Wouldn't any pepperoni curl when cooked?
No, there are 2 kinds of casing options:
1. Artificial (melts out during cooking, ex. vegetarian casings made from glycerin, also good for Halal/Kosher applications) or no casing (ex. comes in a fibrous cellulose casing, which gets removed first)
2. Natural animal intestines (ex. pork), or collagen, which is easier to work with (it gets a bit more nuanced because collagen is
considered an artificial casing, despite being animal-derived, and can be edible or non-edible)
Pepperoni was created just over 100 years ago in 1919! Pepperoni with artificial or no casing tends to either get a bit wavy or stay flat, the latter of which is good for larger pepperonis. I like chorizo for flat & salami for large, however.
Natural & collagen casings create the "cup & curl" effect that bows up the pepperoni, chars the top, and keeps the pepperoni oil in the middle. It's called "cupping" pepperoni or "Old World" pepperoni. This is common in tomato-paste-based Buffalo-style pizza with the "roni cups", which got popular during WWII. The
Ezzo Sausage company started a resurgence of the style about 10 years ago & Hormel made it available at local supermarkets more recently.
Kenji gets into
the science of it. It gets weird because the first thing that matters for curling pepperoni is getting it stuffed in a natural or collagen casing, but the casing can stay on OR get removed before cooking and it will STILL curl! Within the roni cup world, cupping pepperoni can be Cup & Crisp (golden-brown edges on the left below) or Cup & Char (caramelized black edges on the right below). Personally I like it more burnt on top! Not everyone likes the grease tho:
Here's a good short video on cupping vs. lay-flat pepperoni. Both are good, just depends on what style you are in the mood for!
10-minute video on how it's made: