So what about the plastic in the Fermenta Lock cap? Is that a problem?
Plastics have got a lot of bad press lately about potentials to leach chemicals into foods. Some plastics do this worse than other plastics, but with all of them, the rate at which chemicals leach, is very low to begin with. They typically only have significant issues with leaching under two circumstances (and even then, it is still a very small amount):
First, when the food is in prolonged contact with the plastic (hence, the concerns over plastic storage containers).
Second, when food is heated in plastic containers, the leach rate increases many times.
Fermenta Lock lids don’t typically come in contact with the food, and when it does, the contact is brief, and limited to small amounts of food.
Heating is not part of the fermenting process either, so you won’t have issues with increased leaching potentials due to heat.
Fermenta Lock is a lid, which is manufactured for food use. Plastic lids just aren’t an area of significant safety concern.
Rubber products actually leach more into foods than plastic, yet most fermenting systems use some kind of rubber. It is not a concern either, simply because the rubber does not come in contact with the food in typical usage. When it does accidentally, the contact is brief, and not in circumstances that would leach any molecules in sufficient number to be a problem, unless you are hypersensitive to latex, in which case, you are going to have problems with any product which uses rubber at all.
In general, we avoid the use of plastic in our kitchen, where practical, but we use these lids simply because they are safer and more stable for fermenting than metal lids could be.