Pilsner extract kit: why is the original gravity low?

Question

I've just put the extract version of this in the fermenter: http://www.wagonbrewing.co.nz/panhead-port-road-clone?search=pilsner

The gravity is 1.037 which seems way low. Dammit I don't want no weak beer! But I don't understand why it is so low. I did a partial boil (13L once the extract was added), chilled, siphoned to the fermenter and then topped up to 23L per the recipe. Wishing now I stopped at 20L and took a reading. Also aerated it by shaking the fermenter so it is definitely not a mixing issue messing with the reading. There is maybe 1cm of fluid/hops gunk left in the kettle. What happened?

One thing that was not great was the stove I was using was underpowered so it was a weak boil.

Malts:

• 2 x Black Rock Ultra Light Extract 1.7kg Can
• 250g - Weyermann Carahell Malt (Steep)

Answer 1

You say your boil was weak, and you didn't stir much. Extracts are hard to dissolve properly. Even vigorous boil without stirring may fail to do it. And 10-20 minutes of vigorous stir before wort boils may be barely enough. And it gets worse the less water you have in boil, so partial boil brews are most exposed to this risk. Sugars are heavier than water, so if not dissolved enough, they concentrate on the bottom.

What I guess happened is that a lot of sugars stayed at the bottom, with your hop waste. You would lose some of theoretical 1.046 anyway, but you probably lost more than you needed. Your results are consistent with loss of 0.4kg of liquid extract. Within what I can imagine in 1cm of gunk liquid. Especially given the fact that most extracts are already boiled, so you shouldn't have hot break - you shouldn't have any waste except spent hops, but it looks like you think you did.

Answer 2

If you have some dry malt extract (or liquid), you can get the gravity to where you want it now. It's not too late to add the DME, even though it's already in the fermenter.

About 1 pound of DME should get you up to about 1.045, which should give you a roughly 5% abv beer.

Answer 3

To fix the low gravity I followed Jeff's suggestion and added dry Dextrose/Malt mix. Here's the details in case someone else wants to try this procedure for the first time.

I did some calculations in BeerSmith and added 1kg of Dextrose/Malt mix into 1.5L of wort from the fermenter + 1.5L of boiled water. Boiled this then cooled to fermenter temp (24C). This was added 3 days after racking to primary and added 1.017 to the gravity in the fermenter to give me an equivalent OG of 1.054.

The way the yeast responded to the addition was fascinating with the fermentation rate increasing briefly, then almost stopping completely before resuming rigorously all in the space of an hour:

  Time   |   Stage            |    SG   |  Airlock Bubble Interval   
--------------------------------------------------------------------
 0       | Racking to Primary |  1.037  |  0
+36h     | Primary            |  1.017  |  5 seconds
+72h     | Before DME/Dex Add |  1.015  |  23 seconds
+72h 15m | After DME/Dex Add  |  1.032  |  18 seconds
+72h 20m |        "           |    -    |  Almost stopped completely
+73h     |        "           |    -    |  10 seconds
+73h 30m |        "           |    -    |  3 seconds
+8 days  | Primary            |  1.019  |  68 seconds
+10 days | Primary            |  1.016  |  0 (possible slow leak)
+12 days | Fejoa + Dry Hop    |    -    |  0
+13 days | Primary            |  1.013  |  0
+18 days | Final              |  1.013  |  0

It didn't quite attenuate as much as I'd hoped but on bottling the beer smelled and tasted great so I think it has all worked out.