Yeast pitching rates and wort aeration?

[Andre Jaworski]

Hi Guys,

I have a couple of questions regarding yeast pitching rates and wort aeration I'm hoping someone can answer.

Assuming that aerating the wort prior to pitching the yeast is only so that the yeast can multiply,

1. If I pitch the exact recommended amount of viable yeast cells to the wort is aerating the wort necessary? 

2. Am I way over pitching at the moment as I already have the correct cell count in the starter but the cells continue to multiply in the early stages of fermentation?

3. Are there any side effects from over pitching?

 

Cheers in Advance,

 

Andre

[Mike Neilson]

Probablly the most technical question I have seen on this site, and its to late on a Sunday to get into it, especially after a week of festivities!!

[DP]

When yeast goes through its growth phase it produces the majority of its flavour compounds*. If you pitch too little it tends to produce more compounds and more undesirable ones can become detectable as they exceed the taste threshold. If you pitch too much your yeast will tend to get straight on with the job of converting sugars into alcohol and CO2 and you might miss out on some of the desirable characteristics of your yeast. Depending on your yeast, slightly underpitching and keeping the temperature lower should encourage development of more of the desirable flavour compounds. Once the yeast has multiplied and is actively fermenting you can raise the temperature a bit to get it chugging along.

My typical approach to an ale using something like WYeast 1968 in 20L @ 1.040 is to pitch around about 100bn healthy cells at 18deg. I hold it there for the first 12 hours or so until a nice krausen has formed and then gradually ramp it up to about 22deg for five days or more. Once seems to have stopped for three days or so I gradually decrease the temperature down to about 4deg for a week to get it nice and clear and then into a keg where it takes another couple of weeks to start tasting best. I'm still learning but this seems to be a pretty reliable rule-of-thumb for most of the kinds of beers I make on my system.

 

(*most of this info was gleaned -- or misremembered -- from Chris White's book, Yeast. Worth a read)

[Dougal]

Agreed.

From what I've read (or misremembered), recommendations for pitching rates allow for a sufficient growth phase to develop flavour, but not so much that you get stressed yeast flavours like excessive esters. I was reading somewhere that commercial pitching rates are normally higher than homebrew rates, might have been in 'How to Brew'.

So in that case, aeration is necessary as you want some growth (don't ask me about the olive oil thing).

As for the effects of over pitching, Stu's comment in another thread demonstrates it pretty well.    

[Cain Harland]

Digging up an old topic here

What do folks recon the margin is around the recommended pitching rates from the calculators. Is it a case of give or take a few % or is the recommended rate to be treated as the bare minimum which we should try to beat?

Example I'm doing a 40L imperial stout OG 1.092 with mangrove jacks 07 British ale and using the calculator at http://www.yeastcalculator.com/ optimal pitching rate is 660 billion cells. 1 pack of yeast in a 2L starter at 1.055 will give me 610 billion cells. 92% of the recommended cells.

So the decision is do I just pitch the starter 92% or add a second pack of yeast directly to the wort 122%.

[Cain Harland]

How much of an over pitch is required to start getting negative effects on the brew?

My last brew was going to have an OG of 1.092 I knew I was pushing the limit of my mash tun so had to leave out a couple of kilos of base malt bringing the target back to 1.081. Then I got crap 55% efficiency so 1.060. Thats fine rather than being in style (according to Beer Tools Pro) for an imperial stout (with oats) its on track for an oatmeal stout.

The thing I am worried about is a calculated my pitch for 1.092 and was already over pitching by 22%. I didn't think about that and chucked it all in and now have about twice the recommended yeast. Should I be worried?

The yeast is mangrove jacks - British ale and I want clean ferment so less worried than I would be over pitching in a wit or something where the yeast contributed more.