226 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
condensation of an amine with a carbonyl group,
often within a reducing sugar or fat breakdown pro-
duct (Fayle and Gerrard 2002). During the course of
the Maillard reaction, reactive intermediates such as
-dicarbonyl compounds and deoxysones are gener-
ated, leading to the production of a wide range of
compounds, including polymerized brown pigments
called melanoidins, furan derivatives, and nitroge-
nous and heterocyclic compounds (e.g., pyrazines)
(Fayle and Gerrard 2002). Protein cross-links form a
subset of the many reaction products, and the cross-
linking of food proteins by the Maillard reaction
during food processing is well established (Miller
and Gerrard 2004). The precise chemical structures
of these cross-links in food, however, are less well
understood. Thus, surprisingly little is known about
the extent of Maillard cross-linking in processed
foods, the impact of this process on food quality, and
how the reaction might be controlled to maximize
food quality.
In biology and medicine, where the Maillard reac-
tion is important during the ageing process, several
cross-link structures have been identified, including
those shown in Figure 9.2 (Ames 1992; Easa et al.
1996a,b; Fayle et al. 2000, 2001; Gerrard et al.
2002a,b,c; Gerrard et al. 1999; Gerrard et al. 1998a;
Hill and Easa 1998; Hill et al. 1993; Mohammed et
al. 2000). One of the first protein-derived Maillard
reaction products isolated and characterized was the
cross-link pentosidine (Sell and Monnier 1989, Dyer
et al. 1991), a fluorescent moiety that is believed to
form through the condensation of a lysine residue
with an arginine residue and a reducing sugar. The
exact mechanism of formation of pentosidine
remains the subject of considerable debate (Chellan
and Nagaraj 2001, Biemel et al. 2001).
Some of the earliest studies that assessed the
effect of protein cross-linking on food quality exam-
ined the digestibility of a model protein following
glycation. Kato et al. observed that following incu-
bation of lysozyme with a selection of dicarbonyl
compounds for 10 days, the digestibility of lysoz-
yme by a pepsin-pancreatin solution was reduced to
up to 30% relative to the nonglycated sample (Kato
et al. 1986a). This trend of decreasing digestibility
was concomitant with an observed increase in cross-
linking of lysozyme. The increased resistance of
cross-linked proteins to enzymes commonly in-
volved in the digestion of proteins in the body is
unfavorable from a nutritional standpoint. It has also
been reported that the digestion process can be
inhibited by the Maillard reaction (Friedman
1996b).AGE Protein Cross-links Isolated to Date in
FoodInformation regarding the presence of specific
Maillard protein cross-links in food is, to date, limit-
ed, with only a handful of studies in this area
(Biemel et al. 2001, Henle et al. 1997, Schwarz-
enbolz et al. 2000). For example, compared to the
extensive literature on the Maillard chemistry in
vivo, relatively little has been reported on the exis-
tence of pentosidine in food. In a study by Henle et
al., the pentosidine content of a range of foods was
examined, with the highest values observed in roast-
ed coffee (Henle et al. 1997). Overall concentra-
tions, however, were considered low, among most of
the commercial food products tested. It was, there-
fore, concluded that pentosidine does not have a
major role in the polymerization of food proteins.
Schwarzenbolz et al. showed pentosidine formation
in a casein-ribose reaction carried out under high
hydrostatic pressure, which could be relevant to
some areas of food processing (Schwarzenbolz et al.
2000). Iqbal et al. have investigated the role of pen-
tosidine in meat tenderness in broiler hens (Iqbal et
al. 2000, 1999a,b, 1997).
Other Maillard cross-links have recently been de-
tected in food, and attempts have been made to
quantify the levels found. Biemel et al. examined the
content of the lysine-arginine cross-links GODIC,
MODIC, DODIC, and glucosepane in food. These
cross-links were found to be present in proteins
extracted from biscuits, pretzels, alt stick, and egg
white, in the range of 7–151 mg/kg. The lysine-
lysine imidazolium cross-links MOLD and GOLD
were also isolated but were present at a lower con-
centrations compared with MODIC and GODIC
(Biemel et al. 2001).MelanoidinsVery advanced glycation end products that form as a
result of food processing are dubbed melanoidins.
This is a structurally diverse class of compounds,
which until recently were very poorly characterized.
However, a subset of food melanoidins undoubtedly
includes those that cross-link proteins.