TL;DR
Recycled PET is the default “sustainable” answer in beauty packaging right now. We ran the math, looked at the leaching data for postbiotic ferments, and chose glass for the Microbiome Glow Serum. Here is the full reasoning, including the inconvenient parts.
If you read enough sustainability marketing in beauty, you will start to assume recycled PET is the green default and glass is the indulgent choice. We assumed the same thing when we started this project. The data didn’t agree with the assumption.
The result is a 30ml glass bottle. Heavier than it needs to be. More expensive than the rPET version we briefly considered. We don’t regret the choice.
What recycled PET actually is
Post-consumer recycled polyethylene terephthalate. Soda bottles, water bottles, melted down and re-extruded. The recycled content varies; “100% rPET” usually means the new container is made from 100% reclaimed material, not that the resin itself is identical to virgin PET. There is always some virgin content in the food-grade and cosmetic-grade versions because the recycling stream introduces variability.
That variability matters more than the marketing admits.
The leaching question
PET, recycled or virgin, can leach antimony, acetaldehyde, and trace phthalate-equivalent migration products into the contents over time, especially at warm temperatures. The migration is generally small and within food-safety thresholds for water and beverages. It is not zero. And it is not measured for postbiotic ferments specifically.
Our serum contains live-fermentation-derived metabolites that we want to remain unchanged from fill to last drop. The migration question, even at low magnitudes, is one we did not want to leave open.
The carbon math, properly done
This is the part where most brands stop reading the spreadsheet. Glass weighs roughly 4.2 times what an equivalent rPET bottle weighs. Shipping carbon scales with weight. If you only count shipping, rPET wins.
If you count the full lifecycle, the picture changes. Glass is endlessly recyclable without quality loss. PET can be recycled five to seven times before mechanical properties degrade and the resin gets downcycled to fiber. Glass production carbon is high upfront, but the loop closes; PET production carbon is lower per unit, but the loop ends.
The 2022 EU Joint Research Centre packaging study showed that for cosmetic-sized containers used by consumers who recycle at typical municipal rates, the lifecycle difference between glass and rPET is much smaller than either side of the marketing argument admits. Within margin of error, for our specific bottle size.
When the carbon argument is a wash, the contents argument decides.
The contrarian section
Recycled PET is not automatically the better choice. Saying that out loud invites pushback from the sustainability marketing side of the industry, which has spent the last five years training consumers to read rPET as the green default. We respect the brands using rPET responsibly. We just landed somewhere else.
If your product is fragrance-only, water-based with no sensitive actives, and shipped in volumes that make weight the dominant carbon variable, rPET may be the right call. For a postbiotic serum that we ask consumers to use over three months, with sensitive actives that we want stable to the last drop, glass was the answer that survived the audit.
What we picked, specifically
A 30ml violet-glass bottle, sourced from a German manufacturer with verified post-consumer cullet content above 60%. Violet because the glass color filters wavelengths between 450 and 720 nanometers, which slows oxidation of light-sensitive postbiotics measurably in our internal stability testing. Pair that with the airless pump and we get a packaging system that protects the formula from the things most likely to degrade it.
Two-point-three ounces of empty bottle. Heavier than the average. We are okay with the trade.
Where this fits in our wider thinking
We get the question “is this really more sustainable” enough that we wanted to write the answer down once. Sustainability is not a label you apply at the end of formulation. It is a series of small audits, each of which has a wrong answer that’s easy and a right answer that’s harder. We’ve covered the broader version of this argument in our piece on what sustainability actually looks like behind a skincare brand.
Glass. For now. If a postbiotic-stable, non-leaching, lifecycle-cleaner alternative becomes available at commercial scale, we will rerun the math.
FAQ
Is recycled PET dangerous? No. The migration levels are within food-safety thresholds for typical use. We chose glass for a sensitive-active product where we wanted no migration question at all.
Is glass actually recyclable in my city? In most US municipalities, yes. Curbside glass recycling rates have improved over the past five years. Check your local hauler if you are unsure.
Why violet glass and not amber? Violet filters a narrower wavelength range that better protects light-sensitive postbiotics. Amber is fine for many formulations; we tested both and violet won the stability study.
Does the heavier bottle increase your shipping carbon? Yes, by a measurable amount. The lifecycle math still favors glass for our specific use case. The accounting is in the article above.
Will you ever switch to a different material? If a verified alternative passes both the leaching audit and the lifecycle audit, we would consider it. We have not seen one yet at commercial scale for our use case.
Explore the skin science tag hub for more formulation deep-dives.
Sources
- Westerhoff et al., “Antimony leaching from PET bottles,” NIH/PMC review
- US FDA cosmetic labeling guide and packaging considerations
- American Academy of Dermatology, safely storing skin care products