Chemistry Reference
In-Depth Information
lard reactions and will not be considered here. In particular, the measurement
of advanced products of the Maillard reaction provides limited information
about the progress of the reaction because of the existence of induction phases
as a result of the formation of intermediates (Fayle and Gerrard, 2002;
Nursten, 2005). Similarly, the use of microbiological or rat bioassays has
declined as chemical methods have become more refined. Consequently, this
review will concentrate on the chemical methods for monitoring the Maillard
reaction.
7.7.1.
Determination of Available Lysine
Perhaps, the most direct approach to monitoring the Maillard reaction
in food systems is to measure the destruction of reactive amino acids. Clearly,
because of its abundance and reactivity, lysine is likely to be the most
important reactant in milk proteins although the destruction of arginine,
tryptophan and histidine may also become significant, depending on the
processing and storage conditions. A variety of methods are available for
the determination of available lysine in proteins and much experience has
been gained on the use of most of the chemical methods. A disadvantage of
such an approach is lack of knowledge of the amount of 'blocked' or
destroyed lysine obtained by measuring 'available' or reactive lysine. The
simplest approach for the estimation of available lysine in proteins is prob-
ably the dye-binding methods. A variety of anionic dyes combine at low pH
with the basic
"
-NH
2
, imidazole and guanidino groups of lysine, histidine and
arginine residues, respectively, and with the terminal
-NH
2
of proteins.
Binding of the dyes, Orange 12 and Remazol Brilliant Blue R, has been
reported to correlate well with the fluorodinitrobenzene (FDNB) method
for quantifying available
"
-NH
2
lysine groups (Hurrell and Carpenter,
1975). However, although the dye-binding methods are very rapid (Hurrell
et al., 1979), they suffer from lack of specificity and may also underestimate
the damage to lysine due to binding of dyes to some products of the Maillard
reaction (e.g. Amadori adducts).
Several methods are available for the chemical estimation of reactive
"
-NH
2
groups in proteins, including reaction with trinitrobenzene sulphonic
acid (TNBS; Kakade and Liener, 1969), O-methylisourea (Mauron and
Bujard, 1964), borohydride (Hurrell and Carpenter, 1974), [
14
C]succinic
anhydride (Anderson and Quicke, 1984) and fluorodinitrobenzene (FDNB;
Carpenter, 1960; Mottu and Mauron, 1967). Although still used occasionally
to measure the destruction of lysine in food systems, the TNBS method is very
unsatisfactory for this purpose due to reaction of the reagent with the
Amadori adduct (Hurrell and Carpenter, 1974). Of the methods listed
above, the FDNB procedure is probably the most satisfactory, although it
