application of alternative food-preservation - Bentham Science

Chitosan in foods Application of Alternative Food-Preservation Technologies 93
interchangeable in terms of order, depending on the proposed use of chitin. DP and DM steps produce a coloured
product, but if a bleached chitinous product is desired, pigments can be removed with reagents such as ethanol,
ether, sodium hypochlorite solution, absolute acetone, chloroform, hydrogen peroxide or ethyl acetate. This
process is too expensive; however removing the DC step could reduce considerably production cost. It is
important to point out that the use of bleaching agents reduce considerably the viscosity of the chitosan and
sometime cause an undesirable light brown colour; therefore Youn et al. [3] studied an alternative and economic
decolouration method, that yields decolourized chitosan with high viscosity through the use of sun drying.
N-deacetylation involves an alkaline hydrolysis with sodium hydroxide or potassium hydroxide at elevated
temperature under heterogeneous conditions, which can result in an incomplete N-deacetylation and in a
depolymerization to varying extents, thus obtaining chitosans with different molecular weight (high, medium
and low molecular weight).
The degree of deacetylation is defined as the percentage of acetylated monomers referred on the total units; it is a
function of alkali concentration, temperature, size of particles and reaction time. For example, it is well known
that an ordinary reaction time (1 h) leads to a partial deacetylation (about 80%), whereas a reaction time of 48 h
results in a complete deacetylation. However, a high degree of deacetylation (realised in drastic condition) causes
a reduction of molecular weight of polymer.
Yen et al. [4] prepared chitosan from shiitake (Lentinula edodes (Berkeley) Pegler) stipes, a potential source of
fungal chitosan, usually discarded due to their tough texture. They isolated fungal chitin from stipes using
alkaline treatment, followed by a decolourization with potassium permanganate and a N-deacetylation treatment
with a sodium hydroxide solution.
Aqueous base solutions
(NaOH or KOH) are used
for the DP step and the
effectiveness depends on
the ratio shell/solution,
temperature,
concentration of alkali and
reaction time.
Shellfish waste
deproteinization
demineralization
decolouration
chitin
deacetylation
chitosan
DM can be achivied using
diluted HCl (1‐8%) at
room temperature for 1‐3
h, or other acids such as
acetic and sulfuric acids
Figure 2: Simplified flowsheet for the preparation of chitosan, from shellwish waste. (modified from Shahidi et al. [5])
The use of chemicals throughout chitosan preparation has several disadvantages like a complicated recovery of
shell-waste products (proteins, pigments, etc.) or the generation of large quantities of hazardous chemical waste.
Fermentations with proteolytic or chitinolytic enzymes may be an alternative with varying levels of success: for
example, chitin deacetylase from either Mucor rouxii or Absidia butleri and Aspergillus nidulans convert chitin
to chitosan. Hayes et al. [6] reported a detailed review on the methods used to extract and characterize chitin,
chitosan and glucosamine obtained through industrial, microbial and enzymatic hydrolysis of shell waste.
The degree of deacetylation depends on both the raw material, from which chitin has been obtained, and the
procedure influences the fraction of free amino groups, that can interact with metal ions. Chitosan is a waterinsoluble
compound; however, when the degree of deacetylation is larger than 40-50%, chitosan becomes
soluble in acidic media [7]. Although the distribution of acetyl groups along the chain may modify the solubility,