Waste not want not: Waste crab shells can be made into diffraction gratings for optical instruments

Approximately 300 grams of crab shell waste was used in the study to successfully fabricate bioplastic diffraction gratings. (Raphael Guerrero)

By Henrylito D. Tacio

Crabs live in all the world’s oceans, in freshwater, and on land. These crustaceans are generally covered with a thick exoskeleton composed primarily of highly mineralized chitin and have a single pair of pincers. They walk and swim sideways.

Crabs, which vary in size and colors, make up 20% of all marine crustaceans caught, farmed, and consumed around the world. All in all, the world knows over 6,700 species of crabs.

Crabs are packed with protein, which is important for building and maintaining muscle. They also contain high levels of omega-3 fatty acids, vitamin B12, and selenium, all of which play vital roles in improving general health while helping prevent a variety of chronic conditions.

One of the most popular crabs is the blue swimming crab (BSC), known in the science world as Portinus pelagicus. Its common names include blue manna crab, sand crab, blue crab, and horse crag. It is called alimasag in Tagalog, lambay or masag in Cebuano, and kasag in Hiligaynon.

With its sweet, nutty flavor, and evenly textured, moist, firm flesh, BSC makes superb eating. The firm texture and delicate flavor of this kind of crab make it excellent for serving with pasta.

BSC is widely distributed throughout the waters of the Philippines, particularly in nearshore shelf areas. Most of them, however, can be found in the Visayan Sea, which accounts roughly 40% of the country’s BSC output.

Approximately 300 grams of crab shell waste was used in the study to successfully fabricate bioplastic diffraction gratings. (Raphael Guerrero)

Fourth-largest producer

The Philippines is the fourth-largest producer of BSC and the third largest exporter of this species to the United States, according to sustainable seafood organization Seafood Watch. In 2020 alone, experts in the US valued the industry at $45 million.

“The blue swimming crab is a significant sub-sector of marine fisheries in the Philippines,” wrote Josette Emlen Genio, sustainable market consultant at Sustainable Fisheries Partnership. “In 2015, the country produced close to 26,000 metric tons of crabs, most of which are processed into crab meat.”

But not all parts of crabs are eaten. “The global shellfishery industry generates millions of metric tons of waste every year,” said a study.

Crustacean shells are a primary source of this waste, causing serious environmental problems and a significant waste of resources, said a study published in Marine Life Science & Technology.

In the Philippines, more than 250,000 metric tons of seafood waste are produced annually. “In seafood production, 35% to 40% of waste is in the form of discarded shells that, if incorrectly disposed of, cause environmental pollution in coastal areas,” wrote Efren G. Gumayan, Ian Ken D. Dimzon, and Raphael A. Guerrero in their paper that was published in Applied Optics.

Repurposing crab waste

Most of these wastes, however, contain some of the most useful compounds in industrial settings that can be both profitable and sustainable. “Repurposing crab waste as a raw material for bioplastic components shows promise, with shells having chitin content of 10% to 72%, suitable for chitosan extraction,” said a paper that was published by Applied Optics.

The authors – Efren G. Gumayan, Ian Ken D. Dimzon, and Raphael A. Guerrero – have managed to convert an extract from crab shells into a bioplastic that can be used to make optical parts known as diffraction gratings.

Gumayan is a graduate student with the Department of Physics of the School of Science and Engineering at the Ateneo de Manila University (ADMU).  He is also a faculty member of the Iloilo Science and Technology University (ISATU) in La Paz. Guerrero and Dimzon are from the Department of Physics and the Department of Chemistry in ADMU, respectively.

“Diffraction is the bending of light around an obstacle,” explained Gumayan. “A diffraction grating is an optical component that redirects light in specific directions based on its color. The surfaces of CDs and DVDs, with their small micrometer-sized structures, can act as diffraction gratings, resulting in the colors seen on the discs.”

