Caramelization is defined as the thermal degradation of sugars
leading to the formation of
volatiles (caramel aroma) and brown-colored products (caramel
The process is acid or base catalyzed and generally requires
temperatures > 120oC at 9<pH<3.
How is caramelization different from Maillard
Caramelization occurs in food, when food surfaces are heated strongly, e.g.
the baking and roasting processes, the processing of foods with high sugar
content such as jams and certain fruit juices, or in wine production.
Chemistry of the reaction
The generation of flavours and colours in thermally induced caramelization
requires that sugars, normally monosaccharide structures, to first undergo
intramolecular rearrangements. The reaction causes the release of
H+. Thus the pH of the solution undergoing caramelization falls
with time, eventually into the slightly acidic region of pH 4-5.
Caramelization occurs in a sequence of 6 steps:
The initial enolization reaction is of particular importance because it
initiates the subsequent chain of events. These reactions give rise to
aliphatic sugar degradation products which can react further to produce oxygen
heterocyclic and carbocyclic compounds via aldol condensation.
|Step 1: Enolization or de Bruijn van
|Step 2: Dehydration or b
|Step 3: Dicarboxylic Cleaving
|Step 4: Retro-Aldol Reaction
|Step 5: Aldol Condensation
|Step 6: Radical Reaction
The key intermediates of the thermal caramelisation are the
They are a -dicarbonyl compounds
such as 3-deoxyhexosulose. These not only lead to the formation of caramel
colour but also give rise to the important volatile products which are typical
of caramel flavour.
Which reducing sugar below has the highest rate of color
during caramelization browning?
Maltose Lactose Fructose Glucose
The rate of color development of reducing sugars decreases as the pH
decreases; in the case of sucrose, however, the situation is somewhat more
complicated owing to the acid-catalyzed sucrose hydrolysis, which produce
fructose and glucose.
Some typical components of caramel flavour are shown in Fig. 2.
are 4 general categories: furans, furanones, pyrones, and carbocyclics.
The relative proportions of these compounds can be influenced by the
temperature and whether the conditions are acidic or basic. While the
thermally and/or acid-induced dehydration and cyclisation reactions dominate,
there is an increased tendency for cleavage of the carbon chain of the sugar
under basic conditions. Thus, depending on the reaction conditions, one finds
derivatives of furan, such as hydroxymethylfurfural (HMF) and
hydroxyacetylfuran (HAF), furanones such as hydroxydimethylfuranone (HDF),
dihydroxydimethylfuranone (DDF) and the pyranones "maltol" from disaccharides
and "hydroxymaltol" from monosaccharides.
It has been proposed that a structure of the type CH3-CH=CH-CH(OH)-C=O is
responsible for the characteristic caramel flavour (i.e. this moiety is part
of the structure of HDF).