Fermentation Primer Lesson 12
In last month’s lesson we discussed Torulaspora delbrueckii and its potential to be an alternative to Saccharomyces cerevisiae. With this month’s lesson we conclude this informative series by taking a look at the world of continuous fermentation.
Most distillers, small and large, set out to ferment one vessel/tank at a time, transferring the fermented wash/wine from the tank to the still when fermentation is complete. This process is known as “batch” fermentation, because a new “batch” has to be started every time a fermentation tank is emptied.
Q: What happens when fermenting a batch takes considerably longer time than distilling it?
For most distillers, the answer is to have multiple fermentation tanks, each started at a slightly later time than the others. For example, if it takes one shift to distill a tank that has taken 5 days to ferment, then the distiller must have 5 fermentation tanks and must start fermenting a new tank each day. This approach then allows for a distillation shift every single day of the week. What happens if the distillery has to distill two or even three shifts per day? The number ot tanks required goes up proportionally, requiring 10 or even 15 different tanks in this scenario.
Alcohol plants with continuous distillation columns face a similar problem: they must ensure the constant supply of fermented wash to feed their columns, which typically operate 24 hours per day during production season.
Enter Continuous Fermentation
Continuous fermentation, as its name implies, refers to a process designed to address the above scenarios. In continuous fermentation systems, a fresh supply of fermentable liquid is injected into one end of a series of tanks while, on the opposite end, fermented liquid is taken out and transferred to the distillation apparatus (pot or continuous column).
Unlike batch fermentations, which are usually carried out until all the sugar has been processed and which -as a result- often result in yeast death (due to lack of food), continuous fermentations must keep the yeast alive, so that it can be re-distributed across the system, to help ferment the incoming sugars.
Cascade Implementation
The diagram depicts a cascade implementation, with fresh wash being introduced (1) into the system, with interconnected fermenters (5) and a final holding tank (7) that supplies the still. A pump (6) moves yeast-rich liquid back, usually involving a sterilization step, where the yeast is treated with chlorine dioxide or other similar agents, which kill bacteria without damaging the yeast cells.
If continuous fermentation sounds like the perfect solution, why aren’t all distillers employing this approach?
The answer is that, despite their apparent ease to operate, continuous fermentation systems require a great deal of technical savvy, to constantly monitor environmental conditions and yeast health throughout the system. The biggest challenge when operating this type of system is the introduction of contaminating or infectious bacteria which, if left unchecked, could ruin not only one fermenter, but the contents of the entire sytem!
Another disadvantage of continuous fermentation is that, as described earlier, yeast must be kept alive, which means that sugar in the medium is never allowed to drop to zero. For this reason, the liquid taken out for distillation is never fully fermented (always has residual sugars). Batch fermentations, on the other hand, tend to have lower residual sugars, so they are more efficient in that regard.
Final Words
Fermentation
If you’ve been with us since Lesson I, you’ve now emerged from a “deep dive” into the world of fermentation, including yeast evolution, morphology and an overview of commonly-used yeast species and varieties in the alcohol industry.
Which yeast and approach should you use will depend on the equipment you have available, your commercial needs and your environmental conditions. Regardless of which combination of these apply to you, remember that the keys to fermentation success are documentation, instrumentation and perseverance.
Cheers!
The Rum University