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What is Leaven?

Trade card for Horsford’s Self-Raising Bread Preparation, Rumford Chemical Works. Source: Collection of the authors: Karen Weintraub and Michael Kuchta, Born in Cambridge: 400 Years of Ideas and Innovators, The MIT Press, Cambridge, MA, 2022

Where Science Meets Religion by Trent Dee Stephens, PhD, for the Come Follow Me lesson May 29–June 4: Matthew 26; Mark 14; John 13

Before discussing this coming week’s lesson, I want to go back to my blog on the fig tree and add an addendum. Last Sunday, as Kathleen and I were discussing that week’s lesson, which included the fig tree, we were pondering the conundrum of that event. Why did Jesus curse the fig tree? Kathleen suggested that we turn to Jesus the Christ and see what Talmage had to say on the subject. I first read that book while on my mission and have referred back to it many times since. I should have consulted it again before writing my comparatively poor discussion of the fig tree. As with most of Jesus the Christ, Talmage’s discussion is profound and deeply insightful – much better than my attempt. I highly recommend that you revisit his account on pages 489-492 of his inspired book. Also read footnote 1 for that chapter.

Now to next week’s lesson. We read in Matthew 26, Mark 14, and John 13 about the feast of unleavened bread, or the Passover (c.f. Matthew 26:17). What is the leaven that was left out of the bread – and even Jewish homes during Passover? Most of us already know what leaven is, but I thought I’d go into the history in more depth.

Exodus 12:15 states, “Seven days shall ye eat unleavened bread; even the first day ye shall put away leaven out of your houses: for whosoever eateth leavened bread from the first day until the seventh day, that soul shall be cut off from Israel.” Sarah Gray stated in a 2018 article in Time magazine, “This has to do with the story of Passover: After the killing of the first born, the Pharaoh agreed to let the Israelites go. But in their haste to leave Egypt, the Israelites could not let their bread rise and so they brought unleavened bread.”1 That was the story I’ve always heard.

We are told in the Bible Dictionary that, “Anything that in cooking produces fermentation, a lump of old dough being generally used. No leaven was allowed during the Passover Feast... It was probably forbidden because there was associated with it the idea of corruption.” That I hadn’t heard – and I find that quite interesting – but I have found little more on that subject, other than leaven is used as a metaphor in the New Testament for teaching about both the kingdom of heaven (c.f. Matthew 13:33) and corruption of the Pharisees (c.f. Matthew 16:6) – in that only a small amount of leaven is needed to raise the whole loaf.

Merriam-Webster Dictionary defines leaven as: 1a: a substance (such as yeast) used to produce fermentation in dough or a liquid especially: sourdough; b: a material (such as baking powder) used to produce a gas that lightens dough or batter.

When we have a family gathering, Kathleen is often called upon to make her famous baking powder biscuits for breakfast – they are to die for. It takes her only minutes to whip up a batch and about eight minutes for them to bake. Alternatively, I can whip up a batch of my famous pancakes – my mom’s recipe – in even less time. The leavening in both cases is baking powder, which is a combination of three ingredients: baking soda, or sodium bicarbonate; a weak acid, such as cream of tartar or monocalcium phosphate; and a filler and drying agent, such as corn starch. Chemically speaking, baking soda is a base, whereas monocalcium phosphate and cream of tartar are acids. When these bases and acids are combined in water, they react chemically, causing the release of carbon dioxide (CO2), a gas. The CO2 creates pockets of air in the biscuits and pancakes, as well as in cakes and other modern baked goods, making them light and fluffy.2

However, baking powder biscuits and quick and easy fluffy pancakes didn’t exist before the middle on the nineteenth century. During the 1840s, the British chemist, Alfred Bird, combined baking soda with cream of tartar to produce air bubbles (CO2) in dough or batter. However, the two ingredients had to be kept separate to prevent them from reacting prematurely – if they were combined and then got even a little bit wet. Furthermore, in order for the desired reaction to occur, they had to be combined in just the right proportions.3

