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on the yeast strain, once you get about five percent or higher you’ll see a significant decrease in sugar conversion as the yeast slows down a little bit.” Tolerances vary between strains, so it’s crucial for a distiller to know the strain they’re working with. Past a certain point, it’s possible to increase the sugar content of a mash by 30 percent but only increase yield by one percent. Proper nutrient levels and healthy yeast can mitigate this, but adding other stress factors will multiply the deleterious effects. “One of the things that’s true of any yeast stressor: by itself any one stress is bad, but when you combine it with the others, say you have alcohol stress combined with temperature stress combined with acid stress, it’s gonna be way out of control,” Furman said. Nutrients are invaluable not only in higher-gravity situations, but in all fermentations. Options that distillers can consider include yeast nutrient blends, specific individual nutrients such as DAP, or backset, which contains the remains of lysed yeast from a prior batch. A target range of 300 ppm free-amino nitrogen is a good place to start. Oxygen, as the driver for yeast cell growth and reproduction, is just as crucial. Yeast uses oxygen to create fatty acids and sterols to build cell walls. Furman recommends a target of 8-10 ppm of oxygen at the start of fermentation. For distillers running high-gravity fermentations, osmotic stress can cause sluggish fermentations. With too much sugar in solution, yeast has trouble absorbing it through the cell membrane, reducing efficiency and causing the formation of glycerol. Furman said that delaying the addition of glucoamylase until after yeast is pitched could help by reducing the amount of sugar immediately available, leaving less food for bacteria and lower osmotic stress on the yeast. “Basically, you control this by optimizing your glucoamylase dose and optimizing your mash. You want to have the right number of pounds of grain versus water to not shock your yeast.” Other, less common stressors can include dissolved CO2 lowering the pH, very tall and narrow fermenters creating atmospheric stress, high levels of fusel oils, and ions from chemical residue, such as sodium ions from a caustic cycle. A microscope is helpful to detect stress; distillers can look for evidence of budding and healthy, regularly-shaped yeast cells. “Basically, a good, reliable yeast culture is really what you want in any fermentation,” Furman said. “In order to get that, you’ll have to manage these yeast stressors. It can tolerate maybe one of these stresses, but not a combination of several. It’ll drastically reduce your distillery performance. It will cost you time and money.”

Gabe Toth is lead distiller at The Family Jones Spirit House in Denver and Loveland, CO. A former craft brewer, his passion for fermenting and transforming ingredients also extends to sausage and meat curing, cheesemaking, and pickling. He can be found up in the mountains or at gabetoth@hotmail.com. WWW.ARTISANSPIRITMAG.COM  

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Artisan Spirit: Summer 2018  

The magazine for craft distillers and their fans.

Artisan Spirit: Summer 2018  

The magazine for craft distillers and their fans.