As we continue celebrating Pesach this week, and avoiding all things chametz, it is important to take a moment and explore what exactly is chametz? While we spend a tremendous amount of time and effort learning about, and implementing, the various halakhot regarding eliminating chametz, we rarely think about what chametz actually is on the chemical level. If we did know, it would help to clarify what specifically is forbidden, and might save us a great deal of time and effort in our preparations. It would also help us better understand what actually happened in Egypt with our ancestor millennia ago (the answer may surprise you). So, what is chametz?

The critical Torah chapter for understanding it all is Exodus 12. It begins with God’s command to commemorate Nisan as rosh chodashim, the first of the months. Then, God commands that on the 10^th of the month, each household had to prepare a sheep, to be slaughtered on the 14^th. Its blood would then be pasted on the doorposts, and its meat entirely consumed, together with bitter herbs and matzot (12:8). Intriguingly, the Torah has not yet told us anything about chametz! The command to eat matzot preceded the well-known narrative about the dough not rising. Israel already consumed matzot the night before the Exodus!

Several verses later, God commands that henceforth Israel would always celebrate a holiday with seven days of matzah-eating, before which all the se’or (“leaven”) must be destroyed, and during which anyone who dares eat chametz will be spiritually “cut off” from the nation (12:15). We have still not been told what any of these things are, or why they cannot be consumed.

After describing the last plague, with Pharaoh finally relenting and letting the Israelites go, the Torah explains that “the people took their dough before it was leavened, their kneading bowls wrapped in their cloaks upon their shoulders.” (Exodus 12:34) The previous evening they had already eaten the pesach meat together with matzot. Now, in their haste to leave, the Israelites wrapped up their uncooked dough in their kneading bowls and carried them away on their shoulders. It is worth mentioning that Chizkuni (Rabbi Hezekiah ben Manoach, c. 1250-1310) comments here that this is why we have a custom to wrap the afikoman in a towel or cloth!

The Torah says the Israelites then journeyed out of the country, from the city of Ramses (in Egypt) to “Sukkot” (in the Wilderness). Presumably, they stopped to rest and set up their temporary sukkot there. Only at this point, free at last, did “they bake the dough which they had taken out of Egypt into cakes of matzot, for it had not leavened, since they had been driven out of Egypt and could not delay; nor had they prepared any provisions for themselves.” (12:40) This must be repeated: after coming out Egypt, and having already journeyed far away, they stopped to rest and eat and, only then, in the Wilderness, they baked matzot. The reason they couldn’t bake anything fancier is because the dough they had brought out had not risen. This sparks a huge question: How could it be that the dough had not risen? They have obviously been walking for a very long time by this point! Don’t we say that, halakhically-speaking, any flour-water mixture starts to become chametz after just 18 minutes? The Torah clearly tells us that the Israelites had taken prepared dough, already in their mixing bowls, and journeyed far out of Egypt’s borders before using that dough to make matzot. It should have become chametz a long time ago! How do we make sense of this puzzle?

Fungus and Sourdough

Today, we are used to buying purified “active dry” yeast in the supermarket. We assume that when the Torah says se’or, “leaven”, it refers to the same type of yeast. Of course, in ancient times there was no such thing, and no way for people to isolate yeast at all; nor could they know that yeast was actually a microscopic living organism. Yeast, a type of fungus, occurs naturally all over the place. Such fungi are involved in symbiotic relationships pretty much everywhere we look, and are absolutely vital for the survival of all plants, if not all complex life forms. For instance, pretty much every plant on Earth is dependent on a family of fungi called mycorrhiza which lives in the soil, intertwined with the plant’s roots, and provides the plant with vital nutrients. Some mycorrhizae are endosymbiotic, meaning they actually grow into the plant’s cells! They become like one hybrid organism. (Wheat is an example of a plant that has inseparable mycorrhizae growing directly into it.) There are tens of thousands of species of mycorrhizae, and a single plant might have a dozen or more different types that it interacts with and depends on.