Gumayan, Dimzon and Guerrero conducted a study specifically on BSC. They found out that 70% of crab parts is discarded “with decaying waste affecting water sources through the emission of ammonia and nitrates.”

But can this waste be made into something useful?

“We wanted to find an alternative use for crab shell waste, and decided to find out if chitosan from crab shells could be used as a biodegradable replacement for silicone, which we have previously used in our lab to make diffraction gratings,” said Guerrero.

Chitosan-based bioplastic

Chitosan is a sugar that comes from the outer skeleton of crabs, lobsters, and shrimps. It’s used as medicine (useful in bandages to reduce bleeding and as an antibacterial agent) and in drug manufacturing. It can also be used in agriculture as a seed treatment and biopesticide, helping crops to fight off fungal infections.

Bioplastic can also be processed from chitosan. Studies have shown that the bioplastic deteriorates in less than three days.

The ADMU researchers discovered that the chitosan-based bioplastic can be used to make diffraction gratings that work just like commercially available ones.

“Gratings made of chitosan are biodegradable and environmentally friendly while also being very inexpensive since crab shells are generally considered waste,” Guerrero pointed out. “By showing that useful optical components can be made from materials typically considered waste, we hope to help improve sustainability in optical manufacturing and reduce the amount of seafood waste that requires disposal.”

How the idea came to be

The idea is the brainchild of Gumayan. “During my teaching years at one of the island schools in Northern Iloilo, I became interested in the bulk crab shell waste produced in crab meat processing plants,” he recalled.

At one time, while reading articles about bioplastics made from chitosan, Gumayan immediately thought of the crab shells from the island in Iloilo. So, he started his research on extracting chitosan for optics and photonics applications. “This was my first scientific project using crab shells,” he said.

Gumayan was already working on his doctoral dissertation under the supervision of Guerrero and they decided to focus on fabricating chitosan-based diffraction gratings. Dimzon, as a chemist, guided the team in chitosan extraction.

Extraction process

Shell waste from blue swimming crabs was collected from a local crab processing plant in Concepcion, Iloilo. “For this study, we used approximately 300 grams of crab shell waste in order to successfully fabricate bioplastic diffraction gratings,” Gumayan said.

The samples collected were washed with running water and oven-dried for 24 hours. After drying, the shells were ground using a kitchen blender, resulting in a powdered form known as crab meal, which was then stored in a hermetic container.

According to Gumayan, they conducted eight trials to successfully extract chitosan from crab shells, including initial experiments performed at the ISATU Natural Products and Glycochemistry Laboratory. Five trials were needed to efficiently dissolve chitosan, and 5 more trials were required to fabricate working bioplastic gratings using soft lithography.

Soft lithography is a replication process that uses a silicone mold to copy the nanoscale surface features of an object. This involved preparing a silicone cast of a commercially available diffraction grating and then pouring the chitosan solution into it. When the solution hardened, it created a replica of the commercial grating.

“Diffraction gratings are components of optical spectrometers used in chemical analysis,” Gumayan explained. “Currently, commercial gratings are made of heavy materials such as glass. Our bioplastic grating is lightweight and biodegradable. A potential application would be an inexpensive and disposable single-use spectrometer for field measurements such as water quality monitoring.”

Good for the environment

Guerrero said that the low-cost biodegradable gratings they have discovered “are ideal for optics experiments in all levels of education. They will be particularly relevant in public elementary and secondary schools with limited laboratory resources.”

This science breakthrough is good news for those involved in crab raising. “The conversion of shell waste into a valuable product should increase income for the local crab meat industry,” Gumayan said. “This added value to crab shells will hopefully improve the economic status of crab fishermen and their families.”

Aside from its economic benefits, it is also good for the environment. “Using crab shells as the raw material for bioplastic will reduce the amount of waste generated by the seafood industry, and increase sustainability for the environment,” Guerrero said.

Photos courtesy of Raphael Guerrero

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