# Lactose Effect on FG

Discussion in 'General Brewing Discussions' started by Steve SPF, Apr 27, 2021.

1. ### Steve SPF Well-Known Member

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#1
Morning all,

Gave myself a minor heart attack yesterday thinking my oatmeal stout had stalled/stopped at 1.020 when I was expecting 06-10 but then remembered the lactose!

Is there a calculation to help me figure out how much 1kg of lactose would add? Also, as it's added at the end of the boil, what effect will it have on OG as well as FG?

Steve

2. ### dmtaylor Active Member

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#2
Lactose should add to both the OG and the FG roughly 340 gravity points per kg per liter, post-boil volume (I converted this from the American standard of about 41 ppg, which is points per pound per gallon).

So, 1kg in say a 19L batch should add 340 / 19 = 18 points to both the OG and FG. Or if you made 38L, then you've added 9 points, etc.

When you use lactose, it might be best to totally ignore the lactose in any recipe calculations, then after the recipe is designed, add these numbers to both the OG and FG. For example: Let's say you wanted to brew 38L at an OG of 1.069, and the estimated attenuation of your yeast is 75%, and you're going to add 1kg lactose to the boil. Then you know you could design your recipe for 9 points lower, for an OG of 1.060, because you know you'll add 9 more points in lactose at the end. And then with 75% attenuation of the regular sugars present, you can expect an FG of about 1.015 (60 / 4). But finally, with additional lactose, you'll add 9 points to both the OG and the FG, for an OG of 1.069 and FG of 1.024.

It's a little complicated but not too bad. Hope this helps.

3. ### _Stephen_ New Member

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#3
I did this as well the first time I used Lactose lol.

4. ### Donoroto Well-Known Member

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#4
So basically lactose isn't fermented. Right?

5. ### _Stephen_ New Member

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#5
Correct but it still effects your gravity reading even though it won't contribute to ABV.

6. ### Josh Hughes Well-Known Member

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#6
Last edited: Apr 27, 2021
Funny timing I checked my gravity today (10 days) on a milk stout. Supposed to be at 1.018 it was 1.022. I’m going give it a few more days. Sample tasted great but hate to bottle early cannot reminder if my FG was high last time I brewed this. I’m using US05 this time, i used S04. OG of 1.064. Yes unfermentable

7. ### dmtaylor Active Member

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#7
It's close to finished, but with US-05, maybe not yet. How much lactose in how big a batch? If about a pound in 5 gallons (pretty typical) then I think it could come down close to 1.018. But really depends on recipe. And what was your mash temperature and total mash time?

8. ### Josh Hughes Well-Known Member

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#8
8% lactose. 2 gallon batch. Mashed at 151. Fermented mid 60s

9. ### dmtaylor Active Member

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#9
Yeah I think it might still come down a couple more points, hopefully. Waiting a few days is wise.

10. ### Josh Hughes Well-Known Member

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#10
That’s what I figured. Thanks. Not used US05 since last summer

11. ### Nosybear Well-Known Member

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#11
It should be the same as any other sugar, about 46 ppg (can't do the metric conversion in my head). Its effect on OG and FG will be the same since it doesn't ferment.

12. ### Minbari Active Member

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#12
Everything I have read confirms this. Lactose does not ferment

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