Ask Mr. Wizard

Always Question Your Instruments: I


Tom Winiecki — Boulder, Colorado asks,

I made a yeast starter two days before using 1.5 l of water and 151 g of dried malt extract (DME) (about 3 pints water to 3⁄4 cup DME). After cooling the starter wort and adding the yeast (WYEAST 1098), I put it on a stir plate for 18 hours and then into the fridge. I brought it up to room temperature on brew day, decanted the liquid and pitched.

The recipe said to ferment at 68 °F (20 °C), so that’s what I set my Inkbird/Fermwrap to. Within minutes, the temperature shot up to 85 °F (29 °C). I know that temperature rises during fermentation, but is this rapid increase normal and/or a sign of healthy yeast?


My column usually lacks a theme within a single issue and my answers tend to be long, but this short answer is going to contribute to a theme in this issue about trust, as in don’t blindly trust what your instruments are telling you.

To recap your question, your measured wort temperature quickly jumped to 85 °F (29 °C) after fermenter filling and yeast pitching, and you want to know if this makes sense. The healthiest and hungriest yeast pitches are simply incapable of very quickly increasing wort/beer temperature from 68 °F to 85 °F (20 °C to 29 °C). There is a way to estimate the heat of fermentation using ASHRAE (The American Society of Heating, Refrigerating, and Air-Conditioning Engineers) data and if anyone out there wants to send in a question about this, ask! But for now, I am going to bunny hop over this rabbit hole. Perhaps the blended temperature of your wort was around 85 °F (29 °C) and you did not discover this until your Inkbird was turned on.

I tend to be a skeptic about observations that don’t seem to make sense.

When an observation seems to be impossible, for example when a pot of water is put on the stove and the measured temperature jumps from 70 °F to 170 °F (21 °C to 77 °C) in a matter of minutes, the likely cause is a measurement error, such as a temperature probe in contact with the vessel surface or a wonky thermometer, instead of an incredible heating rate. A healthy mistrust of instrumentation readings is a quick way of resolving the improbable.

There are numerous scenarios that could explain what happened to this brew, but running through a list of what-ifs is probably not going to help you much in the future. I am a pipe toucher when it comes to process walks in breweries and food processing plant . . . I quickly touch pipes, tank tops, and vent stacks to get a sense of what is happening. This is no joke. A hand tapped on a surface is very revealing about what is happening in the thing being touched. I think I learned this from watching my dad touch the window of the car on road trips before the advent of the modern automobile cockpit. Back in the old days, a finger on the window was a pretty good indicator of ambient temperature. The 20th century human also used this crude temperature measuring method to estimate the temperature of bottles and cans of beer. Since beer bottles/cans in a cooler of ice are often cold on the surface, but warm in the middle, some folks resorted to shaking cans/bottles to equilibrate the sample for a better reading; this of course lead to the awesome drinking game called Beer Hunter.

Response by Ashton Lewis.