HomeRoast Digest


Topic: "How does sugar carmelize?" (5 msgs / 116 lines)
1) From: Dan Bollinger
I think we bypassed the obvious and stuck too close to coffee roasting when
discussing exothermic reactions.  Taking three steps backward I realized
that the human body stores energy in the form of sugar and oxidizes this to
produce the energy we need to live.  So I went looking for 'sugar'
reactions. Here are just a few of the items I found.
I found this on the Ask-a-Scientist site.  The question was, "How does sugar
carmelize?"
"When sugar (sucrose, C12H22O11) is gradually heated in air it melts into a
clear liquid. Soon afterward (as the temperature is increased) the sucrose
molecules begin to decompose -- first by dehydration (loss of water) and
then later by breaking the bonds between carbon atoms. This latter process
is complex -- partially oxidative and partially degradative. The degradation
products are likewise complex in the variety of intermediate compounds
formed.
It is at this point that caramelization is occurring. The molecular species
that form dissolve in and are dissolved by the molten sugar. The result is a
tasty, brownish syrup which, if further heated, will thence decompose into
some rather unpleasant (bitter tasting) materials. When heating proceeds to
the maximum, the result is a rather crystalline form of impure carbon in
which is dissolved traces of decomposition products. Were it possible to do
the heating in the absence of oxygen, sugar decomposes into carbon and
water." Prof. Hoff
Professor Hoff says that the carmelization of sugar is, in part, oxidative.
We all know that one of the main constituents of coffee beans is sucrose and
that the browning of a coffee bean during roasting is the degree of sugar
decomposition. So, is the oxidation of sugar exothermic?  Let's see:
This webpage says that oxidation of sugar results in the release of energy
and that the energy released is eventually the same regardless of how small
or how tiny the steps are.http://www.accessexcellence.org/AB/GG/cell_Oxid.htmlThis page concurs and says that "vast amounts of heat are release when sugar
oxidizes."http://www.woodrow.org/teachers/chemistry/institutes/1988/gummybear.htmTo me this is sufficient evidence that at least the carmelization of sucrose
in coffee beans during roasting is exothermic and apparently not an
insignificant amount of energy.  Dan
homeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast

2) From: Jim Schulman
Dan, good point. But ... 
When I caramelize sugar in a pan, the reaction never becomes self-sustaining, the heatloss is 
too so great the generated heat cannot compensate. Although, just like in a roast, one has to 
back off on the heat, if one wants the browning to be gentle and controlled.
There are also Maillard reactions, that is, the combination of sugars with amino acids, 
peptides, and proteins. I have no idea what the energy transfers involved in these reactions. 
They may "soak up" the energy from sugar combustion.
Jim
On 11 Sep 2002 at 20:14, Dan Bollinger wrote:
<Snip>
homeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast

3) From: Dan Bollinger
My Masters of Science in Biology step-daughter says that an exothermic
reaction doesn't have to be self-sustaining.  Dan

4) From: C. Marley
Dan Bollinger wrote:
<Snip>
But the definition of exothermic is that it gives off more heat than it
takes in.  Any time I have caramelized sugar, I've had to keep the gas
under it or it stops caramelizing.  It only gives off more heat than it
takes in when it turns black and catches fire.  Also in the body, the
Krebs cycle oxidizes sugar catalytically, and stores the energy of those
reactions in a high energy bond of ATP (adenosine triphosphate). The
series of reactions transfer energy but are not exothermic. 
-- 
For the conservation of the Tibetan Lhasa Apso,
Regards, Cathy http://lists.sweetmarias.com/mailman/listinfo/homeroast">http://www.lhasa-apso.orghomeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast

5) From: Dan Bollinger
<Snip>
Of course, it stops because you removed heat and cooled it below the minimum
temperature required for the reaction.  That's how they put out fires, too.
Add water and cool the reaction to a point where it can no longer react.
homeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast


HomeRoast Digest