Heat treatment not possible? Or ammonia washing?Well, the Thymol eventually evaporates out of the finished, dry coating, but it can help. It is present in packaged sheets and the preservative is certainly present in commercial products. If you open a package of Ilford paper, you can smell the phenol which is what is used as a preservative in their B&W papers. That is so strong and toxic when used in the home darkroom that I don't suggest it, but it is used by Ilford and Schoeller as well in large scale operations. Kodak does not use either Thymol or Phenol. They used equally potent materials. I'll comment on them soon.
However, there is another important fact involved in this that I did not mention earlier.
All batches of fine photo grade gelatins are packed with an assay, just like fine chemicals. Only in this case, it is a bioassay that includes things such as tuberculin bacilli, staph, strep, E. Coli and etc per unit weight. This means that the dry gelatin contains a certain amount (printed on the assay), per specified unit of dry gelatin. Well, when you mix your gelatin in warm water those microorganisms begin to multiply unless you stop them. I don't know about you, but I don't like handling gelatin without some sort of preservative there to retard their possible growth!