BLBS102-c04 BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come
72 Part 1: Principles/Food Analysisand oxidized lipid/amino acid reactions occur simultaneously
(Hidalgo and Zamora 2002).
Radical transfer occurs early in lipid oxidation, and this pro-
cess underlies the antioxidant effect for lipids. In addition,
protein radicals can also transfer radicals to other proteins, lipids,
carbohydrates, vitamins, and other molecules, especially in the
presence of metal ions such as iron and copper (Schaich 2008).
Reactions between proteins and free radicals and ROS suggest
that proteins could protect lipids from oxidation if they are oxi-
dized preferentially to unsaturated fatty acids (Elias et al. 2008).
A study of continuous phaseβ-lactoglobulin in oil-in-water
emulsion showed that tryptophan and cysteine side chains, but
not methionine, oxidized before lipids (Elias et al. 2005).
The interaction between oxidized fatty acids and amino groups
has been related to the browning detected during the progressive
accumulation of lipofuscin (age-related yellow-brown pigments)
in lysosomes of men and animals (Yin 1996). In foods, evidence
of this reaction has been found during storage and processing of
some fatty foods (Hidalgo et al. 1992, Nawar 1996), fermented
alcoholic and nonalcoholic beverages (Herraiz 1996), in co-
coa, chocolate (Herraiz 2000), salted sun-dried fish (Smith and
Hole 1991), boiled and dried anchovy (Takiguchi 1992), cut-
tlefish (Sepia pharaonis) (Thanonkaew et al. 2007), meat and
meat products (Mottram 1998, Herraiz and Papavergou 2004),
smoked foodstuffs such as sausages, cheeses, and fish (Zotos
et al. 2001, Herraiz et al. 2003, Papavergou and Herraiz 2003),
and in rancid oils and fats with amino acids or proteins (Ya-
mamoto and Kogure 1969, Okumura and Kawai 1970, Gillat
and Rossell 1992, Guillen et al. 2005). For instance, interaction
between different carbonyl compounds, mainly aldehydes, de-
rived from lipid oxidation and lysine, tryptophan, methionine,
and cysteine side chains of whey proteins has been shown to oc-
cur in dairy products such as raw and different heat-treated milks
(pasteurized, UHT, and sterilized), as well as in infant formula
(Nielsen et al. 1985, Meltretter et al. 2007, 2008, Meltretter and
Pischetsrieder 2008).
Several studies have been carried out in model systems with
the aim to investigate the role of lipids in nonenzymatic brown-
ing. The role of lipids in these reactions seems to be similar to
that of the role of carbohydrates during the Maillard reaction
(Hidalgo and Zamora 2000). Similar to the Maillard reaction,
oxidized lipid/protein interactions comprise a huge number of
several related reactions. The isolation and characterization of
the involved products is very difficult, mainly in the case of in-
termediate products, which are unstable and are present in very
low concentrations.
According to the mechanism proposed for the protein brown-
ing caused by acetaldehyde, the carbonyl compounds derived
from unsaturated lipids readily react with protein-free amino
groups, following 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 polymeriza-
tion of the intermediate products 2-(1-hydroxyalkyl) pyrroles
has been proposed (Zamora and Hidalgo 1994, 1995). These au-
thors, studying different model systems, tried to explain, at leastR^1 CH 2 CHOR^1R^1 CH 2 CH = N–R'R^2 CH = C – CH = N–R' R^2 CH = C – CH = OH 2 OH 2 O
H 2 O RNH 2Repeated
aldol
condensationsBrown pigmentsR'NH 2R 2 CHOR^1Figure 4.11.Formation of brown pigments by aldolic condensation
(Hidalgo and Zamora 2000).partially, the nonenzymatic browning and fluorescence produced
when proteins are present during the oxidation of lipids (Figure
4.12). 2-(1-Hydroxyalkyl) pyrroles (I) have been found to be
originated 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 formation is always accompanied by
the production of N-substituted pyrroles (II). Compounds de-
rived from reaction of 4,5-epoxy-2-alkenals and phenylalanine
have been found to be flavor compounds analogous to those
of the Maillard reaction. Therefore, flavors traditionally con-
nected to the Maillard reaction may also be produced as a result
of lipid oxidation (Hidalgo and Zamora 2004, Zamora et al.
2006). N-substituted pyrroles are relatively stable and have been
found in 22 fresh food products (cod, cuttlefish, salmon, sardine,
trout, beef, chicken, pork, broad bean, broccoli, chickpea, gar-
lic, green pea, lentil, mushroom, soybean, spinach, sunflower,
almond, hazelnut, peanut, and walnut; Zamora et al. 1999). How-
ever, the N-substituted 2-(1-hydroxyalkyl) pyrroles are unstable
and polymerize rapidly and spontaneously to produce brown
macromolecules with fluorescent melanoidin-like characteris-
tics (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 per-
haps some polymerization finished before maximum color and
fluorescence was achieved.
Although melanoidins starting from either carbohydrates
or oxidized lipids would have analogous chemical structures,
carbohydrate–protein and oxidized lipid–protein reactions are
produced under different conditions. Hidalgo et al. (1999) stud-
ied the effect of pH and temperature in two model systems: (i)
ribose and bovine serum albumin and (ii) methyl linoleate oxi-
dation products and bovine serum albumin; they observed that
from 25◦Cto50◦C, the latter exhibited higher browning than
the former. Conversely, the browning produced in carbohydrate