A Hamburger Today

The Food Lab: Why Does Pepperoni Curl?

It's time for another round of The Food Lab. Got a suggestion for an upcoming topic? Email Kenji here, and he'll do his best to answer your queries in a future post. Become a fan of The Food Lab on Facebook or follow it on Twitter for play-by-plays on future kitchen tests and recipe experiments.

20121211-pepperoni-curl-pizza-lab-01.jpg

[Photographs: J. Kenji Lopez-Alt]

Today's installment of The Pizza Lab presents what is probably the most important work of my career. Nay, my life. It's a story of such unparalleled importance that it makes pressing international issues like comparative baking surfaces and cold fermentation seem trivial in comparison. I'm talking about pepperoni curl. What it is, what makes it happen, and how to maximize it.

It's far more fascinating than you may think.

There are times when I'll head into a bog-standard New York slice joint, see those pre-cooked squares with their flat disks of pepperoni, watch some poor sap order them, and think to myself: Ah, you've fallen victim to one of the two classic blunders, the most famous of which is "never question your pizza toppings in Asia," but only slightly less well known is this: "Never order a Sicilian when you spy flat-laying pepperoni on the line."

To me, flat pepperoni just doesn't fit the bill. It sits there, wan and pliable, its grease spreading over the top of semi-coagulated cheese like an oil spill, dripping off the edges of a slice, making the whole endeavor so treacherous that some folks even resort to blotting with paper napkins.

On the other hand, what you should be having is this:

20121129-Prince-Street-Pizza-1.jpg

As the pizza bakes, the edge of the pepperoni curls upwards, forming a distinct lip. Once exposed like this, the lip cooks faster than the base, which is insulated by the cheese and crust, and thus crisps and renders its fat faster. This fat drips down into the center of the cup. What you're left with is a gloriously flavorful little sip of pepperoni grease, neatly contained within its own, crisp-lipped edible container. The browned lip takes on an almost bacon-like quality—melt-in-your-mouth crunchy. It's one of the true joys of a pepperoni pizza, and once you've experienced it, plain old flat pepperoni just won't do.

I first read the term "grease chalice" in the winning entry of a "pie-ku" contest run by Adam Kuban way back in 2005, before Serious Eats even existed. The winning entry:

crisp pepperoni

edge curled from heat

a chalice of sweet, hot oil

Mr. Sin

Have you ever read anything so beautiful or profound?

A few months back, Serious Eats Community Member ratbuddy charged us with reporting on what makes pepperoni cup when you cook it.

A number of theories were thrown around in that thread, and after talking to a couple of experts, I've heard a few more. The question it, which one is correct?

For the past few months, I've been meticulously testing various types, brands, styles, thicknesses, orientations, configurations, amalgamations, and fibrillations of pepperoni in order to figure it out. Here's what I got:

The Theory Of The Curl

There are a few basic hypotheses that try to explain pepperoni curl. Two seem intuitive, while others require a bit more specialized knowledge. The first two are:

These both seem like very valid, and luckily, very testable hypotheses, so we'll start with them before moving on.

Hypothesis #1: The Thickness

To test this, I cooked slices of pepperoni on a pizza in varying thicknesses. I used a Natural casing Boar's Head pepperoni stick, hand sliced for this test.

20121211-pepperoni-curl-pizza-lab-02.jpg

I used calipers to precisely measure the pepperoni slices, testing 8 different thicknesses that ranged from .25 inches (6.4 millimeters), all the way down to .05 inches (1.3 millimeters)

20121211-pepperoni-curl-pizza-lab-03.jpg

I placed them all on top of a single pie I made using my Basic New York-Style Pizza Dough with a simple sauce and dry mozzarella cheese, then slipped it on top of a pre-heated baking steel.

20121211-pepperoni-curl-pizza-lab-05.jpg

5 minutes later, I had my results.

20121211-pepperoni-curl-pizza-lab-06.jpg

The first thing you'll notice is that the thinnest slice actually got swallowed whole by the cheese. Oops.

