The Torah, “Leviticus” 2:11: No meal-offering, which ye shall bring unto the LORD, shall be made with leaven; for ye shall make no leaven, nor any honey, smoke as an offering made by fire unto the LORD.
“No meal-offering, which ye shall bring unto the LORD, shall be made with leaven” means that it is forbidden to allow our desire to receive to leaven and to rise. It has to remain in its original form.
In the corporeal world, the time for the flour to connect with the water in the process of making the Matzo should be no more than 18 minutes, so that the process of fermentation will not start.
This length of time has a spiritual root, as it is made of nine Sefirot of the Direct Light (Ohr Yashar) and nine Sefirot of the Returning Light (Ohr Hozer), which we can establish without taking the tenth Sefira into account. We cannot correct it and so we just leave it.
From KabTV’s “Secrets of the Eternal Book” 11/27/13