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Several chemicals make water safe to wash in. But only one will also help clean your hair and teeth, keep your sandwiches fresh, strengthen your newspaper and dress your injuries, all without damaging the environment. The chemical in question is chitin, best known as the substance that makes the shells of crustaceans hard. From this unlikely source, a new industry is growing, based on a natural material that some researchers think may eventually prove as adaptable and useful as plastics.

Chitin is the main structural component of the shells of crustaceans, molluscs and insects. It also makes up parts of the jaws and body spines of certain worms, and is found in the cell walls of fungi and in some algae. Henri Bracannot was the first to describe chitin – he called it fungine – as long ago as 1821. We now know that it is a natural polymer that strongly resembles cellulose, the main component of plant cell walls. Chitin is almost as common as cellulose – an estimated billion tonnes are synthesised every year – and this ubiquity holds a clue to some of its potential uses.

At first sight, however, chitin does not look at all promising. Chemically, it is a fairly dull molecule. Like cellulose, it can be broken down by enzymes, but only slowly, and it will not dissolve in most ordinary solvents like water or alcohol. It is usually bound to porteins to form large, complex molecules and its purity varies enormously. Even chitin from the same animal varies in the length of its molecular chain, its cyrstalinity, and in the number of acetyl (CH₃C)O groups hanging off the chain.

But in 1959, a chemist called Rouget found that heating chitin with a very concentrated sodium hydroxide converts it to a related and much more useful chemical, called chitosan. This reaction removes some of the acetyl groups from the molecular chain, leaving behind complete amino (NH₂) groups (see Figure). Increasing the temperature or the strength of the sodium hydroxide solution removes more acetyl groups. In this way chemists can produce a range of chitosan molecules with different properties and applications. Unlike chitin, chitosan dissolves easily in acidic solvents like acetic acid.FIG-mg17555401.jpg

Chitosan’s versatility depends almost entirely on its amino (NH₂) groups. When dissolved in acids, these groups add proton, becoming (NH₃)⁺ and giving chitosan a positive electrical charge overall. This makes the molecule extremely effective for removing negatively charged particles that are dissolved or suspended in water, such as lignosulphates and natural tannins. Chitsan form ionic, or sometimes hydrogen bonds with these molecules, desotabilising the suspension so that they precipitate out as insoluable solids.

One of the earliest uses of chitin was to purify waste water from the processing of shellfish. Processing plants produce contaminated water as well as solid waste, such as shells and viscera. Crustacean fisheries are very wasteful – up to 85 per cent by weight of each animal is thrown away, which amounts to over 3 million tonnes of solid waste every year. Some fisheries already use chitin derived from the solid waste to purify their own waste water. A study of one crawfish processing plant in Louisiana in 1989 showed that chitosan derived from the waste could be used to remove 97 per cent of the solids suspended in waste water this way.

Now some companies are promoting chitin for the clarification and purification of other types of contaimined water. Chitin and chitosan are also good chelators. This means they can bind at several points, rather like the grip of a claw, to metal atoms in solution, especially heavy metals such as mercury, lead and uranium, although no one knows quite how. This useful property could be exploited as the basis of a method for treating waste water that is toxic or radioactive. Japanese firms such a Kurita Industries sell chitosan as a flocculant. So does the Norwegian company Protan, which recommends it for the clarification of swimming pools and spas as its flocculates microbes and removes metals.

Waste-water treatment is only one of many suggested and proven uses for chitin and chitosan. Of these, cosmestics is one of the longest established. Chitin was first used in cosmetics in 1969; more recently, Japanese and German companies have been developing chitosan salts – soluble in water, and formed simply by treating chitosan with acid – for use is cosmetics for skin and hair. The German cosmetics giant Wella has been researching chitosan as hair treatment for 10 years. It has experimented with the film-forming properties of chitosan in hair sprays and nail varnishes and uses its thickening effects in creams and conditioners. In Japan, at least five companies manufacture chitin and chitosan, mainly from crab shells.

