I'm kind of a freak about Kolsch and Alt, so I've done some research on
this.
The answer is yes. You likely won't get exactly the same flavor
profile by substituting a Kolsch yeast for an Alt or vice-versa.... but
there are enough flavor differences within the Kolsch and Alt styles
that I really don't think you'll notice much.
The Weinenstephan Hefebank in Germany also has a yeast that they
promote as being for either style, and I suspect that it is the yeast
used by some of the mega breweries near Cologne that produce both a
Kolsch and an Altbier. Note that the fact that the same brewery
produces both Kolsch and Altbier is fairly controversial, so the
breweries try to hide this... but there are several 'brands' brewed in
the old Kuppers brewery.
So with all due respect to Denny Conn, I'm not going to argue with the
Weinenstephan on the matter. Does that mean that all Kolsch and Alt
yeasts are interchangable? No, but close enough in my opinion.
I spent two days each in both Cologne and Duesseldorf recently drinking
at many different breweries.. and there are some differences.. chierf
among those is that Kolsch is more 'winey' in flavor and Altbier is
dominated by the hop flavors. Yet I am not really comfortable making
some kind of ideological stand on using the 'wrong' yeast.
Besides, I think that the vast majority of homebrewers and beer judges
have never had either fresh Kolsch or Altbier and given that, one
individual's opinion on the 'authentic' taste is somewhat worthless
(including my own).
Maybe if I lived in both towns for a year or so and really learned the
subtle differences of all brands with lots of practice (this is how
they train drug dogs right?) I would feel comfortable having a strong
opinion.
Here's the list:
WLP029 - German Ale Kolsch from PJ Fruh in Cologne
WY2565 - German Kolsch, unknown origin
WY1010 - American Hefe -- Actually a German Kolsch from Paffgen in
Cologne
WLP036 - Duesseldorf Alt - Seasonal, get it now.
WY1007 - German Alt from Zum Uerige in Duesseldorf
WLP320 - American Hefe - actually Widmer's yeast, which is from Zum
Uerige
WY1338 - 'Euro Ale' German Ale Yeast from Wissenschaftliche Station
WLP011 - 'Euro Ale' German Ale Yeast
Hard to find strains:
WLP003 German II Ale - from Duesseldorf - discontinued
BRY335 Alt - same as Weihenstephan 148.
BrewTek CL400 Old German Ale - unknown origion
BrewTek CL-450 Kolsch - unknown origion
EasYeast German Ale
EasYeast Kolsch
Yeast Lab A06 German Ale
DCL Yeast K-97 SafAle German Ale - Not imported to the US market in
homebrew sizes
Note that WY1010 is NOT the same as WLP320, this is contrary to most of
the 'equivalence charts' out there. And unfortunately I can't find ANY
information on Wissenschaftliche Station, and the Weinenstephan
reported no information on the #338 strain. In addition the origin of
WLP011 is conjectured to be the same as WY1338. I have my doubts.
Both the WY1056/WLP001 and Sam Adams WLP008 East Coast Ale might be
worth looking at well.
Many award winning Kolsch and Alt beers have been brewed with the
'wrong' yeast, so if it tastes good then call it good beer.
Good brewing!