Then in 1856, a young chemist at Harvard University, Eben Norton Horsford, who had established the first bona fide chemistry laboratory in the United States, created and patented the first formula for modern baking powder. Instead of using cream of tartar, which was expensive and had to be imported from Europe as a by-product of wine manufacture, he treated animal bones with sulfuric acid to extract monocalcium phosphate, which would then react with baking soda to produce the desired CO2 bubbles. Some time later, Horsford decided to put the two ingredients together into one container. To prevent any moisture from setting off a premature reaction, he added cornstarch as a drying agent.4

After the Horsford breakthrough, rather than a baker, mother, father, grandmother, or grandfather having to purchase two separate ingredients from a pharmacy (where all chemicals were sold at the time), and having to precisely mix them, they could simply pick up a container of baking powder off the grocery store shelf and bake away. A food historian and author of the new book Baking Powder Wars, Linda Civitello, has stated that baking powder is, “…really the first chemical that opens the floodgates for chemicals in food.”5 No self-respecting U.S. kitchen since 1923 has been without its classic round, white can, with red and black print, of Clabber Girl “double-acting” baking powder (with extra baking soda).6 We keep at least one extra can in our food storage for emergencies.

I have friends and relatives who like to make sourdough pancakes – which to me takes way too much time and effort to turn out a pancake that, in my opinion, is inferior in both taste and texture to my baking powder pancakes.

When I was a child, large special family meals always centered on grandma’s chicken-noodle soup and huge dinner rolls. We weren’t allowed to cut the rolls open with a knife – we were told by grandma that that would spoil the taste – so we always had to pull them open by hand; then we could add that great big pat of butter, which melted into the most heavenly taste imaginable. My mouth is watering even now thinking of all that wonderful food. As for her chicken-noodle soup, one of those big, fat noodles was nearly a full, delightful meal in itself. Of course grandma’s noodles and rolls took much more advance planning than our biscuits or pancakes because they required yeast and took time to raise.

It has been assumed for many years that bread-making was first invented by humans at about the dawn of agriculture around ten thousand years ago. In 2018, however, that assumption was shattered by the archaeological discovery of charred bread crumbs in a 14,400-year-old site in northeastern Jordan by Amaia Arranz-Otaegui and colleagues from the University of Copenhagen, University College London, and the University of Cambridge. That bread had been made by someone among the hunter-gatherers living in the area some four thousand years before grains were being planted and tended. The bread was a type of flatbread (without yeast) made from wild einkorn (a primitive type of wheat with very small grains) and the roots of club rush.7

By some ten thousand years later, before 4,600 years ago, someone had discovered yeast. Tomb murals depict bread making in ancient Egypt in great detail, and, in 1991, an AERA archaeological team, which specializes in Giza archaeology, discovered two bakeries, which closely match those depicted in the murals. Both the murals and the archaeological discoveries indicate that baking pots were preheated before a heavy muffin-like sourdough batter, made of ground emmer and barley, was poured in – presumably to facilitate the dough’s rising before it was baked. The first yeast probably came into a mixture of flour and water that was left out on a warm day and the AERA researchers used naturally-occurring yeast from Giza in their experiments of making ancient bread.8 Apparently the Israelites were also making sourdough yeast-bread in Egypt and took the recipe with them when they left, in such a hurry that they couldn’t take their sourdough starts with them, around 1186 BC.9

Saccharomyces cerevisiae is brewer’s yeast and baker’s yeast – it is the same organism. From at least Egyptian times, the brewing of beer and the baking of bread were closely related because they involved the same yeast. Specialized growing vats for the large-scale production of S. cerevisiae were introduced in England in 1879. Soon thereafter, centrifuges were employed in the US to concentrate yeast from slurries.10

