Yeast, But not last

By Miranda Kohout

There’s yeast in every bakery, even if a single loaf of bread never graces the oven. You can find wild yeasts in produce, cheeses and even in the air. A baker or chef who chooses to harness the leavening and fermentation power of yeast, be it wild or commercial, is entering into a contract with a living entity: the chef feeds the yeast and provides favorable growing conditions, and in turn, the yeast leavens breads and other baked goods and gives them unique depths of flavor. An exploration of yeast is, in essence, a look at how to nurture the relationship between baker and these beneficial bacteria.

Yeasts, A Quick Introduction

Yeasts are single-celled microorganisms related to mushrooms, and there are more than 1000 different yeast species. Bakers concern themselves with Saccharomyces cerevisiae, strains of which have been identified as particularly beneficial for baking. These strains are cultivated industrially and processed for commercial distribution.

Yeast leavens bread and other baked goods via fermentation. Put simply, the yeasts consume sugar and starches and then convert them into carbon dioxide and alcohol. Strong and stretchy gluten networks can trap and support expanding bubbles of gas and steam and create a product with good volume and an aerated structure.

How Yeast Works: A Closer Look

Yeast works in doughs via respiration and fermentation, with each process helping to balance the activity of the other. Bakers encourage respiration by stirring, kneading and degassing dough, bringing in oxygen used by the yeast for respiration. In addition to producing our leavening hero, carbon dioxide, respiration produces fermentable sugars, which contribute to flavor and feed the yeast. Yeast also produces some acid during this process, and lactobacillic acid contributes a small amount of carbon dioxide.

This cyclical process continues into the proofing stage and even contributes to oven spring when the sudden high temperature kicks off rapid carbon dioxide production before the yeast dies at 140°F.

Types of Yeast

Commercial Yeast

Commercially produced yeast offers the leavening power of a wild yeast colony without the care and feeding required for a starter. A scoop of beige granules admittedly lacks the romance of a historic and nurtured starter, as well as the intriguing flavors that come with long fermentation times. However, the convenience and long shelf life of manufactured yeast is a better fit for many bakeries, and there are products in which the tang of sourdough is undesirable. You can even freeze commercial yeast for long-term storage. It will lose some viability stored this way, and you should increase the amount used by 25%.

Commercial yeast is available in different formats:

Fresh Yeast – available in cakes and granules. The yeast cells in this format are alive, meaning they you should keep them refrigerated because they have a very short shelf life. Bakers who prefer this type of yeast cite its ability to produce more gas (and therefore better leavening) than other formats.

Active Dry Yeast – available in granules. Dry this yeast is dried at higher temperatures than those used for similar formats. The heating process kills the exterior yeast cells, leaving a protected core of live yeast. The higher heat used means a thicker coating of yeast debris, which is why this yeast must be soaked before use.

Instant Yeast – very similar to active dry yeast, but with some practical differences. Dry this yeast using a gentler process, creating a thinner shell of yeast debris. Instant yeast absorbs water easily and quickly, which means you do not need to soak it. Instant yeast produces carbon dioxide more vigorously than active dry yeast.

Quick Yeast – not often seen outside of home kitchens. This is a fast-acting yeast strain that has been genetically engineered to leaven with less rising time.

Proofing/Proving Commercial Yeast

While some argue that this preliminary bread-baking step is a holdover from when commercial yeasts were unreliable, others maintain that verifying yeast’s viability before proceeding with a recipe is still a necessity. Given the time and effort put into bread production, assurance that one’s yeast is hale and hearty can, at the very least, help eliminate one potential cause of failure.

Before proceeding with a recipe in its entirety, a baker will combine the yeast with a small portion of the recipe’s water (warmed) and flour and wait to see signs of activity. Having proven — or proved — that the yeast is alive and active, the baker then continues with the recipe.

While proofing may be unnecessary, active dry yeast still requires soaking in warm water before use. If you don’t soak the yeast, the thick outer coating of dead yeast will prevent water absorption, resulting in solid yeast granules with their active yeast cores still trapped inside, peppered throughout the dough rather than integrated into it.

You should not soak active dry yeast in cool water. Experts have found that soaking yeast in cool or cold water results in poor yeast activation and causes the yeast to release substances that interfere with gluten formation. The ideal temperature for soaking yeast is 105°F. Activation slows at 120°F, and the yeast will die at temperatures above 140°F. You may be surprised by the temperature of your hot tap water. Be sure to check it if you consistently have issues with seemingly inactive yeast.

Best Practices for Using Commercial Yeast

At warmer temperatures, yeast grows quickly, exhausting its food supply and becoming completely inactive after 18 hours. Cooler temperatures, on the other hand, slow down yeast activity. It will grow slowly and steadily. Allowing a dough to rest for three hours at a warm room temperature results in roughly the same amount of leavening as 18 hours at refrigerator temperatures. The extra rising time required by cooler temperatures allows for the creation of more complex flavors due to the extended fermentation activity.

For best flavor, chefs should avoid using large amounts of commercial yeasts. An excessive amount of commercial yeast not only contributes a distinct and unpleasant flavor; it also accelerates the rising time and does not allow for the development of more complex and desirable flavors.

For an overnight fermentation, .25% of the flour weight (roughly 1 gram of yeast for every 450 grams of flour) is a good starting point.

Wild Yeast

Many cultures throughout history have found ways to attract and nurture yeasts present in the air and put them to work flavoring and aerating different foods. These wild yeasts work in conjunction with different lactobacilli bacteria both to offer a complex fermented flavor and create gasses that leaven baked products.