Chito-Bios of Ancona in Italy sell N-carboxybutyl chitosan, under the trade name EvalsanR, for shampoos, bath foams, liquid soaps, toothpaste, personal-hygiene detergent and face creams. The company uses this derivative of chitin as a replacement for hyaluronic acid, a common component of creams and lotions. It emphasises that chitosan is ‘more than a comestic ingredient’ and could be useful for dressing wounds, for surgery and dentistry.

But perhaps the greatest potential application is paper manufacture. Adding only 1 per cent by weight of chitin to pulp increases the strength of the paper, speeds up the rate at which water drains from the pulp and increases the quantity of fibres retained when making sheets of paper. So manufacturers can use cheaper, weaker fibres, without reducing quality, while saving up to 90 per cent of the energy they use to beat the pulp. Chitin also makes the paper easier to print on.

Paper that incorporates chitin has greatly improved wet strength – an advantage for diposable nappies, shopping bags and paper towels. But these benefits must be offset against the problems of supply. The current world production of chitin from all sources would be overwhelmed by an industry which in 1986 produced 172 million tonnes of newsprint. Any move towards the general use of chitin in paper manufacture would require a huge increase in chitin production. Where would it come from?

Maintaining fisheries of shellfish or molluscs just to harvest their chitin is unlikely to be economic, as they only contain around 1 per cent chitin by weight. This leaves two possible sources of chitin: shellfish waste and fungal fermentation. The pharmaceutical industries of most countries already exploit molecules, including vitamin C and penicillin. The process also produces large quantities of chitinous waste – estimates are difficult to find, but in 1977, one researcher gave a figure of 790,000 tonnes. Unlike shellfish waste, this source of chitin is predictable – a set input will produce a set output – and its quality can be controlled.

Several countries, including the US, Japan, Norway, Italy and India, already have chitin/chitosan plants based on shellfish waste as their source. The little they produce is used by the pharmaceutical industry and in the treatment of waste waters. There are no reliable data for how much chitin should in theory be available from crustacean fisheries, but according to the FAO’s latest figures, in 1987 the world crustacean harvest was 3.69 million tonnes. Assuming chitin forms 1 per cent of the wet weight of a crustacean, on average, we are squandering about 36,700 tonnes of chitin each year as waste from the processing of shrimps, prawns, lobsters and crabs. The main problem is that it would be unecomomic to collect the waste from many small processing plants, so this source can only be tapped where large quantities of crustaceans are being handled.

The largest potential source of animal chitin is the zooplankton that inhabit the upper layers of the sea. But only one crustacean which could be loosely considered a member of the zooplankton is currently being harvested to any significant degree. This is the Antarctic krill. In 1989/90, fishing fleets caught 375,000 tonnes, making it the largest crustacean fishery in the world (‘Who’s counting on krill?’, New ÐÓ°ÉÔ­´´, 11 November 1989). The krill fishery is only marginally economic. Most of the catch is either processed for its tail meat, which is destined for human consumption, or is used whole for aquaculture.

Peeling krill is no easy task and leaves 85 per cent by weight as waste. Of this waste, 85 per cent is recoverable protein. Almost a quarter of the deproteinised waste is chitin, compared wiht 3.2 per cent in the whole animal. About 90 per cent of this chitin can be recovered by conventional extraction techniques. This is half as efficient again as from crab chitin, although the figure would not be so high on board a trawler.

The fishery will probably expand from its current levels and the total krill stock in the southern ocean is now thought to be between about 100 and 400 million tonnes. Several millions tonnes of this could be harvested annually. Such a catch would dominate the total world curstacean catch of about 4 million tonnes and would be a major potential source of chitin. Other sources include squid, whose pens are 40 per cent chitin and largely free of minerals and bivalve molluscs, whose shells oftain contain a high proportion of minerals. (Minerals add to the weight of material and therefore the cost of processing.) Insects have chitin but it is quinone tanned, which makes it difficult to extract, and there is no consistent source.

