88 Part I: Principles
However, in general, the largest losses for noncitrus
foods will occur during heating (Fennema 1976).
The rapid removal of oxygen from packages is an
important factor in sustaining a higher concentration
of ascorbic acid and lower browning of citrus juices
over long-term storage. The extent of browning may
be reduced by packing in oxygen-scavenging film
(Zerdin et al. 2003).
Modified-atmosphere packages (Howard and
Hernandez-Brenes 1998), microwave heating (Vil-
lamiel et al. 1998, Howard et al. 1999), ultrasound-
assisted thermal processing (Zenker et al. 2003),
pulsed electric field processing (Min et al. 2003),
and carbon dioxide assisted high-pressure process-
ing (Boff et al. 2003) are some examples of techno-
logical processes that allow ascorbic acid retention
and, consequently, a prevention of undesirable
browning.
LIPIDBROWNING
Protein-Oxidized Fatty Acid Reactions
The organoleptic and nutritional characteristics of
several foods are affected by lipids, which can par-
ticipate in chemical modifications during processing
and storage. Lipid oxidation occurs in oils and lard,
but also in foods with low amounts of lipids, such as
products from vegetable origin. This reaction occurs
in both unprocessed and processed foods, and al-
though in some cases it is desirable, for example, in
the production of typical cheeses or of fried-food
aromas (Nawar 1985), in general, it can lead to
undesirable odors and flavors (Nawar 1996). Quality
properties such as appearance, texture, consistency,
taste, flavor, and aroma can be adversely affected
(Eriksson 1987). Moreover, toxic compound forma-
tion and loss of nutritional quality can also be ob-
served (Frankel 1980; Gardner 1989; Kubow 1990,
1992).
Although the lipids can be oxidized by both enzy-
matic and nonenzymatic reactions, the latter is the
main involved reaction. This reaction proceeds via
typical free radical mechanisms, with hydroperox-
ides as the initial products. As hydroperoxides are
quite unstable, a network of dendritic reactions, with
different reaction pathways and a diversity of prod-
ucts, can take place (Gardner 1989). The enzymatic
oxidation of lipids occurs sequentially. Lipolytic
enzymes can act on lipids to produce polyunsatu-
rated fatty acids that are then oxidized by either
lipoxygenase or cyclooxygenase to form hydroper-
oxides or endoperoxides, respectively. Later, these
compounds suffer a series of reactions to produce,
among other compounds, long-chain fatty acids that
are responsible for important characteristics and
functions (Gardner 1995).
Via polymerization, brown-colored oxypolymers
can be produced subsequently from the lipid oxida-
tion derivatives (Khayat and Schwall 1983). How-
ever, due to the electrophilic character of the car-
bonyl compounds produced during lipid oxidation,
interaction with nucleophiles such as the free amino
group of amino acids, peptides, or proteins can also
take place, producing end products different from
those formed during oxidation of pure lipids (Gillatt
and Rossell 1992). When lipid oxidation occurs in
the presence of amino acids, peptides, or proteins,
not all the lipids have to be oxidized and degraded
before oxidized lipid–amino acid reactions take
place. In fact, both reactions occur simultaneously
(Hidalgo and Zamora 2002).
The interaction between oxidized fatty acids and
amino groups has been related to the browning
detected during the progressive accumulation of
lipofuscins (age-related yellow-brown pigments) in
man and animals (Yin 1996). In foods, evidence of
this reaction has been found during storage and pro-
cessing of some fatty foods (Hidalgo et al. 1992,
Nawar 1996), in salted sun-dried fish (Smith and
Hole 1991), boiled and dried anchovy (Takiguchi
1992), smoked tuna (Zotos et al. 2001), meat and
meat products (Mottram 1998), and rancid oils and
fats with amino acids or proteins (Yamamoto and Ko-
gure 1969, Okumura and Kawai 1970, Gillatt and
Rossell 1992). For instance, Nielsen et al. (1985)
found that peroxidized methyl linoleate can react
with lysine, tryptophan, methionine, and cysteine in
whey proteins.
Several studies have been carried out in model
systems with the aim to investigate the role of lipids
in nonenzymatic browning. The role of lipids in
these reactions seems to be similar to the role of car-
bohydrates during the Maillard reaction (Hidalgo
and Zamora 2000). Similarly to the Maillard reac-
tion, oxidized lipid–protein interactions comprise a
huge number of several related reactions. The isola-
tion and characterization of the involved products is
very difficult, mainly in the case of intermediate