A couple questions on starters

Ingredients
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My first question is about making a starter for a high gravity beer.  I’ve heard two different schools of thought on this one.  One of them is to just make a moderate grav. starter, say 1.050 regardless of what you’re going to pitch it into.  The other is to try to match the starter grav. to the beer you’re pitching into.  Sounds like it’s a difference between stressing the yeast right away from osmotic pressure, or later when you pitch.  Any thoughts on this?

My second question is a simple one:  foam plug or airlock?

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1.) A starter should always be around 1.035-1.040 regardless of the OG of the beer it’s going in to.  You want to grow lots of healthy yeast, not stress them.  If the OG is really high, then make a multistep starter to grow more yeast.  The key is the cell count.

2.)  I use AL foil

EDIT:OG typo!

For ales, I do about 2 qt.  For lagers, I start with 2 qt., let it ferment out, decant, and add another 2-3 qt.

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And not 0.0001 point lower  ;D

+1 on the aluminum foil or foam stopper.  You want oxygen to be able to get in.

For my bigger ales I do a half liter starter just to wake things up and get a little growth and then pitch it into a gallon of 1.040 or so wort.

For my mid-gravity lagers I do the same thing but add an 2 liter intermediate step in there as well.

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What Denny said.  Low gravity starter, always.

The only exception being if you want to try to isolate your own strain of high gravity yeast, which could be fun if you are both really patient and meticulous.

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Thanks to all for the good advice.  I’m hoping for Santa to bring me a stir plate for Christmas.

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I’m not in agreement with Denny on the starter gravity.  In talking with Chris White (White Labs) many years ago, he said they culture their yeast using a much lower gravity wort.  He indicated about 1.020, but I see that in his new yeast book, he mentions 1.030 as a prefered starter gravity.  The issue of the Crabtree Effect is more pronounced when you increase the wort gravity, which diverts some yeast metabolic effort into alcohol production instead of yeast mass growth.  I use a starter gravity in the low 1.020s and typically add about 700 ml of this wort twice during the starter propagation (I’m mostly an ale brewer).  Most of the spent wort is decanted after the yeast has been dropped by chilling leaving me with a nice yeast slurry for pitching.

I don’t think there is much difference between the use of a foam stopper or foil cover since I don’t believe that there is enough Brownian Diffusion of oxygen into the starter vessel when there is a net exhalation of CO2 during the yeast growth process.  I prefer to pump filtered air into the vessel to assure that the headspace maintains a normal oxygen level.  Over the years, I’ve found that using an airstone is not really necessary since I use a 6L Erlenmeyer flask with a 3-inch stirring magnet and there is a significant wort surface to transfer oxygen.

The other problem I found with using an airstone was that it caused significant foaming problems.  Even with the 6L flask, I would sometimes push foam out of the vessel.  Wheat yeasts are particularly prone to creating a dense foam.  This foaming was a problem, no matter how I adjusted the air flow rate.  So, no airstone needed!

The need for oxygen during yeast growth is well known. But as evidenced here, pure oxygen is not needed during starter preparation.  With beer wort, a good shot of pure oxygen at pitching time will bring the wort’s dissolved oxygen content up much higher than can be accomplished with pumped air or splashing.  Oxygen is fine when you have only one time when you’ll be adding it.  But for the extended yeast growth cycle during starter preparation, constant air supply is a cheaper and equally effective alternative. The primary concern is the cleanliness of that air.

