Stir plate heating yeast starter!?

Hey all,

I'm looking for a bit of info. I did my first yeast starter today. Pitched it around 70 degrees, put it on my stir plate and left it alone. A few hours later I took a look and there were a whole lot of nice little bubbles rising up, and I was pretty happy to see it was all fermenting.

I touched the flask to reposition it and was shocked to see that it felt warm to my touch!

I took the temp and it was 90 degrees!

I felt the top of the stir plate, and I think the motor must be getting warm, because the metal top was warm too.

So...I guess a few questions:

Is this normal for stir plates to warm up that much? I got it used, but it's a really good quality lab "auto mixer."

Is there an easy way to maybe shield the metal top from the glass bottom of the flask?

90 degrees I'm assuming is way too hot for a starter, correct? Should I be shooting for more like 80?

Pretty much any advice, tips, anything to help out, I'd appreciate it. For now, I ran some cold water over the outside of the flask and left it off the stir plate till I figure this out.

After you're done with the starter do a test with tap water and your flask/jug and measure the temperature vs. time to see if it's really heating by immersing the thermometer tip into the water.

Cooling a starter with tap water to 70F takes a lot of patience, my guess is that it was >70F to start. I've measured 5-10F rise in starter temperature with a digital thermometer stuck under a stirred flask (sitting on a silicon hot pad).

I vented my stirplate build by drilling a lot of holes and did the test mentioned above and did not see any temperature rise in days.

Thanks, Dsmith. Only problem for me is I won't be able to really drill it out or anything. It's a pretty heavy duty lab stir plate with a metal top on a weight sensitive mechanism (it turns off automatically if there no weight on it.) I'd be afraid to mess with it too much.

I cooled the starter to around 83 degrees and put an oven mitt on the top of the stir plate. I doubt it will help much, but I'm giving it a shot.

Any other suggestions are more than welcomed, thanks so much!

you sure its not a hot stir plate? lots of labs use stir plates with heaters built in.. i would think there would be an on/off switch for it tho

I think the drilling was unnecessary but was easier to do before my build was assembled.

I recommend doing the experiment to see if the water temperature really does rise over time on the plate. It's much easier to measure when sanitation isn't a concern and when heat isn't being generated by the fermentation.

Yeah, definitely not a hot plate.

Dsmith, I'll do the experiment probably tomorrow. I feel like it's probably mostly the yeast causing the heat as they work, but I figured the stir plate wasn't helping matters.

At what temp should I start worrying that the yeast are dying Over 90?

You could also put a thin piece of styrofoam over the metal, to insulate the flask.

BigRob said:

You could also put a thin piece of styrofoam over the metal, to insulate the flask.

Click to expand...

I think I'll try that next. The oven mitt is working okay so far, but when I put two, it messes with the magnet too much.

I took the top off my stirplate just to show you what it looks like. I felt the sides of the stirplate, and they were hot too, so I'm guessing it's just the motor creating a lot of heat.

I have a newer model fisher scientific as well, it does heat up the yeast a little bit, but mine tops out in the 80s.

BigRob said:

I have a newer model fisher scientific as well, it does heat up the yeast a little bit, but mine tops out in the 80s.

Click to expand...

It's possible I didn't chill it down enough. I took the temp of the wort pre-top off, and it was around 80. Then I topped off with room-temp water, so it may have still been around 75-80 degrees, and then with the slight heat up from the stirplate and yeast activity, 90 seems about right...

Arrrg.

Yeast grows fine in the 80s and 90s, you're not drinking the starter right?

BigRob said:

Yeast grows fine in the 80s and 90s, you're not drinking the starter right?

Click to expand...

Hell yeah I'm drinking it! Just not for another month and a half, and not until it's mixed with some other stuff. :cross:

I know what you mean, and thanks for the heads up on the 90's being okay. I just wanted to make sure I didn't kill the poor little guys.

Either way, the oven mitts are doing the trick. Thanks everybody.

onipar said:

Yeah, definitely not a hot plate.

Dsmith, I'll do the experiment probably tomorrow. I feel like it's probably mostly the yeast causing the heat as they work, but I figured the stir plate wasn't helping matters.

At what temp should I start worrying that the yeast are dying Over 90?

Click to expand...

Did your stir plate generate heat stirring water?

I was making a starter last weekend (1L American Ale yeast) and measured the air temperature (72F) and the starter temperature (thermocouple on the side of a 2L flask with a cloth over it) vs time. The starter wort was cooled to 70F and the pic below shows the greatest temperature difference between the starter and the 72F air temperature. The starter temperature came down to the air temperature within 36 hours and I put it in the refrigerator at about 48 hours.

FWIW, yeast is grown in the lab around 30C/86F. They will start dying around 110-120F.

Dsmith, no I never did the water test. The day I brewed, my table tipped over and dumped the entire finished wort all over the driveway, so I've been sort of not in the mood to do anything brew related for a while because of that. Maybe this weekend.