Why do some yeast die during fermentation or only ferment certain types of sugars? Have you ever thought about why some yeast produce hydrogen sulfide (rotten egg smell) and others don’t? It’s all in the genes. As discussed in other Mālus Trivium posts, the Saccharomyces cerevisiae yeast DNA genome was sequenced in 1996 and about 6,000 genes were identified. In the book Science and Technology of Fruit Wines: An Overview(1), V. K. Joshi and associated summarized some of the key yeast genes known to impact fermentation characteristics. These key characteristics include how yeast manage various elements of fermentation including the following.
- Stress Resilience – Stress can be caused by many different factors including enzymes, sugar, sulfur dioxide, ethanol, and nutrient levels to name a few. The genes a yeast strain has, define how well it is equipped to adapt and survive in these conditions.
- Sugar Utilization – You may think yeast will process all the simple sugars in your juice. While this is generally true about Saccharomyces cervisiae strains, the world has many other types of yeast including Lachancea thermotolerans and Pichia kluyveri, which are known to not process all the simple sugars. Genes define what sugars a yeast can process and how efficient that process will be.
- Nitrogen Utilization – A key compound for yeast growth but some yeast produce hydrogen sulfide if there is too little or too much nitrogen. Also, yeast have preferences for where they get the nitrogen they need. This is demonstrated in the amino acids that are initially broken down by different yeast strains. The genes a yeast have define how it wants to process nitrogen as well as how it reacts when it has too much or not enough.
- Ethanol Tolerance – Ethanol tolerance is often quoted as a reason many yeasts can’t fully ferment all the sugars in a juice. While it may not be as simple as how much ethanol is present, there are definitely genes the make yeast more tolerant or resistant to increased levels of ethanol.
- Cell Wall Structure – It may not be apparent but the genes a yeast has will impact it’s cell wall and membrane structure. Why would this impact fermentation? Yeast have to transport compound through their cell walls and membranes to both remove and assimilate what it needs to survive. Yeast don’t have a mouth like humans and animals to eat. Instead, they absorb what they need and then remove compounds that are toxic, like ethanol. Their genes are what allow them to transport compounds across the cell wall or membrane.
Here are some of the key fermentation genes identified by V. K. Joshi and associates. I’ve used the UniProt.org site to note the roles these genes play in the fermentation process.
- GPM1, HXK1, GSY1, GSY2, TPS1, TPS2 – These are glycolytic genes. Glycolytic means they breakdown glucose (sugar).
- HXT1-HXT18, SNF3, FSY1 – These genes transport hexose across the cell walls and membrane. Hexose are simple sugars like glucose and fructose so these genes are what allow the yeast to consume sugar by bringing inside.
- PUT1, PUT2 – These genes defines how amino acids are broken down. Amino acids are proteins that are linked together in long chains. They contribute nitrogen and other key compounds to the fermentation process.
- PMA1, PMA2 – These genes transport hydrogen ions across the cell wall and membrane. Hydrogen is a common method of transferring electrons that are needed to enable various cellular processes.
- SPR1, SWP1, MNN10, YPS7 – These genes define the cell wall structure. Besides defining how compounds are transported, it also defines how easily the walls shed key compounds like mannoproteins.
- FRO1, FRO2 – These genes catalyze NAD, which is key to yeast metabolism. NAD transfers electrons during the various chemical reactions becoming NAD+ and NADH depending on whether it is collecting or donating an electron.
Genes, we all have them, even yeast. For yeast, their genetic make up defines how they produce hard cider and their fermentation characteristics. There are thousands of yeasts in the world and they all have different capabilities so learn more about yeast genetic characteristics or at least don’t be afraid to try different yeasts to see how they perform in your hard cider. You might be pleasantly surprised.
(1) V.K. Joshi and associates, Science and Technology of Fruit Wines: An Overview, 2017
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