I’ve been trying to better understand yeast and pitching rates and have a few questions…
1) What triggers the end of the yeast growth phase? I know oxygen/UFA/sterols/cell wall health are a major part of this. So do the yeast reproduce while their cell walls are healthy, turn to producing alcohol and then die or go into suspension?
2) Couldn’t an additional nutrient/O2 supplementation encourage greater growth/vitality within the yeast/offspring? I suppose the result would be a beer with higher ester byproducts. This to me would suggest somebody pitching their entire starter might see no advantages over just underpitching.
3) Does an equal yeast growth at two temperatures result in the same amount of ester byproduct? Higher temps=more yeast growth (resulting in greater ester formation and attenuation before cleanup of byproducts)
4) I suppose on the other end: low temps= less yeast growth (resulting in lower final attenuation before yeast inactivity as a potential complication)?
I’ve always wondered if there was a way to treat the fermentation to maximize a low pitch rate. There are many times it would be nice to brew a batch of beer only to find out that you only have a single vial… does this always result in inferior beer to a beer with a proper pitch rate?
Sounds like good questions for Chris White during his spot on Ask the Experts. You an AHA member?
Understanding that I have no background in microbiology, my guesses are that:
Cells stop reproducing when they lack the energy to do so or when the cell walls are sufficiently scarred to prevent budding.
Greater growth isn’t always what you want. More growth = more generations of yeast = lots of unhealthy yeast. Plus if the yeast don’t use all the nutrients, they could have a flavor impact on your beer. But if you’re feeding the yeast all these nutrients and oxygen, wouldn’t it be better if you just fed them properly prepared wort? Depends on whether your goal is growing yeast or making beer.
3/4. Higher/lower temps affect the growth rate not the total growth. Faster acting yeast aren’t necessarily doing their job better. Think Lucy and the candy line. I remember a Chris White talk from MCAB IV, I think, where he said the temperature to maximize yeast growth was somewhere around 90F. But it’s stupid to think that you’d want to make beer at that temperature because they’d be throwing off all these other byproducts that you don’t want. Maximum growth <> optimal flavor. Lower temps might mean the yeast are working slower, so that other beasties could get established. Basically, whenever yeast aren’t operating in their preferred conditions, you can get unwanted fermentation byproducts or fermentation effects. Be good to the yeast and they’ll be good to you. It’s symbiotic.
A single vial doesn’t necessarily mean a low pitch rate. Make a starter, prepare a good environment for the yeast, and have at it. If you need to step up your starter more than once, go ahead and do it. Just because a certain way is optimal doesn’t mean other ways aren’t satisfactory. Sometimes good enough is good enough. People were making good beer way before they counted cells and used spreadsheets.
If you want to run an experiment, split a batch and ferment one with yeast prepared with a calculated pitch rate and ferment the other with conditions like I described. Taste the resulting beers and see if you can tell a difference. Do a triangle test with trained tasters if you want the best results.
I agree with Gordon, both in his answers (mostly) and that these will be good questions for Chris White, but I’ll take a stab at some of them . . .
1. In a normal fermentation the yeast stops growing when the O2 is depleted, because they are unable to produce the necessary cell membrane components to continue dividing. Under lab conditions, yeast typically divide about 25 times, that is the extent of their “replicative lifespan”. But keep in mind that yeast will only double about 1-3 times in the wort, so that isn’t really a factor. Especially when you consider that 50% of all of the cells in the wort have never divided. 25% have only divided once. 12.5% have divided twice, etc. By the time you get out to 20+ divisions, those old mother cells make up a very small percentage of the yeast in the batch.
2. Yes, adding O2 and nutrients can help the yeast divide more, and that technique is sometimes used in really big beers to help fermentation. However, Gordon makes a good point that more growth is not necessarily what you want, and if you add O2 at the wrong time you’re risking oxidation.
3/4. Ester formation is more complex than just cell division. And again, I agree with Gordon.
If you want to make a beer and only have a single pack of yeast, you have options. You can make a lower gravity beer so that one pack is enough. You can make a starter like Gordon mentioned. You can also keep a few packets of S-04 or US-05 on hand so you don’t run into that problem, they last a long time.
Your beer won’t always be inferior if you underpitch for the strain/style, but you need to think about why you are brewing - if it’s for you to have on tap and you like it, then no problems. If it’s for competitions then maybe it’s not ok. I think you should try to control what you can and not stress about the rest.
