Yes of course you have to. Otherwise it may still have the same cnsequences as it is supposed to have when the powder is kept not at the respective temperature even when it isn't diluted. Small petty things like this can seriously and I mean seriously get you in danger if not taken proper attention. Hygen especially in this regard has to be taken with special care in order for reaping maximum benefits. So next time you dilute something make sure it is kept and stored at an optimum temperature.