From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 7

the product hard to wash. Therefore, ethanol is the best medium for deacetylation. Ethanol
with a certain polarity and penetrability can efficiently penetrate into chitosan to increase
reaction efficiency. When the reaction occurs at 80°C with ethanol as the medium for 3 h,
DD can reach 90% if the weight ratio of chitosan to NaOH to ethanol is 1:3:16, while DD is
80% in the traditional method [20].
Microwave radiation heating greatly reduces alkali treatment time compared with the
traditional heating technique for preparing chitosan, and makes chitosan have high DD and
solubility. In 1979, Peniston tried to prepare chitosan by treating chitin with microwave radi-
ation in the normal alkali solution method for the first time [21]. Chitosan with 85% DD can
be formed by the normal alkali solution method by reacting in 50% NaOH solution at 100°C
for 10 h. When microwave is used, only 80°C and 18 min are enough for chitosan with over
80% DD. The semi-dry microwave method can also be applied for chitosan preparation,
comprising the following steps: uniformly mixing a concentrated alkali solution with chitin,
which is milled into a certain granularity in advance to form a paste, deacetylating in a
microwave oven, washing the product using hot water to neutral, steeping the product in
methanol, and drying the product in vacuum to form white or yellow grains [22].
The microwave radiation greatly shortens deacetylation time and lowers energy con-
sumption. But radiation also seriously breaks the chitin chain, making the product low in
relative molecular weight. Hence the method is particularly suitable for making chitosan
with high DD and low relative molecular weight. Microwave treatment increases the reac-
tivity of chitin and the reaction rate of deacetylation; hence the reaction time is shortened
and the alkali amount is reduced. It is a good way of saving material and lowering energy
consumption, so that the product cost of chitosan is saved. The industrial microwave reac-
tor will definitely bring remarkable benefits if it is developed.
The alkali solution catalysis method is suitable for chitosan with high DD and high
relative molecular weight. This method uses thiophenol and dimethylsulfoxide in addi-
tion to NaOH. The thiophenol is transformed into sodium thiophenol with deoxidizing
and catalytic functions in NaOH solution. Therefore, the reaction is accelerated and chain
breakage is prevented. The reaction medium is an alcohol-water solution of NaOH. The
phase transfer catalyst is cheap and harmless polyethylene glycol with good human com-
patibility (no need to remove the catalyst after reaction). The reaction condition is moder-
ate and with high DD, which can be obtained when concentration of NaOH is 35%, reaction
temperature is 90°C, reaction time is 3 h, and concentration of the phase transfer catalyst
is 5%. This technique can remove protein and prevent the degradation of chitin when
alkali concentration is low. It lowers acid and alkali consumption and shortens the pro-
duction period [23]. But it can only be used for preparing small amounts of samples in the
laboratory.
1.2.2 enzyme Method
Chitin deacetylase can hydrolyze acetyls of chitin, and so it may replace the hot concentrated
alkali method for producing high-quality chitosan. Chitin deacetylase was first found in
Mucor rouxii of zygomycetes in 1974 [24]. Electrophoretically pure chitin deacetylase from
Mucor rouxii with specific activity 13.33 U/mg can be formed by immune affinity chroma-
tography, and the yield is 29.1% [25]. Chitin deacetylases from different sources differ with
respect to relative molecular weight, isoelectric point, optimum pH, inhibitor, and distri-
bution, leading to different physiological functions. The reaction mechanism of the chitin
deacetylase from Mucor rouxii is multipoint attack mode; specifically, the enzyme system-
atically hydrolyzes acetyls from the nonreducing end of the binding site after it binds to a