The Knead to Know: The Rise of Sourdough

Fall 2023

We are all familiar with the Torah’s directive to rid our homes of chometz in order to prepare for Pesach. The Torah uses two terms when instructing us in the cleaning process: chometz and se’or.

Ask any talmid or talmida: “What is chometz,” and you will get a very erudite response. Ask the next question: “What is se’or,” and seven out of ten will shrug and say, “I dunno,” while the other three might venture, “Sourdough?” When you follow up and ask them to define sourdough, most – if not all – will say, “I dunno!”

Today, more and more homes are seeing sourdough boules find their way into their bread baskets. With its “rise” in popularity, it is only natural that we look into this new trend and ask, “Ma nishtana sourdough bread from its conventional counterpart? Are there halachic and practical differences?”

Wheat and Bread Making Fundamentals

Let’s begin by reviewing the basic bread making fundamentals. Bread combines five basic ingredients: flour, water, sugar, salt and yeast. The number one ingredient is flour, the basic bread starting point. Another great question that elicits a quizzical response is, “How is flour produced?” The answer is that the Ribono Shel Olam produces flour in every kernel of wheat; it is the milling process that unlocks the flour from its housing.

A wheat kernel is comprised of three basic components: the outer cover called the bran, the powdery white innards called the endosperm, and a small nutritional component called the germ. Milling separates the endosperm from the bran and germ through a process of grinding and sifting. Mills typically use over twenty sifting screens to separate any large particles and enable the fluffy white powdery endosperm to be transformed into flour. There are numerous varieties of flour, including all-purpose, bread, pastry, whole wheat, semolina and rye.

In bread making, mixing water and yeast with flour jumpstarts the leavening process, and the dough begins to rise. How exactly does this miraculous process happen?

We know through the wonders of the Ribono Shel Olam that once water mixes with flour, the enzymatic conversion of starch into sugars begins. When yeast – which is a single-celled living organism – is added to the dough, it reacts with the water and begins to feed on the sugar in the dough.

As a fungus that depends on outside sources for nourishment to live, yeast is the very engine that effects changes in the dough to create the leavening process. In the course of feeding on the sugars, it releases carbon dioxide gas, causing the dough to rise. This gas is trapped by a complex gluten network within the dough, preventing it from escaping. For this reason, the best flour for bread making is high-gluten.

Gluten is a protein found in flour and composed of two parts – gliadin and glutenin. It is present in the starch of the endosperm of wheat, barley, rye and spelt grains. The gluten helps the dough rise more effectively when it is kneaded and re-kneaded, as the yeast molecules work harder to grow, multiply and release the CO2 contained in the gluten network. It is the gluten that holds the bread together.

The Miracle of Yeast

We have now explained that in order for dough to rise, the leavening process relies on yeast. The most popular strain of yeast used for baking breads and cakes in conventional baking is Saccharomyces cerevisiae, grown and propagated using molasses. But what is the source of sourdough breads, which do not use commercial yeasts?

In a word, nature!

Hashem Yisborach has filled the briah with a myriad of wild, natural yeasts. Wild yeasts are everywhere – in the air, on our skin and, most significantly for the purpose of baking sourdough, throughout wheat fields. (This is why matzah is an 18-minute activity: to keep the wild yeasts on the wheat from activating!) Once milled, the yeasts on the wheat end up in the flour, and later serve as the catalysts to create a viable sourdough starter.

Before yeast was commercially produced in the late 19th century, homemakers would set aside a piece of the previous day’s risen dough and use it to leaven the next day’s batch. Today, sourdough starters harness those natural yeasts through a long process of propagating and concentrating. Making a “mother” sourdough starter begins with combining flour and water in a clean covered jar at room temperature and allowing the natural bacteria to begin the natural leavening process.[1] The wild yeasts consume the flour and multiply, until they exhaust the nutrients in the flour and need to be fed again. As the starter grows, more and more natural yeasts are generated. A happy starter will be fluffy because the yeasts are sated. After about seven days, the mother has propagated enough yeasts to make a good sourdough starter.

Sourdough Starter Implications on Pesach

The rise in sourdough’s popularity can be attributed to multiple factors, among them its unique flavor, crusty exterior and a host of nutritional benefits not found in conventional breads. The leavening process produces natural lactic acid, which aids digestion, as well as many natural vitamins and minerals. Furthermore, its lengthy fermentation helps to break down the bonds in the gluten network, helping those with gluten sensitivities to enjoy sourdough bread more easily than conventional breads.

Clearly, sourdough breads have many nutritional and culinary benefits. But they also have significant halachic ramifications.

It is written that “se’or lo yimatzei b’vateichem” – sourdough should not be found in your homes on Pesach. Unlike conventional yeast, which is not chometz, a sourdough starter very much is. The very feeding of a sourdough starter is a 100% chometz process. Hence, a sourdough starter must be removed with the rest of one’s chometz.

Those who sell their chometz must sell their starter, as well. Those who don’t use sold chometz after Pesach, or who do not purchase chometz products that were sold, may not use any sold sourdough or consume sourdough breads baked with the sold starter.

Some sourdough bakers prize starters that have survived for generations and were produced from pedigree mother starters – some over one hundred years old! But from a halachic standpoint, sourdough bakers who do not sell their chometz must discard their starters before Pesach and “start over” once Yom Tov ends.

Additionally, if one feeds a yoshon sourdough starter with chodosh flour, the sourdough bread will be chodosh. If one sells chometz after Pesach, when the chometz is repurchased, the chodosh starter will now be yoshon.

Sourdough harnesses the special gifts that the Ribono Shel Olam has naturally provided us, since – as we have noted – “se’or nimtzah b’chol bateinu.” We are grateful to have “re-discovered” something which has taken bakers, bakeries and our pocketbooks by storm, yet has been Hashem’s gift to mankind since long before Yetzias Mitzrayim.

Ma rabu ma’asecha, Hashem!

The author wishes to thank Mrs. Esti Rossberg for her assistance with this article. Mrs. Rossberg is a veteran sourdough baker and educator and has published articles on the topic. Her six-year-old sourdough starter is archived in the sourdough library at

[1] The exact process of creating and maintaining a starter is beyond the scope of this article.