Sacharomyces Cerevisiae is the most commonly used yeast for wine, beer, bread, and cider maker. When Saccharomyces cerevisiae DNA was sequenced in 1996, there were around 6,000 genes identified. These genes, which are located in the 16 chromosomes, are what define and regulate biological information and pathways in the hard cider making process. For example, the ability of a given strain of yeast to ferment sucrose is defined by not just one but numerous genes. In research done by Wesley Marques and associates (1), they assessed the impact of 3 genes on sucrose fermentation. Genes AGT1, MALx1, and SUC2 were assessed through various experiments.
In one, AGT1, was deleted from a yeast strain that was also unable to break down sucrose through other channels. The result was the yeast could no longer grow on a sucrose rich media. This demonstrated AGT1’s impact as a key transporter of sucrose into the yeast cell. If you wanted to only ferment fructose and glucose and leave all the sucrose in your hard cider, this type of modification would enable that.
They also tested it on a yeast that can process maltose through the MALx1 gene. While maltose isn’t naturally found in hard cider. The MALx1 gene can still impact the cider fermentation process. The research found that The MALx1 gene could process sucrose but at a lower level. The result was that the yeast consumed less overall sucrose resulting in more residual sugar and lower alcohol. This demonstrated that there are other pathways besides AGT1 that can process sucrose, though not as effectively. This modification could be a means of having some naturally residual sweetness in a hard cider.
The last experiment was the elimination of the encoding sequence in the SUC2 gene that allows sucrose to be broken down outside of the cell. It forces the yeast to only process the sucrose internally. This was done to increase the production of ethanol. In other words, they were able to make the yeast a more efficient producers of ethanol. This genetically engineered strain of Saccharomyces cerevisiae could produce 11% more ethanol from the same amount of sugar. This yeast would enable you to have a naturally higher %ABV.
These experiments showed the importance of genes that support the transportation of sucrose within the yeast. They also highlight the adaptability and evolutionary capacity of yeast and bring more understanding to the mutated condition of many beer yeasts due to selective cropping and reuse over centuries. Finally, it offers us interesting means to customize how yeast function and the resulting hard ciders we could develop.
(1) Wesley Leoricy Marques and associates, Sucrose and Saccharomyces cerevisiae: a relationship most sweet, FEMS Yeast Research, 2016, Vol. 16, No. 1.
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