The complex world of polysaccharides edited by Desiree Nedra Karunaratne

 

Is Chitosan a New Panacea? Areas of Application 

 

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Chitin is a big molecule composed of –beta-1,4-N-acetylglucosamine (GlcNAc) monomers. 

There are three forms of chitin: α, β, and γ chitin. The α-form, is mainly obtained from crab 

and shrimp. Both α and β chitin/chitosan are commercially available [5]. 

3. Sources of chitin 

In the book “Chitin” published by Muzarelli in 1977, we can find a complete list of 

organisms that contain chitin: Fungi, Algae, Cnidaria (jellyfish), Aschelminthes (round 

worm), Entoprocta, Bryozoa (Moss or lace animals, Phoronida (Horseshoe worms), 

Brachiopoda(Lamp shells), Echiruda, Annelida (Segmented worms), Mollusca, 

Arthropoda and Ponogophora [6]). Herring, P.J in 1979 wrote that chitin is the main 

component of arthropod exoskeletons, tendons, and the linings of their respiratory, 

excretory, and digestive systems, as well as insects external structure and some fungi. It is 

also found in the reflective material (iridophores) of both epidermis and eyes of 

arthropods and cephalopods (phylum: Mollusca) and the epidermal cuticle of the 

vertebrate Paralipophrys trigloides (fish) is also chitinous [7,8]. The main commercial 

sources of chitin are the shell wastes of shrimp, lobsters, crabs and krill. There are three 

forms of chitin: α,  β, and γ chitin. The α-form, is composed of alternating antiparallel 

polysaccharide strands and is mainly obtained from crab and shrimp. α -Chitin is by far 

the most abundant; it occurs in fungal and yeast cell walls, krill, lobster and crab tendons 

and shells, shrimp shells, and insect cuticle. The rarer β -chitin is composed of parallel 

strands of polysaccharides, is found in association with proteins in squid pens [9,10] and 

in the tubes synthesized by pogonophoran and vestimetiferan worms [11,12]. It also 

occurs in aphrodite chaetae [13] as well as in the lorica, built by some seaweeds or 

protozoa [14,15,16]. And 2 parallel chains alternating with an antiparallel strand 

constitute gamma chitin and are found in fungi [15]. 

 

Figure 1.

 Chitinous structure of worm and insects 

4. Chitin from crustacean 

Currently most commercial production of chitin is based on extracting it from the 

exoskeleton of shrimp, prawn, crab and other crustaceans. This source contains a high 

percentage of inorganic material, primarily CaCO

3

 and a rough calculation indicates that for 

every tonne of chitin produced, 0.8 tonne of CO

2

 is released into the environment. In view of 

current concerns about global warming this cannot be considered to be a truly 

environmentally friendly process [3]. 

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The Complex World of Polysaccharides 

 

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Another source of chitin that is more environmentally friendly, although much more limited 

in volume, is squid pen. This waste contains very little in the way of inorganic material and 

very little, if any CO

2

 would be released in the extraction and purification process. Another 

and perhaps more sustainable source in the long run is vegetable chitin from fungal sources 

such as waste mycelia. There is extensive literature on the topic, but it is only recently that it 

has become commercially available [3].  

 

Figure 2.

 Exosqueleton of crustacea, this is the source of commercial chitin 

5. Chitin from fungi 

Chitin is widely distributed in fungi, occurring in Basidiomycetes, Ascomycetes, and 

Phycomycetes, where it is a component of the cell walls and structural membranes of 

mycelia, stalks, and spores. The amounts vary between traces and up to 45% of the organic 

fraction, the rest being mostly proteins, glucans and mannans. However, not all fungi 

contain chitin, and the polymer may be absent in one species that is closely related to 

another. Variations in the amounts of chitin may depend on physiological parameters in 

natural environments as well as on the fermentation conditions in biotechnological 

processing or in cultures of fungi [4]. 

