Partial Extract Brewing, DMSO and Boiloff

I have made two batches of brew, an amber ale and a kolsch style ale. For both I put all the extract, (LME) in at the beginning of the boil. The original boil volume was 3.75 gal. I had boil off of 27% in each case, (75 min total boil)

My questions: If I use the partial extract method do I need to worry about DMSO? If not, can I cover my boiler to reduce liquid loss? Would covering have any effect on the hops, like adding off flavors or reducing/increasing IBUs?

I think my boil might be too vigorious so I'm going to reduce it for the next batch. I also plan to add make up water to the boiler several times.

I'm still interested in the partial extract method, though.

Any hepl/thoughts will be appreciated. thanks.

I would not fool around with covering. Too much bad drops going back into the boil but I have no scientific evidence if this. Obviously you are worried that by the time you are done you will have 1 pint of condensed liquid left LOL.

I would put only maybe 1/3 of the extra into the initial boil and add the rest with 15 or 20 minutes to go. This will ad more liquid into the boil. Then once finished just top off with cold water when you have cooled your wort.

I've read two schools of thought on rapid boiling. One is you need rapid boils and the other is that a boil is a boil! I would, in your case, just make it a moderate normal rolling boil, not a heaving cauldron of splashing wort.

My understanding is that liquid malt extract has already had most of the DMS already boiled off since that is how they condense it into the syrup. However, I would still not try to cover the pot since that can lead to boil overs and large messes. My advice would be to either account for the boil off at the beginning of the boil(add more water to start) or reducing the heat to slow the boil a bit.

Full boils are always a better way to go, if you can do a full boil anyway. A full boil gives better hops utilization which means less hops for the same effect and it can also mean not having to "top up" your wort and possibly introduce contaminants!

You really do not need to boil extract for more than about 45 minutes. With that you should not lose so much to evaporation.

I do not feel it is a good idea to keep the boil covered at all once you come to a boil but you would have to make that call based on your results!

Good luck!