84 Part I: Principles
acids as catalysts, higher yields of HMF are pro-
duced from fructose than from glucose. Also, only
the fructose moiety of sucrose is largely converted to
HMF under the unbuffered conditions that produce
the highest yields. The enolization of glucose can be
greatly increased in buffered acidic solutions. Thus,
higher yields of HMF are produced from glucose
and sucrose when a combination of phosphoric acid
and pyridine is used as the catalyst than when phos-
phoric acid is used alone (Fenemma 1976).
In alkaline media, dehydration reactions are slow-
er than in neutral or acid media, but fragmentation
products such as acetol, acetoin, and diacetyl are
detected. In the presence of oxygen, oxidative fis-
sion takes place, and formic, acetic, and other organ-
ic acids are formed.
All of these compounds react to produce brown
polymers and flavor compounds (Olano and Mar-
tínez-Castro 1996).
In general, caramelization products consist of
volatile and nonvolatile (90–95%) fractions of low
and high molecular weights 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 composition of the system (pH
and type of sugar), caramelization reactions may
also play an important role in color formation in sys-
tems heated at lower temperatures. Thus, some stud-
ies have been conducted at the temperatures of
accelerated storage conditions (45–65°C) and pH
values from 4 to 6 (Buera et al. 1987a,b). These
authors studied the changes of color due to car-
amelization of fructose, xylose, glucose, maltose,
lactose, and sucrose in model systems of 0.9 aw and
found that fructose and xylose browned much more
rapidly than the other sugars and that lowering the
pH inhibited caramelization browning of sugar solu-
tions.
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 monosaccharides
than in the disaccharides. In all sugars studied,
increase of browning was greater at aw 1 than at
aw 0.75.
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 increased from 80
to 110°C, reaction rate was greatly increased. With
respect to pH, the optimum value for caramelization
was 10.
Although most studies on caramelization have
been conducted in model systems of mono- and di-
saccharides, a number of real food systems contain
oligosaccharides or even polymeric saccharides;
therefore, it is also of great interest to know the con-
tribution of these carbohydrates to the flavor and
color of foods. Kroh et al. (1996) reported the break-Figure 4.8.The Lobry de Bruyn-Alberda van Ekenstein
transformation.Figure 4.9.1,2 Enolization and formation of hydroxymethyl furfural (HMF).