I measured the amount of cuppage on each slice visually, and by measuring the height to which the highest point of the cup curled up beyond the interior base of the cup, which would correspond to the original height of the cup edge.

Turns out that thickness does have an effect on pepperoni curl, but not all that much. The very thinnest slices showed a conservative amount of curling, not really picking up until the .1 inch (2.5 millimeter) range, but after that, cuppage was excellent all the way until we got to the very thickest slice, which was simply too bulky and thick to be able to curl properly. You could see it trying, but failing:

20121211-pepperoni-curl-pizza-lab-07.jpg

While the test may answer a few questions, I've certainly seen slices of pepperoni in that thickness range that don't do any curling at all, so there has to be more to the story than just thickness.

Let's move on.*

*For an interesting computer simulation visualization of this model, please jump to the bottom of this post!

corollary to hypothesis #1: heat source direction

Part of the thickness hypothesis posits that a directional heat source is required. That is, the pepperoni curls only when heated from one side, and it curls in the direction of the heat source. To confirm this, I did another quick test, frying six slices of pepperoni in a skillet (they were placed in different orientations so that some faced up while others faced down relative to each other).

20121211-pepperoni-curl-pizza-lab-17.jpg

As expected, they all curled downwards (towards the heat source), confirming that there is something to the first hypothesis after all.

Hypothesis #2: The Casing

I've seen pepperoni sold in three forms: natural casing, collagen casing, and casing-free. To get a bit more info on these style, I spoke with Eric Cherryholmes of the Ezzo Sausage Company of Columbus OH, one of the finest pepperoni makers and wholesale distributors around (their product is not available retail, but you've probably had it on a pizza before).

According to Eric, the cupping has everything to do with the casing. As he said to me, "our Classic pepperoni is stuffed in a fibrous casing that gets stripped before slicing and lays flat when cooked. The GiAntonio [their brand name] is stuffed in collagen casing and gets sliced in its casing and applied. The casing shrinks when cooked, causing the cupping of the product."

I had Eric send me a few sticks of his pepperoni (and man, was it tasty!), asking him to leave all of the casings—including the fiber casing—intact so that I could get a look at them.

20121211-pepperoni-curl-pizza-lab-16.jpg

Next I cooked them side-by-side on a pizza. Indeed, the collagen-casing sticks do shrink more than the fiber casing sticks, which tend to lay completely flat, even limping a bit to conform to the contours of the crust and cheese.

But the question I had was this: Is it specifically the casing that shrinks, causing it to cup, or is it perhaps something to do with the nature of a sausage that's already been stuffed inside a natural casing? In other words, once the pepperoni is stuffed and cured, does the casing make any difference at all?

20121211-pepperoni-curl-pizza-lab-10.jpg

I peeled the casings off of an ezzo stick, as well as adding in a stick from Boar's Head, which uses a natural pork casing. I baked them all side by side with slices that still had their casings intact.

Guess what? Every single slice of pepperoni curled, regardless of whether or not the casing was left on.

So that's interesting. You need to make your pepperoni with a natural or collagen casing to get it to curl, but once it's been stuffed, that casing no longer plays a role. What the heck? What's special about that casing?

My next clue came from our very own Community Member Meat guy, who, if you've been around these parts, is a great authority on all things sausage and meat related, having spent his life in the field. According to him:

The meat, if stuffed using a smaller than desired stuffing horn for the casing, (casings that are in sticks generally the horn is about 1/3 of the diameter of the casing) and it is held about 1 inch back from the end of the horn. This causes the meat to flow into the casing in a U shape, so when you slice the meat, that is the pattern that is reinforced as it cooks and shrinks,causing the cup to form. When I worked for another major Pepperoni producer, our cup proof pepperoni was hand stuffed using casings that were as close to the diameter of the stuffing horn a possible creating a straight line flow and no discernible flow pattern, and the product never cupped. Other companies have cuts and holes drilled into the end of the horn to change the dynamics.

A-ha!

