It has to do with pH, innoculation (how clean the water is, the bucket is, the feeder is), how strong the syrup is (2:1 keeps much better), how warm the syrup is (it ferments rapidly up in the 90s and slower as you get colder). I make 5:3 syrup and add ascorbic acid and it keeps a long time. When you make 1:1 with no acid it does not keep very long at all.