If you have your own sourdough starter, kept alive by carefully feeding it on a regular basis, you are growing your own micro organisms. Micro organisms are all around us, in the air and on most surfaces. In most cases we don’t want them to grow out of fear for it spoiling our fruit for instance.
However, in some cases you do want it to grow. To make that sourdough starter for instance (or when making yogurt). And even though maintaining your own sourdough starter is a great way to bake bread, it isn’t always a feasible option. Luckily, nowadays you can bake your bread without having your own starter.You can just buy yeast.
But where does that yeast come from? And which one should you use? Instant, active, fresh or maybe something else?
A quick note, we’re focusing on baker’s yeast in this article. Even though brewer’s yeast and others work in a similar way, their use and manufacturing is somewhat different.
What is yeast
Yeast is a microorganism, made up of just a single cell. Yeast cells grow in a very different way and a lot faster, than animals (like humans) do. They can grow through a process called budding in which a small yeast cell grows on the outside of a mature one, until it is fully grown and ready to separate. In order for yeasts to grow in this or other ways, they need sufficient food (mostly sugars) and agreeable conditions. The temperature should be agreeable as should the pH (a measure for acidity) and the presence (or absence) of oxygen.
We use yeast to make a lot of different foods. Wine, beer and a lot of breads wouldn’t exist without yeast. Yeast can convert sugars into alcohol through a process called fermentation. Yeast can also produce carbon dioxide which ensures that your bread rises while proofing.
There are a lot of different yeast species. The yeast species that you use to make bread, baker’s yeast, is called Saccharomyces cerevisiae. Nowadays manufacturers can grow this yeast species in a very controlled manner, giving you easy access to the useful ingredient.
If you want to learn more about the basics of yeast, have a look at our lesson on the topic.
How yeast is made
Yeast is a living microorganism. As such, manufacturing isn’t a simple standard chemical process, or a separation. Instead, in order to make yeast, you have to grow it. Even though you can control the growth quite well, the process is less stable and harder to control than that standard chemical reaction.
The good thing of growing yeasts is that you don’t need a lot to get started. You don’t need to ‘make’ yeast. Instead, you start off with an existing yeast culture. Manufacturers can get these cultures from various places. In most cases they get the yeast from previous batches. These strains are carefully maintained and taken care of to provide an ideal starting point for yet another batch.
Saccharomyces cerevisiae exists naturally around us, so it can also be isolated from a product containing it. You use this method of growing yeast when making your own sourdough starter.
Growing the yeast – starting small
Yeast manufacturers have extensive supplies of the yeast species they need to grow. When they need to manufacture more of a certain type, they start by taking some cells of that strain.
Manufacturers know exactly when this yeast grows well and under which conditions it doesn’t. They place the yeast cells in a flask, at this point still quite a small flask. They add plenty of water and food, generally sugar beet or sugar cane molasses. Ensure the yeast gets enough oxygen and keep it at the yeast’s preferred temperature. From there, the yeast starts to grow and fill that flask full of yeast cells.
When growing yeast this way it is important for the yeast to have enough oxygen. If there isn’t enough oxygen, it will start to ferment. Fermentation will result in the formation of alcohol, which is not good for further growth of the yeast.
Once that first flask contains enough yeast, the yeast is transferred to a larger flask. It is fed again and left to grow. From here on, the yeast is moved into ever larger containers to grow in. The final tank can contain thousands of liter of liquid full of yeast.
Don’t be fooled though, that tank is mostly water at this point. Only a few percent of the total weight is made up of yeast. Yeast needs plenty of water, oxygen and food to grow, so as soon as they become too concentrated, they won’t grow anymore.
So you might be wondering why you can’t just place the yeast in this small tank to start with. That is because it is really challenging for the manufacturer to create the ideal growing conditions for a tine amount of yeast in a huge tank. By scaling it up step wise, the control and efficiency of the process is far greater.
Challenges of yeast manufacturing
When you’re growing yeast you want to create ideal conditions for them to grow. Unfortunately, under those conditions there are a lot of micro organisms that love to grow as well. When making yeast it is very important to ensure those undesirable bacteria don’t even get a chance to enter the facility. It is why the food for the yeast, the molasses, is sterilized before being used. Apart from that, overall hygiene is essential during the entire process.
Sourdough starters – growing your own
Some bakeries get their yeast from these large tanks. But others make their own by maintaining a sourdough starter. This starter doesn’t just contain yeast, it also contains plenty of other microorganisms such as bacteria. It is great for making bread.
What you’re doing when maintaining a starter though in essence is the same as what you would do when growing yeast in tanks. Instead of growing your container though, you continuously throw out part of the starter. In essence, you’re doing the same thing, keeping the growing conditions as ideal as possible for the microorganisms.
In that last final tank the yeast is thriving. It is very active and growing happily. It would be ready to use for making bread or an other leavened product. However, once the food in the tank runs out, the activity of the yeast can slow down quickly. At some point, the yeasts will die down.
In order to keep those yeasts active, manufacturers dry the yeast. You can either dry them off completely, which will make them shelf stable for months or years. Or you can concentrate them just enough to keep them alive while transporting them to the factory where they’ll be used to do their work.
The first step in concentrating the yeast “broth” is centrifugation. Centrifuges use a difference in density to separate the yeast cells from part of the liquid. It is a similar system to what is used for separating cream from milk.
After centrifugation, the amount of yeast in the mixture is still quite low, <20%. This is cream yeast. It can’t be bought in regular stores and supermarkets. Instead, it is directly transported to bakeries. The yeast is still very active, so it can’t be kept for very long. By transporting it in refrigerated conditions it can be kept for long enough until it’s used.
