Yeast and Sourdough Starters

One of the unexpected consequences of the pandemic has been a surge in home bread-baking and sourdough starters, partly due to a shortage of bread and yeast in stores. Many people sheltering at home began creating their own yeast, or sourdough starters, for bread-baking. It was also a lovely way to spend time with family during the lockdown.

Although during lockdown people began to bake their own bread and create their own sourdough starter many were unclear exactly what starter is and more fundamentally what yeast is. What is it about this flour and water paste that comes to life and magically makes bread rise? It all boils down to yeast.

So what is yeast and sourdough starters?

Yeast is a microscopic fungus made of single oval cells. The cells reproduce by budding and can convert sugar into alcohol and carbon dioxide. When yeast divides and multiplies it produces more carbon dioxide (this is what makes bread rise). Roughly 1500 species of yeast are found worldwide in the soil and on plant surfaces, such as the skins of fruit and berries.

There are four kinds of yeast

There are four common kinds of yeast: bakers yeast for bread, brewers yeast for beer, distillers and wine yeast, which metabolizes sugar and produces alcohol as a byproduct and nutritional yeast, a deactivated yeast sold commercially as a food product popular with vegans and vegetarians as an ingredient in cheese substitutes.

All four of these yeast products are made from the exact same strain of yeast: Saccharomyces cervisiae. The difference is how each is made and the additional ingredients that are added. For example bakers yeast is made to take a small amount of added sugar and along with some flour, slowly rise to make the tiny trapped bubbles in bread. Distillers yeast is made to ferment quickly and likes a lot of sugar to ferment into a lot of alcohol. For beer you add malted barley and hops. Nutritional yeast is deactivated and converted into flakes for use in food.

Yeast Extract

You might also be familiar with strong-flavored yeast extract - which isn’t a yeast - that comes in the form of a dark paste known commercially as Vegemite or Marmite. It’s used as a spread or to add umami flavor to savory dishes.

Yeast and Sourdough Starters for Bread

Regular bread is made using store-bought yeast that reacts with gluten to make dough rise; whereas sourdough bread is made from wild yeast (and bacteria) that grows inside a paste of flour and water. This paste is called sourdough starter (also known as levain - the French word for leaven). Starter is also known as mother yeast (pasta madre or lievito madre in Italian, literally translated to “mother of dough”- usually a firm sourdough starter).
Yeast breads take time and usually one or two risings, each of which might take up to an hour or several hours. So-called quick breads are another thing altogether; they are made with a non-yeast leavening agent, such as baking powder or baking soda, which is mixed into batter and baked without waiting.
If you decide to buy yeast for your home-baked bread, you can choose either cake yeast - also known as wet, fresh or compressed yeast - or dry yeast.

Cake yeast - (wet, fresh, compressed yeast)

Cake yeast must be kept refrigerated and lasts only two weeks, is 70% moisture; and it’s usually packaged in 1- to 2-inch cubes. It must be dissolved in tepid water, often with a very small amount of sugar, and then added to flour. Cake yeast is often used in Europe, but much less commonly used in the United States.
Dry yeast (roughly 8% moisture) is sold in packets with a shelf life of several years. In the United States you can buy active dry yeast and instant yeast and two of the better known brands are Fleischmanns and Red Star.

Dried yeast

Active dry yeast must be activated by dissolving in liquid before it’s incorporated into other ingredients, whereas instant yeast can be mixed directly into dry ingredients. They are both 100 percent yeast. The difference is that instant dry yeast has porous granules so it can quickly absorb moisture and oxygen so it doesn't need activation.

Making Yeast and Sourdough Starters

You can make your own yeast starter, or sourdough starter, at home and all you need is a paste of flour and water. When left loosely covered at room temperature, the flour and water paste breathes in the air, and bacteria, from the environment. Over the course of a few days it begins to bubble, and your sourdough starter comes to life. There are 50 million yeast and 5 billion lactobacillus bacteria in every teaspoon of starter. Now you have to feed it—more flour and water. In the beginning, it needs to be fed daily, but eventually you can store it in the refrigerator and feed it once a week, or even less. If taken care of, a sourdough starter can last for years, and you can share your starter with friends and family. And as they maintain the starter it will become their own with a distinct flavor based on the bacteria in their own kitchen environment. Here's some of my sourdough starter.

Pre-ferments

When you are ready to bake a loaf of bread you must prepare a pre-ferment. For a pre-ferment you take a small amount of your starter from the refrigerator (about ⅓ cup) along with some flour and water and let it sit until it doubles in volume. There are a number of terms for a pre-ferment: a biga is a dry and thick pre-ferment used in Italian baking, especially for breads like ciabatta, that yields a soft and porous interior and a crunchy and crispy crust. A poolish, or sponge as it’s often referred to in the United States, is a wetter pre-ferment.
For a good guide to bread-baking, yeast and sourdough starters, and baking terminology, plus great bread recipes, The Perfect Loaf is a great resource. This is a great Weekday Sourdough Bread recipe for beginning bread bakers that I now make all the time.

I love baking bread with whole grains and this delicious Farro Nut Bread has been a favourite of mine for many years.

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