Yeast is something you might think you know, but then there’s always a bit more about yeast that will rock your world. It is a living organism, as many know. It is technically in the fungus family. It’s also practically everywhere; it’s in the air, it’s underwater, it’s in the intestines of animals and you can bet it it somewhere in, if not closely around you. The best thing you probably didn’t know about yeast? It’s one of the very first organisms that was domesticated by humans, and we didn’t even know it was an organism at the time. In fact, people didn’t really understand yeast until around the 1850’s. Until then they believed that fermentation was magic, divine or chemical.

The term yeast refers to a ton of different organisms that act and react similarly but they are generally classified into two variations; bottom-feeding yeast and top-feeding yeast. Both of these classifications break down into numerous, numerous strains; not all of them safe for humans to be around. The kind we are going to talk about feeds on sugars and starches, in turn producing carbon dioxide and alcohols, like ethyl/ethanol. This process is known as fermentation, and it is used for various applications from baking bread to making ethanol for alcohol.

Granulated, dry yeast is a relatively modern invention, and until its advent everyone was pretty used to cultivating their own yeast. Today very few would know where to start if they had to whip up some yeast from scratch. As a micro-organism, yeast is alive. It needs to be kept alive and fed for it to survive, and surviving is exactly what yeast plans on doing. As type of fungus, yeast reproduces sexually and asexually via spores; this is often how it is floating around in the air and in water, until it can find a medium or place to land and start a culture. As a culture, the yeast will spread, consume and grow so long as it has a source of food and a home that is within an ideal temperature range. Yeast likes things room-temperature to warm temperature. Different strains of yeast have different ideal temperatures, but generally too hot will kill it, and too cold will slow it down substantially.

To cultivate yeast (as humans have been doing for thousands of years), all you need are the things yeast wants; an ideal (warmish) temperature, some kind of sugar to consume (including the glucose found in starch), water (don’t use chlorinated water) and some kind of medium for it to take over and set up shop on. If you build it, the yeast will come, because again, it’s everywhere. If it’s not in the ingredients you are using, it will float by on the air.

For simplicity’s sake, let’s use a potato.

A potato has starch in it, which has some glucose sugar, but it may need more to help yeast win the battle for domination. The potato’s skin (like many fruit skins) contains some yeast already, so if you really want to jump start yeast’s bid for domination, add more skin. A lot of people use grape skins (part of how wine gets made). You can replicate this process with many other ingredients instead of a potato including fruit and flour. Starting with something which you know has yeast in it (like bread, or even actual yeast) will make things exponentially easier, but if you already have bread and yeast then what do you need this for? The purpose of this article is to teach you how to cultivate yeast.

  1. Start with boiling the potato in water until the potato is soft enough to eat.
  2. Remove the potato from the water, and mush it up. Save the potato water.
  3. Add some sugar to the potato mush. If you don’t have sugar, try fruit/fruit skins.
  4. Get yourself a large (and sterile) jar that you can seal up eventually.
  5. Add the potato/sugar mush to the jar, then use some of your leftover potato water to top the mixture up at about 1/3 volume. If your potato was bigger than the jar, get a bigger jar, or use less potato. Mix it well.
  6. Allow this mixture to sit covered with paper towel or a cheesecloth (do not seal it, it releases gas).
  7. Let the mixture in the jar sit for 2 full days in a warm area.
  8. Inspect the jar’s contents; if it is starting to foam, you’ve got the start of cultured yeast. If you don’t, wait longer, or try again. The contents should start taking on an odor similar to bread.
  9. Feed the yeast; add some of your original potato water (if you still have it) as well as more potato. If you were using normal water and flour, add more of that at this point. There’s no exact or perfect amounts here, but try to stick to something around 2 parts water, 3 parts potato (or fruit, or flour). Leave enough room in the jar for the yeast to grow. It could double in size, but make sure it has about 1/3 the volume of the jar to grow into.
  10. Continue to feed the yeast on a daily basis. Remove most of the mixture to dispose of, leave at least 1/2 cup of yeasty stuff, and feed the remains just like you did previously in step 9. While the waste contents will contain yeast, it likely still contains other organisms as well which may not be ideal for human use.
  11. Keep this daily process up for a week or so – the contents may start to produce yellowish liquid or start to smell like alcohol, but just keep repeating step 10 and it should start to smell more and more like bread. If the contents produce a dark liquid at the surface, the yeast is trying to tell you that it’s starving and it needs more food or food more frequently.
  12. When the yeast culture begins to consistently double in size everyday, and seems to smell only of bread and no longer produce any other smells or liquids; the yeast has won the battle for domination of the jar. Any other organisms within that had been struggling to gain a foothold have been snuffed out and systematically removed with your help. Continue daily feeding (step 10) for one more week (or at least three days) just to make sure that what you are dealing with is pure yeast culture. All in all, this could still actually take a month in some scenarios, so be patient.
  13. Seal the jar up and start refrigerating it. You will still need to feed your yeast culture, but since it will slow to a crawl you should only have to feed it about once a week. Continue to remove contents to prevent overflow, but now the excess foamy paste will be yeast that you can use to ferment stuff and leaven bread. In its refrigerated state, a yeast culture can survive for months or years so long as you continue to feed it.
  14. So pat yourself on the back, you have domesticated yeast and isolated it in a jar. When it’s time to use it for something, take out what you need and let it “bloom”/activate. Live, refrigerated yeast will need to be brought back to room temperature (or warmer) for about a full day before it is ready to be used. Knead (or mix) your yeast starter right into any dough or liquid you plan to use it in.

 

You can experiment with various ingredients to create different yeast cultures and starters, but once you understand the basics, yeast is relatively simple to cultivate. In some cases (like brewing), malts (germinated grains) are used instead of (or in addition to) yeast in order to increase the sugar content and fermentation activity. You can however still use a simple potato-derived yeast to brew beer or make any alcohol.

Different strains of yeast vary in rarity and function. Some are more desirable for differing applications in baking and brewing. Some may encourage slight variation to fermentation, promote different flavors or thrive in different temperatures. As a result many yeast cultures around the world are prized and coveted for their performance. Some brewers pay top dollar for specific and rare yeast cultures (which they can turn into more yeast cultures in addition to using for actual fermentation).

Yeast can often perform very differently depending on what strains you caught and what circumstances you cultivated it under. It may take you a try or two, but once you got a good yeast culture that fits your needs, keep it alive and use it forever!