tl;dr version: for good results use champagne yeast, or bread yeast if there's no homebrewing stores nearby, with a mixture of 2/3-3/4 cups of light brown sugar per 16 oz. water. For optimal results and the science behind it, read on...
I've seen lots of uncertainty, misconceptions, and flat-out false information dealing with yeast in regards to DIY CO2 production. My kegging equipment made a very easy conversion to pressurized tank CO2, but I've recently had to go DIY for space considerations. I'd like to share some of what I've learned through years of brewing studies to hopefully help everyone DIY easier, and clear up some misconceptions. A lot of what makes yeast tick is very easily related to other things in the hobby.
Warning - I tend to feel the need to explain something to the fullest degree of useful detail, so this is going to be deep. My honors students love that about me...the others, not so much. Regardless, the application conclusions are easy to do, regardless of the understanding of why.
There are different species of yeast that will anaerobically digest sugar to produce CO2 (your body does the same thing in the absence of oxygen, but produces lactic acid instead of CO2). The yeast we want is Saccharomyces Cerevisiae
. It's readily available and does a fantastic job of doing what we want it to.
However, there are many different strains of S. Cerevisiae
out there, all selectively chosen for the characteristics desired for a certain application. The yeast used for bread, beer (ale, that is - lager yeast is a different species, but requires colder temperatures and is slower), wine (and, by extension, champagne, sherry, mead, cider, etc.) , and distilled beverages all use this same species. So which is best? Let's first look at what inhibits yeast performance.
Temperature - too low and the yeast grind to a halt, too hot and it dies. Temperatures in the 90-100 degree (F) range are optimal for yeast growth and activity, but room temperature is fine, and preferred for our application.
Salt - something I haven't seen mentioned with DIY. Small amounts of salt (.5%-1%) will help yeast grow (nutrients!). More than that will slow yeast. Some bakers use this to their advantage...just the right amount of salt in a dough will cause it to rise slowly, keeping the yeast from getting out of hand. I haven't had the chance to experiment with salt in DIY yet, but around 2% should help the CO2 production stay slower and steadier. The big question would be if it makes the yeast stop at a lower alcohol percentage (AKA "crapping out")
pH - yeast prefer acidic environments. It takes very low pH to greatly inhibit yeast, lower than a DIY mixture will reach. The addition of a little baking soda as a buffer couldn't hurt, but probably does little to help. If anything it's likely slowing the yeast down rather than helping it thrive. Which isn't necessarily a bad thing.
Nutrients - something else I've not seen mentioned in DIY. Yeast are much like your plants...they have their main need (sugar instead of light and macros/carbon), but they also do better with traces. In baking and beer and winemaking it's not super-important to worry about nutrients, because the other ingredients provide most if not all of the necessary nutrients, much like fish provide carbon and macros. But if we're talking about a mixture of sugar and water, there's 0 nutrients in that for the yeast. Yeast will be much more healthy with yeast nutrients (a couple of bucks from a homebrew store, and it will last a long time). Definitely something to consider.
Complexity of sugars - yeast can only "eat" simple sugars - to ferment more complex sugar chains they must first break them down. They release enzymes that do this, just like your body does. However, like your SAE that won't eat algae if you keep putting food in the tank, the yeast will go after the easiest-to-consume sugars first.
Density - yeast can't work in solutions that are too dense. That's why honey is shelf-stable basically forever...at ~85% sugar there's no space for the yeast to do its job. Or why fake maple syrup is shelf-stable after opening but the real stuff typically isn't....the fake stuff has a higher percentage of sugar.
Pressure - this is of concern to those using fine-bubble diffusers. Champagne yeast is really the only yeast selectively chosen to be able to perform at higher amounts of pressure. How much difference that makes at this point is purely speculatory without controlled testing. However, switching to champagne yeast might be a possible fix for someone having trouble getting enough pressure in the system to get their DIY working.
Alcohol percentage - this is where the different types of yeast are going to be differentiated for our purposes. Different strains of S. Cerevisiae can tolerate from anywhere between 4% (some ale yeasts) to upwards of 20% (distiller's yeast).
You can, of course, get DIY with white sugar and baker's yeast. This is simple to do with readily-available ingredients. The problems with this are -
1) yeast tear through the available sugar quickly, causing co2 production in a curve rather than steadily.
2) frequent changes required - not only because of the quick fermentation, but also due to the somewhat lower alcohol tolerance of bread yeast
Our ideal DIY mixture would have these characteristics - a steady, dependable flow of CO2 that goes as long as possible between bottle changes. We need a high supply of sugar, ways to limit how quickly it's fermented, and a yeast that can handle higher amounts of alcohol (and possibly pressure). I'm relatively new to DIY so I haven't done extensive testing, maybe we can get some crowdsourced experimentation done.
