Sourdough Starter vs. Yeast: What’s the Difference?

Baked goods generally require a rising agent or leavening. Think of whipped egg whites in a dreamy souffle, baking powder for your favorite biscuits, or baker’s yeast for pizza dough. You often hear about sourdough starter as well. What’s the difference between sourdough starter and yeast? Are they the same thing? 

Yeast is a naturally occurring fungus used as a leavening or rising agent in baking. Generally, bakers use the word ‘yeast’ to mean commercially produced and sold yeast. However, sourdough starter is a traditional homemade leavening agent that uses the natural or ‘wild’ yeast in the air around us. 

It can be confusing for the new baker to know where to begin: when recipes call for yeast, does this mean the sachets of instant or active dry yeast you see in the supermarket? But sourdough bread is trending everywhere. Is it better to use a sourdough starter? And If shop-bought yeast and sourdough starter are kinds of yeast, can you use them interchangeably? Let’s look at the differences between these leavening agents so that you can get baking.

What is yeast?

Most baking requires a leavening agent to make the bread or pastry rise. The most ancient and still most common rising agent for bread is yeast. Yeast is actually a fungus – of the same broad family as the mushroom – and is, therefore, a living organism. When you ‘activate’ yeast with warm water and then feed the yeast on flour, it produces carbon dioxide, making the dough rise. Apart from making the bread rise, yeast also develops the delicious flavors and aromas we associate with bread.

What is commercially available yeast?

When recipes refer to yeast, it usually means the regular, commercially available yeast you can buy in the supermarket. Shop-bought yeast is a particular strain or type of yeast produced under factory conditions, allowing the dough to rise uniformly and within two hours. The obvious advantage of commercially produced yeast is that you can buy it conveniently and use it immediately. 

Commercially produced yeast comes in three forms.

Active dry yeast

Active dry yeast is the yeast most home bakers use and keep in the pantry: the little sachets, envelopes, or tubs open to reveal dull, beige granules.  You add this dehydrated yeast to about a cup of warm water (no warmer than 115⁰F/45⁰C or the yeast will be killed), with a teaspoon of sugar or so for the yeast to feed on, which reactivates it within five minutes. You then add the liquid to the dry ingredients and continue with the recipe. Keep any leftover dry yeast in an airtight container in the refrigerator. 

Instant dry yeast

People who bake with bread machines tend to use this kind of yeast, also known as rapid rise yeast. You find instant dry yeast in readily measured tubs or sachets. Instant dry yeast looks very much like active dry yeast, but it doesn’t require activation because it is powdered. You simply add it to your flour, and the liquid in the dough will react with it. Instant dry yeast is the most effortless kind of yeast to use. However, the quick-rising process reduces flavor, so recipes that use instant yeast often have oil, butter, sugar, or honey in them to develop more taste.

You can use the different kinds of dry yeast interchangeably, allowing for the activation time for active yeast before adding it to your ingredients.

Fresh yeast

Sometimes called cake yeast or compressed yeast, fresh yeast is sold in small whitish blocks that look and feel somewhat like moist clay. You can find these in the refrigerator at the supermarket, specialty store, or bakery. To use, you simply break a piece off and add it as per the recipe. There is no need to activate it. 

Fresh yeast has a moisture content of around 70 %, so it needs to be kept refrigerated and kept fresh. Keep track of the expiry date because fresh yeast only lasts for about a week. Because of the short storage period of fresh yeast, most modern recipes call for dry yeast instead.

You can swap fresh yeast for dry or instant yeast, using a ratio of 7 g of dry yeast (usually one sachet) to 2/3 ounce fresh yeast and 4 cups of flour.

What is a sourdough starter?

A sourdough starter begins as a mixture of flour and water that you leave at room temperature to ferment. The fermentation process cultivates the natural bacteria and yeasts in the air, creating a leavening agent that you can use when baking bread. Because a sourdough starter contains wild yeast, you do not have to use commercially produced yeast in the dough. In fact, sourdough baking is a traditional process that has been around for thousands of years, before quick rising, commercial yeast was invented.

