I did a search on the forum, but I didn't find any info on this real basic question. Do I have to worry about contamination from air lock water even after it has been blasted by fermentation. What is the best fluid to avoid contamination of air lock water back flowing into the beer? I haven't had any problems yet but I like to take precautions.

I personally either use some of my StartSan sanitizer solution or vodka. Both will stay sanitary so that any liquid sucked in will not cause an infection.

Cheers.

Proper airlock fill level can play a role here too.

That's a good point, I have rarely had any 'sucked in'. More often the issue and reason I use sanitizer or vodka is the blow over from active fermentation.

Sometimes when I crash cool, it does draw back in.
When fermenting lagers, I press on the pail lid to get a little negative pressure to get the water higher in the tube under the bubbler, to see if it is done fermenting. I usually use DW with a little shot of star san.

+1 I always use vodka.

Thanks for all the replies!

I always fill my airlocks with starsan just in case this does happen. A good tip when crash-cooling a beer is to use a single s-shaped airlock (not the 3 piece ones) and put just a small amount of starsan in it. That way when the beer cools and the pressure difference changes, the liquid can't be sucked back into the fermenter into your finished product. This will happen if you use the 3 piece airlocks though. I always learn my tips the hard way of course.