I think my example is compatible with my understanding of the term 'introduce', but whatever, we can raise another objection that doesn't have this problem: In enzymatic browning, enzymes in a food react to oxygen in the atmosphere. This does not make the apple unnatural to the general intuitive sense of the idiots who care about whether food is 'natural'.

We could also twist at your objection from the other direction by pointing to plant byproducts that are generated without the 'introduction' (by your definition) of any new substances but are considered 'unnatural' by the same idiots.