R & D Overview What is Chitin? Molecular Structure of Chitin Applications of Chitin

Chitin is a white, horny substance found in the outer skeletons of crabs, and lobsters and in the internal structures of other invertebrates. It is a polysaccharide consisting of units N-Acetyl Glucosamine and its highly crystalline structure accounts for its poor solubility.

The seafood industry produces in excess of 3.5 million tonnes of solid waste each year and with this figure rising, there is a subsequent waste disposal issue. The utilisation of chitin currently represents a relatively small industry but through research into its properties and structure it is hoped that this abundant material may find greater uses. Estimation of molecular weight is one aspect of chitin specification.

The aims of this R & D project were to investigate:

1. the performance of two lithium chloride-tertiary amide solvent systems 

2. the rheology of chitin samples in order to establish molecular weights. 

Two types of chitin were investigated; the first ('Fluka' chitin) was extracted using the current industrial 'chemical' process based upon demineralisation by acid treatment and deproteination by alkali treatment. The second, referred to in this study as 'University chitin' was produced from waste scampi within the Chemical Engineering Department at Loughborough University on the laboratory scale, using a lactic acid fermentation process (Hall, G.M., et. al, 1994).

The two solvent systems investigated were: 

• Lithium Chloride / N,N-Dimethylacetamide

• Lithium Chloride / N-Methyl – 2 - Pyrrolidinone. 

The latter of the two proved to be the most effective solvent, although both required vigorous agitation of the chitin in the solution and neither were capable of dissolving 100% of the chitin added. The swollen chitin that remained was removed via a filtration stage and from this the chitin present in the solution calculated.

The molecular weights were determined using u-tube viscometry and applying the Sakurade-Houwink equations. This method produced results that were consistent with literature and repeatable. The average molecular weight for the Fluka chitin was 350000, in comparison to 650000 for chitin produced via the fermentation process. Neither solvent system was capable of dissolving the un-treated University chitin, and so the molecular weights recorded for the University chitin were obtained only after the completion of a purification stage. The difference in molecular weights can perhaps be accounted for by the extent of de-polymerisation occurring in the two different extraction processes.

Chitin, pronounced KITE-IN, was first isolated in 1811 by Henry Braconnot and along with Cellulose and Starch, it is one of the most abundant polysaccharides in nature. It is found naturally in the shells of crustaceans, insect exoskeletons, fungal cell walls, microfauna and plankton. It is found in association with proteins and minerals such as Calcium Carbonate and it is estimated that approximately ten gigatonnes of chitin are synthesised and degraded each year in the biosphere.

When chitin is isolated from an appropriate source, it is an off white, non-toxic, biodegradable, tasteless powder and in its purest form contains less than 0.5% protein and less than 0.6% Calcium Carbonate.

 

Molecular Structure of Chitin

Chitin is a polymer formed primarily of repeating units of beta (1-4) 2 - acetamido-2-deoxy-D-glucose (or N-acetylglucosamine). Its structure is similar to the structure of Cellulose (Figure 1), except that acetylamino groups have replaced the hydroxyl groups in position 2. Approximately 16% of naturally occurring chitin units are deacetylalated.

Figure 1: The molecular structure of chitin

The various sources of chitin differ in their structure and chitin content. There are three polymorphic forms of chitin, a, b and g. The most abundant form of chitin is a-chitin where the polymer bonds are tightly bonded in an anti-parallel arrangement that leads to a highly crystalline structure; hence chitin has poor solubility and reactivity.

Applications of Chitin

The properties of chitin have made it attractive for use in a wide variety of applications including: -

1)       Agricultural

Chitin and chitosan have anti-fungal properties that can be used to protect seeds from soil fungi by coating the seeds with chitin or chitosan. Chitin and chitosan can also be used as an antinematode agent in soil and both are biodegradable.

2)       Cosmetics

Chitin and chitosan are non-toxic and non-allergenic which means that the body won't reject them as foreign invaders; hence they can be used in the production of emulsifiers, anti-static agents and emollients to extend the cosmetic product shelf life.

3)       Medicinal

Chitin and chitosan have anti-bacterial and anti-viral properties that have led to them being used as wound dressings, surgical sutures and in cataract surgery and Periodontal disease and burns treatment.

4)       Food Industry

Chitin can be used to recover proteins from food processing wastes to be used in animal food production. Mycrocrystalline Chitin (MCC) has been used as a thickening/gelling agent in the binding, stabilising and texturing of food.