Even though sourdough starter is a yeast product, it cannot be swapped out for yeast in a recipe. The reason is that the fermentation process requires a different kind of bread preparation and proofing method.

How to make sourdough starter

The best way to understand how the yeast in sourdough starter works is to make your own. Making your own sourdough starter is a simple process, but does require commitment and attention to detail. Fermentation is also a slow process, so making a sourdough starter can take up to a week. Some bakers argue that this is a disadvantage of sourdough baking. However, this very process creates the unique flavors and healthy bacteria that make sourdough bread both delicious and nutritious.

Follow these steps:

Day 1 

1. Combine ½ cup of flour and ¼ cup of water in a large mason jar.
2. Mix until it has a smooth, paste-like consistency. Add more water if necessary.
3. Cover the mason jar with plastic wrap or a lid.
4. Put the jar in a warm place for 24 hours.

Day 2

1. Are any bubbles forming? These are signs of fermentation and show that the starter is activating.
2. If there are no bubbles yet, don’t worry.
3. If you can see a dark liquid developing in or on the starter, don’t worry – this is normal.
4. Let the jar rest in a warm place for another 24 hours.

Day 3 

1. Pour off any dark liquid that has developed. This liquid is called hooch and is a side-product of the fermentation process. It may smell a bit funky, but that is normal. Hooch tells you that your starter needs to be fed.
2. Remove about half of the starter with a spoon. The texture will now be quite stretchy. You can discard this as you are building a more robust starter.
3. You are now going to feed your starter to help it grow and keep it healthy. Add ½ cup flour and ¼ cup warm water to the starter and mix until smooth. The texture will be looser again, a bit like a thin pancake batter. Add more water if necessary.

Days 4-6

Follow the feeding process every 24 hours:

1. Remove about half of the starter with a spoon and discard.
2. Feed the starter with ½ cup flour and ¼ cup warm water.
3. The starter should now develop bubbles and start rising in the jar. These carbon dioxide bubbles show that yeast is growing in the starter.

Day 7

1. The starter should now have doubled in size.
2. The texture should be thick, fluffy, spongy, and bubbly.
3. The starter should smell yeasty and pleasant.
4. To test if it is ready to use, put a small dollop of starter in a glass of water: it is ready to use if it floats to the top.
5. Transfer the starter into a clean jar. 
6. Your starter is now ready for use.
7. Use your sourdough starter only in recipes that call for a sourdough starter.

Day 7 onwards

1. Once you have created a starter, you have to keep feeding it daily (as described for Days 4-6 above) so that it survives and flourishes.
2. Keep your starter at room temperature to have a ready supply of leavening agent.

Note: If you are looking for advice concerning the maturity of your sourdough starter, make sure to check out my article “When Does Sourdough Starter Mature?“

Sourdough starter vs. yeast: a comparison

Sourdough starter and commercial yeast are similar in that both are leavening agents for baking. However, they are very different in a few significant ways.

Availability 

It is convenient and straightforward to purchase yeast at your local supermarket and start baking immediately. Yeast is always available, even from small local stores. Fresh yeast can be obtained from your local baker if you can’t find it in the supermarket.

You cannot buy sourdough starter from a store. It is possible to purchase sourdough starter online, particularly dehydrated starter, but this is not ideal for a newbie baker. You might be given a jar of sourdough starter by a baker friend. But, for the best results, sourdough starter needs to be homemade from scratch, which means you can’t bake until you have a batch of sourdough starter prepared. 

Portability

It is easy to carry dry yeast with you if you are going on vacation or visiting. It is possible to travel with a sourdough starter, but it is far more sensitive to heat and humidity and can spill more easily.

Maintenance 

Sourdough starter requires nurturing and feeding so that you always have a healthy starter available. Maintaining a sourdough starter is a commitment and is a little like having a pet rather than a pantry ingredient!

Commercially available dried yeast requires no maintenance, although it does have an expiry date you should note. Fresh yeast is more sensitive – it can be killed if you add too much salt to the recipe. It needs to be refrigerated. Even so, fresh yeast will last only about a week.

