1

Fig. 11.4 Mechanism of the Strecker degradation of amino acids.

Below pH 3 and above pH 8 or at temperatures above 130°C (caramélisation), sugars will degrade in the absence of amines (Ledl and Schleicher, 1990). Ring opening followed by 1, 2 or 2, 3-enolisation are crucial steps in ARP transformation and are followed by dehydration and fragmentation with the formation of many very reactive dicarbonyl fragments. This complex of reactions is considered the intermediate stage of the MR.

Maillard observed also the production of CO2, which is explained by a process named the Strecker degradation (Fig. 11.4). The mechanism involves the reaction of an amino acid with an a-dicarbonyl compound to produce an azovinylo-gous b-ketoacid 4, that undergoes decarboxylation. In this way amino acids are converted to aldehydes containing one less carbon atom per molecule. These are very reactive and often have very peculiar sensory properties. The aldehydes that derive from cysteine and methionine degrade further to give hydrogen sulfide, 2-methylthio-propanal, and methanethiol: that means that the Strecker degradation is responsible for the incorporation of sulfur in some Maillard reaction products (MRPs). Another important consequence of the Strecker reaction is the incorporation of nitrogen in very reactive fragments deriving from sugars, such as 5.

CHO CHOH CHOH CHOH

C=N-R reverse aldol CH-NH-R

  • RNH2 CHOH reaction CHOH 5
  • CHOH —- CH=O

CHOH CHOH

Fig. 11.5 Possible pathway for the formation of glycolaldehyde aLkylimines proposed by

Namiki and Hayashi (1986).

CHO -OH

R-NH

_NR 3-deoxy _ —OH aldoketose route

  • OH -OH CH2OH
  • OH —OH CH2OH

ch2oh beta-dicatbonyl route

3,4-dideo: aldoketose route

OH [ CH2OH

C6-pyrroles C6-furans

  • OH -OH CH2OH
  • OH —OH CH2OH

3,4-dideo: aldoketose route

Pyrroles Amadori

beta-dicar-bonyl route

3,4-dideoxy aldoketose route i y

beta-dicar-bonyl route

c"h"20h

3,4-dideoxy aldoketose route polymers polymers

Fig. 11.6 Transformation of hexoses and pentoses to C5- and C4-pyrroles and -furans. (Reproduced with permission from Tressel et al, 1998a)

However, in the last two decades other mechanisms have been proposed. For example, starting from the experimental observation of free radical formation at the start of the MR, Hayashi and Namiki (1981; 1986) proposed a reducing sugar degradation pathway that produces glycolaldehyde alkylimines without passing through the formation of ARPs (Fig. 11.5).

Very recently, on the basis of extensive experiments with 13C- and 2H-labelled sugars, a detailed reaction scheme was proposed by Tressel et al (1995 and 1998a): the formation of various C6-, C5-, and C4-pyrroles and furans from both intact and fragmented hexoses and amines could be unambiguously attributed to distinct reaction pathways via the intermediates A-C without involving the

Table 11.1 Composition of the primary fragmentation pools

Type of pool

Constituents

Amino acid

Amines

fragmentation pool

Carboxylic acids

{A}

Alkanes and aromatics

Aldehydes

Amino acid specific side chain fragments: H2S (Cys), CH3SH

(Met), styrene (Phe)

Sugar fragmentation

C1 fragments: formaldehyde, formic acid

pool {S}

C2 fragments: glyoxal, glycoladehyde, acetic acid

C3 fragments: glyceraldehyde, methylglyoxal, hydroxyacetone,

dihydroxyacetone, etc.

C4 fragments: tetroses, 2, 3-butanedione, 1-hydroxy-2-butanone,

2-hydroxybutanal, etc.

C5 fragments: pentoses, pentuloses, deoxy derivatives, furanones,

furans

C6 fragments: pyranones, furans, glucosones, deoxyglucosones

Amadori and Heyns

C3-ARP/HRP derivatives: glyceraldehyde-ARP, amino

fragmentation pool

acid-propanone, amino acid-propanal, etc.

{D}

C4-ARP/HRP derivatives: amino acid-tetradiuloses, amino

acid-butanones

C5-ARP/HRP derivatives: amino acid-pentadiuloses

C6-ARP/HRP derivatives: amino acid-hexadiuloses, pyrylium

betaines

Lipid fragmentation

Propanal, pentanal, hexanal, octanal, nonanal

pool {L}

2-Oxoaldehydes (C6-9)

C2 fragments: glyoxal

C3 fragments: CHOCH2CHO, methylglyoxal

Formic acid, acids

Amadori rearrangement (Fig. 11.6). These pyrroles and furans polymerise very easily to highly coloured compounds that may be involved in the formation of melanoidins.

By means of experiments showing that sugars and most amino acids also undergo independent degradation (Yaylayan and Keyhani, 1996), a new conceptual approach to the MR has been proposed recently by Yaylayan (1997). He suggested that in order to understand the MR better, it is more useful to define a sugar fragmentation pool {S}, an amino acid fragmentation pool {A}, and an interaction fragmentation pool {D}, deriving from the Amadori and Heyns compounds (Table 11.1). Together they constitute a primary fragmentation pool of building blocks that react to give a secondary pool of interaction intermediates and eventually a very complex final pool of stable end-products.

However, most foods contain also lipids that can degrade by autoxidation (Grosch, 1987) giving reactive intermediates, mainly saturated or unsaturated aldehydes or ketones and also glyoxal and methylglyoxal (in common with the polymers

Interaction pool heterocycles

Primary fragmentation pool dimers

Fig. 11.7 Conceptual representation of the Maillard reaction: generalogy of primary fragmentation pools, interaction pools (containing self-interaction pools as well as mix-interaction pools) and end-products.

Primary fragmentation pool dimers

Fig. 11.7 Conceptual representation of the Maillard reaction: generalogy of primary fragmentation pools, interaction pools (containing self-interaction pools as well as mix-interaction pools) and end-products.

Maillard reaction) and malondialdehyde (Table 11.1). These belong to a fourth pool, the lipid fragmentation pool {L} (D'Agostina et al, 1998) and in this way the scheme proposed by Yaylayan (1997) was revised to include it (Fig. 11.7). Clear interconnections between the MR and lipid autoxidation have been extensively studied in the case of food aromas, where many end-products deriving from lipids and amino acids or sugars are very well documented (Whitfield, 1992), but certainly they may be relevant also for other sensory aspects, such as colour or taste, or for nutrition, although these research areas have been almost completely neglected until the present.

Depending on food composition and heating intensity applied, thousands of different end products may be formed in the advanced stage of the MR: they are classified here according to their functions in foods (Fig. 11.8). Very volatile compounds, such as pyrazines, pyridines, furans, thiophenes, thiazoles, thiazolines, and dithiazines are of interest, when considering aroma; some low molecular weight compounds relate to taste (Frank et al, 2001; Ottiger et al, 2001), others behave as antioxidants and a few are mutagenic. Polymers (melanoidins) that in sugar/amino acid model systems and some foods such as coffee, roasted malt, or chocolate are the major MRPs, and determine the colour of the food.

This review will discuss only the mechanism of formation of MRPs that have some nutritional significance or may be used as molecular markers for quantifying the MR in foods. A very detailed description of the pathways leading to most Maillard reaction products may be found in an excellent review by Ledl and Schleicher (1990).

Volatile compounds

Antioxidants

Metal chelating agents

Maillard reaction

Metal chelating agents

Maillard reaction

Toxic compounds

Tasty compounds

Brown compounds

Fig. 11.8 Functional classification of Maillard reaction products.

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