Ever since we started making whisky in 2018, I've had a fermentation problem that appears without warning and then disappears again. Seven years later, I think I might finally be getting close to the answer.
All whisky starts life as an unhopped, unpasteurised beer known as wash.
Some distilleries outsource this part of the process to a brewery. We always knew we wanted to keep it in-house because it allows us to use our own spring water and run open fermentations. There are downsides though. I often joke with visitors that around 80% of the distillery infrastructure and 99% of the mess comes from the brewing side of the operation!
Ever since we started making whisky in 2018, I've had a sporadic fermentation problem that I've never quite managed to get to the bottom of.
The strange thing is that it doesn't happen all the time.
Sometimes the yeast performs perfectly, consuming virtually all the sugar available and producing an excellent alcohol yield. Other times it seems to leave more sugar behind than it should. The result is lower efficiency and, more importantly, an unanswered question that has been irritating me for years.
To understand the problem, we first need a little background.
After mashing, I measure the sugar content of the sweet liquid, known as wort. This is called the Starting Gravity. Once the yeast has finished its work, I take a second reading called the Final Gravity.
When everything goes well, the Final Gravity drops to around 1001, indicating the yeast has consumed almost all available sugar.
When the problem occurs, the Final Gravity remains significantly higher, suggesting the yeast has left valuable sugar behind.
The challenge is that there are many possible causes.
Over the years I've considered:
• Cleaning and sanitation issues
• Ferments getting too hot and stressing the yeast
• Ferments getting too cold and slowing the yeast down
• Problems rehydrating the dried yeast
• Unfermentable sugars created during mashing
What has made diagnosis particularly difficult is that the issue appears sporadically. A ferment can behave perfectly one week and then underperform the next.
For a long time I suspected temperature. Then I ruled it out because the problem occurred in both winter and summer.
After years of eliminating other possibilities, I've found myself coming back to temperature once again.
My current theory is that it's not the daytime temperature that matters most, but the temperature swing between day and night.
Three factors influence the temperature inside a fermentation:
• The temperature of the wort when it enters the fermenter
• The heat generated naturally by the yeast itself
• The ambient temperature inside the distillery
What if a cool night allows the ferment to shed enough heat to keep the yeast operating in its ideal range?
Or conversely, what if a series of warmer nights prevents that cooling and causes the yeast to become stressed?
The next step is to stop guessing and start measuring.
I'll be logging fermentation temperatures throughout the entire cycle and combining that data with historical weather information and production records. I'm even planning to throw some AI at the problem to see whether it can identify patterns that I've missed.
It's one of those reminders that even after seven years of making whisky, the process still has plenty of mysteries left to solve.
I'll let you know what we discover.