Hot Flavor in my Beer

The last few batches, I've noticed a very hot flavor in my beer.  It gives me a headache.  The strange thing is when I put it in my keg, it tastes great.  After about 1 week, the off flavor appears.  My last batch, I put some in a bottle before adding it to my keg (to see if it was the kegging system), and I still got the flavor.

Any ideas?

Might help if we had your recipe before we start making guesses.

What temp are you fermenting and what yeast?
You're describing fusels. You may be fermenting too warm.
You said one week. Might be green beer that off flavors may diminish with age.

The problem is it has been across my last 3 batches (IPA, high gravity christmas ale, and an ESB).  I don't have temperature readings for the actual fermentation, but they did ferment well (the christmas ale exploded).

I did not taste any of this when I kegged the beer, just shortly after.  Is it normal to not taste the fusel alcohols at first, only to taste them later?  Maybe the cooler temperature of the beer had something to do with it?

Like the post above says, it is most likely that you are fermenting your beer at a temperature that is too high.  When this is the case it stresses the yeast and they produce fusel alcohols.  The classic descriptors for fusels are hot or spicy tasting, warming of mouth and throat, and headaches.  It can be caused by under pitching yeast, but the most common cause is fermenting too warm.  Another indicator of this the explosive fermentation you described.

There are a number of ways to better control fermentation temperatures.  The cheapest is a partially submerging your fermentation vessel in water, wrapping with wet towels, etc.  Remember that it is the temperature of the fermenting beer that you are concerned with, not the air temperature of the room.  Fermenting yeast will produce heat which can raise the temperature of your beer by as much as 10*.

reim0027 said:

The problem is it has been across my last 3 batches (IPA, high gravity christmas ale, and an ESB).  I don't have temperature readings for the actual fermentation, but they did ferment well (the christmas ale exploded).

I did not taste any of this when I kegged the beer, just shortly after.  Is it normal to not taste the fusel alcohols at first, only to taste them later?  Maybe the cooler temperature of the beer had something to do with it?

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How long do you allow beer to ferment before kegging?  It's possible that other stuff is masking enough of the "taste" of fusels during the first week.  Did you still get the headache even though you did not taste the fusels?  But for root cause, everyone is correct........short, warm and explosive fermentations are ripe for creating fusels. 

The first 2, I left it in the primary for about 3 weeks, then it went into the keg.  The last one, it was in the primary for 1 week, then into a secondary for 2 weeks.  All 3 of them tasted great when going into the keg (that is what was so frustrating).

I'm not positive, but I do not recall fusels being one of the issues (e.g., diacetyl, green-apple) that occur in every beer, and if allowed time, the yeast self-clean.  Most sources say fusels rarely diminish [much] over time.  The delay in your detection of the fusels could be that other "green beer" flavors mask them until later. 

The transition from wort to beer is quite complicated, and takes longer than many sources may quote, esp. recipe kits that recommend "one week primary, secondary, then package" types of timelines. 

So prevention is the only solution, and fermentation temperature control, pitching big healthy yeast starters, and wort aeration would each help produce cleaner fermentations.  If you don't have a freezer, nor room for one, then you could try the water bath, wet t-shirts, and fans that some use, and also the big starters and oxygen to alleviate yeast stress wherever possible.