You order some pizza from your favorite Kosher pizza store, which has conveniently partnered with Uber Eats. The pizza gets delivered to your front door and you waste no time getting started. Half way through your third slice you stop and wonder to yourself: "How do I know that this is really the pizza I ordered? Maybe the delivery guy ate my kosher pizza and then replaced it with Domino's?"
This gets us onto the topic of chosamos (seals). Chazal decreed that when kosher food is sent through a non-Jew, the food needs to have a seal that serves as proof that the food was not tampered with. The logic is that the inconvenience and difficulty of removing the seal and then having to replace it in an identical manner will deter the delivery person from interfering with it.
The source for this halacha is from the Gemara in Avodah Zara (39a). The Gemara defines two groups of items that need to be transported with a seal: "CHaVIS" and "CHaMPaG". CHaVIS stands for "chatichas dag" (a piece of fish), "basar" (meat), "yayin" (wine) and "techeiles" (the blueish dye used on tzitzis). Some say that the first item is not chatichas dag, but rather "cholov" (milk). CHaMPaG stands for "chiltis" (a type of pungent vegetable), "muryas" (fish brine), "pas" (bread) and "gevinah" (cheese).
The halacha is that the items in CHaVIS require two seals when being transported, while CHaMPaG only requires one seal.
What distinguishes the items in the strict group CHaVIS from the more lenient group CHaMPaG? There is a three-way disagreement regarding this.
The Rashba says that the items in CHaVIS are all items that if swapped with a non-kosher equivalent, would result in you transgressing a prohibition from the Torah. Non-kosher fish or meat are both forbidden from the Torah. Yayin nesech, which is wine that was poured out in a libation for avodah zara, is forbidden from the Torah. Techeiles allows you to wear tzitzis that contain a mixture of wool and flax. Without it, you would be transgressing the Torah prohibition of sha'atnez.
Conversely, the Rashba holds that CHaMPaG represents items that are only rabbinically prohibited. We are concerned that the delivery person will swap the kosher bread with pas akum, which is only assur m'derabbanan. With regards to non-kosher cheese, the non-kosher part of it is that they would coagulate it with non-kosher rennet. Since the non-kosher element within it is only giving it structure and not flavor, this is only assur m'derabbanan.
The Tur writes that the distinction between CHaVIS and CHaMPaG is that the items in CHaVIS are all more expensive (and therefore more exclusive) than the items in CHaMPaG. Therefore the delivery person would be more tempted to help himself to the expensive items and so they require two chosamos.
The Ran writes in the name of the Yerushalmi that with CHaVIS, the prohibition is in the body of the food item itself, while with CHaMPaG, the prohibition is only from a ta'aroves (a mixture), which makes it a more lenient type of issur and therefore only needs one chosam. But didn't we just say that bread is one of the elements of CHaMPaG? Bread is not part of a mixture?! The answer is that since it is only assur m'derabbanan it is also sufficient with only one chosam.
The Shach says that one should be concerned for any one of the three opinions i.e. if an item is either d'oraisa, or expensive, or the issur is in its body, then it needs two chosamos.
In conclusion, Chazal required kosher food to be secured with a seal when sent through a non-Jew. Some foods require only one seal, some require two. Next week we will begin the Shulchan Aruch and start looking more in depth.