In 1868, Charles and Maximilian Fleischmann emigrated from Austria-Hungary to the U.S. – but the bread here was not as good as the bread back home – with a better variety of yeast. They partnered with an American businessman, James Gaff, who financed the construction of a yeast-production plant in Cincinnati, Ohio. There they produced and patented the foil-wrapped compressed yeast cakes that were to revolutionize baking in their new homeland. “The new yeast had excellent leavening power, delivered consistent quality and made great-tasting bread. The Fleischmanns had created America’s first commercially produced yeast, which soon became the country’s best-selling yeast.” Only eight years after arriving in the U.S., they introduced their new, freshly-baked breads at the Philadelphia Centennial Exposition and soon Fleischmann’s yeast became a household name. As World War II dawned, the Fleischmann research laboratories invented a process for making dry yeast, which required no refrigeration and was activated quickly in warm water, so that our GIs could enjoy home-baked bread wherever they were in the world. Fleischmann’s “Active Dry Yeast” soon made its way into nearly every American household.11 But during Passover, it may be assumed that even the Fleischmanns’ households went without yeast for a week.

Rob Eshman, publisher and editor-in-chief of TRIBE Media Corp./Jewish Journal, wrote a wonderful piece in the Jewish Journal for Passover, 2017. It is very well written and extremely entertaining. I highly recommend reading the entire piece – here I excerpt a few points. He created a sourdough starter in the summer of 2016 by putting some flour and water, uncovered, out on his kitchen counter. After about a week, wild yeast, which are always floating around in the air, had landed in the mixture and began to reproduce, metabolize the sugars in the flour, and produce CO2, causing the mixture to bubble. Then he lovingly nurtured and cared for his yeast starter over the next nine months. He said, “…our ancestors ate sourdough bread because they didn’t have a choice. If you want to know why the Israelites couldn’t wait for their bread to rise, it’s because natural leavening takes a long time to do its magic.” Then he continued his own story, “And now comes Passover, when we are commanded to forgo any leavened thing. In our kosher home, that means all yeast products, all flour, anything with leavening, must go.” That included his precious nine-month-old yeast starter. But Eshman solved his dilemma, by taking his starter to a non-Jewish neighbor who tended it for him until Passover had passed. 12

One wonders if Jews throughout history may have done something very similar – rather than having to start a new batch of starter from scratch every year – perhaps they took a bit of starter to their shops, or barns, or back yards until Passover was finished. After all, the commandment in Exodus only says that the leaven must be removed from “your houses.” It doesn’t say that the precious starter, leaven has to be put out of your life – only set aside for one week.

In our last Where Science Meets Religion meeting we discussed: The Creation of Man and The Creation of Woman. We will not meet during the summer, but will resume discussions the first Thursday in September. Please join us then.

Trent Dee Stephens


1. Gray, Sarah, The History Behind 7 Passover Traditions; Time, 15 March 2018;; retrieved 21 May 2023

2. Weintraub, Karen and Michael Kuchta, Born in Cambridge: 400 Years of Ideas and Innovators, The MIT Press, Cambridge, MA, 2022

3. Panko, Ben, The Great Uprising: How a Powder Revolutionized Baking, Smithsonian, June 2017;; retrieved 23 May 2023

4. Panko, Ben, The Great Uprising: How a Powder Revolutionized Baking, Smithsonian, June 2017;; retrieved 23 May 2023

5. Ibid


7. Arranz-Otaegui, Amaia, et al., Archaeobotanical evidence reveals the origins of bread 14,400 years ago in northeastern Jordan, Proc Natl Acad Sci U S A, 15:7925-7930, 2018

8.; retrieved 26 May 2023

9. Knohl, Israel, Pinpointing the Exodus from Egypt, Harvard Divinity Bulletin, 2018,; retrieved 27 May 2023

10. Litchfield, John, Single-cell proteins, In, Marx, Jean, editor, A Revolution in biotechnology. Cambridge, UK: Cambridge University Press. Chp 6, p. 71, 1989

11.; retrieved 28 May 2023

12. Eshman, Rob, What sourdough taught me about Passover, Jewish Journal,; retrieved 22 May 2023

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