Starter is Higher Gravity than Wort

[3 Dawg Night]

I'm going to be brewing a 5 gallon batch of Dark Mild this weekend. My calculated OG is 1.036, which by my calculations means I need 128B cells. I'm not a fan of underpitching, so I'm going to build a small starter. This is the lowest OG beer I've brewed, and I'm curious if a starter is even needed. My starter gravity (1.040) will be higher than my wort gravity (1.036), so what am I getting from the starter that I wouldn't be getting from direct-pitch? Is it just a quicker start to fermentation?

 

[GilSwillBasementBrews]

A larger yeast population.

 

[hotbeer]

Of course you could pitch more yeast directly and not worry about the starter. Or make a starter and save some expense.

These are just two of several different ways to achieve what will basically be the same finished product.

 

[day_trippr]

Interestingly, 1.036 is more favored for starter SG than 1.040...

Cheers!

 

[Steveruch]

What yeast?

 

[3 Dawg Night]

What yeast?

WLP002 English Ale

 

[TheMadKing]

Volume

Yeast grow to the size of their food supply (in simple terms), so when you do a 2L starter at 1.040 they go through their complete life cycle, multiply and flocculate so that there are more cells and they are ready to go when food is more available.

If you pitch too few cells, even into a 1.036 mild, they will need to grow by more generations on the same oxygen supply in order to fill the size of their container. So all you're doing when you're pitching an "adequate pitch" is preventing the yeast from multiplying so many times before they reach maximum population density that they hurt themselves and produce off flavors

 

[3 Dawg Night]

@TheMadKing has the best explanation I've seen. That's exactly what I was looking for. Thanks!

 

[Stormcrow]

@TheMadKing has the best explanation I've seen. That's exactly what I was looking for. Thanks!

I know right? I've heard people drone about this for hours and felt like I got it, but that was a great succinct explanation.

 

[Miraculix]

Volume

Yeast grow to the size of their food supply (in simple terms), so when you do a 2L starter at 1.040 they go through their complete life cycle, multiply and flocculate so that there are more cells and they are ready to go when food is more available.

If you pitch too few cells, even into a 1.036 mild, they will need to grow by more generations on the same oxygen supply in order to fill the size of their container. So all you're doing when you're pitching an "adequate pitch" is preventing the yeast from multiplying so many times before they reach maximum population density that they hurt themselves and produce off flavors

I think the truth lies somewhere in between. AFAIK, yeast needs oxygen to multiply. It can store some of that oxygen in a tricky way so that there will be multiplication after there's already no oxygen left, but on the long run, with no oxygen being present, yeast will stop to multiply. This means that the cell count being present continues to live and to produce alcohol etc. As long as there are the right circumstances (food, pH, alcohol levels ....). But this also means that they won't grow till they "fill the volume", instead they grow as long as they have oxygen and food. If one of these is missing, they will stop the reproduction. So we got an actual interest to keep the starting amount of yeast high enough, because multiplication in the beer just works to a certain degree (ie. Until oxygen is used up). In a starter it's different. It's an open ferment with continuous introduction of oxygen, so there's going to be multiplication until the food is gone.

I have somehow the feeling that some yeasts can multiply without oxygen, I did kveik beers with such a ridiculously low cell count and it finished in no time and the slurry was hundreds of times more then what I introduced at the beginning.

 

[VikeMan]

The limiting factor in yeast growth (besides carbon source) is cell wall material, particularly sterols. Think about a cell, having to divide over and over again, the walls getting thinner with each budding. If it starts out "thick" (i.e. with lots of sterols), it can divide more times. Yeast "know" when to stop, to avoid their walls getting dangerously thin and causing autolysis. If they can't reach enough mass (population), each cell has to do more work in fermenting the remaining sugars, which can cause issues like excessive esters.

This is where oxygen comes in. During the lag phase, yeast take up the available O2 and use it to build sterols.

As an aside, yeast don't always have to build the sterols. If they are available in the environment, they can assimilate them. (That was the idea behind the New Belgium olive oil experiments.) There are also some sterols in malt, but generally not enough to do the job.

 

[TheMadKing]

I think the truth lies somewhere in between. AFAIK, yeast needs oxygen to multiply. It can store some of that oxygen in a tricky way so that there will be multiplication after there's already no oxygen left, but on the long run, with no oxygen being present, yeast will stop to multiply. This means that the cell count being present continues to live and to produce alcohol etc. As long as there are the right circumstances (food, pH, alcohol levels ....). But this also means that they won't grow till they "fill the volume", instead they grow as long as they have oxygen and food. If one of these is missing, they will stop the reproduction. So we got an actual interest to keep the starting amount of yeast high enough, because multiplication in the beer just works to a certain degree (ie. Until oxygen is used up). In a starter it's different. It's an open ferment with continuous introduction of oxygen, so there's going to be multiplication until the food is gone.

I have somehow the feeling that some yeasts can multiply without oxygen, I did kveik beers with such a ridiculously low cell count and it finished in no time and the slurry was hundreds of times more then what I introduced at the beginning.

We are pretty much saying the same thing but I just made a logical leap that volume = food supply. 10 gallons of 1.030 wort is more food than 1 gallon of 1.030 wort, so in order to consume all of that volume of sugar, a given population of yeast will need to multiply more times and use up more of its sterol reserves in the process. Yeast can certainly multiply once the oxygen supply is used up, but each bud reduces the sterol reserves and they are less able to replenish it after that point. So the oxygen depletion essentially starts the "clock ticking" on how many healthy generations you can get within a beer.

I intentionally oversimplified to cut to the chase of what the OP was asking, Vikeman expanded on it nicely

I'll also add the yeast "sense" when food supply (sugars) is running low in an aerobic environment (i.e. a starter on a stir plate) and fatten up their sterol stores using oxygen so you get more cells with nice thick cell walls and then flocculate so that they are basically in hibernation and ready to go in happier times