Fresh yeast / Cake yeast
Next, the yeast is concentrated even further. Manufacturers can using vacuum drying equipment for this for instance.
At this point the yeast content has increased up to 30-35% (from <20% in cream yeast). By pressing this together into blocks, it makes what you would consider fresh yeast. Fresh yeast is sold at bakeries and some supermarkets. It is less active than cream yeast, but it still has to be stored in the fridge to keep it alive for long enough. Depending on the yeast type the total shelf life is only about 4 weeks.
Since that is generally still not dry enough, the yeast is next dried even further. A way to do this is using granulator and air drier. This yeast, also called semi-fresh yeast, contains only 20% moisture. This yeast is still very active and stored in the freezer to ensure it stays active for sufficiently long.
Drying the yeast even further gives dried yeast. It’s the yeast you’ll find in supermarkets. The yeast is dried into tiny granules and you can scoop it from the cup quite easily. The moisture content at this point has dropped down to <5%. This yeast can generally be stored for up to two years at room temperature.
Nutritional yeast doesn’t really belong in this list, but we’re adding it just to be clear. This type of yeast is no longer active and alive. It has been deactivated on purpose. As such, you cannot use it for proofing breads, etc. Instead, you use nutritional yeast to add flavour to your food. It provides depth by adding umami to your food.
Choosing your yeast
If you’re not a professional bakery chances are very small you’ll ever use cream yeast or semi-dried yeast. Your options are either fresh or dried yeast.
Fresh yeast will work fine, however, it only lasts a few weeks in the fridge. If you don’t bake frequently it is hard to use it in time. For most recipes you can exchange fresh and dried yeast. Keep in mind that fresh yeast contains a lot more water than dried yeast. Therefore you need a lot more fresh yeast. As a rule of thumb, use three times the amount of fresh yeast as you would of dried or follow the guidelines in your recipe or on your yeast package.
When using dried yeast, there is another choice to make. There are several types to choose from. In the US the most common ones are active dry, instant and rapid rise (or a similar name, these are generally brand names). Outside of the US, instant yeast is most common. By reading the instructions on how to use your yeast and the guide below, you should be able to tell which of the three it is.
Active dry yeast
Active dry yeast needs to be activated before you can use it. You do this by dissolving some of it in luke warm water. The yeast rehydrates and starts bubbling, which means it’s ready for use.
The major reason for rehydrating active yeast is its large granule size. Those large pieces make it harder for the yeast to fully rehydrate and absorb enough water. By placing it in some water in advance, it easy access to enough moisture.
Active dry yeast is less stable than more modern varieties. As such, proofing the yeast on forehand is a good test to see whether it still is sufficiently active. If it doesn’t bubble up, it isn’t anymore.
Even though this is the general advice given when using active dry yeast, several of the current active dry yeast varieties work perfectly fine without any pre-hydration. You can use them in the same way as you would instant yeast.
Over the years drying technologies for yeast have improved a lot. This is what enabled the production of instant yeast. It is similar to active dry yeast, however, it does not have to be activated on forehand. Instead, you can just add it to the rest of your ingredients.
Yeasts all have their own ideal growing conditions. Some yeasts don’t grow well when there’s a lot of sugar, whereas others, called osmotolerant yeasts, don’t mind the sugar at all. It is why you might find different types of instant yeasts being sold. Generally speaking, only specialized (online) stores have these varieties in stock. When scaling up having different types of yeast may be helpful. At a small scale you can often fix the issue by extending leavening time or adding more yeast.
An even more recent development is that of Rapidrise or Quickrise yeasts. These yeasts are very active, as the name says. As such, they can rise a product pretty quickly, but, they really only work well for one rise. So, if you’re baking a bread that needs to proof twice, this yeast won’t be suitable. It won’t be able to get that second rise going.
Using baker’s yeast
Yeast is a live microorganism. In order to ensure is actually leavens your breads or cakes you need to treat it well.
Temperature: baker’s yeast grows best at temperatures around 30-35°C. It’s why recipes tell you to put your dough in a warm spot to proof. However, don’t become too enthusiastic. At temperatures above roughly 40°C starts dying off. It’s why recipes will never ask to add hot water to a dough that needs proofing, it would kill off the yeast.
At temperatures below the optimum, yeast will still grow. However, it will just grow more slowly. For some recipes this is actually desirable, the slower longer process also results in the formation of more flavour, made by the yeast cells.
Salt: yeast doesn’t grow well in the presence of a lot of salt. As you can see in the image above, salt slows down growth considerably. You will need to balance the need for flavour from the salt with the need for a fast (or slow) rise.
Sugar: sugar is a great food source for yeasts. Yeasts can grow from carbohydrates present in wheat flour, but pure sugar is more easily accessible and can really accelerate growth.
Concentration: if you start with more yeast, you will get a large volume of yeast more quickly. Also, larger volumes produce more gases, which leaven your bread. However, add too much and the yeast runs out of food too quickly and with a bread that’s easily over proofed.
Whether it’s your sourdough starter, fresh or dried yeast. In all cases you’re dealing with a live microorganism that helps you make great food (and drinks). Treat it well and it will treat you well, which should be easier now that you know where it comes from.
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Monika B., Production of Baker’s Yeast Industrially: Process, Principles and History | Industrial Microbiology, Biology discussion, link
Food and agricultural industries, Chapter 9, section 9.13.4 Yeast production, link
Lallemand, Yeast Production, Volume 3, number 4, link
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Stella Parks, All About Dry Yeast: Instant, Active Dry, Fast-Acting, and More, Nov-4, 2019, Serious Eats, link