Home Environment Turning waste crab shells into diffraction gratings

Turning waste crab shells into diffraction gratings

by Admin-Phmp

Text by Henrylito D. Tacio

Photos courtesy of Dr. Raphael A. Guerrero

Human beings throw away what they cannot use. That’s elementary. So, it is no wonder the world’s seas are now awash with plastics floating all over.

Filipinos, for instance, use a staggering 163 million sachets a day, according to a study done by environmental group The Global Alliance for Incinerator Alternatives.

That’s for sachets alone. What about those plastic bottles we throw away after drinking water? The Philippines has no statistics for that but in the United States, more than 60 million plastic water bottles are being thrown away – and most of them end up in landfills or as litter.

But it’s not only plastics that people discard every now and then. Even food like rice, meats, and vegetables. When a person is already full, he quits eating them. If these foods are not recycled – as food for animals, particularly swine – they end up thrown away in landfills, rivers, or other bodies of water.

Not all parts of what we eat are edible and should be consumed. Take the case of crabs, which are generally covered with a thick exoskeleton and have a single pair of pincers.  There are 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. These nutrients, reports webMD.com, play vital roles in improving general health while helping prevent a variety of chronic conditions.

Although a crab is not a fish, crabs are among the most popular seafoods. They are esteemed for this sweet, succulent flesh.

Perhaps one of the most in-demand crabs is the blue swimming crab (BSC), known in the science world as Portunus pelagicus. It is called alimasag in Tagalog, lambay or masag in Cebuano, and kasag in Hiligaynon. With their sweet, nutty flavor, and evenly textured, moist, firm flesh, they make superb eating.

Our country is the fourth-largest producer of BSC and the third largest exporter of this species to the United States, according to the Seafood Watch. In 2020 alone, experts in the US were valued 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.”

Genio said shells and appendages make up 60% to 70% of the total weight of the BSC, “so for every ton of crabs processed in a day, some 300-400 kilos of shells are dumped.” All these wastes end up in landfills and some into the sea.

Yet, these wastes 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.”

Diffraction gratings are oftentimes used in lasers, wavelength division multiplexing (a fundamental building block of modern telecommunications) and spectrometers. “It is the latter hardware family that these organic diffraction graters could have the biggest impact,” hailed the nybreaking.com.

The outer skeleton of crabs contains chitosan. “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.

“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.”

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 told us in an exclusive interview.

At one time, while reading articles about bioplastics made from chitosan, Gumayan immediately thought of the crab shells from the island in Iloilo. The Visayan Sea produces around 40% of the country’s BSC picking stations, which supports an estimated 10,000 workers.

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.

Shell waste from BSC 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.

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.

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