Food Biochemistry and Food Processing

4 Browning Reactions 89

products, which are unstable and are present in very
low concentrations.
According to the mechanism proposed for the
protein browning caused by acetaldehyde, the car-
bonyl compounds derived from unsaturated lipids
readily react with protein-free amino groups, fol-
lowing the scheme of Figure 4.11, to produce, by
repeated aldol condensations, the formation of brown
pigments (Montgomery and Day 1965, Gardner
1979, Belitz and Grosch 1997).
More recently, another mechanism based on the
polymerization of the intermediate products 2-(1-
hydroxyalkyl) pyrroles has been proposed (Zamora
and Hidalgo 1994, Hidalgo and Zamora 1995). These
authors, studying different model systems, tried to
explain, at least partially, the nonenzymatic browning
and fluorescence produced when proteins are present
during the oxidation of lipids (Fig. 4.12). The 2-(1-
hydroxyalkyl) pyrroles (I) have been found to origi-
nate from the reaction of 4,5-epoxy-2-alkenals
(formed during lipid peroxidation) with the amino
group of amino acids and/or proteins, and their for-
mation is always accompanied by the production of
N-substituted pyrroles (II). These last compounds
are relatively stable and have been found in 22 fresh
food products (cod, cuttlefish, salmon, sardine, trout,
beef, chicken, pork, broad bean, broccoli, chickpea,
garlic, green pea, lentil, mushroom, soybean, spin-
ach, sunflower, almond, hazelnut, peanut, and walnut)

(Zamora et al. 1999). However, the N-substituted

2-(1-hydroxyalkyl) pyrroles are unstable and poly-

merize rapidly and spontaneously to produce brown

macromolecules with fluorescent melanoidin-like

characteristics (Hidalgo and Zamora 1993). Zamora

et al. (2000) observed that the formation of pyrroles

is a step immediately prior to the formation of color

and fluorescence. Pyrrole formation and perhaps

some polymerization finish before maximum color

and fluorescence are achieved.

Although melanoidins starting from either carbo-

hydrates or oxidized lipids would have analogous

chemical structures, carbohydrate-protein and oxi-

dized lipid–protein reactions are produced under

different conditions. Hidalgo et al. (1999) studied

the effect of pH and temperature in two model sys-

tems:(1) ribose and bovine serum albumin and (2)

methyl linoleate oxidation products and bovine

serum albumin; they observed that from 25 to 50°C

the latter system exhibited higher browning than the

former. Conversely, the browning produced in the

Figure 4.11.Formation of brown pigments by aldolic

condensation (Hidalgo and Zamora 2000).

Figure 4.12.Mechanism for nonenzymatic browning

produced as a consequence of 2-(1-hydroxyalkyl)pyr-

role polymerisation (Hidalgo et al. 2003).