Do I want my wort to evaportate during boil?

[critterz]

I am very new to making beer. I am on my 9th batch of all-grain. In the past I have let the wort slow boil and have had a good quantity evaporate out. Today, after the foam explosion, I put the lid on the pot and tilted it just about 1/4 inch and hardly any wort evaporated. Is this good or bad? My OG was off target by .005 (lower).

With the no-lid boil, I topped off one of my batches in the fermenter and I don't know if it is coincidence or not, but that batch kind of tasted watered down.

I keep thinking of making soup.... lid off, hardly any soup left; lid on, lots of weak soup; lid tilted, just right.

 

[doby]

You want the lid off, this lets certain unwanted things escape. Most of what boils off is water and unwanteds the fermenalbes remain. With ag you need to account for this with your water volumes and not use top off water

 

[latium]

What doby said. You're primarily wanting to boil off a compound called DMS, which is converted at high temperatures from the SMM (a different compound) in the malt. If you don't boil vigorously and drive it off, or if it condenses on the lid and drips back in, it can impart a cooked vegetable or corn flavor.

It's okay to have the lid partially on, but you definitely want to boil off a fair amount of the water, I think about 5-10% of it (somebody correct me if I'm wrong). You don't need to boil the crap out of it, but aim for a nice rolling boil, and if you use the lid during the boil itself (covering while getting to a boil is fine), keep it no more than probably half on.

 

[BryceL]

Definitely lid off. If your put is big enough to do full boils, you should account for boil off. With my system I boil off about 1.25 gallons pet hour. That means if I want 5.5 gallons in my fermenter, my pre boil volume should be 6.75. Topping off is also fine if you are unable to do full boils, add long as you get the volumes right it shouldn't water down your beer.

 

[TriageStat]

You want to do your boil with the lid off. When preparing your recipe you want to take into account your evaporation losses.

 

[Jon73]

I lose about a 1.5 gallons per hour. I did a 6 gallon boil for one hour of straight h20 to figure it out.