I’ve found that 0.45 micron disposable filters that are intended for groundwater sampling are great for filtering air.  Even though they are touted as disposable, they can be used for many years if you are diligent in keeping them sanitary.  They have huge filtering capacity and should handle millions of cubic feet of air flow through them.  There are 5 and 10 micron filters available too, but you need 0.45 micron or smaller in order to filter bacteria and other airborne debris from the air flow.  You’ll see an example of the filter I use here:
 
http://www.geotechenv.com/disposable_filter_capsules.html

These are not cheap at about $15 each, so you will want to make sure it lasts a long time.  I would still be using my original filter except that I allowed sanitizer to backflow into the filter once.  I found out that you have to keep those filters dry!! Fortunately, my engineering company performs groundwater sampling and I was able to sweet talk another filter out of our stock.  I use a regular aquarium air pump and vinyl tubing.  The tubing will actually fit tightly into the filter housing, so no special fittings are needed.  I am very particular about keeping the output side of the filter clean and sanitary.  I remove the outlet tubing and place tape over the filter outlet while its not in use.  The other end of the filter stays hooked up to the tubing and pump.

This has proven to be a robust and reliable yeast ranching method.  If you’re looking to improve your starter preparation, this will get you there.

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Way back when before my joining, there was the Yeast Culture Kit Company that was run by a guy in our club.  His advice to some in the club was to use a 1.020 starter.  Some still use 1.020 starters.  I am usually in the 1.028 to 1.030 range, as I had read that was best for growth a long time ago on the HBD.

Not being a microbiologist, I will be interested in what others say.

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Martin and Jeff, I’m in agreement with you about 1.020 starters.  I know that’s what the yeast cos. do and I think it’s a great idea.  It’s just not what I do and I’ve been happy with my results.  So, I’ll modify my statement and say “anywhere from 1.020-1.035”.  Which still excludes the high OG starters asked about originally.

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Martin’s comment about keeping the flask headspace purged with air makes a lot of sense to me.  I would think that even with foil or foam plug you would otherwise tend to have a nice blanket of CO2 on top of the liquid.  A follow-up questions re. the foam plugs:  I know that these are advertised as boilable.  How about soaking them in StarSan and then wringing them out real well?  Does anyone know if the polymer will stand up to the phosphoric acid?

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I’ve been soaking a foam plug in Star San for the last 7 months and I make 2 starters every 3 weeks (Yah, I really need to start rinsing and reusing yeast!) . It seems to be holding up fine.

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I use the 10-1 method because it’s easy… 200grams of DME in a 2000ml starter (2Liters)

it’s been a long time since I checked the gravity but if I just did the math right assuming 43pppg for DME, 200G =.44pounds, and 2L = .53Gallons you get a starter of 1.036 gravity… I have never had a problem at this level and since I can my starter wort, measurments are fast (if not optimal).

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AFAIK all the yeast suppliers culture at <1°P (1.004) to avoid the Crabtree Effect entirely. The only reason that isn’t a viable approach (pardon the pun) for most brewers is that you need to be able to add precise quantities of wort, and remove precise quantities of alcohol, continually through the entire culturing phase.

I did a sort of one-off experiment to see if that was the case, and it seems that you do get more growth with foil (or foam, presumably) than with an airlock. So I believe that diffusion is able to carry at least some O2 into the headspace to replace CO2.

Aeration and Yeast Starters

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I’m not a microbiologist, but, do yeast produce a significant amount of CO2 during aerobic metabolism? 
If not, then if the wort remained aerated (by virtue of a stir plate), would enough O2 be getting into the starter?
If yes, then the air pump idea makes sense (I assume you set the hose down into the starter wort in order to bypass the blanket of CO2 on the surface)

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Yes, it’s actually the same respiration mechanism as nearly every other aerobic species, including us (the Krebs Cycle). The net reaction for glucose is:

C6H12O6 + 6O2 → 6CO2 + 6H2O

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It is my understanding as well that the Crabtree Effect already becomes effective at fairly low gravities. 1.020 is already too high. However, like Sean pointed it out, there is a practicality aspect to the starter wort gravity. Too high and the yeast gets stressed too much and too low and you’ll need large starter volumes.