The chitin in fungi possesses principally the same structure as the chitin occurring in other 

organisms. However, a major difference results from the fact that fungal chitin is associated 

with other polysaccharides which do not occur in the exoskeleton of arthropods. The 

molecular mass of chitin in fungi is not known. However, it was estimated that bakers' yeast 

synthesizes rather uniform chains containing 120 ± 170 GlcNAc monomer units which 

corresponds to 24,000 ± 34,500 Daltons [4]. 

6. Chemical methods to prepare chitin  

Several procedures have been developed through the years to prepare chitin; they are at the 

basis of the chemical processes for industrial production of chitin and chitosan. Various 

methods are reported in Muzzarelli’s book such as: Method of Rigby (1936 and 1937); 

Hackman (1954); Foster and Hackman (1957); Horowitz, Roseman and Blumenthal (1957); 

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Is Chitosan a New Panacea? Areas of Application 

 

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Whistler and Be Miller (1962); Takeda and Abe (1962); Takeda and Katsuura (1964); 

Broussignac (1968); Lovell, Lafleur and Hoskins (1968); Madhavan and Ramachandran 

(1974) [6]. There is also a review that summarizes methods of preparation of various chitin 

and its conversion to chitosan [17]. 

7. Enzymatic methods to prepare chitin  

A new process for deproteinization of chitin from shrimp head was studied [18]. Recovery 

of the protein fraction of the shrimp waste has been widely studied by enzymatic hydrolysis 

method [19,20].The enzymatic deproteinization process has limited value due to residual 

small peptides directly attached to chitin molecules ranging from 4.4% to 7.9% of total 

weight [21]. As these processes are costly because of the use of commercial enzymes, there is 

now a need to develop an efficient and economical method for extracting proteins from 

shellfish waste. One interesting new technology for extraction of chitin that offers an 

alternative to the more harsh chemical methods is fermentation by using microorganisms. 

Fermentation has been envisaged as one of the most ecofriendly, safe, technologically 

flexible, and economically viable alternative methods [22-28]. Fermentation of shrimp waste 

with lactic acid bacteria results in production of a solid portion of chitin and a liquor 

containing shrimp proteins, minerals, pigments, and nutrients [26,29]. Deproteinization of 

the biowaste occurs mainly by proteolytic enzyme produced by Lactobacillus [30]. Lactic 

acid produced by the process of breakdown of glucose, creating the low pH condition of 

ensilation; suppress the growth of microorganisms involved in spoilage of shrimp waste 

[31]. The lactic acid reacts with calcium carbonate component in the chitin fraction leading 

to the fermentation of calcium lactate, which gets precipitated and can be removed by 

washing. There is now a need to develop an efficient, simpler, eco- friendly, economical, and 

commercially viable method. 

8. Chitosan 

Despite the wide spread occurrence of chitin, up to now the main commercial sources of 

chitin have been crab and shrimp shells. In industrial processing, chitin is extracted from 

crustaceans by acid treatment to dissolve calcium carbonate followed by alkaline extraction 

to solubilized proteins. In addition a decolorization step is often added to remove leftover 

pigments and obtain a colorless product. These treatments must be adapted to each chitin 

source but by partial deacetylation under alkaline conditions, one obtains “chitosan” [16]. 

Chitosan is the most important derivate of this naturally occurring polymer being one of the 

most abundant polysaccharides after cellulose. Chitosan is a copolymer composed of N-

acetyl-D-glucosamine and D-glucosamine units. It is obtained in three different ways, 

thermochemical deacetylation of chitin in the presence of alkali, by enzymatic hydrolysis in 

the presence of a chitin deacetylase, or naturally found in certain fungi as part of their 

structure. In chitosan part of the amino groups remain acetylated. It is generally accepted 

that N-acetylglucosamine residues are randomly distributed along the whole polymer chain. 

In an acid medium, amino groups are protonated and thus determine the positive charge of 

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