Quantity 

Most recipes call for around 7-14 g of dry yeast and 2/3 – 1 1/3 oz fresh yeast, making it relatively cheap and straightforward to use. For a similar amount of dough, you will need up to a cup of sourdough starter. However, since sourdough starter continues growing and reproducing, this difference in quantity is not a problem.

Ingredients 

Although you may need more sourdough starter to produce a loaf, you will need fewer other ingredients. Because sourdough bread is naturally flavourful and rich, you need not use oil, butter, or sugar. Sourdough bread is made from three ingredients: flour, water, and salt – and that includes the starter itself. 

Bread made with yeast often requires oil, butter, sugar, honey, and even milk to enrich it and add to the flavor. So although little yeast is needed, the other ingredients make the bread more expensive.

Preparation time

The most significant difference between using a sourdough starter and commercial yeast is the time taken for preparation: first, you need time to create the sourdough starter. However, once you have made a sourdough starter, it is available for baking immediately. Then, you require time to proof the dough or allow it to rise. This process takes much longer for dough with sourdough starter.

	Sourdough starter	Commercial yeast
Creation of yeast	Up to 7 days	Available immediately in shops; may need five minutes for activation
Proofing time	Up to 12-24 hours	Approximately 1-4 hours
Baking time	Around 1 hour	Around 1 hour

Predictability

Instant and active dry yeasts are commercially produced and, unless expired, will behave predictably and produce relatively uniform products. Sourdough starter is far more volatile and influenced by environmental conditions, particularly heat and humidity, and how well the starter has been fed and maintained, so you may get different results, despite using the same starter. 

Flavor of bread

Another difference between using a sourdough starter and ordinary yeast is the flavor of the product. Although bread baked with fresh commercial yeast has an appealing, sweetish taste, nothing compares to the richness and depth of flavor of sourdough bread. This taste is why bakers take the time and effort to make a sourdough starter and follow the slow process of sourdough bread baking. Generally, the longer dough proofs, the more flavor the final product has.

Health benefits

Apart from not requiring ingredients such as sugar or butter, sourdough bread is also healthier because of how it is made. The sourdough starter fermentation process and the long proofing time produce beneficial bacteria that break down the flour proteins more quickly, making sourdough bread more digestible and healthier.

Sustainability

A well-maintained sourdough starter will last indefinitely, so you need never purchase a leavening agent again. It is, therefore, a very green, renewable product. You can also share your sourdough starter with others, making it a lovely sustainable gift.

Once you have used a packet of commercial yeast, you will need to purchase more. You also need to check the expiry dates of dry yeast, as they will not be efficacious after that.

Should I use a sourdough starter or commercial yeast?

Deciding which leavening agent to use will depend on a couple of factors:

• Skill level: If you are a novice baker, it is easier and more convenient to begin with active or instant dry yeast.
• Time available: If you want to bake a quick loaf on the spur of the moment, commercially produced yeast is readily available and convenient to use.
• Desired product: Sourdough starter always produces a more delicious, flavorsome, and nutritious product. However, it’s a good idea to experiment with different yeasts to find out which is your favorite. 
• Baking approach: If you are a traditionalist or have a slow approach to baking, it is worth investing time and energy in creating a sourdough starter for baking. You will enjoy the sense of pride and achievement that goes along with participating in this traditional craft.

Can I use sourdough starter and yeast interchangeably?

Although both are leavening agents, sourdough starter and yeast work slightly differently and cannot be used interchangeably.

To replace yeast or sourdough starter in a recipe requires calculation as ratios and percentages of all the ingredients will change. This substitution of ingredients should be tackled only by experienced bakers. 

Instead, look for recipes that use the leavening agent you have at hand. 

Note: If you want tips on how to change from yeast to sourdough starter, read my article “Want to Convert your Recipe to Sourdough? Here’s How!“

Conclusion

Sourdough starter and commercially available yeasts are both yeast products you can use to leaven your baked goods. While commercial yeasts are quicker and more convenient to buy and use, bread baked from sourdough starters is more delicious and healthier for you. It is worth experimenting with different recipes to find out what works for you and your family.