Calculating the correct amount of DME to add to the boil

[pmiddy]

I'm about to brew my first Milk Stout (ready for the Aussie winter):

Ingredient	Kgs​
Light Dry Malt Extract	2.700​
450g Lactose Powder	0.450​
450g Simpsons Black Malt	0.450​
350g Joe White Dark Crystal Malt	0.350​
250g Pale Chocolate Malt	0.250​

I'm doing a partial mash/steep and I intend steeping all the grains first, then adding DME (and maybe lactose) to bring the boil gravity up to match the fermenting OG. I've read that this is what you should do.

So I'm trying to calculate how much DME I should add to the boil. Initially, I thought I'd add half the lactose and then top up with the DME, just to keep the chemisty similar to fermentation. By my calculations, this meant adding only ~270g of DME for boil, with the remaining 2.5kg going in at the end (to Pastuerise):

Ingredient	% of total mass added to boil/steep	Kgs
Light Dry Malt Extract	10%	0.270
450g Lactose Powder	50%	0.225
450g Simpsons Black Malt	100%	0.450
350g Joe White Dark Crystal Malt	100%	0.350
250g Pale Chocolate Malt	100%	0.250

The alternative is to leave the lactose out from the initial boil and add at the end. This results in adding 520g DME to the boil.

Ingredient	% of total mass added to boil/steep	Kgs
Light Dry Malt Extract	19%	0.520
450g Lactose Powder	0%	0.000
450g Simpsons Black Malt	100%	0.454
350g Joe White Dark Crystal Malt	100%	0.340
250g Pale Chocolate Malt	100%	0.227

Note:

Questions:
1. Can anyone verify my calculations are correct?
2. Is it advisable to rely so heavily on the grains for the boil gravity?
3. Does the fact that lactose is unfermentable make any difference at all?

Thanks heaps.

 

[VikeMan]

What matters for OG is what you want the gravity to be at the end of the boil. For that, it doesn't matter at what point in the boil you add the lactose. And you shouldn't change the amount of DME based on the timing of the lactose. For OG target calculations, include the lactose, regardless of when you plan to add it.

As you said, lactose is unfermentable. So, it increases the OG, but its full gravity contribution remains after fermentation, in the FG.

Also, just a nit about terminology. None of your grains have any diastatic power, i.e. enzymes for a mash, so this won't be a partial mash (or any kind of mash). It will be a steep, which is fine, because all of the grains you listed are steepable.

 

[BrewnWKopperKat]

I'm about to brew my first Milk Stout (ready for the Aussie winter):

Welcome to Homebrew Talk!

 

[pmiddy]

Thanks, @VikeMan. I understand that the lactose can be added at any time. And that I want the OG to be at the target (1.051) at full volume (~21L) before I pitch the yeast.

What I don't understand is why I need to try and match the partial boil gravity with the OG (I have an inkling this is because of the effect on the hops in the boil), and whether I need to consider what ingredients I use in the partial boil to achieve that. Or does it not matter as long as the boil gravity is ~1.051?

Thanks again.

 

[VikeMan]

What I don't understand is why I need to try and match the partial boil gravity with the OG (I have an inkling this is because of the effect on the hops in the boil), and whether I need to consider what ingredients I use in the partial boil to achieve that. Or does it not matter as long as the boil gravity is ~1.051?

IMO, It doesn't really matter. Yes, hop utilization is to some extent a function of gravity. But not because of the amount of sugars/carbs in solution while the alpha acids are being isomerized (as was once thought), but because gravity is roughly proportional to the proteins in the wort, and the proteins (and yeast) drag some of the isomerized alpha acids along with them when they settle out as trub. More proteins = more Iso AAs removed.

 

[pmiddy]

Thanks again, @VikeMan. I'll assume that selecting boil ingredients based on their protein contribution is pointless (if for no other reason than because of the variability in my process) and just get on with it!

Thanks again.