Dr. Tim, I believe that virus, bacteria and fungi are all consider microbes, that said, what types of microbes are you finding in the sap? I should reword that, are you finding more than bacteria and can it be assumed that natural yeast is present? We typically hear of sap containing bacteria but you using the microbes term is throwing me off. I am fairly certain that there are no virus in the sap but after this past year nothing should surprise us about a virus.
I believe what Steve is looking for and would be great to have is a graphic calculator (I can not remember the correct term for a line graph typy calculator with three axis) saying at a particular sugar concentration and a particular temperature, sap will stay good for this amount of time before the bacteria makes the sap spoil. Sounds easy, I have no idea how to do it though.