Chemistry Reference
In-Depth Information
moisture content has an advantage in that it avoids difficulties caused by the
dependency of a
w
on temperature. However, moisture content as a predictor
of browning may be equally flawed if a phase change occurs in the system,
which may be possible at constant moisture content.
7.4.5.
Miscellaneous Factors
Phosphate, citrate and phthalate buffers have been shown to accelerate
the Maillard reaction (Bobbio et al., 1973). However, it is unclear if such
anions had a catalytic effect per se or if the effect was simply pH related due to
differences in buffering capacity. At constant pH in the range 5-7, phosphate
has a dramatic effect on reaction rate which increases up to 15-fold over
that of a phosphate-free system (Potman and van Wijk, 1989). Chan and
Reineccius (1994) showed that the activation energy for the formation of
2-acetylfuran and di(H)di(OH)-6-methylpyranone in a model system was
approximately halved by conducting the reaction in phosphate buffer.
Potman and van Wijk (1989) concluded that phosphate acts as an acid-base
catalyst during the Amadori rearrangement and does not react directly in the
Maillard reaction. Increasing the concentration of phosphate or citrate
increased the rate of browning in cows' or buffalo milk on sterilization
(Gothwal and Bhavadasan, 1991). When citrate or phosphate levels were
increased by 50%, there was an increase in the browning index of
17-23%
in buffalo or cows' milk, the increase being slightly greater in the latter.
Although the pH values following sterilization were similar, the addition of
phosphate or citrate salts led to a change in the initial pH (up to
0.2 pH units
for a 50% increase). It is likely that the increase in initial pH was responsible
for the increase in browning. The study of Gothwal and Bhavadasan (1991)
illustrates clearly the extreme sensitivity of browning reactions in milk sys-
tems to even small changes in pH. Citric acid has a marked catalytic effect on
the rate of 5-HMF formation from fructose at constant pH (3.5) (Lee and
Nagy, 1990).
The overall Maillard reaction and the rate of browning of Amadori
products are accelerated by Fe(II), Fe(III) and Cu(II) (Patton, 1955; Kato
et al., 1981b; Pilkova et al., 1990). Some of the effects of metals on the
Maillard reaction may be due to pH effects, which is the case for aluminium
and zirconium compounds (Powell and Spark, 1971). There is evidence that
complex formation between intermediates in the Maillard reaction and the
metal ion is a prerequisite for the promoting activity of Fe and Cu on the rate
of browning (Kato et al., 1981a). The presence of copper ions (as CuCl
2
)
promoted the rate of browning in glucose-glycine, glyoxal-glycine and
5-HMF-glycine systems (Rendleman and Inglett, 1990). The activity of
copper in this study also appeared to be associated with its ability to form
