Contents

Treasure from Trash

Chitin Breakdown:
The Bacterial Way

Marine Ornamentals
and Aquaculture

By Merrill Leffler

April marks the beginning of blue crab season in the Chesapeake. Crabs are already moving out of the deep channels and trekking north from winter grounds near the Bay's mouth and, in response, watermen start setting their traps. By May, processing plants on Maryland's Eastern Shore are in full swing, more than 30 of them, picking crab meat and discarding the leftovers.

Those leftovers are not insubstantial. For every pound of picked crabmeat, some six pounds of shell and runny chum are left behind, says Andrew Tolley, who owns Toddville Seafood, near Cambridge. In an average year on Maryland's Eastern Shore, he estimates that waste can add up to eight million pounds. Four thousand tons - a small smelly mountain of potential problems.

Decaying crab wastes release ammonia and nitrates that evaporate in air and seep through soil; in large enough concentrations, they can pollute the slowing moving ground waters below and, with them, freshwater wells that provide drinking water and shallow aquifers that eventually feed into the Bay. It is for such reasons that Dorchester and other Eastern Shore counties have been closing down landfills to the burial of crab waste.

And yet within that discarded scrap heap lies a potential mine of profit. The ore in this case is chitin, a substance that makes up some 20 percent of the shell (another 60 percent is calcium carbonate and 20 percent is protein). Chitin is a polymer - a large natural molecule composed of repeating units of simple sugar molecules; next to cellulose, it is the most abundant polymer on earth. The shells of crustaceans - like crabs, lobsters and shrimp - of zooplankton, of insects, and the cell walls of fungi such as mushrooms, all are composed of chitin.

Two U.S. companies - one on the west coast and one in New Jersey - are already extracting chitin from crab waste; so are companies in Asia and Europe. They modify chitin to produce chitosan, a compound that offers applications ranging from medical sutures and seed coatings to dietary supplements and coagulants for waste treatment. Chitosan is hailed by many for its extraordinary versatility.

So what is the problem in the Bay region? With the largest crab harvests on the east coast, why is there no chitin-processing industry on the Eastern Shore? Such an industry would seem to be a win-win situation: it would create new jobs and it would convert crab waste from trash to treasure.

Raising the Bottom Line

"The reason is simple," says Pat Condon - costs. "To build a full-scale facility to extract chitin and process chitosan," he says, "you're talking millions." That's because stripping chitin from tightly-bound protein in shells requires heavy-duty treatment with acids and bases - not only are these consumables expensive, but they present potential environmental hazards and require careful treatment. "Because of their corrosive nature," says Condon, "they also eat up equipment," all of this driving up costs.

Condon heads New Earth Services, a company that composts poultry and vegetable wastes, as well as crab waste, marketing it to gardeners and others as Chesapeake Blue. But crab compost is a low-value product and cannot be manufactured profitably, says Condon. For several years, Dorchester County helped subsidize that composting as a way of meeting its recycling requirements, says Andrew Tolley; but the county no longer does so.

Chitosan is hailed by many . . . so what is the problem in the Bay region?

The bottom line, says Tolley, is that processors would have to subsidize that operation, in addition to paying transportation costs of the waste to New Earth Services. That is something many either cannot afford or are unwilling to do, especially if there is still land to receive waste.

While the prospects of such heavy investment in chitosan processing are daunting, says Condon, he has nevertheless been exploring markets for different chitosan products to determine if the prospects of return can justify such investment.

Researchers believe that if chitosan can be produced economically from the chitin in crab shell, its chemical properties could be tailored to create a diverse range of new products which are naturally degradable, from coagulants for use in waste treatment plants, like the one shown at left, to new kinds of wound dressings and casings for the controlled release of drugs.

Though there are numerous applications in which chitosan outperforms competitive products, the margins of profit appear to be too low, or product costs are not competitive. Chitosan's use as a coagulant in waste treatment processes is a case in point, says Susan Murcott, a research environmental engineer at the Massachusetts Institute of Technology. Chitosan performs more effectively than synthetic petroleum-based polymers and is a naturally biodegradable substance. That is the upside. Unfortunately, the cost for chitosan is twice as much as the synthetics, she says. If those production costs could be brought down, or if new products could be developed, then this natural polymer could fare better in the marketplace.

This is the catch-22, says Greg Payne, a chemical engineer at the University of Maryland Baltimore County and the Center for Agricultural Biotechnology. "Chitosan's high production costs," he says, "have limited the development of new products," while "the low margins of existing products have not justified the investment for industry to develop more innovative methods for extracting chitin and processing chitosan."

Payne believes that because of its unique chemical properties, chitosan has untapped commercial potential. If we can demonstrate how chitosan's potential can be released, he says, that could stimulate industrial innovation, so that even manufacturing costs for products such as coagulants could come down considerably and better compete with synthetic polymers.

