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For food storage I mainly use mason jars with stainless steel lids and food-grade silicone ring seals to keep them airtight. Twice while storing different types of loose-leaf tea, the silicone seals became oily and swollen even though they were not in contact with the tea at all. I'm assuming the silicone is somehow attracting the essential oils from the teas, but I thought silicone was inert and I can't figure out why it's doing this.

The first time it was earl grey tea, which I know has bergamot oil. After several weeks, I noticed that the silicone ring was covered in an oily film on both sides. It had also changed color (becoming more transparent) and expanded in size so that it no longer fit comfortably in the lid. The seal had never touched the tea, and neither the glass jar nor the steel lid had the oily residue on them. I washed the oil off the seal and let it air dry for a while. After a few days away from the tea it reverted back to its original color and size and feels normal again.

I didn't think about it much more until the same thing happened again while storing a loose-leaf herbal tea. This one had basil and chive oil in it. After a few weeks, similar experience: the silicone seal was covered in an oily film. Once again none of the tea had been touching the seal, and there was no oil on anything else. This time I didn't notice much size or color change, but it may not have been stored as long. Once again the silicone went back to normal after being cleaned and separated from the tea.

I assume the essential oils in the tea must be evaporating and then the silicone must be attracting and absorbing them somehow. Does anyone know why this happens? Should I be worried that it is damaging the silicone or causing it to leach?

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    Welcome to SA! That is highly peculiar; silcone is non-absorbent, period. And ... other than Earl Grey, most tea leaves contain no oil at all.
    – FuzzyChef
    Aug 28, 2022 at 17:52
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    Do you know if the seals are truly pure silicone? Are they labeled food-safe? My first thought was that it is more likely not coming from the tea, but that the rubber is slowly sweating out a plasticizer.
    – rumtscho
    Aug 28, 2022 at 20:10
  • It's a good question and that's what I've been kind of worried about. But it seems unlikely to me? I bought the seals years ago from a seemingly reputable company, and they were specifically advertised as food safe. I guess there's no way to know for sure - though they do pass the silicone "pinch" test in that they don't turn white at all when pinched or stretched. I should also add that I have had a bunch of these seals for many years and none of the other ones have ever had any kind of oily film like this.
    – Dan B
    Aug 29, 2022 at 0:58
  • Is it possible that oil is coming in from outside? For example, are these jars being kept on a shelf near where food is being fried?
    – dbmag9
    Aug 30, 2022 at 6:08
  • I don't think so. The jars are stored in a cabinet far away from the stove and are not exposed to light or unusual heat (beyond just normal humid summer weather).
    – Dan B
    Aug 30, 2022 at 15:10

1 Answer 1

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This won't be a complete answer as I'm not enough of a chemist at all to go into the intricacies of the chemistry. Note that in this answer I'm using the word "organic" to mean carbon-based chemistry, as this is how the term is used in chemistry. It has nothing to do with commonly used "organic" practices for food production.

Silicone cookware is properly known as a polysiloxane. These are chemicals similar to plastics, but with a inorganic silicon-oxygen (-Si-O-Si-) backbone instead of an organic (carbon-based; -C-C-) backbone. They are synthesized through reactions between various monomeric precursors and a whole bunch of different organic compounds, making them technically organo-silicone compounds because of the organic side-groups of varying composition (-R1R2Si-O-SiR2R1-, where R = organic side-group), which provide the cross-linking so that you can get solid substances, much like plastics.

Exactly how the silicones are synthesized (which type of polysiloxane they are) and which organic reactive groups are added to make the particular silicone you have will alter any potential chemistry that is going on in your case.

What I suspect is happening is that some chemical residue in the silocone is reacting with a volatile compound from your "tea", producing a gas and a chemical residue. Exactly which chemicals might be doing that are impossible to say, but it could possibly be something that reacts with hydrochloric acid (HCl) as this seems to be used a lot in the creation of silicone cookware, though I would expect this to react and be used up in the generation of the final silicone product.

Polysiloxanes are gas permeable, this means that whatever is coming off your tea could well be entering the silicone through the pores in it, and then reacting with whatever liquid/solid is chemically available, thereby producing a volume-filling gas, which would account for the change in volume you are seeing. As these gasses diffuse out again (this will be in the order of <1 micrometer/hour, so very slow), the volume should revert to approximately the original volume. It could also be a chemical reaction with the siloxanes or side-groups on them resulting in a chemical volume change.

Colour changes could also be the result of chemical reactions, possibly through oxidation of an oil or perhaps through a state change (gas, liquid, solid) in the siloxane itself or in the microstructure of the pores.

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  • Thank you so much for this detailed and helpful answer! That explanation makes sense to me. Do you think I should be concerned about re-using the seal once it's cleaned and seems normal again? Also does this indicate that I should be concerned the other seals have some unreacted chemical additives?
    – Dan B
    Aug 30, 2022 at 14:29
  • @DanB I don't know the answers to those questions. All I know is that the substances produced are very similar to plastics in their physical properties and chemically similar too. This isn't particularly surprising - Si and C are in the same column of the periodic table so will have similar chemistry. The relevant authorities say they are safe; they also said that about plastics, but now the scientific consensus is changing. The rate of leaching out will be very slow too - as slow as it was to swell and shrink.
    – bob1
    Aug 30, 2022 at 19:55

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