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Old 09-13-2012, 09:10 PM   #1
onipar
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Default 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.

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Old 09-13-2012, 09:19 PM   #2
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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.
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Old 09-13-2012, 09:24 PM   #3
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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!

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Old 09-13-2012, 09:29 PM   #4
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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

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Old 09-13-2012, 09:35 PM   #5
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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.

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Old 09-13-2012, 09:40 PM   #6
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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?

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Old 09-13-2012, 10:04 PM   #7
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You could also put a thin piece of styrofoam over the metal, to insulate the flask.

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Old 09-13-2012, 10:09 PM   #8
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Quote:
Originally Posted by BigRob View Post
You could also put a thin piece of styrofoam over the metal, to insulate the flask.
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.



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Old 09-13-2012, 10:10 PM   #9
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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.

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Old 09-13-2012, 10:38 PM   #10
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Quote:
Originally Posted by BigRob View Post
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.
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.
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