Other fungi grow on plant stems, leaves, and fruits. They are everywhere, and impossible to get rid of. We have all seen our fruits become covered in fungus when we leave them out too long—even fruits we have washed thoroughly. Why does this happen? Where does the fungus come from? It is already there embedded deep within the fruit’s skin, and even washing won’t help. Eventually, the fungus will grow out, like it or not! The same is true for wheat. Long before it is harvested, the wheat is already infested (enhanced?) with yeast. So, as soon as you add some warm water to flour, you are beginning the process of activating those yeasts. It takes about a week, and usually closer to two weeks, to get a rich amount of yeast turned on. The result is called starter dough, or sourdough. In ancient times, this was the only way to make fluffy bread. They didn’t have “active dry yeast” in a convenient package from the store. Either you wait a couple of weeks for your dough to rise or, since no one wants to wait that long, you keep a supply of starter dough, se’or, from which you can take a chunk and add it to a new batch of dough.

With that knowledge, everything becomes clear: the Israelites had new batches of dough, but no starter dough, for they hadn’t the time to “prepare any provisions…” (Exodus 12:40) So, when they grabbed their new dough and wrapped it up, and walked for many hours, the dough had not yet sufficiently risen, and they could only bake matzot! Based on this, I believe there is a better way to read the verse cited previously: “the people took their dough before it was leavened, their ‘kneading bowls’ [משארתם] wrapped in their cloaks upon their shoulders.” (Exodus 12:34) Most of the major commentators, including Rashi, read משארתם as misharotam, translating it as “kneading bowls”. However, it could also be read mi’se’orotam, “from their starter doughs”. In other words, the dough that the Israelites took out of Egypt was without added starter dough, which is why it did not rise despite the long journey.

Making Sense of Yeast

With the above in mind, it becomes clear that yeast itself is not the problem. Yeast is everywhere, and our ancestors did not have purified yeast like we have today. When our Sages spoke of the se’or sh’b’isah, the “leaven in the dough”, that metaphorically causes us to sin, they were referring to starter dough, not yeast. Yeast is inseparable and pretty much indestructible. In fact, recently scientists were able to extract yeast from ancient Egyptian pottery dating back over 4000 years—and then bring it back to life and bake with it! Yeast itself is not chametz, and cannot be, since we consume plenty of yeast on Pesach. In fact, we couldn’t have four cups of wine at our seders since wine is fermented with yeast, too.

Similar to breadmaking, winemakers today add yeast to speed up the fermentation process. In ancient times, however, the crushed grapes fermented on their own with the yeast that is already naturally growing all over them. Leave it longer and it will keep fermenting and turn into wine vinegar. This is mentioned in the Torah (Numbers 6:3), where it is called chometz yayin (חמץ יין). Interestingly, it is believed that hummus comes from the word chametz, too, as we read in Ruth 2:14, where Boaz tells Ruth: “Come over here and partake of the meal, and dip your pita in chometz.” Keep in mind that the letter tzadi was pronounced sa’adi back then (and still is in many Sephardic and Mizrachi communities, see ‘Shabbat or Shabbos: What’s the Correct Pronunciation?’ in Garments of Light, Volume II). So, Boaz would have told Ruth to dip her pita in chomes, ie. hummus!* This could have been either regular vinegar, or perhaps fermented beans of some sort, hence the hummus etymology.

Of course, wine and wine vinegar are kosher for Passover, so there is no issue with chometz yayin. Some might therefore argue that drinking chametz is not forbidden, since there is no prohibition on fermented grapes. After all, the Torah only speaks of achilah, “eating”, and not drinking (as in Exodus 12:15-16, 19, 13:3, and Deuteronomy 16:3). What does that mean for beer, vodka, and whiskey made from fermented grains, which are not kosher for Passover?

Drinking Chametz

How do we distinguish between drinking fermented grapes, which is allowed on Passover, and drinking fermented grains, which is forbidden? The simple answer, of course, is that fermented grains are directly related to fermented dough. Fruits are not an issue, and have nothing to do with the Exodus. Since the Torah prohibits eating fermented grains, namely dough, we can extend that prohibition to drinking fermented grains, too. The extension is certainly appropriate, for in olden times (and today) people make certain types of beer from old bread. As my mother recalls, her mother used to take old bread and dry it in the oven, then soak it in barrels of water and let it ferment to make kvass. This type of beer is made directly from old bread, undoubtedly chametz. There is no question that beer, in general, should be forbidden.

What about vodka and whiskey? These beverages are usually made from fermented grains, too. However, they are distilled, meaning that the ethanol (alcohol) is extracted from the fermented mixture. There is little left of the original grains, which is also why vodka and whiskey are gluten-free (while beer is not). In other words, the chametz would have been removed from the vodka or whiskey in the distillation process, leaving only ethanol, water, and perhaps tiny traces of flavour compounds. In the case of whiskey, it is when the liquid is later aged in casks that it develops its true flavour and colour.

