Which O.G. to Use for Yeast Starter Sizing When Using Lactose

[hafmpty]

So weird question...probably not the first person to wonder or ask.

I brewed a Sweet Stout last weekend (Triple-X from BCS). When I used Mr. Malty and the Brewers Friend yeast calculators, I used the O.G. of the wort with the added lactose sugar being added in. I added it in the boil so the gravity I used was the gravity of the wort.

But we all know that lactose is UN-fermentable. So...in the future, I'm wondering if I should use the O.G. of the wort based on it's fermentable sugars vs. it's unfermentable sugars...especially the lactose as that is a known quantity of unfermentable sugars I'm adding to the wort.

I know there is a lot more than the lactose that isn't fermentable, but with this at least I know it's going to be in there but the yeast aren't going to be able to do anything about it.

An Analogy
I invite 100 people to a party. I order enough food for 100 people. But...I purposefully ordered 25% rotten food that no one would be able to eat. Now everyone gets less than they need and the party isn't as good.

That's kind of what I did with my yeast. I made a yeast starter big enough for a 1.060 wort, but my wort was really 1.052 as far as "fermentable sugars."

Thoughts?

 

[billl]

You are overthinking this. Yes, you are correct that sizing the starter based on the gravity pre-lactose would be perfectly fine. The part that you are skipping is the the margin of error in your yeast count is much higher than the gravity difference you are talking about. These are all ROUGH approximations.

On the plus side, as long as you get reasonably close to a proper pitch, yeast are pretty forgiving. You want to avoid underpitching by 50% obviously, but a 10% overpitch isn't going to negatively impact your beer.

 

[joshesmusica]

I mean, I guess you might be technically over-pitching. But that lactose is still an environment the yeast have to survive. In my opinion, it's always good to think about why the yeast act the way they do on an evolutionary scale. It's all about survival. Your wort still has all of those sugars, including the lactose. That is an environment the yeast need to adapt to before they start munching away at the type of sugars they can metabolize.

It's more like you invited 100 people to a pool party. The only thing is that you shoved those 100 people into the pool while it was still empty. You then provided enough food for 75 people, but enough liquid for 100 people... all at once. All of the guests had to survive the inundation of water into the pool first before they could eat. Because of the billions of years of evolution that they have endured, they were able to survive the pool-flood. They all escaped to their dinner tables, and ended up splitting up the food so that everyone got their share.

But really that's a bad analogy, because pitch-rate calculators take growth into consideration. That's why you'll start off fermentation with 220 billion cells and end up with 600 billion.

Either way, as Bill said, you're overthinking this. Go with what the calculators say according to your OG.

 

[hafmpty]

The part that you are skipping is the the margin of error in your yeast count is much higher than the gravity difference you are talking about.

Good point. Maybe if I was using a large percentage of unfermentable sugars it would make sense to adjust. But with the normal "brewing" amounts, it probably doesn't.

I mean, I guess you might be technically over-pitching. But that lactose is still an environment the yeast have to survive.

I was thinking about this too. The lactose is still a factor, making the density of the liquid higher and so more difficult for them to survive in (osmotic stress I think it's called).

 

[joshesmusica]

Good point. Maybe if I was using a large percentage of unfermentable sugars it would make sense to adjust. But with the normal "brewing" amounts, it probably doesn't.



I was thinking about this too. The lactose is still a factor, making the density of the liquid higher and so more difficult for them to survive in (osmotic stress I think it's called).

In going from a no-sugar environment to a high sugar environment it's called hyperosmotic shock. If the reverse happens, it's called hypo-osmotic shock. It happens all the time with yeasts and bacteria in the wild, and it's why they've evolved to become so great at adapting. It's why if you just pitch a single vial without doing a starter, you will see a longer lag time than if you had made a starter. The yeast are adapting to their environment. Same with rehydrated vs. hydrated dry yeast.

 

[hafmpty]

In going from a no-sugar environment to a high sugar environment it's called hyperosmotic shock. If the reverse happens, it's called hypo-osmotic shock. It happens all the time with yeasts and bacteria in the wild, and it's why they've evolved to become so great at adapting. It's why if you just pitch a single vial without doing a starter, you will see a longer lag time than if you had made a starter. The yeast are adapting to their environment. Same with rehydrated vs. hydrated dry yeast.

Cool. Good to know. Thanks.