Help tracking down infection

I've had my last several extract beers turn out a little off, an off flavor that doesn't totally ruin the beer but its borderline not drinkable. In retrospect, I'm convinced it's origin is post fermentation. I use healthy yeast and temperature controlled fermentation chamber. About 3-4 weeks in primary and then 2-3 weeks in keg.
The gravity sample before racking has tasted pretty decent, and then once it's cold and carbed its got the off flavor. I believe these are the possible culprits:
1. Sample thief
2. Racking cane and/or hose
3. Keg and/or o-rings

What's the easiest, most cost effective way to rule any of these out? For instance, I believe I can rule out the sample thief because if it had a bug wouldn't it infect the beer at the onset of fermentation after taking the OG sample? Also, I can replace the o-rings in the keg and clean the keg again. Looking for suggestions what I should do so I don't have another ruined batch.

Thanks,
Andy

I would say do a batch with store bought bottled water to rule out any water chem issues. If that gives the same results start cleaning/renewing your gear. Do you have a two piece plastic spigot on your FV? Those can have hard to clean nasties.

I've been using filtered water and I treat with 1/4 campden tablet. I'm using a glass carboy without spigot for fermentation vessel and I siphon it out.
Any other ideas?

Thanks

What kind of temperature control do you use during fermentation and what temps do you set it at? How long at each temp is somewhat important too.

Since you say it's happened over several brews I'm guessing you have used different kegs so I wouldn't think O rings are the problem.

Are there any hairline cracks in the sample thief? When I used Starsan to clean mine I noticed that it became brittle and had hairline cracks appear in it.

I have heard/read about some CO2 causing problems. Never experienced it but just something to consider.

RM-MN said:

What kind of temperature control do you use during fermentation and what temps do you set it at? How long at each temp is somewhat important too.

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I have a Haier mini-fridge with an STC-1000 controller. For brews with US-05, I hold at 62 for about 4-5 days and then slowly ramp up the temp (about 1 degree per day). Confirm with fermometer.

raysmithtx said:

Since you say it's happened over several brews I'm guessing you have used different kegs so I wouldn't think O rings are the problem.

Are there any hairline cracks in the sample thief? When I used Starsan to clean mine I noticed that it became brittle and had hairline cracks appear in it.

I have heard/read about some CO2 causing problems. Never experienced it but just something to consider.

Click to expand...

I don't see any hairline cracks in the thief. If it were the culprit, wouldn't the beer be infected from the beginning of fermentation? And then the sample post-fermentation would taste terrible?
I'll look into the co2, thanks.

I've read about people putting plain water in a keg and adding CO2. Then taste the water and you will know if the CO2 is the problem. I've also heard of beer backing up into the regulator and causing lots of flavor issues.