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Author Topic: QotW - 12.01.15 - Why does caramel cooking temperature rise in steps?  (Read 3723 times)

Offline thedoc

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Jeff Brewer asked the Naked Scientists:

Hi Chris,

I made several batches of caramels over the holidays, using a recipe that combines corn syrup, brown sugar, condensed sweetened milk and butter in a sauce pan. The mixture is heated over a medium heat for about 40 minutes until it reaches a final temperature of 244 degrees Fahrenheit. This year I was using a new digital thermometer and was surprised to observe that rather than rising at a constant rate throughout the entire cooking time, the temperature would rise steadily for a few minutes, then remain constant for several minutes and then rise again, repeating  several times while making each batch of caramels. I expected a slow, constant-rate rise in temperature from beginning to end. Why would the temperature rise so inconsistently?

Love the show,

Thanks

Jeff Brewer
« Last Edit: 09/01/2012 14:03:55 by BenV »


 

Offline thedoc

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We'll have the answer on the 15th January Naked Scientists Show.  Until then, what do you think?
« Last Edit: 09/01/2012 14:04:21 by BenV »
 

Offline eterman

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I hypothesize that there is an ongoing endothermic chemical reaction that stalls the continual rise of temperature.  The question is which of the ingredients are reacting and transform the sugars to caramel.
Eric Terman
 

Offline blind Pete

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Maybe a phase change.  Consider what happens when you melt ice.
 

Offline Sprool

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Combination of phase changes in ingredients plus energy absorbed during breakdown of starch and sugars will absorb heat energy during the process, so the temperature increase will not be steady during these transitions it will appear as a series of steps.
 

Offline Bored chemist

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Do you know how they set "medium heat" on an electric hob?
They switch the heating element on and off.
 

Offline RD

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Do you know how they set "medium heat" on an electric hob?
They switch the heating element on and off.

According to wikipedia several times a minute ...
Quote
Typically switchings have to be done several times a minute in an electric stove
https://en.wikipedia.org/wiki/Pulse-width_modulation

so maybe too frequent to account for the observed phenomenon. 
« Last Edit: 15/01/2012 18:14:09 by RD »
 

Offline Bored chemist

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Interesting. I'm sure my mum's cooker was slower than that (mine is gas so I can't check).
What happens if you record the temperature as it cools?
If it's a phase change it should be more or less reversible. If it's a chemical reaction then it should be effectively irreversible (because the lost water won't be replaced).
If it's a dodgy thermometer I won't be at all surprised.
How do you fancy repeating the experiment with, for example, a can full of tin or solder? That should give one, clear phase change at a known temperature (and then you can buy a new thermometer because you probably won't want solder in your caramel).
 

Offline Sprool

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does it state he used an electric hob?
 

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