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There are many benefits to making a yeast starter, and a plethora of threads on this forum discussing their merits and the techniques used to make them. It was through my involvement in one such thread I discovered that some people consider making a starter in an Erlenmeyer to be potentially dangerous. I hold the view that with correct materials and methods, making a starter directly in the flask is arguably the simplest and most sanitary method one can employ. The purpose of this article is to illustrate this straightforward process, whilst highlighting some important steps along the way eliminating any messy or potentially injurious errors. As with any task, having the right tools for the job is imperative. Let's look at what's needed.

Items Needed Before Getting Started
A lab-grade borosilicate flask
These are readily available from an array of reputable homebrew and laboratory supply stores and come in a variety of sizes. From personal experience (and much to my chagrin) owning a trifecta of flasks in 1, 2, and 5 liter sizes, I would advise against buying one smaller than 2 liters.
A stir-bar (optional)
Using a stir plate is highly recommended. Constantly stirring the starter optimizes oxygen absorption into the wort and keeps the yeast in suspension. The result is a more rapid completion of the starter and greater yeast growth-rates.
A non-airtight stopper
During fermentation of the starter, a free in-flow of air while at the same time preventing ingress of airborne particulate is desirable. This allows an oxygen-rich, sanitary environment to be maintained inside the flask. A sanitized foam stopper or aluminum-foil will serve this purpose effectively. Airlocks are contra-indicated.
A weighing scales with gram resolution
A starter gravity approximating 1.037 is desirable. To achieve this 1g of dry malt extract is needed for every 10ml of the planned starter size.
A heat source capable of bringing the starter volume to a boil
I imagine most folks will use their kitchen stove when making a starter. A gas stove is not required, but is arguably preferable, as any required adjustments to the intensity of the heat can be made with immediate effect.
Dry malt extract (DME)
This is an incredibly hydrophilic fine powder. It is imperative to keep it stored in a sealed container or zip-lock bag and to use it in the absence of water vapor to prevent a sticky mess from forming. Albeit an extreme example, the less than desirable results of trying to add DME to already heating water can be seen in this thread.
Assuming the water is potable, tap-water is all that's required. There is no need to consider mineral or pH adjustments for a starter.
Yeast Nutrient (optional)
Yeast nutrient can be added in small amounts if there are concerns about the viability or health of the source-yeast.

Gram Scale, Fermcap, Stir Bar And Stopper
Fermcap S
This product is an emulsion of the active ingredient, dimethylpolysiloxane. This prevents foam formation by reducing surface tension. It has long been approved by the FDA and WHO for human consumption.
A thorough outline of the science supporting its safety was compiled in 1975 by The World Health Organization's International Program on Chemical Safety. In addition to being a safe and effective anti-flatulence medication, frequently given to infants, the active ingredient in Fermcap S can be found in many foods and beverages. The effective medicinal dosages are orders of magnitude higher than the amounts used in homebrewing.
To put it bluntly, you can ingest lots of dimethylpolysiloxane to stop you from farting so much. It goes in one end and out the other unchanged. No worries.

Steps To Making A Starter In An Erlenmeyer Flask
Making the starter
1. Calculate the desired starter size and gravity
There are plenty of incredibly useful and free online tools with which to calculate starter size needed for any given batch of homebrew. I am a fan of Homebrewdad's yeast starter calculator as I like to overbuild and harvest yeast for future brews directly from the starter. The projected size of a starter will vary depending on a multitude of factors including starting gravity and volume of the batch, the planned pitch rate, and the projected viability of the yeast being used.
2. Place the flask on the weighing-scales and zero it out
Place the required weight of DME into the flask and subsequently add cool tap-water to reach the desired volume. In order to target the planned specific gravity it is important to add the DME first and then the water. If this is done in the opposite manner, i.e. water first, DME second, the volume will be larger than planned and the gravity reduced accordingly.
3. Carefully add the stir-bar
Tilt the flask and insert the bar, allowing it to gently slide down the flask-wall. I have never broken a flask, but imagine that repeatedly dropping a stir bar into one, could, at least in theory, potentially weaken the glass. Perhaps not.
4. If using yeast nutrient in the starter, now is the time to add it
Adding a stir-bar or powdered yeast nutrient to near-boiling or boiling wort is likely to cause a sudden and violent boil-over to occur. The result is a sticky mess and a potentially painful experience. DO NOT ADD ANYTHING TO THE HEATING OR BOILING WORT.
5. Add one drop of Fermcap S to the starter
This is an incredibly useful product and is in my view indispensable if making starters in an Erlenmeyer flask. One drop weighs approximately 0.05g of which only a portion is dimethylpolysiloxane.
6. Heat the starter wort
Heat the wort while occasionally swirling the flask to completely mix the DME and water.
7. Bring the starter to a boil and boil it for the desired time
It would seem there is no definitive duration for which this needs to be done. In order to render microbe-laden water water safe to drink, the United States' Center for Disease Control advocates boiling for one minute. Three minutes is recommended above an altitude of 6,500 feet. Boiling for one minute will sanitize water killing the vast majority of water-borne pathogens. Is this boil duration applicable to sanitizing a starter wort and flask? I'll let the reader decide. I boil my starter for 10 minutes but have no good basis for why. I just let it boil as I do other things.

