I read that matzahs for Passover need to be baked within an 18-minute timeframe. Why is that? Who came up with that magic number? And when do the 18 minutes start?

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On Passover, there is both a mitzvah to eat matzah and a prohibition against eating any chametz (leavened food). “Kosher for Passover” matzah is made of flour and water that has not fermented or risen. If the dough does rise or ferment, it becomes chametz,and not only don’t you fulfill the mitzvah of eating matzah, you actually transgress the prohibition of eating chametz. As the somewhat ironic (but true) saying goes, “the closest thing to chametz is matzah.”

Although one usually adds yeast to help the dough rise, it is possible for it to rise even without adding anything to it. Yeast is essentially a type of fungus, and depending on the environment, there is naturally occurring airborne yeast that is especially attracted to the dough when it is warm.

This brings us to the baking of matzahs for Passover. Not only is it prohibited to use yeast, it is also prohibited to let the dough rise on its own (even just a minute amount) due to the yeast in the atmosphere.1

So how long does it take for the dough to start rising or fermenting on its own?

Time to Rise

The sages of the Talmud explain that under regular conditions, dough that is left alone can begin to rise after the amount of time it takes to walk the distance of a mil (a Talmudic-era measurement).2 There is a debate whether this measurement of time is 18, 22.5, or 24 minutes. Practically speaking, especially when dealing with something as serious as chametz on Passover, we generally follow the stricter opinion—18 minutes (see exception below).3

According to halachah, as long as the dough is actively being worked on, it does not rise. Thus, lechatchilah—as an initial preference—it is forbidden to leave the dough dormant for even one moment without working it.4

Dough can also become chametz in less than 18 minutes when it is warmed—either as a result of the kneading, rolling, etc., or by being placed somewhere hot.5

What Is Done?

Taking these factors into consideration, today all matzahs are baked within “18 minutes” (so simply advertising that matzah was made in “18 minutes” without any further elucidation isn’t really saying much).

However, not all “18 minutes” are created equally.

In most U.S.-based handmade matzah bakeries, the 18 minutes are counted from when the water is first poured into the flour to create the first batch of dough at the start of the matzah baking process.

The entire process from when the flour and water are mixed and the dough is kneaded, rolled, perforated and fully baked actually takes about 5-8 minutes (depending on the methods). Thus, there are multiple batches of dough made in each 18-minute segment.

After each 18-minute segment, everything, including the bowls, tables and rolling pins, are thoroughly cleansed of any residual dough, and only then is the process restarted. Otherwise, this residual dough, which was not worked on, can become chametz.

Many other matzah bakeries, taking into account the fact that dough that is actively being worked on does not technically become chametz, start the 18-minute clock only after the dough is kneaded and brought to the table to be rolled, gaining a few valuable minutes to make more matzah. However, these bakeries usually make sure that the entire process (from the mixing of the flour and water) is still done within a 22.5- or 24-minute timeframe, in line with the other Talmudic opinions. They then pause, clean everything, and restart the process.

(This, incidentally, is an advantage to having a simpler method of matzah production, where everything can be thoroughly cleaned every 18 minutes. A huge machine with many parts can be more challenging to clean thoroughly enough to ensure that no speck of dough is left anywhere.)

Matzah Is Almost Chametz—But Worlds Apart

Indeed, “the closest thing to chametz is matzah.” When we look at the Hebrew words "chametz" (חמץ) and "matzah" (מצה), the spellings are very similar.6 The only difference is the letters chet (ח) and hei (ה), which look almost identical, except the letter hei has a small opening on the top left side.

The Lubavitcher Rebbe explains that there is a thin line between chametz—dough that has fermented and risen, symbolizing one's sense of self and inflated ego—and matzah—dough that doesn’t rise, representing humility. The opening in the hei represents openness to receive, to be guided by others in bettering yourself. Visually, one can imagine that little opening at the top of the hei as a way to “climb out” of the mess you're stuck in.

On Passover, we are extremely careful about chametz, making sure that we don’t become egocentric or too self-important, which can lead us to all sorts of negative places. Luckily, if we’re in an ego-induced mess, all we need to do is make an opening, let out the hot air and enjoy our tasty matzah.7