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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,
94 Application of Alternative Food-Preservation Technologies Campaniello and Corbo the solubilization occurs by protonation of the NH2 groups on the C2 position of the D-glucosamine units according to the equation (1). Because of the positive charge on the C2 of the glucosamine monomer at pH < 6, chitosan is more soluble and has a better antimicrobial activity than chitin [8]. Chitosan-NH2 + H3O + ↔ Chitosan-NH3 + + H2O (1) Due to the presence of free amino groups, chitosan (pka = 6.5) is a cationic polyelectrolyte at pH < 6.5; consequently, this property along with the chelating ability of amine groups of macromolecule is used for the most of the applications of chitosan [8]. Chitosan preparations commercially available possess a degree of deacetylation (DD) > 85% with molecular weights between 100 kDa and 1000 kDa. They are usually complexed with acids, such as acetic or lactic acids [9]. Different studies focused on the possibility of obtaining reproducible and straightforward depolymerization methods for generating low molecular weight chitosan (LMWC) from high molecular weight chitosan (HMWC), through enzymatic or oxidative degradation, acidic cleavage and ultrasonic degradation. Liu et al. [10] reported that NaNO2 showed better performances during the depolymerization of chitosan if compared to H2O2 and HCl and these results were confirmed by other authors [11]; however no detail on the procedure was provided. To obtain low molecular weight fragments, Mao et al. [12] performed a depolymerization of chitosan through an oxidative degradation with NaNO2, thus producing a large series of chitosan with desired molecular weights by changing chitosan/NaNO2 molar ratio, chitosan concentration and reaction time. In a recent work, Baxter et al. [13] investigated the influence of high-intensity ultrasonication on the molecular weight and degree of acetylation of chitosan. In particular, the aim of their research was to develop a reaction kinetic model as a function of ultrasonic processing parameters to predict degree of acetylation and polymerization of ultrasonicated product; they concluded that high-intensity ultrasound could be a convenient and easily controllable methodology to produce this important functional carbohydrate. They observed that in presence of an acidic solvent neither power level (16.5, 28.0 and 35.2 W/cm 2 ) nor sonication time (0, 0.5, 1, 1.5 15 and 30 min at 25°C) altered the degree of deacetylation of chitosan molecules. Applications Properties such as biodegradability, low toxicity and good biocompatibility make chitosan suitable for use in biomedical and pharmaceutical formulations, for hypobilirubinaemic and hypocholesterolemic effects, antiacid and antiulcer activities, wound and burn healing properties (Fig. 3). Furthermore, applications of chitosan include wastewater purification, chelation of metals, coating of seeds, to improve yield and protection from fungal diseases and drug delivery system [13]. FOOD INDUSTRY removal dye, suspended solid preservative colour stabilization anticholesterol and fat binding flavour and taste Figure 3: Commercial applications of chitosan. BIOTECHNOLOGY enzyme immobilization protein separation cell recovery chromatography cell immobilization CHITOSAN MEDICAL bandage blood cholesterol control controlled release of drug skin burn contact lens AGRICOLTURE COSMETICS seed control fertilizer controlled agrochemical release moisturizer face, hand and body cream bath lotion WASTEWATER TREATMENT removal of metal ions flocculant/coagulant (protein, dye, aminoacid)
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Page 20 and 21: Essential Oils in Foods. Applicatio
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