BLBS102-c04 BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come
4 Browning Reactions 73R^1R^3N NN NMelanoidin-like polymersNR^1
nR^3R^3R^1R^1R^3 NH 2CHOOOH
(I) (II)R 3PolymerizationR^1 R^3Figure 4.12.Mechanism for nonenzymatic browning produced as a
consequence of 2-(1-hydroxyalkyl)pyrrole polymerization (Hidalgo
et al. 2003).model system was increased at temperatures in the range of
80–120◦C. The effect of pH on browning was similar in both
oxidized lipid–protein and carbohydrate–protein model systems.
Some of the oxidized lipid–amino acid reaction products have
been shown to have antioxidant properties when they are added
to vegetable oils (Zamora and Hidalgo 1993, Alaiz et al. 1995,
Alaiz et al. 1996). All of the pyrrole derivatives, with differ-
ent substituents in the pyrrole ring, play an important role in
the antioxidant activity of foods, being the sum of the antioxi-
dant activities of the different compounds present in the sample
(Hidalgo et al. 2003). Moreover, others studies have suggested
that tetrahydro-β-carbolines derived from oxidation of trypto-
phan might act as antioxidants when they are absorbed and accu-
mulated in the body, contributing to the antioxidant effect of fruit
products naturally containing these compounds (Herraiz and
Galisteo 2003). Likewise, Hidalgo et al. (2006a), in a study car-
ried out with amino phospholipids (phosphatidylethanolamine
(PE) and phosphatidylcholine (PC)), lysine (Lys), and mixtures
of them in edible oils, demonstrated thein situformation of
oxidized lipid–amino compound reaction products (PE–Lys and
PC–Lys) with antioxidative activities. More recently, studies
have shown that antioxidative activity of such carbonyl-amineproducts may be greatly increased with the addition of toco-
pherols and phytosterols such asβ-sitosterol (Hidalgo et al.
2007, Hidalgo et al. 2009). Alaiz et al. (1997), in a study on
the comparative antioxidant activity of both Maillard reaction
compounds and oxidized lipid–amino acid reaction products,
observed that both reactions seem to contribute analogously to
increase the stability of foods during processing and storage.
Zamora and Hidalgo (2003a) studied the role of the type of
fatty acid (methyl linoleate and methyl linolenate) and the pro-
tein (bovine serum albumin)–lipid ratio on the relative progres-
sion of the process involved when lipid oxidation occurs in the
presence of proteins. These authors found that methyl linoleate
was only slightly more reactive than the methyl linolenate for
bovine serum albumin, producing an increase of protein pyrroles
in the protein and an increase in the development of browning
and fluorescence. In relation to the influence of the protein–lipid
ratio on the advance of the reaction, the results observed in
this study pointed out that a lower protein–lipid ratio increases
sample oxidation and protein damage as a consequence of the
antioxidant activity of the proteins. These authors also concluded
that the changes produced in the color of protein–lipid samples
were mainly due to oxidized lipid–protein reactions and not as
consequence of polymerization of lipid oxidation products.
Analogous to the Maillard reaction, oxidized lipid and pro-
tein interaction can cause a loss of nutritional quality due to the
destruction of essential amino acids, such as tryptophan, lysine,
and methionine, and essential fatty acids. Moreover, a decrease
in digestibility and inhibition of proteolytic and glycolytic en-
zymes can also be observed. In a model system of 4,5(E)-epoxy-
2(E)-heptenal and bovine serum albumin, Zamora and Hidalgo
(2001) observed denaturation and polymerization of the protein,
and the proteolysis of this protein was impaired as compared
with the intact protein. These authors suggested that the inhibi-
tion of proteolysis observed in oxidized lipid-damaged proteins
may be related to the formation and accumulation of pyrrolized
amino acid residues.
To date, although most of the studies have been conducted
using model systems, the results obtained confirm that there is
an interaction between lipid oxidation and the Maillard reaction.
In fact, both reactions are so interrelated that they should be
considered simultaneously to understand their consequences on
foods when lipids, carbohydrates, and amino acids or proteins
are present and should be included in one general pathway that
can be initiated by both lipids and carbohydrates (Zamora et al.
2005a, 2005b). The complexity of the reaction is attributable to
several fatty acids that can give rise to a number of lipid oxidation
products that are able to interact with free amino groups. As
summary, Figure 4.13 shows an example of a general pathway
of pyrrole formation during polyunsaturated fatty acid oxidation
in the presence of amino compounds.Nonenzymatic Browning of AminophospholipidsIn addition to the above-mentioned studies on the participation
of lipids in the browning reactions, several reports have been
addressed on the amine-containing phospholipid interactions
with carbohydrates. Because of the role of these membranous