Food Biochemistry and Food Processing (2 edition)

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