Overnight Starter: What Are my Yeast Doing?

[RCCOLA]

I brew mostly lagers and pitch a lot of harvested yeast. On older slurries(> 1mth) I will pull 1 gallon of wort on brewday, rapidly chill it to 50F, pitch my slurry to it with DAP and yeast energizer, and put it in my chamber set at
50F.

The rest of my wort gets chilled overnight and the 1 gallon starter gets pitched the next day into the 50F wort. The starter is usually fermenting balls-out the next day.

My last batch got 200ml of very clean 1.5mth old slurry and was way underpitched according to Mr Malty. However, it fermented as strongly as any lager I have ever brewed.

This got me to wondering--Am I getting yeast growth with the overnight starter or am I just reactivating the present yeast?

 

[DoWBrewer]

If you are an AHA Member (and you should be) go and listen to Neva Parker from White Labs lecture on yeast growth from 2012 NHC. There are quite a few variables such as the gravity of your wort and your pitch rate. She also has a video on making a yeast starter that is good. I was creating yeast starters with too many fermentables before watching her video and getting the appropriate ratios of water to DME.

 

[WoodlandBrew]

Yes, quite a bit of growth. How much depends on a few things. If the yeast has a sufficient glycogen reserve then the lag phase may be less than an hour. If the pitch rate is high enough you could easily double cell count in 24 hours. My blog and book have quite a bit of detail on this.

I use the same method as you on a regular basis. It makes me wonder why people bother with starters.

Here are details:
http://woodlandbrew.blogspot.com/2012/12/no-more-wasteful-yeast-starters.html

 

[DoWBrewer]

Yes, quite a bit of growth. How much depends on a few things. If the yeast has a sufficient glycogen reserve then the lag phase may be less than an hour. If the pitch rate is high enough you could easily double cell count in 24 hours. My blog and book have quite a bit of detail on this. I use the same method as you on a regular basis. It makes me wonder why people bother with starters. Here are details: http://woodlandbrew.blogspot.com/2012/12/no-more-wasteful-yeast-starters.html

That is an interesting method. It certainly seems like a viable alternative to yeast starters. My only concern besides infection would be two lag phases and the potential for unintended by-products from two growth phases. It would be interesting to do side by side tests.

 

[WoodlandBrew]

Yes, you do have to make sure the wort is sanitary.

Actually, there is the same amount of cell growth independent of when the sugar is added, and it occurs mostly during the same time as traditional methods. Yeast growth is directly proportional to sugar consumption.

The change is osmotic pressure when the second wort is added will force the yeast to adjust to the new environment, (a second lag phase) but I haven't noticed any off flavors from this method.

 

[RCCOLA]

Thanks for the info Woodland. Makes me feel better that I'm increasing my cell count.
I'll continue doing this with repitched older yeast and may incorporate this as a step-up for starters on first pitched lagers---2qt chilled and decanted starter followed by my overnight starter.

 

[WoodlandBrew]

You're Welcome,

Recently I've been developing a fermentation model to create some graphs for my book. I made some adjustments to display your technique which is very similar to the "Two Step" method in my book.

This is showing a 5 gallon batch of 12°P wort pitched with 100 billion cells. The first graph is pitching into 1 of the 5 gallons and adding the remaining 4 after 24 hours. The second is adding everything at the same time.

The thing I see is that with under pitching the cell density is less than 30 billion per liter for about a day, where with the two step method the cell count only dips below 30 billion for short periods of time.

 

[RCCOLA]

I found this statement particularly interesting from Kai's blog:

Narziss reports that this technique is beneficial to the attenuation and the ester levels of the final beer. The latter are reduced because the yeast is kept longer in its growth phase during which it consumes the ester precursor acetyl CoA [Narziss, 2005]

The last 2 beers I used this technique on attenuated farther than the previous batches pitched directly. I don't have an opinion on the ester levels of either batch yet.

 

[WoodlandBrew]

Agreed. I've seen a similar correlation between pitch rate and attenuation. Unfortunately this model doesn't show that.