3666 J. Agric. Food Chem., Vol. 54, No. 10, 2006
Haghighat Khajavi et al.
Figure 4. Arrhenius plot of the rate constant, k, for the hydrolysis of
maltotetraose. Symbols: (]) k4 3, ([) k4 2, and (4) k3.
f f
1.1 × 102 kJ/mol, respectively. The E value for k4f3 was lower
than that for k4f2, indicating that the energy barrier for the
cleavage of the exo-site bond would be lower than that for the
cleavage of the endo-site bond.
Figure 3. Hydrolysis of maltotetraose at (a) 200, (b) 220, (c) 240, and
(d) 260
) maltose, (
eqs 2a d and the rate constants estimated in this study. The dotted lines
°
C and 10 MPa. Symbols:
(]
) maltotetraose, (4) maltotriose,
(
0
3
) glucose, and ([) pH. The curves were drawn using
−
connect the experimental results.
Conclusion. The hydrolysis of maltooligosaccharides with
DPs of 3-6 in subcritical water consecutively proceeded. The
rate constants for the cleavage of each bond were evaluated by
applying first-order kinetics to the experimental results at short
residence times. A weak site specificity was found for the
hydrolysis, and the rate constants for the exo-site glucosidic
bonds were slightly greater than those for the endo-site bonds.
The activation energy for the cleavage of the exo-site bond was
smaller than that for the cleavage of the endo-site bond for the
hydrolysis of maltotetraose.
In our previous study (7), it was demonstrated that the rate
constant for the hydrolysis of various disacharides could be
correlated to the electrostatic potential charge of the glucosidic
oxygen atom. For the site specificity for the hydrolysis of the
maltooligosaccharides used in this study, the electrostatic
potential charge might also be an important factor. On the other
hand, an increase in the DP induces the flexibility of fluctuation
(structural hindrance) of the maltooligosaccharide molecule. This
fluctuation might also induce the difference in the probability
of the proton attack on the glucosidic bond. Further investigation
into the importance of electrostatic potential change and chain
flexibility during hydrolysis in subcritical water, while beyond
the scope of this present study, might provide useful insight to
enhance our understanding of these phenomena.
LITERATURE CITED
(1) Clifford, T. A single substance as a supercritical fluid. In
Fundamentals of Supercritical Fluids; Clifford, T., Ed.; Oxford
University Press: New York, 1998; p 23.
Hydrolysis of Maltotetraose at Different Temperatures.
Figure 3a-d shows the changes in the concentration versus
the residence time of maltotetraose and its hydrolysates at
temperatures of 200-260 °C and 10 MPa. At every temperature,
the concentration of the saccharides with a shorter DP increased
at the longer residence times. This tendency was in agreement
with that reported by Carvalheiro et al. (16). The concentration
of G1, which is a constituent monomer of the maltooligosac-
charide, was higher for the longer residence times and higher
temperatures. The rate constants, k4f3, k4f2, and k3, were
evaluated by methods similar to those described previously.
Although the k3 value was estimated for the hydrolysis of
maltotriose in the previous section, it was treated as a parameter
to be determined as well as the k4f3 and k4f2 values.
(2) Miller, D. J.; Hawthorne, S. B.; Gizir, A. M.; Clifford, A. A.
Solubility of polycyclic aromatic hydrocarbons in subcritical
water from 298 K to 498 K. J. Chem. Eng. Data 1998, 43, 1043-
1047.
(3) Savage, P. E. Organic chemical reactions in supercritical water.
Chem. ReV. 1999, 99, 603-621.
(4) Haghighat Khajavi, S.; Kimura, Y.; Oomori, T.; Matsuno, R.;
Adachi, S. Degradation kinetics of monosaccharides in subcritical
water. J. Food Eng. 2005, 68, 309-313.
(5) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K.
Kinetics of glucose epimerization and decomposition in sub-
critical and supercritical water. Ind. Eng. Chem. Res. 1997, 36,
1552-1558.
(6) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K.
Glucose and fructose decomposition in subcritical water: De-
tailed reaction pathway, mechanisms, and kinetics. Ind. Eng.
Chem. Res. 1999, 38, 2888-2895.
The temperature dependence of a rate constant, k, is in many
cases expressed by the Arrhenius equation.
(7) Oomori, T.; Haghighat Khajavi, S.; Kimura, Y.; Adachi, S.;
Matsuno, R. Hydrolysis of disaccharides containing glucose
residue in subcritical water. Biochem. Eng. J. 2004, 18, 143-
147.
(8) Bobleter, O.; Bonn, G. The hydrothermolysis of cellobiose and
its reaction product D-glucose. Carbohydr. Res. 1983, 124, 185-
193.
(9) Kabyemela, B. M.; Takigawa, M.; Adschiri, T.; Malaluan, R.
M.; Arai, K. Mechanism and kinetics of cellobiose decomposition
in sub- and supercritical water. Ind. Eng. Chem. Res. 1998, 37,
357-361.
k ) k0 exp(-E/RT)
(5)
where k0 is the frequency factor, E is the activation energy, R
is the gas constant, and T is the absolute temperature. Figure 4
shows the Arrhenius plots for the k4f3, k4f2, and k3 values. For
every rate constant, the plots lay on a straight line on a
semilogarithmic scale. The k0 and E values for k4f3, k4f2, and
k3 were evaluated to be 2.2 × 108 s-1 and 9.9 × 101 kJ/mol,
3.6 × 1010 s-1 and 1.2 × 102 kJ/mol, and 7.6 × 109 s-1 and