Cool The Starter Wort Before Adding Yeast
8. Cool the starter
This can be easily and safely done by taking the flask off the heat, placing a sanitized foam stopper or aluminum-foil and immediately immersing the flask in cold water. A lab-grade borosilicate flask is designed with much greater temperature swings in mind.
9. Add Yeast to the starter wort
Once cooled, remove the stopper and pitch the yeast. Star san or any no-rinse sanitizer can be used to sanitize the outside of the yeast pack/vial or jar prior to pitching. Replace the sanitized sponge stopper once yeast has been added to the starter.
10. Allow the starter to ferment to completion
I usually allow 48 hours for this to occur. If using a stir-plate, this is in all likelihood far more time than is needed.
11. Chill the starter
I routinely chill a starter in the refrigerator for 3-4 days prior to using it. This allows ample time for the majority of yeast strains to flocculate and form a dense layer of yeast in the bottom of the flask. This is particularly important with large starters where adding excessive volumes of spent starter-wort could have an impact on the characteristics of the final beer.
12. Decant wort and pitch yeast
On brew-day, decant the spent wort and pitch the yeast. To prevent stressing the yeast it is desirable to have the yeast and wort at similar temperatures. The wort is cooled to a temperature slightly below that at which initial fermentation will occur and the yeast removed from the refrigerator a few hours prior to pitching with this goal in mind. Differing opinions on pitching temperature exist and I am certainly no authority on the topic. This is the method I favor.

