Mason jar silicone seal mystery

Question

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?

Answer 1

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.