What’s the most sustainable skillet?
Many years ago, when I began my work as a food physicist, I became intrigued with the range of choices available when buying a new skillet. Just check out the skillet isle next time you’re in a store with a kitchen section - it’s impressive! From the tried-and-true cast iron, raw or enameled to stainless all-clad to the latest “eco” non-stick, to anodized aluminum & solid copper. How is one to know the best option??
A good skillet is a critical component of any kitchen. And skillets are unique kitchen tools in that they are typically “thermally dominant.” In other words, when cooking things in a skillet there is typically a low food-to-pan ratio by mass and so the skillet material becomes very important to dictate the performance and the cooking experience. In contrast, when cooking in a stock pot, there is typically a very high food-to-pan ratio by mass and so the material of the pan is less important, the food (usually water) dominates the heat transfer and thermodynamic behavior.
What’s your “go-to” skillet?
To help inform my quest for the best skillet, I turned to the experts. I started asking chefs and people I knew who were passionate about cooking to describe their favorite skillet. Overwhelmingly, people who loved to cook LOVED cast iron skillets. So – as a physicist, I went to the thermodynamic property tables (‘cause I’m a geek) to find the source of this passion, but became even more confused. When comparing thermal conductivity of commonly available skillet materials, Iron is one of the lowest. How quickly the material moves heat (thermal conductivity) is a good thing for a skillet, right?
To further my curiosity, others who overheard our conversation would comment that they tried cast iron but became frustrated with it because things tended to burn. Well, as a scientist trying to improve the energy efficiency of our cooking, my immediate thought was that this was great news. If things are burning, the burner simply needs to be turned DOWN. You must need to use less energy as compared to whatever skillet you were used to cooking on before converting to Cast Iron to prevent burns. But again – how could this be since the conductivity was low in comparison to cast iron’s competitors?
The super-woman physics tool … Infrared Thermography
This is when I decided to employ physics “super woman” powers….The infrared camera. Thanks to Appalachian State University, I had access to the diagnostic tool that allows us to “see” heat by converting the emitted wavelength to a color. Different temperature objects emit radiation at a different wavelength (hot objects with a shorter wavelength than colder objects). IR cameras allow us to “see” a temperature profile of objects. Teaming up with my material scientist husband, we tested 8 different skillets and studied their temperature profiles through the thermal transients of heating and cooling.
If you want to seriously geek out and read all the research details [it’s actually very cool (well, really hot) stuff!] It can be found here. https://doi.org/10.18848/2160-1933/CGP/v10i01/1-17) .
If you want the “Cliff Notes” – here’s the summary….
This study sought to test 8 different skillet materials to determine the most sustainable option based on thermal and non-thermal considerations. Here’s a photo of the 8 we chose:
What impacts the heat distribution?
It ends up that there are 3 thermal variables that are particularly important to the performance of a skillet:
· Thermal Conductivity – While it seems that the best skillets would have the highest thermal conductivity (ability to move heat via conduction), the IR images helped us to notice that a lower thermal conductivity (cast iron) was slow to transport the heat from the bottom of the skillet, where the burner was located, up the walls of the skillet. When cooking in a skillet, the temperature of the walls does not impact the cooking experience – they’re mainly there to contain the food, not to actually transfer heat. So, a skillet with high conductivity, such as solid anodized aluminum (like Calphalon), for example, is great at creating a homogeneous temperature from edge to edge, but this results in wasted energy since the edges aren’t cooking, they are simply radiating to the room. The lower conductivity of cast iron kept the heat where the food it – at the bottom of the skillet. This improves its energy efficiency.
· Heat Capacity – Heat Capacity is a combination of the mass of the skillet and the specific heat of the material. The heat capacity is a measure of a material’s “thermal sluggishness”. The higher the value, the more heat must be added (from the burner) to raise its temperature. The flip side of this is that the higher the heat capacity, the longer the skillet will take to cool down when the heat input is removed. Skillets with a high heat capacity are thermally resilient, which allows you to turn off the burner before you’re actually done cooking and let the skillet “passively” complete the cooking. Of the skillets we chose, Cast Iron and Anodized Aluminum were the highest. These images show that after 3 minutes of cooling from an initial 450F temperature, the anodized aluminum has a slightly higher temperature, but they are both still able to “cook” without additional heat input at this point.
· Emissivity – Lastly – the ability of a surface to emit radiation (emissivity) is also important. If there are any gaps between the surface of the skillet and the food you’re cooking, the skillet can no longer “conduct” to transfer heat. Through an air gap we rely on another mode of heat transfer called radiation. Some surfaces are great at emitting heat via radiation (they have a high emissivity) and some surfaces are terrible (low emissivity). Pans with a low emissivity are typically good reflectors and terrible emitters. Again - cast iron, with its near-perfect black, high emissivity surface was a clear winner and is able to “reach” food to cook it through an air gap. It’s shiny counterparts like stainless steel and solid copper weren’t so lucky. Stay tuned for more research in this area…
What other factors impact skillet performance and sustainability?
There are also several non-thermal factors that are worthy of noting when comparing skillet materials from a sustainability perspective.
· Longevity – While a non-stick skillet is impressively non-stick out of the box, it is typically fairly useless in 5-10 years after the nonstick surface has degraded and sent to the landfill. Cast iron and carbon steel pans are passed down for generations! Annodized Aluminum are more resilient than nonstick but with use, the anodized surface degrades and this is not something that can be “fixed” at home.
· Toxicity – Cast Iron skillets develop a natural non-stick surface with proper seasoning. There is no concern about toxic non-stick material fumes or questionable reactivity between the metal and our food as there is with aluminum, and non-stick surfaces.
· Embodied Energy – When comparing the resources necessary to mine the virgin metals and form them in to a pan, cast iron was the lowest in our study for energy and water consumption, having the lowest manufacturing carbon footprint. Anodized aluminum was the highest carbon footprint, almost twice as much as any other.
· Cost – At under $20 for a 10-inch skillet, cast iron was the least expensive material we tested. Compare this to its $200 copper skillet of the same size!
So, there you have it. It ends up there is good, solid science to back up this collective passion for a cast iron skillet. The only valid draw-back to cast iron is its weight. While this is a thermal advantage, increasing its above-mentioned heat capacity, it can be tough to toss around. Ten-inch or smaller pans are not a problem but for pans 12-inch diameters and greater, this can be problematic. Hey – no one’s perfect!