So it seems that the major source of chitin in the future will probably be biotechnology rather than seafood waste. The infant chitin/chitosan industry will probably develop by using cheap supplies of waste materials, but if demand increased sufficiently, manufacturers could develop genetically engineered microorganisms to produce these useful molecules. Cultured strains of microorganisms will be able to produce chitin with desired properties under controlled conditions and in fixed quantities. This would sever the link between chitin production and the widely fluctuating market for protein. Chitin is easily extracted from fungal hyphae and some species even produce up to 14 per cent by weight of chitosan. Culturing chitosan-producing strains would eliminate the deacetylation step that converts chitin to chitosan. Although this step is fairly simple, it makes chitosan nearly twice as expensive to product as chitin.

Certain algae produce pure chitin in the form of extracellular fibres which can be between 10 and 15 per cent of the dry weight of the cells and can be readily separated from the non-chitinous structures with a yield of 80 per cent. But these algae grow only slowly under normal conditions. Researchers hope that advances in biotechnology will give them fast-growing strains that retain large amounts of chitin.

Chitin and its derivatives are shaping up to be as versatile as plastics. Unfortunately, although chitin and its derivatives can do many things well, there are few functions that they alone can carry out. Chitin-based products usually have to compete with those produced by established biochemical technologies. On the other hand, a ‘natural’ material that uses up waste, is biodegradable and does not damage the environment may have a bright future.

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Industry shells out for chitin

Many chemical, medical and pharmaceutical companies are now researching and in some cases developing and patenting chitin-based products. Protan, a Norwegian company, has been producing and selling chitin and chitosan from shellfish waste since 1984. It lists 13 broad areas for its products, from ‘personal care’ to detoxification of industrial waste.

Other applications include treatment of sewage, dairy waste, paper mill effluent, food-factory waste, liquid radioactive waste and purfication of drinking water. In Japan about 500 tonnes of chitin are used every year as a water purifier, and the US Environmental Protection Agency rates chitosan as acceptable for the purfication of drinking water.

Using chitosan to remove suspended solids from food-processing wastes, such as cheese whey, has an additional benefit. As well as purified effluent, the method yields coagulated by-products rich in proteins which can be added to feed for domestic animals. This seems to make the feed more digestible.

Chitin and its derivatives also have some very useful properties in the medical field. Between 1968 and 1975 researchers working for the American pharmaceuticals company Lescarden of Goshen, New York, filed five patents for the use of chitin and chitosan to accelerate wound healing. They found the chitin mats, fibres, sponges, sutures and films were much better than standard cartilage-based ones. The pharmaceuticals company Katakurachikkarin based in Hokkaido makes an artifical skin – a chitosan-collagen composite – that appears to enhance recovery from surgical wounds or burns. In 1983, doctors working for the Veterans Administration Medical Center in Omaha discovered that chitosan could also speed up blood clotting and used it to reduce the loss of blood following blood vessel grafts.

Chitosan can be produced in numerous forms – powder, paste, solution, film, fibre or spray – giving manufacturers huge scope for incorporating it into bandages, dressings, salves, sutures or disposable contact lenses. The body does not seem to reject these and they break down slowly to harmless carbohydrates, carbon dioxide and water. Because chitosan is absorbed completely in the body, it is an ideal carrier for drugs that must be released slowly. After tests on rats in 1978, some Japanese researchers claimed that chitosan reduces serum cholestrol. In Japan you can now buy biscuits and noodles sold for the alleged benefits of the chitin they contain.

The food industry is developing ways of exploiting the emulsifying properties of chitosan to make mayonnaise and peanut butter. Chitosan could eventually find its way into an area where non-toxic, high strength films are required, form sausage casings to oven wraps and food packaging.

Some researchers even think these chemicals will be the basis of a biodegradable plastic. Technics, the hi-fi manufacturer, of Schizuoka in Japan has even made the vibrators of flat-panel speakers from chitosan, an idea which is supposedly based on the acoustic properties of crickets’ wings.

Stephen Nicol is a senior research scientist with the Australian Antarctic Division, Kingston, Tasmania.

Further reading Chitin in Nature and Technology, edited by R Muzzarelli, C Jeuniaux and G W Gooday, Plenum, 1986; Chitin and Chitosan, edited by G Sjak-Braek, T Anthonsen and P Sandford, Elsevier, 1989.