My first helles was pitched with only one packet of w-34/70
which I gather from this forum is underpitching. It seemed
to taste OK and fermentation at 50F took right off.
Aside from a slow start, what other bad effects can underpitching
have?
There are off-flavors that can develope due to underpitching. Basically you would be playing with the growth phase of the yeast.
However, I don’t know if I would be that concerned in your case. I fermented a beer recently with the same yeast, at a similar temp, and technically underpitched as well. The only off-flavor I detected was a heavy alcohol burn, which went away with an extra week of cold conditioning. Otherwise ended up with a very clean beer.
Underpitching can also cause incomplete fermentation if the yeast has to overexert itself and drops out early. But again, I don’t see this happening in your case.
You should be fine.