In addition to that, in an aerated environment, oxygen is not the growth limiting factor. At least in brewer’s wort it is nitrogen and you may have to add nitrogen to the wort to reap the benefit of the more yeast growth when you manage to suppress the Crabtree Effect enough that the the yeast actually starts to metabolize sugars aerobically.
All we want in our starters is for the yeast to build up as many sterol reserved as they can by supplying unlimited amounts of oxygen. Unless we can handle very low gravity worts I doubt that we can prevent them from fermenting the sugars.

For a while now I’m propagating my lager yeast like this:

The Carboy contains 10-12l of 2 Plato wort and is constantly aerated with an aquarium pump through and air stone. I don’t like using foam control which is why I blow off excess foam into a pot. The aeration provides a constant movement in the starter beer which helps in keeping the yeast in suspension. I have been able to raise very well performing yeast with this set-up but recent measurements of the new growth per gram of extract showed less than 1 Billion/g which is not as good as the results that I get on a stir plate. There I can get 1.3-1.6 Billion/g.

If I had a conical I’d love to give this set-up a try:

Yeast harvesting in this set-up would be much easier.

But this is a subject that interests me very much and we should keep discussing this. Maybe we can find a conclusive answer to the question regarding Crabtree Effect and the wort gravity at which is starts.

Kai

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Kai has an interesting approach, but I’m concerned with the foaming and loss of yeast volume.

On page 130 of White & Zainasheff’s Yeast book, there is a story that illustrates the number of yeast cells in foam.  Its hard to determine the true yeast numbers in the foam from the story, but it implies to me that there was a huge number sequestered in the foam since the cell count went from 6 million to 35 million per mL after the foam was mixed in.

Blowing off foam is OK for beer making, but its too valuable when making starters.

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The yeast loss is minimal in lagers. This is a different story for ales. I cannot propagate the latter that way and for those I use a 2l flask on a stir plate.

Kai

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I would think that you could just insert an air tube into the small side of a carboy cap (the 6.5 gal caps fit a 2L flask, the 5 gal caps fit 4000 and 6000ml flasks) and blow filtered air onto the surface of the starter wort.  That will mix things up and tend to displace the CO2 so more O2 would be available to the yeast coming to the surface.  You could also try submerging the end of the tube and just allow a slow flow into the wort, that should alleviate most of the foaming problems caused by stones.

My $.02

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Catabolite repression is caused by glucose, not maltose, and our worts are primarily maltose.  However, maltose is converted to glucose (but not in the presence of glucose), so if the rate of that conversion is high enough maybe it could trigger the Crabtree effect.  Maybe conversion is so fast that even 100% maltose will still trigger it in brewing strains?  Typical brewing strains are likely to more maltase than standard lab strains, which typically come from bread making.  This is good to keep in mind with any published research.

The Crabtree threshold will also be dependent on the expression of various enzymes that will tend to push it toward the TCA cycle instead of fermentation.  So it seems to me the threshold can be strain dependent even within brewing strains.

Anyway, I don’t have access to the primary research (and it’s in Polish), but it was reported by DeDeken (1966) that Slonimski (1956) puts the level of glucose at 6 mM.  That’s ~1g/l, or 1.0004.  Also in 1956 Ephrussi et al reported that 3% glucose (SG = 1.012) triggers the Crabtree effect.

As far as I know, we still don’t understand exactly what causes the catabolite repression, meaning what pathway does the level of glucose trigger that causes aerobic fermentation.  But for what it’s worth, standard lab yeast growth media uses 2% glucose (SG = 1.008). I asked my PI years ago why 2% and if that avoided the Crabtree effect.  He didn’t know what I was talking about, just shrugged and said that’s what is standard.  :-\

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I’m curious.  Would lager foam contain the same quantity of yeast cells as ale foam when measured on a weight basis?  In other words, does a gram of lager foam have a similar cell count as a gram of ale foam?

As Kai wrote, I would assume that a top-cropping ale yeast would more typically produce a foam stand than a bottom-cropping lager yeast.  But, is that always true?  If it is true, then I can agree that yeast cell losses from a lager starter are lower.