Based on this information, vodka and whiskey probably shouldn’t be classified as genuine chametz. That does not mean they can be consumed on Pesach, since we are extremely stringent when it comes to this holiday, and don’t want to take even the slightest risk of contamination. After all, the Torah warns that one who consumes chametz will be karet, spiritually “excised”. Therefore, it is best not to consume something derived from fermented grains, even though the chances of it having real chametz is minimal.

So, what is chametz? As we’ve seen, it isn’t yeast itself, which we consume on Pesach. Starter dough is chametz, but has a special title in its own right, se’or. Can we pinpoint a specific chemical compound that is chametz? Well, chametz is what makes the dough rise. It isn’t the yeast itself that makes the dough rise, it is what the yeast generates: carbon dioxide (CO₂). In bread, the carbon dioxide gas bubbles are released as a waste product by the yeast cells, and these gasses are trapped within the gluten proteins of the dough, forcing the dough to rise.

Alternatively, one can add baking powder, which has a combination of baking soda and some weak acid. As is well-known, when a base (like baking soda) meets an acid, they undergo a neutralization, “cancelling” each other out. When baking soda is neutralized, it releases CO₂. Thus, adding baking powder to your cake can also make it rise, in lieu of (or in addition to) the yeast. Amazingly, the Tanakh already spoke of the neutralization reaction long ago:

In Proverbs 25:20, King Solomon teaches that trying to cheer up a sad person by singing them happy songs is like disrobing them on a cold day, or like pouring chometz al nater, “vinegar on natron”. Natron is a natural mixture containing baking soda (NaHCO₃) that was mined even in ancient times. Of course, adding vinegar to natron is like adding vinegar to baking soda, causing them to neutralize each other, and making a vigorous reaction with lots of bubbles. Those bubbles are carbon dioxide.

Can we say that CO₂ is the real chametz? After all, it is the CO₂ bubbles that make the bread rise. Upon further thought, carbon dioxide cannot be chametz since, once again, it is found all over the place, including within our bodies and in the air around us. Among many other things, it makes the bubbles in soft drinks, which we are permitted to drink on Pesach. Having said that, CO₂ is definitely part of the chametz equation.

Perhaps we can come to the following conclusion: since what makes dough rise is the CO₂ trapped within the gluten, the combination of both makes chametz. Therefore, something like wine, which is gluten-free, cannot be chametz, and neither can Coca-Cola, which has CO₂ but not gluten. Beer has both gluten and CO₂ so it would be chametz. Matzot generally have gluten, being made of grain flour, but no CO₂, so they are not chametz. Defining chametz chemically as gluten+CO₂ works well in every case I could think of.

(It is interesting to note that rice and corn also have proteins from the gluten family, so there may be something substantial to the prohibition of kitniyot besides tradition. Having said that, the types of gluten in rice and corn are not the same and can be consumed by people who are otherwise gluten-intolerant.)

A concluding thought: plants “breathe” in carbon dioxide and use it—with help from some other compounds absorbed through the roots—to make all kinds of macromolecules like sugars and proteins (gluten included). Humans, on the other hand, consume plants with their sugars and proteins and gluten, digest them and metabolize them into carbon dioxide that we then breathe out (and some other compounds excreted elsewhere). In short, what we do with carbon dioxide and gluten (chametz) is the exact reverse of what plants do with them. And so, it is quite beautiful that when we read the word chametz (חמץ) in reverse we get tzemach (צמח), a “plant”! Plants can teach us how to truly get rid of our spiritual chametz which, as our Sages teach, represents greed, ego, and “lawlessness”. Be like a plant: life-giving, oxygen-providing, food-distributing, sheltering, beautifying, purifying, peaceful and pleasant. This is how we reverse the corruption of chametz. And with this we can further understand why Mashiach—who will usher in a world with no spiritual “chametz” of any kind—is called by God to be ‘avdi tzemach, His “plant-like” servant (Zechariah 3:8).

Chag sameach!

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*In multiple places in the Tanakh, the letter tzadi is replaced with samekh or sin, further indicating that tzadi was pronounced with an “S” sound. One such place is Psalm 71:4, where David asks God to save him from the “unjust and lawless”, the latter spelled חומץ, which Rashi says is the same as חמס, meaning “lawlessness” or “violence” throughout Scripture. This gives all the more meaning to removing חמץ from our homes!