Picarro Spotlight: Counting Every Carbon to Reveal the Full Biodegradability Story of Compostable Plastics

Picarro Spotlight is a blog series showcasing important scientific work from our customers around the world. Each blog is selected and summarized by our team. Enjoy!

Compostable plastics are meant to biodegrade in industrial composting facilities. Biowaste bags, fruit stickers, coffee capsules and tea bags made from polyesters such as PBAT are chemically designed to be fully metabolized by microbes in industrial compost, leaving only CO₂ and microbial biomass. But can we advance a detailed picture of this process?

Biodegradation measurements are central to compostability specifications such as EN 13432, ASTM D6400 and ISO 17088, alongside requirements for disintegration and compost quality. However, these measurements have two limitations: high polymer loadings are needed to distinguish polymer-derived CO₂ from compost’s background respiration, and CO₂ measurements alone cannot reveal whether non-mineralized carbon remains in residual polymer or has become microbial biomass or other organic matter.

A new open-access study in Environmental Science & Technology by Flora Wille, Michael Sander and colleagues at ETH Zürich, Eawag, Nestlé and BASF tackles both limitations.

Following the label

The team synthesized the biodegradable polyester PBAT and its partly biobased cousin PBSeT with ¹³C-labeled butanediol, then incubated them in industrial compost at 58 °C, at concentrations about 20 times lower than in standard tests. Thirty-six bottles ran in an ECHO Instruments ER Series respirometer, with a Picarro G2201-i isotopic analyzer in series measuring ¹²CO₂ and ¹³CO₂. Each line was sampled roughly every eight hours, producing continuous, highly reproducible mineralization curves.

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At set time points, individual bottles were removed for further analysis. EA-IRMS quantified the non-mineralized ¹³C left in the compost, and solvent extraction with ¹H NMR measured residual polyester. Combined with the CRDS data, the ¹³C mass balance closed at 90–102%.

Figure 1. Tracking the fate of carbon in compostable plastics. Researchers used carbon-13 isotope tracking to measure how much plastic carbon became CO₂ or microbial biomass, and how much remained as polyester.

What they found

Concentration matters. In incubations run at low, more realistic loadings of PBAT and PBSeT (experimentally accessible by ¹³C-labeling), mineralization showed shorter lag phases and faster relative mineralization than in standard incubations run at higher loadings. High polyester concentrations in standard tests thus may overwhelm compost microbes and underestimate biodegradation rates at lower, more realistic polyester loadings.
Microbes build biomass from plastic, not just respire it. Early in the incubations, up to about half of the metabolized polyester carbon went into microbial biomass. After four days, ¹³C-PBSeT was about 81% biodegraded, while mineralization measurements alone suggested only 47% of biodegradation.
Incomplete mineralization does not mean persistent plastic. After 18 days, ¹³C-PBSeT mineralization stood at 75%, yet only 2% remained as polyester; the remaining 23% was in compost microbial biomass. Mineralization-only data could have mistaken that 25% for undegraded plastic.

Why it matters

As regulators and manufacturers look for a deeper scientific insight and a fundamental understanding of the biodegradation of compostable products, methods that close the carbon mass balance become essential. This workflow can add weight of evidence to certification and help link polymer chemistry to biodegradation behavior. In addition, the use of stable isotopes in biodegradability research will allow assessing biodegradability assessment in even more active inoculums, such as “young” compost or wastewater sludge, or mixed inoculum cases, such as soil and compost.

The role of Picarro

Pairing a multichannel respirometer like ECHO's with continuous, high-precision ¹³CO₂ measurement by cavity ring-down spectroscopy makes low-concentration labeling experiments practical. It isolates the polymer-derived ¹³C from a carbon-rich compost background, automatically and over weeks, so researchers can test biodegradability of plastics at lower concentrations much closer to their real-world occurrence.

For more information:

Wille, F. et al. (2026). Environ. Sci. Technol.60(30), 21190–21200. https://doi.org/10.1021/acs.est.6c00032

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