Caramelization
During a trip to the woods, a group of students poured some
sugar onto the fire. This sugar soon produced a sweet smell and, we think, may
have turned into caramel. So, what is the process of caramelization?
Caramelization is the oxidation of sugar. It is a
non-enzymatic browning reaction. The reaction involves the removal of water and
the breakdown of sugars. The reaction depends on the type of sugar. Sucrose and
glucose caramelize at around 160˚C and fructose caramelizes at around 110˚C.
(3)
Caremelization often occurs when sugars are heated, either
dry or in concentrated aqueous solutions. When the sugar is heated, dehydration
and self-condensation reactions occur, producing volatiles (2-hydroxymethylfurfural),
pigments (melanoidines). (1)
(5)
The process of caramelization:
The process of caramelization:
Decomposition of sucrose –
Sucrose breaks down into glucose and fructose. This
decomposition occurs at around 180˚C.
Decomposition of fructose and glucose into aroma
molecules –
When fructose
and glucose start to break down into smaller, more volatile compounds, the
brain detects that new molecules are present. Some of the molecules produced are:
furans, diacetyl, maltol and ethyl acetate. The breakdown of fructose and glucose
into these molecules is acid catalysed. Warm solutions of sugar are more acidic
than cold solutions. Aromas will develop by starting the caramelization process
in water. If this is an acid catalysed reaction, aromas will form at a lower
temperature of water.
Oligomerization of fructose and glucose –
In the oligomerization reactions, the brown pigmentation and
the textures of caramel begin to form. The overall reactions occurring at this
stage are still unknown to chemists. However, recent research has shown that,
first, the individual sugars dimerize (two sugars come together to form one
molecule) into a new form, containing two rings attached by a third central
ring. When this happens to fructose, the structure formed is called
di-D-fructose Dianhydride. These difructose dianhydride molecules react further
on three different pathways:
·
One molecule loses 12 water molecules, forming
caramelan (C12H18O9).
·
A second type of molecule made by difructose
dianhydride is caramelen (C36H50O25).
·
Difructose dianhydride also forms caramelin (C125H188O80)
by the combination of two difructose dianhydrides and the removal of 27 water
molecules.
Chemists still do not have a firm idea as to what these
molecules are. However, they do know that they have ring structures, like
sugars, and that there are free radicals in the system, making the caramel
sticky. (2)
The caramelization process can also be looked at as:
·
Equilibration of anomeric and ring forms
·
The breakdown of sucrose to glucose and fructose
·
Condensation (two molecules combine to form a
larger molecule and lose a water molecule). (3)
(4)
·
Intramolecular bonding (interactions between one
molecule and a neighbouring molecule).
·
Isomerisation of aldoses to ketoses (one
molecule is transformed into another molecule containing the same atoms but the
atoms are arranged in a different structure).
·
Dehydration reactions (the loss of water
molecules from the reacting molecules).
·
Fragmentation reactions (breaking down radicals
into smaller ions or radicals).
·
Unsaturated polymer formation (3)
By Lauren Watmough
No comments:
Post a Comment