Making a starter in an Erlenmeyer flask is simple, hassle free and can be completed while doing other things. I usually make my starters while cooking dinner on Monday evenings in preparation for a weekend brew. By eliminating the risk of boil-overs, the process becomes entirely uneventful, requiring minimal oversight. For your next starter, why not give it a try?
Very good article.
I pretty much already follow this process. idk why people would get a flask and not use it to boil the wort.... the only issue i see is the boil over which fermcaps mitigates.
Thx for posting - what are you doing in the pic to the right of the ice bath?
Well written with good pictures! One point for discussion. I have been under the impression that you should not place an Erlenmeyer Flask directly onto the burner of an electric stove to boil the wort, and have been blindly following that proscription. I would love to find out that this is not true.
I heard 'foam stopper' on the forums and thought it was the Fermcap. But it is literally a stopper made of foam.
Nice write-up.
Great write-up, I've been utilizing this method for a couple years now. The only dangerous thing is grabbing the top of the flask after you're done boiling (it's hot btw). I have seen some "knock-off" flasks, so make sure you're using a lab grade one and you're good to go.
Any comment on using fermcap to prevent blow off while the starter is fermenting? I just tried to do a 1.5L starter in a 2L flask with 2 drops of fermcap added after chilling, the krausen still climbed out of the flask and onto the kitchen counter. More fermcap next time?
About the 4th time I used an Erlenmeyer flask on my electric stove it shattered. What a mess. Do not use a flask on an electric stove! I boil and cool in a stainless steel pot now, pitch the yeast then transfer to the flask.
I'm pouring some sanitizer over the jars of yeast prior to pitching them into the starter. Probably overkill from a santitation point of view. Spraying the area would be equally as effective I suppose.
Very nice. Could you add something about the process for collecting the overbuild? I am guessing that overbuilt yeast is what you are getting ready to pitch in the picture mentioned in skydvr's question.
This is something I have never done so I am not in a position to comment either way. I would defer to others on this one. I omitted any words on the use or non-use of an electric stove as I have no experience using one with a flask.
I've no idea if fermcap-S will help reduce foaming from fermentation. I see less foaming with a stirred starter than a non-stirred.
Using the calculator from homebrewdad an extra number of cells is planned and the starter is made. Check out the linked calculator. It's really handy.
After the starter has fermented out and completely homogenous, the calculated volume of starter is poured off into a sanitized container(s). Fill them right to the top.
These are loosely capped, labeled and refrigerated. The loose capping allows them to off-gas for a few days and then I tighten the caps down.
Two half pint mason jars nets me 90-100 billion yeast cells to use next time. Nearly free yeast. (approx cost is 50g of extra DME in the starter to meet the volume demands of the overbuild).
Hope that helps.
Great write up. Can you explain what you are doing in the picture in which you are pouring water on canning jar lids? Are you somehow sealing the lids on the jars before putting the screw on bands on the jars or what?
Good article!
A hint for cooling the starter if you are using a stir plate:
Stand the flask in a plastic bowl/container with a thin base and then put that on your stir plate. The stir plate should still be able to drive the stir bar, at least at a low speed. Add ice water to the container, and the stirring will help cool the starter to pitching temps much faster without you having to stand a swish the flask around. You can change out the ice water a couple of times to speed chilling.
Letting the flask sit off of the heat for a couple of minutes before adding the ice water helps avoid the risk from exposing the possibly superheated flask directly to ice water.
Nice write up.
I too have been saving an extra 125ml from my starter to reuse later. My take on it is that this yeast is pure, clean yeast to use for my next starter versus trying to "wash" yeast from a fermented batch of beer. from the starter, you're saving just pure yeast and not other fermentation and wort byproducts. Additionally, those yeast are probably stressed after going through a much stronger fermentation than the starter.
@Wichitalineman try taking a piece of wire hanger bent into a triangle. Set the flask on the wire. It keeps the flask from having direct contact with the burner, but still gives even heating. I use this with a glass coffee percolator.
This is absolutely well done! I've never made a starter but following this really is great. Thanks!
Very well done. Could someone please take this tutorial another step further and explain how to build up a starter for lagers and/or big beers, please ? I seem to have a mental block with that procedure, although I think that it is quite similar.
I've only done one for a Cali common, but what I did was do one cycle of wort/fermentation/chill/decant and then add another 1.5L of wort onto the existing yeast cake. I don't remember my exact volumes for my steps. Most yeast calculators will have multiple "steps" for creating large volumes of yeast.
thank you great
just in time just getting ready to do my first starter. will follow you instructions.
thanks again
As far as the boiling to cold water transition, even in an actual chem lab we'll have Pyrex flasks or beakers crack eventually due to heat stress.
@marcopolo google billybrew step up starter and click on the homebrew academy link. the best and simplest explanation ive seen
@Wichitalineman: For electric burner you need a heat difusser. As said earlier a coat hanger, which you need to get the coating off somehow before using or a SS from Amazon or local kitchen store.
And BTW Gavin a very good write up, thanks!
I did my first flask on the stove and will definitely buy some Fermcap as I had to stand over it watching for boil overs
Pyrex sold in the USA is not borosilicate glass. It is made of tempered glass and as such not recommended for direct heating as I outline. It will also break easily if you cool it rapidly. You need to be sure you are getting a borosilicate glass flask. Pyrex in the USA and borosilicate glass are not one and the same. DO NOT USE PYREX if following this process.
Thanks for this write-up for the simple steps. I haven't done any starters yet and have been fine, but I've been saving all of my yeast and am going to want to make sure its healthy before I use it again. Once I pick up a stir plate I will surely be coming back here for a refresher!
One of the best articles I've seen on this where I fully understood everything you did from beginning to end. Thank You.
greater starter on starters! but what does decant mean? That is the scariest word and why I haven't tried a starter yet. I should probably just google it...
Fantastic article, this is how they all should be. Great content, clear, concise, well written, good pictures. Bravo
I have always let the krausen for, sprayed the tip of the fermcap bottle and added a few drops. It breaks the krausen down pretty quickly and in my opinion allows more oxygen to the surface of the starter. No infection so far.
Only downside is the krausen can sneak up on you while you are sleeping.
decant is a fancy word for pouring off the water/liquid and leaving the solid (yeast) mass
I use fermcap for boiling of yeast starter, fermenting of yeast starter, boiling wort in brew pot, and in my glass carboys for fermentation with out any problems. I just follow the dosing amount per the bottle instruction. Great product for brewing.
Thanks Gavin a great write up, I picked up a few pointers to stream line my process of yeast starters, and cant agree more with you on making extra at this point to save for the next brews.