232 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
-casein, which lacks cysteine, but could not ac-
count for the extent of aggregation observed, sug-
gesting the presence of other cross-links that remain
undefined (Pellegrino et al. 1999).
The glycation approach has also been employed
as a tool to improve gelling properties of dried egg
white (Handa and Kuroda 1999, Matsudomi et al.
2002), a protein that is used in the preparation of
surimi and meat products (Chen 2000, Weerasinghe
et al. 1996). Some have suggested that the polymer-
ization may occur via protein cross-links formed as
a result of the Maillard reaction, but disulfide bonds
may still play some role (Handa and Kuroda 1999);
the exact mechanism remains undefined (Matsu-
domi et al. 2002).
The effect of the Maillard reaction and particular-
ly protein cross-linking on food texture has received
some attention (Gerrard et al. 2002a). Introduction
of protein cross-links into baked products has been
shown to improve a number of properties that are
valued by the consumer (Gerrard et al. 1998b,
2000). In situ studies revealed that following addi-
tion of glutaraldehyde to dough, the albumin and
globulin fraction of the extracted wheat proteins
were cross-linked (Gerrard et al. 2002b). Inclusion
of glutaraldehyde during bread preparation resulted
in the formation of a dough with an increased dough
relaxation time, relative to the commonly used flour
improver ascorbic acid (Gerrard et al. 2002c). These
results confirm that chemical cross-links are impor-
tant in the process of dough development, and sug-
gest that they can be introduced via Maillard-type
chemistry.
The Maillard reaction has also been used to mod-
ify properties in tofu. Kaye et al. reported that fol-
lowing incubation with glucose, a Maillard network
formed within the internal structure of tofu, result-
ing in a loss in tofu solubility and a reduction in tofu
weight loss (Kaye et al. 2001). Changes in tofu
structure have also been observed in the author’s
laboratory when including glutaraldehyde, glycer-
aldehydes, or formaldehyde in tofu preparation
(Yasir, unpublished data).
The covalent polymerization of milk during food
processing has been reported (Singh and Latham
1993). This phenomenon was shown to be sugar de-
pendent, as determined in model studies with -
casein. Further, pentosidine formation paralleled
protein aggregation over time at 70°C (Pellegrino et
al. 1999).
The Maillard reaction has been studied under dry
conditions in order to gain an understanding of the
details of the chemistry (French et al. 2002, Kato et
al. 1988). Kato et al. reported the formation of pro-
tein polymers following incubation of galactose or
talose with ovalbumin under desiccating conditions:
an increase in polymerization, relative to the protein
only control, was observed (Kato et al. 1986b). This
study was extended using the milk sugar lactose, as
milk can often be freeze dried for shipping and stor-
age purposes. In this study, it was shown, in a dry
reaction mixture, that ovalbumin was polymerized
following incubation with lactose and glucose (Kato
et al. 1988). In model studies with the milk protein
-lactoglobulin, the formation of protein dimers has
been observed on incubation with lactose (French et
al. 2002).
Proof of principle has thus been obtained in sever-
al systems, demonstrating that reactive cross-linking
molecules are able to cross-link food proteins within
the food matrix, leading to a noticeable change in the
functional properties of the food. However, much
work remains to be done in order to generate suffi-
cient quantities of cross-linking intermediates dur-
ing food processing to achieve a controlled change
in functionality.ENZYMATICMETHODSThe use of enzymes to modify the functional proper-
ties of foods is an area that has attracted consider-
able interest, since consumers perceive enzymes to
be more “natural” than chemicals. Enzymes are also
favored as they require milder conditions, have high
specificity, are only required in catalytic quantities,
and are less likely to produce toxic products (Singh
1991). Thus, enzymes are becoming commonplace
in many industries for improving the functional pro-
perties of food proteins (Chobert et al. 1996, Pou-
tanen 1997).
Due to the predominance of disulfide cross-
linkages in food systems, enzymes that regulate
disulfide interchange reactions are of interest to food
researchers. One such enzyme is protein disulfide
isomerase (PDI). PDI catalyzes thiol/disulfide ex-
change, rearranging “incorrect” disulfide cross-links
in a number of proteins of biological interest (Hill-
son et al. 1984, Singh 1991). The reaction involves
the rearrangement of low molecular weight sulf-
hydryl compounds (e.g., glutathione, cysteine, and