Ironically, the food that can most easily become Hametz is Masa – the food that the Torah obligates us to eat on Pesah. Masa is made from wheat flour and water, the ingredients which can become Hametz, and thus extreme caution must be taken by those working in Masa production facilities to ensure that the dough does not become Hametz.
The Shulhan Aruch presents the guidelines for baking Masa in Siman 459 (Orah Haim), where he explains that as long as the dough is being handled, it cannot become Hametz. If a person kneads a piece of dough without stopping, then it cannot become Hametz, even if the person kneads it the entire day without baking it. This is why workers in Masa factories are constantly working the dough, without leaving it unhandled. However, it is practically impossible for the dough not to be left unhandled at all. For example, after the dough is kneaded and formed into a circle, it is placed on a table that has on it the machine that makes the perforations in the Masa. Sometimes, the dough remains on the table for a full minute before it is taken for the perforations to be made. And, the person standing by the oven needs a few moments to collect the raw Masot on the long stick before placing them into the oven. The Shulhan Aruch writes that dough which is left unhandled for the amount of time needed to walk a "Mil" becomes Hametz. If the dough is left unhandled for a shorter period, then it does not become Hametz. The accepted view is that this refers to a period of 18 minutes, and so as long as the dough has not been left unhandled for 18 minutes, it is permissible for consumption on Pesah.
This is why many Masa companies advertise that their product is "18-minute Masa." This means that they stop production 18 minutes after the time the water is poured into the flour to make dough. When the bell rings – usually 16 or 17 minutes after the water is poured – the process is stopped, and all the dough which has not yet been placed in the oven is discarded. Then a new run with new dough is begun. This procedure ensures that no dough used for Masot was left unhandled for 18 minutes.
However, the Shulhan Aruch adds that if the dough is warm, it could potentially become Hametz as soon as it is left unhandled, even within 18 minutes. Usually, during the production process, the workers’ hands become warm, which causes the dough to become warm. The question thus becomes whether the Masa is permissible, given that, as mentioned, it is impossible for the dough not to be left unhandled for a few moments at various stages of the production process.
This question was addressed by Hacham Bension Abba Shaul (Jerusalem, 1924-1998), in his Or Le’sion (11:5). He draws proof from earlier sources that the Shulhan Aruch cannot mean that the dough must not remain unhandled for even a minute. For example, the Ramban (Rav Moshe Nahmanides, Spain, 1194-1270) writes that he used to knead the dough for Masa inside his home, and then bring it to his oven outside to bake. Furthermore, the Rashba (Rav Shlomo Ben Aderet of Barcelona, Spain, 1235-1310) writes explicitly that it is acceptable to knead the dough in one place and then bring it to the oven somewhere else as long as the oven is not too far away. Hacham Bension thus concludes that if the dough is left unhandled for one or two minutes, the Masa is permissible, even though the dough is warm. If the dough is left unhandled for five minutes, Hacham Bension writes, then it must be considered Hametz, and if it is left unhandled for three minutes, then it is questionably Hametz. But if it is left unhandled for just 1-2 minutes, it is permissible.
It should be noted that there is greater room for leniency nowadays, when most Masa is made very thin, which makes it less likely to ferment, and thus the dough would need more time to become Hametz. Moreover, the process today is generally done on metal tables, which remain cold and do not become warm. Nevertheless, one should purchase one’s Masa for Pesah from factories that ensure not to leave the dough unhandled during the production process for more than 1-2 minutes.
Summary: Dough which is left unhandled for even a brief period of time could potentially become Hametz. Therefore, one should purchase one’s Masa for Pesah from factories that ensure not to leave the dough unhandled during the production process for more than 1-2 minutes.