Some artisan bakers continue to make bread with wild yeast, creating and nurturing a sourdough starter or levain. Unlike commercial yeast, a sourdough starter requires care, needing to be fed and maintained at an amenable temperature to keep its resident yeasts alive and maintain a proper pH balance. Some bakeries streamline this process by adding a small portion of the previous day’s dough to the current day’s bake, thus keeping the yeast and bacterial culture alive and active while simultaneously using it in production. A bit of pâte fermentée (old dough) typically contains 10,000 to 1 million yeast cells per gram of dough.

On occasion, bakers will opt to add a little “insurance” to a recipe or boost their wild yeast by adding a small quantity of commercial yeast to their sourdough recipes.

Maintaining and Using Wild Yeast

A sourdough starter should be fed frequently for maximum activity. The more it is divided and refreshed, the more leavening power it will have. It is best to feed a sourdough starter when it has reached or just passed peak activity. Timing feedings to produce maximum results can range from tricky to extremely inconvenient, and bakeries that maintain a starter often opt to feed the starter on a set schedule, sometimes adjusted for seasonal temperature fluctuations.

Yeast, particularly wild yeast, thrives at close to ideal room temperatures (68-78°F), so keeping starters away from ovens, windows and vents can be paramount. For peak performance, wild yeasts in doughs generally perform better at slightly cooler temperatures than commercial yeasts. For best results, seeking out or creating an ideal location for bulk rises may be necessary.

Ingredients That Affect Yeast Performance

Salt:

We are all familiar with the edict that salt should never touch yeast. While it is true that a large quantity of salt mixed with yeast and allowed to sit for several hours would kill the yeast, the amount of salt used for baking is not enough to affect yeast performance.

Sugar:

When proofing yeast, many bakers add a small amount of sugar as food for the newly awakened yeast. However, in doughs, the hygroscopic nature of sugar causes it to pull moisture away from yeast and inhibit its activity. This is why sweet doughs require a larger proportion of yeast than doughs that contain little to no sugar. Sweet doughs suffer volume loss at quantities of sugar greater than 10 grams of sugar per 125 grams of flour, and bakers should adjust their yeast quantity as necessary.

Acid:

Yeast fermentation is most efficient at a pH between 6.0 and 8.0. This level hovers right around neutral and is not something most bakers typically need to consider. Still, it is good to note that if the pH of the dough is too low or too high, it can affect yeast performance, so additives like buttermilk, lemon juice or even very hard water should be used carefully.

Malted Barley:
Malted barley is excellent food for yeast, so much so that it is even added to some flours. It contains live enzymes that convert starches to sugars that act as food for yeast. Adding malted barley syrup or non-diastatic malt powder to a dough can help boost yeast performance.

Spices:
Other ingredients, such as small quantities of cardamom, ginger, nutmeg and thyme, have been found to increase yeast performance. Cinnamon can boost yeast performance up to a point. At quantities greater than 1 gram of cinnamon per 1 gram of yeast in a recipe, the leavening quality of yeast will start to be negatively affected.

Minerals:
Milk’s minerals promote yeast growth, making it a multi-tasking ingredient. Distilled water lacks these minerals and should be avoided.

Other Inhibiting Factors

It can be hard to determine if poor yeast performance is due to ingredient interference or poor methods. How a baker handles the dough can affect how well yeast performs.

For example, yeasts multiply in dough in clumps. As these clumps grow, the yeast in the center of the clump is cut off from oxygen and food. Degassing, stretching and folding help distribute yeast and form gluten. Degassing also splits larger, weaker bubbles into smaller, more stable bubbles, resulting in a strong network of many bubbles.

A strong gluten network can support these gas bubbles as they expand, allowing them to hold their shape until the dough has risen and set in the oven. In poorly developed doughs, these bubbles will rupture before setting, yielding a dense product.

Substituting types of yeast

A percentage of the weight of dry yeast is made up of the outer coating of dead yeast cells. Active dry yeast has a thicker layer of this debris than instant yeast, so less of the weight of active dry yeast is made up of live yeast cells. Therefore, roughly 25% more yeast will be necessary to achieve the same leavening effect when substituting active dry yeast for instant yeast.

For example, the equivalent of 1 gram of instant yeast is 1.25 grams of active dry yeast, and the reverse is true when substituting instant yeast for active dry yeast.

Fresh yeast has no coating of yeast debris – 100% of its weight is live and active – so when substituting dry yeast in a recipe that calls for fresh, 40% more active dry yeast is necessary to create the same leavening effect. Reduce this adjusted quantity by 25% if you substitute instant yeast for fresh. 

Substituting wild yeast for commercial yeast is a bit more complicated, and converting a recipe to use a sourdough starter will likely require some experimentation. You need a much greater volume of sourdough starter in order to provide close to the same amount of active yeast bacteria found in a commercial product, and it brings with it additional flour and water, which you will need to take into account. A good place to start is by replacing 5 grams of yeast with 100 grams of sourdough starter and reducing the amount of flour and water in the recipe by 47 grams each, assuming the starter’s hydration is 100%.

Trivia

You may know someone in possession of a “vintage” sourdough starter, one that was first set out to develop a wild yeast colony over 100 years ago. This makes for a great story, but is there anything more to it? Somewhat sadly, science has determined that while the idea of a heritage starter makes for a great conversation topic, none of the original microbes reside in the current version of that starter.

Further romance-killing research found that sourdough “terroir” is also a myth, and species of lactobacilli and yeasts are not limited by geography. For example, you can find L. sanfranciscensis, the eponymous San Francisco sourdough bacteria, throughout Europe. The conditions in which a bacterial culture lives have more impact than geography.

But don’t toss your heirloom starter just yet! Successful business owners know that a good story will win out over molecular analysis any day. Besides, for many of us, our heirloom starter is more than an ingredient; it’s part of the team.

(This article appeared in the Fall 2024 issue of Pastry Arts Magazine)