Food Biochemistry and Food Processing (2 edition)

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BLBS102-c04 BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come


4 Browning Reactions 63

Aldose

+Amino compound

–H 2 O

–3H 2 O –2H 2 O

+H 2 O

C

A

C

Sugars

Reductones Dehydroreductones
Aldehydes

+Amino acid

Strecker
degradation

–CO 2
–2H
Aminocompound +2H

Schiff’s base of
hydroxymethylfurfural
or furfural

B

N-substituted glycosylamine

Amadori
rearrangement

D

Amadori rearrangement product (ARP)
(1-amino-1-deoxy-2-ketose)

E

E

F

G G G

Melanoidins (brown nitrogenous polymers and copolymers)

Aldols and nitrogen-free
polymers

G

Hydroxymethylfurfural
or furfural
+Amino
compound +Aminocompound +Aminocompound +Aminocompound +Aminocompound

G

F

F

F

Fission products (acetol, diacetyl, pyruvaldehyde, etc.)

Figure 4.4.Scheme of different stages of Maillard reaction (Hodge 1953, Ames 1990).

The 3-deoxyglucosone participates in cross-linking of proteins
at much faster rates than glucose itself, and further degrada-
tion leads to two known advanced products: 5-hydroxymethyl-
2-furaldehyde and pyraline (Feather et al. 1995).
The final stage is characterized by the production of
nitrogen-containing brown polymers and copolymers known as
melanoidins (Badoud et al. 1995). The structure of melanoidins
is largely unknown, but in the last few years, more data became
available. In particular, it was shown that they can have a dif-
ferent structure according to the starting material. Melanoidins
have been described as low-molecular weight (LMW) colored
substances that are able to cross-link proteins viaε-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 advanced stages of the Maillard
reaction and linked by polycondensation reactions (Martins and
van Boekel 2003). Chemical structure of melanoidins can be
mainly formed by a carbohydrate skeleton with few unsaturated
rings and small nitrogen components; in other cases, they can
have a protein structure linked to small chromophores (Borrelli
and Fogliano 2005).
In foods, predominantly glucose, fructose, maltose, lactose,
and to some extent reducing pentoses are involved with amino
acids and proteins in forming fructoselysine, lactuloselysine or
maltuloselysine. In general, primary amines are more impor-

tant than 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 products) (Ledl 1990).

Factors Affecting Maillard Reaction

The rate of the Maillard reaction and the nature of the products
formed depend on the chemical environment of food, including
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 processed foods, 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 guidance 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 sys-
tem; it generally increases with pH(Namiki et al. 1993, Ajan-
douz and Puigserver 1999). Bell (1997) studied the effect of
buffer type and concentration on initial degradation of amino
acids and formation of brown pigments in model systems of
glycine and glucose that are stored for long periods at 25◦C. The
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