Hello,
generally no chemicals at all should contact any container, glas, bottle or other equipment which is intended for food. Heating is only necessary up to about 50-60° C for quicker dissolution of chemicals. The best way is to buy a small magnetic stirrer with controlled heating. Look for a used one on e-bay, it is quite useful for preparing all photographic solutions. TEA is not very toxic and is used even in cosmetic formulations for direct skin contact. It has a high flash point and high boiling point.