Towards such ends, Payne, working with support from Maryland Sea Grant, has been studying how the structure of chitosan can be controllably altered so that manufacturers can tailor its chemical, mechanical and biological properties for a number of uses, uses that might include biocompatible materials for medical implants, wound dressings and casings for controlled drug release.

Tailoring the Chitosan Polymer

Payne's interest in chitosan grew out of innovative studies in which he had shown that enzymes could be used to remove phenols - a broad class of natural and synthetic compounds - from liquid mixtures. The problems he focused on involved industrial operations in which phenols appear as undesired contaminants in process streams. Although present at low levels, these contaminants had to be removed if manufacturers were to recover and reuse the desirable materials in the fluid. According to Payne, there are a surprising number of cases in which the success of an industrial waste minimization program requires the removal of a phenolic contaminant.

The innovative two-step approach of Payne's group was the use of the enzyme tyrosinase to "recognize" and react with the phenolic contaminant, while the product of this reaction underwent a subsequent reaction with the chitosan polymer. The product of the second step is a solid which can readily be filtered from the process stream. Based on these studies, his group was awarded a U.S. patent which is currently licensed by the University of Maryland to a local company.

Only one functional group's difference (circled in this illustration) separates the formulas of these chemical cousins:   (1) chitin   (2) chitosan   (3) cellulose

More recently, Payne's group has been examining how tyrosinase can be used to systematically modify the chitosan polymer. Although their function in nature is not fully understood, tyrosinase enzymes are ubiquitous - they are found in bacteria, fungi, plants and animals - and have the ability to react with phenol-type compounds. Since the products of these tyrosinase-catalyzed reactions undergo further reaction with chitosan, it is possible to use this approach, says Payne, to anchor various chemical groups to the chitosan surface, an approach that has the possibility for tailoring the physical, chemical and biological properties of chitosan.

More recently, Payne's group has been examining how tyrosinase can be used to systematically modify the chitosan polymer. Although their function in nature is not fully understood, tyrosinase enzymes are ubiquitous - they are found in bacteria, fungi, plants and animals - and have the ability to react with phenol-type compounds. Since the products of these tyrosinase-catalyzed reactions undergo further reaction with chitosan, it is possible to use this approach, says Payne, to anchor various chemical groups to the chitosan surface, an approach that has the possibility for tailoring the physical, chemical and biological properties of chitosan.

There are numerous possibilities, says Payne, of how such control could then be used to tailor chitosan for specific commercial products, products as diverse as pharmaceuticals and paper. For instance, an oral drug designed to treat the intestine must be able to withstand stomach acids that could prematurely release the drug. Because the surface chemistry of chitosan can be designed to resist acid conditions but hydrolyze under neutral pH, the drug can be coated in the altered chitosan. In this way, it would pass through the stomach, then once in the intestine's neutral pH, the coating would become hydrophilic, allowing the drug to be released.

Already, small quantities of chitosan are being used in paper making to increase the mechanical strength of fibers. The chemical methods for strengthening the chitosan itself - a process called cross-linking - employ somewhat harsh chemical processes, says Payne. The proposed use of tyrosinase could provide a "cleaner" alternative for such cross-linking.

Payne's research is ongoing with support from the National Science Foundation and Sea Grant. In addition, he is collaborating with Pat Condon and New Earth Services - with funding from the University's Maryland Industrial Partnerships program - in first-phase studies that will explore whether chitin processing from Eastern Shore crab waste is practicable. That means, says Condon, whether it can be profitable.

There are many technical, economic and social questions.

Chitin extraction and chitosan processing, for example, can produce different grades of purity depending on the treatment with acids and bases. What are the costs and benefits of different levels of purity, with regard to marketable products? Furthermore, is it even possible to produce chitin profitably from crab waste when it is only available on a seasonal basis? And is it cost effective to work with a wet crab chum from processors or will a dry, dehydrated product be required?

Will processors, or an independent operator, be willing to shoulder the costs for transporting and drying out crab scrap? Again, says Condon, that will depend on whether it is profitable to the processors: if a chitin processing plant is set up on the Eastern Shore, then it could be; on the other hand, unless there is a ready supply of dehydrated chitin, it may not be profitable enough for industry to invest in a chitosan processing operation.

Do environmental and public health concerns over land application of crab waste justify public investment for chitosan processing if it appears to be commercially viable? With nutrient runoff from farm animal manure a major threat to Chesapeake Bay water quality, government currently provides subsidies to poultry and dairy farmers to construct waste containment facilities. Does crab scrap deserve the same?

While there may be a promise of chitosan treasure in crab waste, turning that promise into reality will take even more than lowering extraction and processing costs. Only the demand for products derived from chitin - based on the availability and costs of alternatives - will determine whether its extraction and processing can be profitable. Until then, mountains of crab waste may remain an untapped treasure.

Maryland Sea Grant
Research, Outreach & Education Programs
Coast, Bay & Watershed Issues

This page was last modified May 23, 2002

http://www.mdsg.umd.edu/MarineNotes/Mar-Apr97/

Accessibility Statement