There is a sign in the bakery that says, "All our goods are baked on premises as yosan". Although the word "yosan" is supposed to be transliteration of a Hebrew word, it is still misspelled. The word is commonly spelled "yoshon", which in Jewish law refers to when certain types of grains are planted and take root in relation to the calendar year.
The entire movie turns on the bakery being kosher, i.e., that what is baked in the bakery be baked in accordance with Jewish religious law and hence appropriate for consumption by religiously observant Jews. At the end of the movie, Nat Dayan goes on vacation. In his absence Nat has the Moslem Ayyash Habimana run the bakery. Unfortunately, under Jewish religious law the lack of Jewish oversight to the baking process would render all of the resulting baked goods and the bakery itself non-kosher.
After Ayyash is forced to dump some pot when police come into the bakery and it gets mixed into some dough, he waits for the mix to finish and looks through the dough hoping to recover the pot. As the mixer finishes, the dough ball is on the bottom of the bowl and not wound around the dough hook attachment as it would need to be to actually get mixed.
When Nat goes to bake goods to replace those Cotton bought and planned to test for cannabis, a muffin tray was one of the first things he grabbed--even though, as Ayyash soon points out, he had no idea what Cotton bought (which was muffins).
When Nat and Ayyash go to switch out the pot-laced goods Cotton bought, they accidentally knock the old and new muffins together and don't know which are the new ones they ought to leave. Yet they don't really need to switch them out--they can simply steal them, and deprive Cotton of goods to test for pot. Of course, having the goods come back clean helps them more, but it's not worth the risk.
Mr. Dayan says the bakery has been in the family for over a century, but the sign in the shop says since 1946.