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GMO jeans: Using biotech to make indigo-dyed denim a bit more green

Each year some 40,000 tons of waste discharge flows through factories turning out 3 billion pairs of jeans.

Sometimes it seems the world is awash in genetically modified organisms.

You have your landscape-scale examples, like transgenic salmon or those GMO corn and soybean crops, made to be pest- and pesticide-resistant (but capable of stimulating other resistance in the marketplace).

Then you have your tiny (and less controversial) ones, like bacteria with their genes edited to make them into factories producing insulin, vaccines, biofuels and other useful chemicals.

Now comes news from a bioengineering lab in Berkeley, via the prestigious journal Nature Chemical Biology, of a high-tech venture to get a more earth-friendly indigo dye for denim out of modified E. coli.

I suppose you could call it jeans editing. And it has the potential, at least, to address ghastly impacts of  current manufacturing practice that might make you think twice about that new pair of 501s.

Though jeans are still dyed with indigo, the natural, plant-derived pigment was used for less than 30 years after Levi Strauss started putting his famous denim pants together with rivets. It was expensive and, at industrial scale, kind of scarce.

Since roughly 1900, according to a Berkeley Research announcement posted last week, denim has been made blue with synthetic dyes that contain no natural indigo at all, and are produced from benzene, formaldehyde, hydrogen cyanide, caustic sodas and other noxious chemicals.

This requires what the announcement calls “a dirty chemical process” that produces waste “toxic to fish and some other aquatic life,” not to mention corrosive to wastewater treatment equipment. Which is  putting the problem politely.

Environmental cost of cool

As you might guess, the volume of synthetic indigo used in making blue jeans and various matching garments and accessories is not at all small. Nor is the waste discharge.

Each year, according to the Berkeley lab, some 40,000 tons of it flows through factories turning out 3 billion pairs of the formerly bohemian pants that, nowadays, are worn by the average American on four days out of seven. (The paper itself, which is available here but must be purchased, puts the dye volume at 50,000 tons per year and the garment count at four billion.)

At what environmental cost? Discolored and chemically tainted rivers across China and India, especially, but also in Turkey, Brazil, Pakistan and other places with significant denim production.

It’s hardly the only pollution problem associated with textile mills, but denim’s downside has begun to get special attention in recent years, mostly outside the mainstream media. There’s a 2016 documentary out of Canada, RiverBlue, Or consider this passage from Hakai Magazine:

Like tanneries, denim factories are often located in the developing world — out of sight, and out of mind of Western consumers. One of the world’s biggest producers of denim is a gritty looking city in China’s Pearl River Delta — Xintang. According to one 2016 study, Xintang’s factory workers produce a whopping 200 million pairs of jeans a year, but they and their neighbors pay a steep price for the industry.

The local river has turned a strange black color from the dumping of untreated dye water from the factories. And scraps of denim and other factory sludge rot along riverbanks. In 2010, Greenpeace researchers conducted a study in Xintang and neighboring Gurao, sampling water and sediments from the river. Nearly 80 percent of the samples were contaminated with heavy metals. One sample contained a level of cadmium, a heavy metal toxic to humans and aquatic life, that was 128 times greater than the limit set by the Chinese government.

Indigo's natural deficits

Natural indigo pigment is no good as a dye without a lot of processing. Because it’s not water soluble, explains a Nature editorial accompanying the Berkeley research paper, it can’t penetrate cotton or other textile fibers.

So it has to be converted into a form called leucoindigo, also known as white indigo — although it apparently is kind of yellow until it comes into contact with oxygen, which turns it that beautiful blue.

To make leucoindigo, the plant extracts are typically treated with a sodium bleaching compound that degrades to corrosive sulfates and sulfites; this discharge, like its synthetic counterpart, corrodes pipes and other equipment in both the dye mills and treatment plants that receive their waste.

And so, the paper observes, “many dye mills avoid the additional cost of wastewater treatment by dumping the spent dye materials into rivers.”

But the Berkeley team saw a way to solve the problem with a sort of biomimicry. As the editorial explains, in language somewhat more accessible than the paper’s:

Inside plant leaves, the unstable indigo precursor indoxyl is combined with glucose and stored as a colourless molecule called indican. The researchers mimicked this by adding genes to Escherichia coli bacteria to make them secrete indican.

To dye material with this biosynthetic indican, the scientists dissolved it in water and applied the solution alongside an enzyme that stripped away the glucose to re-form indoxyl. This indoxyl then spontaneously oxidized to leucoindigo. When removed from the liquid, the leucoindigo reacted with the air and turned to indigo.

The clever mechanism goes further than previous attempts to clean the process, because it kills two polluting birds with one stone. First, it does away with the wasteful chemical synthesis of indigo. Second, unlike previous indigo biosyntheses, this project removes the damaging bleaching stage that converts indigo to leucoindigo.

A clever solution experimentally, yes, but is it practical at industrial scale? The paper doesn’t address that question directly. John Dueber, who headed the Berkeley research team, told Smithsonian magazine that “we feel pretty confident” in the method’s potential, “but there’s always work to be done going from the lab scale to an industrial scale.”

Another waste problem

An expert in textile engineering pointed out to Smithsonian that even if the chemistry were changed to make the waste stream from denim dye mills less toxic, the volume of blue discharge could still be vast.

That’s because of another interesting property of indigo dye: It only stains the surface of the fibers, wearing away to create  that cool fade over years of wear — or in a hurry as the mill does its final processing.

Peter Hauser, a retired prof from North Carolina State University, told the magazine that “a large amount of the pollution in denim manufacture is caused by rinsing the denim after it’s dyed and abrading it to get the desired worn-in look. Denim died with bacteria-produced indigo would still produce dirty wastewater by being washed.”

“Indigo is actually a terrible dye,” Hauser says. “But because of that property, it fades and washes off, and that’s why people like it.”

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