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General Science => Question of the Week => Topic started by: thedoc on 09/01/2012 14:02:17

Title: QotW - 12.01.15 - Why does caramel cooking temperature rise in steps?
Post by: thedoc on 09/01/2012 14:02:17

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

Title: {question_type:eq:QOTW:QotW - {pubdate:date:%y.%m.%d} - :}{tx_naksciforum_title:istrue:{tx_nakscifor
Post by: thedoc on 09/01/2012 14:02:17

We'll have the answer on the 15th January Naked Scientists Show.  Until then, what do you think?

Title: Re: QotW - 12.01.15 - Why does caramel cooking temperature rise in steps?
Post by: eterman on 09/01/2012 14:54:36

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

Title: Re: QotW - 12.01.15 - Why does caramel cooking temperature rise in steps?
Post by: blind Pete on 11/01/2012 02:25:28

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

Title: Re: QotW - 12.01.15 - Why does caramel cooking temperature rise in steps?
Post by: Sprool on 13/01/2012 16:06:56

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.

Title: Re: QotW - 12.01.15 - Why does caramel cooking temperature rise in steps?
Post by: Bored chemist on 15/01/2012 12:51:03

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

Title: Re: QotW - 12.01.15 - Why does caramel cooking temperature rise in steps?
Post by: RD on 15/01/2012 18:04:42

Quote from: Bored chemist on 15/01/2012 12:51:03

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. 

Title: Re: QotW - 12.01.15 - Why does caramel cooking temperature rise in steps?
Post by: Bored chemist on 15/01/2012 20:51:22

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).

Title: Re: QotW - 12.01.15 - Why does caramel cooking temperature rise in steps?
Post by: Sprool on 06/02/2012 12:27:47

does it state he used an electric hob?