If you took the same juice and fermented it with different yeasts, why would they have different aromas or flavors and even unique mouthfeel and sensory characteristics? Why would one be slightly sweeter or more acidic? It’s all in the gene’s. 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 associates summarized some of the key yeast genes known to impact fermentation characteristics. These key characteristics include how yeast impact aromas and sensory characteristics including the following.
- Terpenoid Release – Terpenoids or sometimes referred to as terpenes are organic chemicals known for their antioxidant properties and their aroma producing ability. Some examples of aromas involving terpenoids are ginger, cinnamon, turmeric, and clove. Genes that release terpenoids will creates hard ciders with more of these preservative properties and aromas.
- Volatile Esters – Esters are the largest volatile compound impacting aromas and flavor. They are formed from organic acids and alcohols and can range from fruity and spicy to just unpleasant. Genes within a yeast strain define what aromas will likely be developed. You can look at beer yeasts and how saison variants will tend to produce phenolic esters while English ale yeast producers fruitier esters.
- Fusel Alcohol – Fusel alcohols are another volatile compound impacting aroma and flavor. Too much fusel alcohol can create negative perceptions but the addition of some fusel alcohols are important to aroma complexity and flavor. Genes support the creation of fusel alcohols. These are often part of the Ehrlich Pathway. They support oxidation and reduction reactions that convert fusel aldehyde to fusel alcohols or acids.
- Glycerol Production – Saccharomyces cerevisiae is more efficient at ethanol production than most other yeast genera. This efficiency is cause by the genes found in the yeast. Non-Saccharomyces cerevisiae yeast tend to be less efficient at ethanol. Instead of ethanol, they tend to produce glycerol. This is related the the genes they possess and how the yeast process the amino acids present in the juice to create glycerol. It’s good to remember that glycerol improves mouthfeel and increases residual sweetness.
- Acidity – Some yeasts can deacidify hard cider, which can be done by converting malic to lactic acid. Other can acidify cider by creating various organic acids, often lactic acid. Lachancea thermotolarens is an example of a yeast with genes that can create lactic acid and in some strains convert malic to lactic acid.
Here are some of the key aroma and sensory genes identified by V. K. Joshi and associates. I’ve used the UniProt.org site to note the roles these genes play in generating aromas and sensory characteristics.
- END1, EXG1, CEL1, BGL1, PEL5, PEH1, ABF2 – These genes impact the cell wall structure. Weaker cell walls will shed more compounds and generate more aromas and volatile esters.
- ATF1, IAH1 – These genes support the creation of volatile esters and fusel related products.
- ILE, LEU, VAL – These genes support the processing of amino acids.
- GPD1, GPD2, FPS1, ALD6 – These genes support the creation of glycerol.
- MAE1, MAE2, mleS, LDH1 – These genes impact the creation and processing of organic acids, like the conversion of malic to lactic acid.
This is another example of how yeast is such a critical element of your hard cider process. Wild or natural fermentations will offer a wide range of yeasts and therefore, genes that are capable to making all types of aroma and sensory variations to your hard cider. Some of those may be great while others may be undesirable. Knowing and understanding your yeast and the aromas and flavors they create will allow us to make great cider. This is why some people like to inoculate with known yeast. They want to have more control over the sensory characteristics of their hard cider. However, if you like your wild yeast, you could always capture and propagate it. Try harvesting your yeast if you like how a batch turns out.
(1) V.K. Joshi and associates, Science and Technology of Fruit Wines: An Overview, 2017
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