90 Part I: Principles
carbohydrate model system was increased at tem-
peratures in the range 80–120°C. The effect of pH
on browning was similar in both oxidized lipid-
protein and carbohydrate-protein model systems.
Some 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 different sub-
stituents in the pyrrole ring, play an important role
in the antioxidant activity of foods, being the sum of
the antioxidant activities of the different compounds
present in the sample (Hidalgo et al. 2003). Alaiz et
al. (1997), in a study on the comparative antioxidant
activity of Maillard reaction compounds and oxi-
dized lipid–amino acid reaction products, observed
that both reactions seem to contribute analogously
to increasing 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 protein (bovine serum albumin)–
lipid ratio on the relative progression of the process
involved when lipid oxidation occurs in the presence
of proteins. These authors found that methyl lino-
leate was only slightly more reactive than the methyl
linolenate for bovine serum albumin, producing a
higher increase of protein pyrroles in the protein and
increased 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 pro-
teins. 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 a consequence of polymerization of lipid oxida-
tion products.
Analogous to the Maillard reaction, oxidized lipid
and protein interaction can cause a loss of nutrition-
al quality due to the destruction of essential amino
acids such as tryptophan, lysine, and methionine and
essential fatty acids. Moreover, a decrease in di-
gestibility and inhibition of proteolytic and glycolyt-
ic enzymes can also be observed. In a model system
of 4,5(E)-epoxy-2(E)-heptenal and bovine serum al-
bumin, Zamora and Hidalgo (2001) observed denat-
uration and polymerization of the protein, and the
proteolysis of this protein was impaired as com-
pared with the intact protein. These authors suggest-
ed that the inhibition of proteolysis observed in oxi-
dized 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
point out the importance of lipids during browning,
and in general, it is possible to suggest that the role
of lipids during these reactions could be similar to
the role of carbohydrates in the Maillard reaction or
phenols in enzymatic browning. 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 a
summary, Figure 4.13 shows an example of a gener-
al pathway for pyrrole formation during polyunsatu-
rated 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 addressed amine-containing
phospholipid interactions with carbohydrates. Due
to the role of these membranous functional lipids in
the maintenance of cellular integrity, most of the
studies have been conducted in biological samples
(Bucala et al. 1993, Requena et al. 1997, Lertsiri et
al. 1998, Oak et al. 2000, Oak et al. 2002). The gly-
cation of membrane lipids can cause inactivation of
receptors and enzymes, cross-linking of membrane
lipids and proteins, membrane lipid peroxidation,
and consequently, cell death. Amadori compounds
derived from the interaction between aminophos-
pholipids and reducing carbohydrates are believed
to be key compounds for generating oxidative stress,
causing several diseases.
In foods, this reaction can be responsible for dete-
rioration during processing. Although nonenzymatic
browning of aminophospholipids was detected for
the first time in dried egg by Lea (1957), defined
structures from such reactions were reported later by
Utzmann and Lederer (2000). These authors demon-
strated the interaction of phosphatidylethanolamine
(PE) with glucose in model systems; moreover, they