4. Conclusion
Waxy maize starch amylolysis catalyzed by -amylase from
Bacillus licheniformis was studied in terms of the quantitative evolution
of the hydrolysis reaction, as well as in terms of morphological
and crystallinity characteristics of partially hydrolyzed starch granules.
The two methods used to determine reaction kinetics showed
a change of reaction rate at approximately 6 h of reaction, delimiting
two hydrolysis stages which have previously been attributed
to the hydrolysis of starch regions with different crystallinity.
Besides, for the mentioned hydrolysis time, the use of two different
methods for CI calculation showed a maximum of crystallinity
in residual starch granules. Scanning microscopy images illustrated
the pattern of -amylase action over starch granules, with
the most interesting feature being the generation of clear layered
hydrolyzed/non-hydrolyzed interior structures, which suggested
different susceptibility to enzyme attack of alternating growth
rings. Image analysis allowed finding very good estimations of the
growth rings thicknesses which agreed with values found in the
literature.
All three type of analysis performed herein (kinetic data, SEM
and XRD analysis), contributed to the validation of the hypothesis
that under the assay conditions chosen, in the initial hours of reaction
amylolysis effectively proceeds preferentially and at a higher
rate in the less ordered regions of starch. Moreover, results showed
that if reaction is stopped at 6 h of hydrolysis, micrometric starch
granules with increased crystallinity obtained by a pure enzymatic
protocol can be easily produced.