Our ingredients list should, therefore, take the following factors into consideration -
Yeast - ale yeasts can handle between 4%-12%, depending on the strain. Wine yeasts between 10%-13%. Bread yeast around 12%. Champagne yeast around 16%. Distiller's yeast around 20%. Again, for simplicity, bread yeast will work fine. Champagne yeast can be bought cheaply at a homebrew store. Distiller's yeast (AKA turbo yeast) is usually a bit harder to find, but is likely the best option. Champagne yeast and turbo yeast will also multiply slower than bread yeast, which as been selectively chosen to work as quick as possible.
Sugar/density - again, white sugar alone is just TOO easy for the yeast to go through. A better situation is to have a solution where some of the sugar is instantly and easily available so the yeast can get started quickly, but also have sugars that are harder to get to. Light brown sugar provides some more complex sugars that the yeast will have to work at. Further pondering of the use of jello makes me wonder if it slows down fermentation due to sugar molecules being held by the jello, which seems to be the prevailing thought, or if it's simply due to the jello raising the density of the solution; pure jello being, of course, too dense to ferment at all, the watering-down step taken is necessary in this method of DIY. As the sugar is fermented, density is lost due to the loss of sugar as well as the production of lower-than-water density alcohol. By the time the solution becomes markedly less dense the other limiting factors (alcohol percentage and amount of sugar available) kick in. I believe this is probably what is actually happening, or at least a combination of this and the "binding" theory.
Other additives - baking soda may help inhibit overly-fast yeast growth at the beginning by raising pH, and help buffer pH as time goes on. However the effects are likely minimal. Nutrients added will lead to yeast with stronger cell walls and overall better health...if growth is kept in check this will help the yeast ferment a higher amount of sugar before crapping out, and provide more dependable and consistent performance. Finally, the addition of salt may keep yeast growth in check, but testing is needed to see if this affects more than just growth and instead also shortens the yeast's long-term viability. The correct balance of yeast health and inhibitors should make it so we don't have to try and add starches to our mix. Rice and flour are common ingredients but are undependable at best.
What's important to add is that you can't control the yeast growth simply by not adding as much. Yeast are like pond snails...given ample food they'll readily multiply more rapidly than you can keep them under control, and they'll reproduce to a number that can best thrive on the given supply. A given yeast colony under average conditions could multiply 64 fold
in one day (doubling every 4 hours, less in more ideal conditions).
So, with everything under consideration I would consider the following an ideal recipe:
Per 16 oz. (2 cups) water -
1/4-1/2 teaspoon distiller's yeast (or champagne or baker's), properly re-hydrated...seriously, you don't need much
1 cup brown sugar (or white sugar + 1 6.5 oz. package of jello (experimentation needed for the correct amount, this is based on DIY jello threads), mixed and cooled, then brought back to room temperature before the yeast is added) (if using champagne yeast cut back to 3/4 cup, and if using baker's yeast cut back to 2/3 cup)
1.5 teaspoon salt (yeast inhibitor/nutrient, experimentation needed to test for usefulness)
a pinch of yeast nutrient
1/4 teaspoon of baking soda (experimentation needed to test for usefulness and amount)
Preferably diffused with a nano ceramic diffuser (one that can "handle" DIY)...the pressure will be a further inhibiting/stabilizing factor.
Again, I present a lot of this based on theory instead of tested practice, so skepticism is expected. That being said, I'm going to be testing a lot of this out in the coming months. I fully understand that this is way more complicated than most people care to get, and the need to reinvent the wheel is seen to be unnecessary. But for the smaller percentage of people interested in optimal, consistent performance and the desire to advance the hobby, I think this is a useful starting point.
Links to supplies...all from Midwest Supplies, which is an amazing company. -
Distiller's yeast - $8 and you'll get enough to leave to your grandchildren after you die - http://www.midwestsupplies.com/homeb...bulk-pack.html
Yeast nutrients - a pinch will do you, so even 1.5 oz. will last a while - http://www.midwestsupplies.com/homeb...nt-1-5-oz.html
Fermcap S - this stuff is AMAZING. If you have trouble with the solution bubbling and expanding and getting all in your tubes and check valves and bubble counters and diffusers, a few drops of this will stop it...and, again, since you're using so little, this small bottle will last quite a while - http://www.midwestsupplies.com/homeb...itor-1-oz.html