Biodegradable plastic bags, cutlery and coffee cup lids may seem like a win for the environment, but they often introduce more problems than solutions.

Despite being touted as "green", many of these plastics take just as long as their conventional counterparts to break down in home composts and landfill, leading to more pollution in soils and waterways. 

Many are also not recyclable, and can only be broken down by industrial composting under high temperatures. 

Now, a team of researchers at the University of California, Berkeley have finally created a biodegradable plastic that disappears almost entirely in household compost within a matter of days, just by adding heat and water.

The new material includes built-in enzymes that chew the plastic down to non-toxic molecules without leaving behind traces of harmful microplastics.

The researchers' study has been published today in Nature.

"Enzymes are really just catalysts evolved by nature to carry out reactions," materials scientist and study co-author Ting Xu said.

"If you want to get a material to become a part of nature, we should go with what nature has already developed." 

Biodegradable does not equal compostable

A 2015 study estimated that just 9% of the world's plastics are recycled, with most of it ending up in landfill.

Australia is marginally better, with about 18% of our plastic waste ending up at recycling facilities.

Biodegradable plastics — which break down into water, carbon dioxide or organic material with the help of microorganisms — have been proposed as an environmentally friendlier alternative to petroleum-based varieties. 

Many of these plastics are made from polyesters, such as polylocatic acid (PLA) and polycabrolactone (PCL).

They comprise tightly packed chains of molecules, called polymers.

This makes them durable, but also difficult for water and soil microbes to penetrate enough to degrade them. 

While the chemical make-up of these traditional materials is technically biodegradable, they can only be broken down in industrial composting facilities under tightly controlled temperatures and conditions, said materials scientist Hendrik Frisch at the Queensland University of Technology, who was not involved in the study.

"Under other conditions such as soil or marine environments, these materials often display a similar durability as their conventional fossil-fuel-based counterparts, causing significant environmental damage and pollution," Dr Frisch said.

In response to this problem, the federal government launched a National Plastics Plan earlier this year that aims to phase out plastics that "do not meet compostable standards".

The power of enzymes

Professor Xu has been exploring how to use enzymes to tackle pollution and make materials more biodegradable for more than a decade.

In 2018, Professor Xu and her team created fibre mats with embedded enzymes that break down toxic chemicals, found in insecticides and chemical warfare agents, in water.

In their new study, Professor Xu and colleagues dispersed billions of polyester-eating enzymes throughout PLA and PCL beads, which are used early in the manufacturing process to create plastic products. 

After melting these beads down, they shaped the material into filaments and sheets for testing. 

Plastics pull a disappearing act

When the team added their enzyme-studded polyesters to household soil compost with a little tap water, 98% was converted into their individual building blocks in just a few days. 

The small molecules left behind were harmless, with the enzymes turning PLA into lactic acid, a food source for soil microbes. 

The enzymes ate away at the plastics even faster under industrial composting conditions, with PCL breaking down in just two days at 40 degrees Celsius, and the PLA disappearing within six days at 50C. 

Below | The modified plastic contains enzymes that break it down after just three days in household compost, and entirely after two weeks.

Story By | Gemma Conroy