NB: Perhaps even more interesting than the statement above is the following: "the reason chains like it cup proof is so they don't have some yahoo suing them because they burned their mouth on the 450 Degree grease pocket in the pepperoni cup. Sort of like spilling coffee in your lap." What has society come to that we live in a world where the joy of cupped pepperoni is trumped by the fear of litigation?!

Hypothesis #3: The Fluid Dynamics of Stuffing

I called up Eric at Ezzo to inquire about Meat guy's statement, and he confirmed. "When we stuff our collagen casing pepperoni, it doesn't stretch as much, so the meat is forced down the center and sticks more to the sides. The fiber casing stretches as you stuff it, so you get an even stuffing density."

I took a length of collagen casing pepperoni generously donated by Vermont Smoke and Cure*, and sliced it in half lengthwise. According to Meat guy, I should be able to see a U-shaped meat pattern inside.

*My new favorite brand

20121211-pepperoni-curl-pizza-lab-18.jpg

And there it is: you can clearly see a U-shaped pattern in the meat and fat striations (in the picture above, the U dips downwards in the center of the sausage), as opposed to a fiber casing pepperoni, which shows a more homogenous mixture:

20121211-pepperoni-curl-pizza-lab-19.jpg

Could this be the answer I was looking for? I still had a couple of doubts, the main one being this: if the U-shaped flow pattern of meat and fat in a stick of pepperoni is what causes pepperoni to curl, how come the curl of pepperoni is not directional? That is, if we randomly place a bunch of slices of pepperoni on a pizza and bake it, shouldn't some of the slices curl up, and others curl down if the curl is based on meat flow patterns?

Or perhaps it would work that way, except for the fact that the heat differential discussed in hypothesis #1 ends up overriding its natural tendency to curl in one direction or another.

To test this, I placed slices of pepperoni on top of a paper towel-lined plate and simply microwaved them. Microwaves heat via charged electromagnetic waves that cause water molecules to vibrate. The waves can penetrate through a few millimeters of meat fairly easily, so microwaved slices of pepperoni will cook evenly throughout their volume and thus should curl in the direction they're naturally inclined to curl in, with no heat differential to get in the way.

20121211-pepperoni-curl-pizza-lab-14.jpg

On the plate above, slices marked with an X are facing down, while slices marked with an O are facing up. I microwaved the plate for 30 seconds.

20121211-pepperoni-curl-pizza-lab-15.jpg

Lo and behold, the pepperoni does curl differentially! I repeated the test several more times. 48 slices of microwaved pepperoni and a shot of pepto-bismol later, I noted that every single one curled in the predicted direction, indicating that there is a good degree of truth in Hypothesis #3 as well, though heat differential overrides curl direction.

Corollary to Hypothesis #3: Drying and Density

There's another factor that should be considered when talking about cured meats: they are dried after stuffing. Since cured meat products all dry out from the outside inward, the outer layers of the stick should be dryer, and therefore denser than the center of the stick. Eventually, after the stick leaves the curing room and gets stored in a moisture-sealed plastic package for transport and storage, this moisture level will even out to a degree. I confirmed this by meticulously punching out the centers from 50 slices of pepperoni and comparing the density of the centers to that of the edges. They were virtually identical.

However, not all moisture is moisture, as it were. Moisture that is contained within a well-emulsified sausage is bound by protein, making it difficult to escape. The moisture that migrates to the outer layers of a cured sausage during storage, however, is not bound as tightly. So upon cooking a slice of pepperoni, even though the relative density of the center and edges may be identical to start, moisture will evaporate from the edges faster than from the center, causing those edges to shrink, like a belt being cinched around your waist.

Again, microwaving the separated edges of the slices side by side with the centers of the slices and measuring their relative weight loss confirmed this.

This final factor on its own is not enough to cause significant curling—otherwise we'd see the fiber casing pepperoni curling as well—but it certainly exacerbates a slice that is already naturally inclined to curl.

Final Analysis and Conclusion

20121211-pepperoni-curl-pizza-lab-13.jpg

So where do we stand in the final analysis? We know of three factors that definitely affect curling: