80 Part I: Principles
proteins with the carbonyl group of reducing sugars
(aldose), with loss of a molecule of water, leading,
via formation of a Schiff’s base and Amadori rear-
rangement, to the scalled Amadori product (1-
amin1-deoxy-2-ketose), a relatively stable interme-
diate (Feather et al. 1995). The Heyns compound is
the analogous compound when a ketose is the start-
ing sugar. In many foods, the -amino group of the
lysine residues of proteins is the most important
source of reactive amino groups, but due to block-
age these lysine residues are not available for diges-
tion, and consequently the nutritive value decreases
(Brands and van Boekel 2001, Machiels and Istasse
2002). Amadori compounds are precursors of num-
erous compounds important in the formation of
characteristic flavors, aromas, and brown polymers.
They are formed before the occurrence of sensory
changes; therefore, their determination provides a
very sensitive indicator for early detection of quality
changes caused by the Maillard reaction (Olano and
Martínez-Castro 1996).
The intermediate stage leads to breakdown of
Amadori compounds (or other products related to the
Schiff’s base) and the formation of degradation prod-
ucts, reactive intermediates (3-deoxyglucosone), and
volatile compounds (formation of flavor). The 3-
deoxyglucosone participates in cross-linking of pro-
teins at much faster rates than glucose itself, and fur-
ther degradation leads to two known advanced
products: 5-hydroxymethyl-2-furaldehyde and pyr-
raline (Feather et al. 1995).
The final stage is characterized by the production
of nitrogen-containing brown polymers and copoly-
mers known as melanoidins (Badoud et al. 1995).
The structure of melanoidins is largely unknown,
and to date, there are several proposals about it. Me-
lanoidins have been described as low molecular
weight (LMW) colored substances that are able to
cross-link proteins via -amino groups of lysine or
arginine to produce high molecular weight (HMW)
colored melanoidins. Also, it has been postulated
that they are polymers consisting of repeating units
of furans and/or pyrroles formed during the ad-
vanced stages of The Maillard reaction and linked
by polycondensation reactions (Martins and van
Boekel 2003).
In foods, predominantly glucose, fructose, malt-
ose, lactose, and to some extent reducing pentoses
are involved with amino acids and proteins in form-
ing fructoselysine, lactuloselysine, or maltulosely-
sine. In general, primary amines are more important
than the secondary ones because the concentration
of primary amino acids in foods is usually higher
than that of secondary amino acids (an exception is
the high amount of proline in malt and corn prod-
ucts) (Ledl 1990).Factors Affecting the Maillard ReactionThe rate of the Maillard reaction and the nature of
the products formed depend on the chemical envi-
ronment of food including the water activity (aw),
pH, and chemical composition of the food system,
temperature being the most important factor
(Carabasa-Giribert and Ibarz-Ribas 2000). In order
to predict the extent of chemical reactions in pro-
cessed foods, a knowledge of kinetic reactions is
necessary to optimize the processing conditions.
Since foods are complex matrices, these kinetic
studies are often carried out using model systems in
which sugars and amino acids react under simplified
conditions. Model system studies may provide guid-
ance regarding the directions in which to modify the
food process and to find out which reactants may
produce specific effects of the Maillard reaction
(Lingnert 1990).
The reaction rate is significantly affected by the
pH of the system; it generally increases with pH
(Namiki et al. 1993, Ajandouz and Puigserver 1999).Figure 4.6.Effect of phosphate buffer concentration on
the loss of glycine in 0.1M glucose/glycine solutions at
pH 7 and 25°C (Bell 1997).