all grain - Which is worse: a weak boil or boiling with a lid partially on? - Homebrewing Stack Exchange
I finished Papazian's book The Complete Joy of Homebrewing last weekend and learned that a vigorous boil is necessary to isomerize the alpha and beta acids in hops which makes these bittering acids soluble in water-- both the boiling temperature and the physical action of the boil are mechanism by which the hop acids under go isomerisation.
I've also learned on this stackexchange that Dimethylsulphide is boiled off during the boiling period. Leaving a lid on the pot or a lid partially on the pot to help boiling will ultimately lead to some of the DMS condensing on the lid and falling back into the solution, leaving off flavors behind.
Well, I'm starting my first all grain today and my stove just doesn't have the capacity to boil 6.5 gallons of wort beyond the most pathetic boil.
This leads me to ask -- which is worse, boiling with a lid partially covering the pot, therefore increasing the vigor of the boil (substantially), but also increasing the DMS in solution, or boiling with a very weak boil?
You really just need a nice, rolling boil. There's a good discussion on boil vigor in the DMS episode of Brew Strong (starts at 38:36, but the whole thing is worth a listen). thebrewingnetwork.com/shows/485
I would highly recommend How To Brew by John Palmer. The book is more up to date and covers basic to very advanced topics.
I brewed 3 batches of 20L (5 gallons) at a common stove. The boil ins't vigorous, but I can see some bobbles popping up, I left the pot uncovered all the time, and I cannot observe hop utilization problems even residual DMS, until now (comparing with the beers I already made using a dedicated stove). PS: I used BIAB. The boil temperature in my altitude is 97.6C (207.68F).
Typically, people either do extract/partial boil/late addition or smaller batches if they are using a stove top. For a full boil, you pretty much need to use a stand alone burner or some other system that produces a good rolling boil. Commercial brewers use techniques like a calandria to reduce DMS while boiling off only 4%, but a stand alone burner is more economical for home brew.
Having said that, I think Pepi's answer is correct in that a longer boil time will yield the same volume and DMS reduction. Whether or not you'd get proper hop utilization I'm not so sure about. At my altitude, where 200F is the boiling point, I only get 81% utilization with a rolling boil. I would imagine if you were at sea level, then you'd probably be able to get decent utilization with a low boil, but I have no idea how you'd measure it or factor for it.
The other alternative is to boil two batches. It would be more complicated, but provided your burners and kettles were equivalent, you would be able to just split any additions, and boil the two side by side.
Why do you say that you'd need a stand alone burner for an all grain boil? First, why does it matter whether it's all grain or not? IMHO only the amount of wort should matter, not how you made it. That said, I never have problems boiling my wort (for a 5-gallon / 22l batch) on a regular stove.
I didn't. Read the post again.
The temperature is most critical for isomerisation of the alpha acids. 100C for 90 min results in the maximal conversion of alpha acids; The Handbook of Brewing 2nd Ed p209. The vigor of the boil is not a major factor. The book goes on to say that breweries operating at high altitude can struggle to get good isomerisation rates.
Depending on the malt and style of beer DMS/leaving the lid on can be OK. I have made the mistake before of leaving the lid fully on for 50min of a 60 in boil. I removed it for the last 10min and kicked it to a rolling boil to ensure the DMS was evaporated out and the solution properly cycled round. The brew was fine with no bad effects. It was an IPA.
You can compensate for a weak boil by boiling longer. The 'standard' boil is usually 60 minutes, many experienced brewers say 90 is better for removing DMS. I'd say plan on a two hour boil and keep boiling till you get down to the volume you want.
BTW The hop acids will isomerize pretty well despite a weak boil. I have heard that (don't quote me here) most isomerization happens in about 20 minutes, and that 180F/80C is enough.
Am I right to paraphrase you as saying that, given a weak boil, a longer boil will be helpful for removing DMS but not for the isomerization of hop acids?
DMS removal will be slower, and helped by a long boil, but isomerization will be at the normal rate. In other words isomerization will be complete even if you hold the wort just below boiling for an hour.
Hop utilization is lower at altitude because the boiling point is lower. At 200F boiling point, this results in 81%; requires ~25% more bittering hops. This makes me think 180F can't possibly be enough.
A function of boiling rate is the amount of surface area.
I have some really large surface-area pots (tuna-can shaped). A trick I do to increase the boil rate without increasing heat is float a stainless steel bowl or pot on top to decrease surface area. I maybe reduce surface area 25-35%. If it is a case where you are borderline this trick may bump you over the edge.
This also reduces evaporation rate with the reduced surface area so you may end up boiling longer anyway. I just compensate for the lowered evaporation rate with my initial volume of water.