BLBS102-c04 BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come
68 Part 1: Principles/Food AnalysisHC O HC OH
HCOH HC OHHCOHHOCH HOCH
HCOH
HCOH
CH 2 OHHCOH
CH 2 OHGlucose 1,2-EnodiolCHO–H 2 O –2H 2 O
H 2 C CHOHMFOHCO
CH 2
HCOH
HCOH
CH 2 OHOFigure 4.9.1,2 Enolization and formation of hydroxymethyl furfural (HMF).glucose and sucrose when a combination of phosphoric acid and
pyridine is used as catalysts than when phosphoric acid is used
alone (Fennema 1976).
In alkaline media, dehydration reactions are slower than in
neutral or acid media, but fragmentation products such as ace-
tol, acetoin, and diacetyl are detected. In the presence of oxy-
gen, oxidative fission takes place, and formic, acetic, and other
organic acids are also formed. All of these compounds react
to produce brown polymers and flavor compounds (Olano and
Mart ́ınez-Castro 2004).
In general, caramelization products (CPs) consist of volatile
and nonvolatile (90–95%) fractions of LMWs and HMWs that
vary depending on temperature, pH, duration of heating, and
starting material (Defaye et al. 2000). Although it is known that
caramelization is favored at temperatures higher than 120◦C and
at a pH greater than 9 and less than 3, depending on the com-
position of the system (pH and type of sugar), caramelization
reactions may also play an important role in color formation
in systems heated at lower temperatures. Thus, some studies
have been conducted at the temperatures of accelerated storage
conditions (45–65◦C) and pH values from 4 to 6 (Buera et al.
1987a, 1987b). These authors studied the changes of color due
to caramelization of fructose, xylose, glucose, maltose, lactose,
and sucrose in model systems of 0.9awand found that fruc-
tose and xylose browned much more rapidly than other sugars
and lowering of pH inhibited caramelization browning of sugar
solutions.
In a study on the kinetics of caramelization of several
monosaccharides and disaccharides, Diaz and Clotet (1995)
found that at temperatures of 75–95◦C, browning increased
rapidly with time and to a higher final value, with increasing
temperature, this effect being more marked in the monosaccha-
rides than in the disaccharides. In all sugars studied, increase of
browning was greater ataw=1 than at aw=0.75. Likewise,
results of a more recent study on the kinetics of browning de-
velopment showed that at temperatures of 75–95◦C and pH 3.0,
color development increased linearly with a first-order kinetics
on fructose concentration (Chen et al. 2009).
The effect of sugars, temperature, and pH on caramelization
was evaluated by Park et al. (1998). Reaction rate was highest
with fructose, followed by sucrose. As reaction temperature in-creased from 80◦C to 110◦C, reaction rate was greatly increased.
With respect to pH, the optimum value for caramelization was- In agreement with this, more recently, Kim and Lee (2008b)
reported that degradation/enolization of sugars rapidly increased
at a high alkaline pH (10.0–12.0) and with increasing heat-
ing times, fructose being degraded/enolized more rapidly than
glucose.
Although most studies on caramelization have been conducted
in model systems of mono- and disaccharides, a number of
real food systems contain oligosaccharides or even polymeric
saccharides; therefore, it is also of great interest to know the
contribution of these carbohydrates to the flavor and color of
foods. Kroh et al. (1996) reported the breakdown of oligo-
and polysaccharides to nonvolatile reaction products. Homoki-
Farkas et al. (1997) studied, through an intermediate compound
(methylglyoxal), the caramelization of glucose, dextrin 15, and
starch in aqueous solutions at 170◦C under different periods
of time. The highest formation of methylglyoxal was in glu-
cose and the lowest in starch systems. The authors attributed
the differences to the number of reducing end groups. In the
case of glucose, when all molecules are degraded, the con-
centration of methylglyoxal reached a maximum and began to
transform, yielding LMW and HMW color compounds. Holl-
nagel and Kroh (2000, 2002) investigated the degradation of
malto-oligosaccharides at 100◦C throughα-dicarbonyl com-
pounds such as 1,4-dideoxyhexosulose, and they found that this
compound is a reactive intermediate and precursor of various
heterocyclic volatile compounds that contribute to caramel fla-
vor and color. More recently, it has been observed that galac-
tomannans of roasted coffee infusions are HMW supports of
LMW brown compounds derived from caramelization reactions
(Nunes et al. 2006).
Perhaps, as mentioned above, the most striking feature of
caramelization is its contribution to the color and flavor of cer-
tain food products under controlled conditions. In addition, it
is necessary to consider other positive characteristics of this
reaction, such as the antioxidant activity of the CPs. This prop-
erty has been found to vary depending on temperature, pH,
duration of heating, and starting material, main variables affect-
ing the caramelization kinetics. Thus, several studies (Benjakul
et al. 2005, Phongkanpai et al. 2006, Kim and Lee 2008b) have