Ask Mr. Wizard

Always Question Your Instruments: III



I received this phone call a little over a year ago from a colleague. This sticks in my mind as a perfect example of a technical problem that is presented with just enough bias to lure the most suspicious fish:

I just had a question from a customer about a major problem with a whiskey fermentation. The mash was made of 45% distillers malt and 55% flaked corn, which the distiller boiled just to be sure the starch was gelatinized, the mash thickness was 3.2:1, and the mash was held at 152 °F (67 °C) for 90 minutes before cooling to 80 °F (27 °C) into the fermenter when yeast was pitched. The problem is that the fermentation seems to be over, but the final gravity is way high. The distiller is pretty sure the malt was low in enzymes. Any other ideas?


On the surface, this question looks like a no-brainer. Of course the enzymes in the mash were deficient, right? 55% enzyme-free adjunct, longer than normal mash at a moderate temperature perfect for producing highly fermentable wort. And the distiller even cooked the flaked corn just to be sure that the corn starch was gelatinized before bringing down to mash temperature and adding the malt.

The first part of this discussion began by talking about the malt. Distillers malt with a diastatic power (DP) of 260 and alpha amylase level (DU or dextrinizing units) of 60 was used for this mash. Was the adjunct ratio too high? At 55% adjunct, the blended DP was 117 and the blended DU was 27. A value of 60 is the lowest DP that brewers consider manageable and DU should be at around 30 at the minimum. Distillers will reach for exogenous enzymes when the mash is below these general guidelines. So no major concern with the DP in this mash, which is a measure of both the beta and alpha amylase activity. But the DU level, at 27, is a touch low. Too little alpha activity can limit starch breakdown during mashing. One viable suggestion for this and future mashes is to add some fungal alpha amylase to boost the alpha from the malt. But what did distillers do before this was an option? Hmmm, let’s keep that idea in the parking lot for the time being.

Here’s another legitimate question: Are the DP and DU values from malt specifications or from malt “COAs” (certificates of analysis)? The difference between the two designations is that a malt specification defines the typical analytical values for a malt type produced by a given maltster, whereas a malt COA is the actual laboratory data associated with a specific lot of malt. This is like guessing your kid’s height and weight based on your family’s history versus actually measuring your kid’s height and weight once they’re fully grown. In this case, we were looking at a malt COA. Furthermore, when the COA for this particular lot of malt was compared to previous lots of the same type, the lab values were similar. Probably not a problem with the values.

A healthy mistrust of instrumentation readings is a quick way of resolving the improbable.

Onto another usual suspect in fermentation issues, the yeast. Always a great question and in this case the distiller actually performed a forced fermentation to confirm that the final gravity was really it for this fermentation. Although not too common at home, forced/accelerated fermentations are easy to perform by simply over-pitching a wort or beer sample and fermenting the sample on a stir plate. The purpose of the method is to identify the final gravity ahead of the brew being fermented so that there is no guessing about the end of fermentation. In this case, what appeared to be the FG was indeed the FG.

Our advice ended up being pretty simple; drop an amyloglucosidase depth charge into this batch and move on. After all, this was a distiller’s wash and any differences between this spirit and others could be later blended. But the lingering question about what actually happened remained. Being the skeptic, I suggested delicately asking the distiller to consider shining a flashlight on the measured temperature. Blaming instruments can come across as pretty desperate, but sometimes the elephant in the room must be addressed. My colleague explained that this was a new operation with very high-zoot equipment that cost a small fortune. New, expensive pieces of kit normally are pretty trustworthy when it comes to instruments, so probably not the problem. With not having much more to add, it was time to move on to other business.

About two months pass and my phone rings while cruising around in Chicago while searching for a parking spot, and it’s the same colleague who brainstormed with me about the high FG wash. “Hey, Ashton, this is going to put a smile on your face. Remember the distiller? Well, his thermometer was way off and his 152 °F (67 °C) mash was more like 165 °F (74 °C)!” The primary moral of the story is to always question instruments. Always. And the secondary moral is to beware of bias introduced from questions. In this case, the distiller gave us his assessment along with the problem; sometimes the original assessment is the only thing considered and the problem is never solved.

Response by Ashton Lewis.