in the food and fermentation industries, killer yeasts have been used to combat contaminating wild-type yeasts which can occur during the production of wine, beer and bread. Killer yeasts have also been used as bio-control agents in the preservation of foods, in the bio-typing of medically important pathogenic yeasts and yeast-like fungi, in the development of novel antimycotics for the treatment of human and plant fungal infections and finally in the field of recombinant DNA technology.
Yeast Cells
Pichia kluyveri
According to “The ecological role of killer yeasts in natural communities of yeasts” (Starmer, W. T., P. Ganter, V. Aberdeen, M. A . Lachance, and H. J. Phaff. 1987), the distribution of killer toxin-producing yeasts in naturally occurring yeast communities have shown that such yeasts are relatively common, especially in decaying fruit, in which killer toxin-producing strains make up 27% of the yeast community.
One common fruit yeast, Pichia kluyveri, was found to display genetic variability for its ability to kill other yeasts and to be polymorphic for this trait in natural populations. Observations on the co-distribution of P. kluyveri and other yeast species suggest an ecological role of this killer species in excluding other yeasts from particular communities.
Pichia kluyveri has been used in the production of low alcohol or alcohol free beers, thanks to its fast glucose fermentation rate and its ability to produce positive flavor compounds. Some light/neutral alcohol distillers have also used this species to increase the production of higher alcohols from residual glucose or when market conditions demand it.
Yeast activity graph
According to a research paper published by the American Society for Microbiology, some chocolate producers have also been fermenting cocoa beans using P. kluyveri since the congeners produced by it are considered desirable in that industry, adding to the depth and richness of the final product (Meersman, Steensels, Struyf, Paulus, Saels, Mathawan, Allegaer t, Vrancken and Verstrepena, 2015).
Join us again next month as we continue to explore this fascinating topic.