1. Why "Biodegradable" PLA Plastic Stays in Soil
Polylactic acid (PLA) first reacts with water to become small molecules in high-temperature, high-humidity environments, such as industrial composting, where the temperature is above 60°C and humidity is above 60%. Microorganisms then decompose these small molecules.[1]
However, in low-temperature environments like soil or seawater at room temperature, this first stage hardly progresses. The term "biodegradable" does not mean it decomposes anywhere; it means decomposition happens only under specific conditions.[1]
A living thing was found that can jump over these conditions at room temperature. It was living on the surface of rice leaves.
2. What Rice Leaves and PLA Plastic Have in Common
Plant leaves are covered on the surface by a substance called cutin. Researchers noticed that the chemical structure of cutin is very similar to biodegradable plastics. Both are solids at room temperature, made of organic acids connected by ester bonds.[2]
This led to a hypothesis: if microorganisms living on leaf surfaces have enzymes (cutinase or lipase) that break down cutin, they might also be able to break down plastics with a similar structure.[2]
In fact, strains of the yeast *Pseudozyma antarctica* isolated from rice leaves and rice husks strongly decomposed PBS and PBSA biodegradable plastic films at 30°C.[2]
It is important to note that the yeast did not evolve to eat plastic. The explanation is simply that cutin-decomposing enzymes also work on plastics with similar structures. So, what is this enzyme?
3. The Shape and Size of PaE, the Yeast's Plastic-Eating Enzyme
PaE is a small protein enzyme similar to cutinase. Its size measured by electrophoresis is about 22 kDa, while the expected value calculated from its sequence is about 20 kDa. The slight difference in numbers is due to different measurement methods.[2]
PaE works by randomly cutting the ester bonds in the long chains of plastic at various points along the chain.[3] Imagine it turning long chains into short chains. However, it is not a pair of scissors; it is a protein that facilitates a chemical reaction.
Solid films of PBS, PBSA, polycaprolactone, and PLA are targets for this enzyme.[2] The shortened chains become a size that other organisms can use more easily.
However, there is a form of PLA that even PaE struggles with.
4. Why Crystalline PLA Breaks Down More Slowly
It has been reported that PaE can decompose non-crystalline (amorphous) PLA.[4]
On the other hand, films of crystalline PLA decomposed at only about 1/1000th the speed of PBSA films.[3] Under the conditions of this test, decomposition was very slow. The comparison was made against PBSA film.
Therefore, caution is needed when saying "there is a yeast that decomposes PLA at room temperature." It does not mean that all PLA decomposes quickly. Crystalline parts still present a large barrier.
Still, some people have tested whether this enzyme can be used in actual fields.
5. What Happened When Yeast Enzyme Liquid Was Sprayed on Field Mulch Film
In agriculture, mulch films are laid on fields to suppress weeds. The research team is developing a technology to weaken the film with the yeast's enzyme at the timing when it is finished being used.[7]
They sprayed a filtered culture liquid of the yeast onto commercial films containing PBAT laid in a field, and then plowed the soil the next day. It is reported that the size and weight of the remaining film pieces decreased greatly.[5]
The tests were conducted at two temperatures: early summer (about 31°C) and autumn (about 14°C). When plowed the next day, the strength of the film decreased in both conditions. The fragments recovered after plowing became smaller, and the total weight decreased.[3] However, these results are for biodegradable mulch films. This does not mean that PLA was decomposed at 14°C.
So, how are these enzyme liquids made, and how long do they last?
6. How to Make and Store the Enzyme Liquid
If enzyme liquid is left as is, it can deteriorate, or microorganisms can grow. Researchers investigated enzyme liquid made with 20% ethanol. After more than 6 months at room temperature, about 60% of the enzyme liquid's activity remained, and microorganisms did not grow. The yeast itself completely lost the ability to grow in ethanol concentrations of 20% or higher.[6]
Another problem is that PaE is broken down by proteases (protein-decomposing enzymes) produced by the yeast in the culture liquid. In normal yeast culture liquid at 25°C for 48 hours, activity dropped to about 55%. When a strain with two protease genes disrupted was used, 94% of the activity remained.[4]
By combining these improvements, it seems possible to produce a liquid that weakens film in the field.
7. Things About PLA Plastic You Can Check Yourself
Look at the back of plastic bags or cups at home. See if any are labeled "PLA" or "biodegradable." If you find one, read the conditions written on it. Does it say it disappears quickly in soil at room temperature, or does it decompose only when conditions are met?
When walking past a field, look to see if black film is laid down to suppress weeds. Do not enter the field; view it from the road.
Information about this research is available on the websites of the National Agriculture and Food Research Organization (NARO) and the National Institute of Advanced Industrial Science and Technology (AIST). Some paper abstracts are free to read.