I’m far from an expert. What I know is what I’ve experienced, heard and read. I don’t make starters usually because dry yeast is too easy. Plus my beers usually are about 1.045-50. Not too much for a 11g pack of hydrated yeast to handle.
That being said, your questions is are excellent and should be addressed by the experts.
The question about temp and growth is interesting. Worry more about temp first then pitch rate? For outstanding results what my experience has taught me is that pitching an appropriate population of healthy actively fermenting yeast produces the best beer. Hands down.
One vial of yeast? How about a two gallon batch and then top-crop and pitch into your five or another 2 gallons. I think the idea is that you don’t want much yeast growth but healthy activity at the right temp.
For #4 Lagers or ales. I think at ale temps under-pitching will still net you your attenuation. However, under-pitching a lager will give you problems at a cool temp according to John Palmer. A tidbit from the member’s only content…
Thanks for the answers, these have been very helpful. And yes, I am an AHA member, there should be something by everybodies name that is…might even encourage more membership.
So when you plate a yeast to isolate a particular strain (not sure if this is the right terminology), the single cell or cells yous isolate are presumably already very scarred? Doesn’t scarrage just get even worse as this is grown up to a pitchable size rate or is there some means of yeast reproduction that results in yeast that fresh and unscarred?
Maybe a reread of CW and JZ’s yeast book is in order for me.
The yeast offspring don’t have scars. When the yeast offspring bud and break off the mother yeast cell, it leaves a scar (on the mother). So, if that mother cell reproduces 20 times (average) in her lifetime, she ends up looking pretty scarred up, eventually sloughs her mortal coil, and drops to the bottom of the fermenter
Once the yeast offspring “grow up and become adults”, then the cycle starts anew. As long as you do starters (or rinse your yeast off the trub), you’ll always have more healthy yeast than old, scarred, senecent yeast.
These would definitely be great questions for the “Ask the Experts” Q & A session with Chris White from White Labs.
I am not a microbiologist so I can’t answer your questions with any educated responses.
However I can respond from my experiences with homebrewing.
I tend to agree with Gordon as well. The bottom line for me is the end product. I have read and heard that pitching calculated quantities of healthy yeast into an oxygenated environment is the best way to go. This has always resulted in good quality beer for me. When I first started brewing I had limited knowledge of yeast pitching techniques and didn’t follow any good practices in this regard and my beer suffered from it. Now that I have experience, through trial and error I have come to believe that good yeast pitching practices are required for making quality beer IMO.
Again it all comes down to ones own tastes… let the beer be your guide.
This seems to be the take-home message, I am trying to oversimplify something that should not be.
By carefully controlling yeast activity through temperature and maybe with a little nutrient manipulation I always wonder if I could essentially perform an extended yeast growth in the main wort that would result in a similar beer profile to a larger pitch rate via multiple tubes or starter.
My thinking is lower pitch temp = less esters during growth phase, so by extending the growth phase (maybe with a little extra O2) I end up with the same amount of total yeast after the growth phase to ensure complete attenuation/cleanup.
I know I risk contamination with less vigorous yeast activity, but the cooler temp might offset this.
The other factor I am not considering yet is of course fusel production of underpitched yeast, what is the main mechanism for this? Is it just overworked, nutrient-lacking yeast?
I practice good pitching rates with starters and always keep S05 on hand for last minute brews and am very happy with it. Sometimes though it would be nice to do a last minute brew when I only have a single vial of my desired yeast without having to drive across town to the brew shop to pick up a couple of extra vials. I guess I’ll try a side by side sometime and see if it is acceptable to my palate- this is the only real way I will know if this is ok for me.
This also leads me to the rant, why doesn’t Wyeast or Whitelabs produce a larger pitchable size for more typical brewing conditions (SG:1.05). Per Mr Malty’s website a single vials is really only pitchable to 1.03 (I know this is debatable and subjective but following it has lead to some decent beers IME). I’d happily pay more…
This might be THE question for some. Why not double/triple the capacity of the vial? Charge an extra $2. Educate homebrewers that this is indeed enough yeast for most beers (ales). Chris has to harvest more yeast, but it would set him above the competition.
I’ve heard Chris defend his one vial no starter product when interviewed by Jamil. Obviously, he’s in the business of yeast selling but maybe he isn’t as concerned with under-pitching. It might be his response too. I’d put the question to Jamil as well and see how the answers match up LOL.
