Mashed Potato Volcano

November 30st, 2020

What are mashed potatoes?

Mashed potatoes are one of the most self-explanatory dishes that I’ve encountered in my life, and certainly the most self-explanatory of what I’ve covered so far, though there are some nuances. The different ways of prepared mashed potatoes all involve cooking the potatoes, then mashing them into fine pieces in the presence of some fluid that lubricates the pieces and causes them to adhere to each other. There are generally 2 categories of potato based on their texture after cooking. Mealy varieties like Russet potatoes tend to absorb a lot of water and the plant cells that compose the potato can be separated from each other relatively easily after cooking. Waxy varieties like “new” potatoes (ones harvested earlier in their lifecycle) retain their shape even after cooking, because of this, waxy types are generally used for dishes like salads and gratins. Because the plant cells readily fall apart into small aggregates, mealy types are generally more common for mashed potato dishes. Waxy types are still used in some recipes but requires much more mashing to obtain a fine texture. One exception to this that I saw were many recipes calling specifically for Yukon Gold potatoes which reportedly have a texture intermediate to mealy and waxy.

From a material structure perspective, at a basic level, mashed potatoes seem to be a granular suspension. The granules are the aggregates of plant cells that jam against each other, lubricated by some interstitial water or oil, usually in the form of butter or cream. Rheologically speaking, the jammed granules provide the structure necessary for the material to have a yield stress and hold its shape. In preparing the dish, I was curious about how the interstitial fluid would affect the properties and so I did some experiments.

Making the mashed potatoes

Recipes for mashed potato dishes cover a wide range of complexity; I chose to go with a very simple two-ingredient mashed potato to make it easier for me to try and determine the effects of the ingredients and their processing. In the field of rheology, we often analyze the mechanical behavior of “model materials” which display all the relevant phenomena we care about (like a yield stress, for example), but are simple enough that we can begin to understand the role that one ingredient or preparation step might be playing.

To start, my wife and I peeled nearly a kilogram of potatoes and submerged them in cold water before bringing it to a boil. As was called for in the recipe, we went with mealy-type Russet potatoes. The cold water, and thus longer boiling time, was emphasized in a few recipes I took a look at; my presumption is that in order for the potato cells to separate effectively, ample time must be given for the absorption of water. After allowing the potatoes to simmer for a while, they indeed became quite soft and easy to pierce and separate.

I began by coarsely mashing the potatoes using a spoon, then I used a wire whisk attached to a hand blender to produce finer particulates while I gradually added in some of the water I had reserved when draining the potatoes. This process reminded me of some material preparation protocols like for emulsions of oil droplets in water; oftentimes in the lab we will begin with a coarse mixing process just to disperse a fluid or solid particles in another fluid, then follow it up with a “homogenizer” tool which produces a finer, more uniformly sized dispersion.

After a few minutes of blending, I was satisfied with the uniformity of the texture of the potatoes. Here I separated out a small portion of the mash to try and understand the impact of the second of the two ingredients in this recipe, sour cream—itself a yield-stress fluid. I made sure to mix in the sour cream gently so as not to further break down the potato particles which would affect the texture of the mixture.

In terms of mouthfeel, the mashed potatoes without sour cream felt noticeably softer, though they both seemed to have a yield stress of around 75 to 100 Pascals. This was a little unexpected to me since I thought that the increased interstitial fluid would mean that the concentration and packing of potato particles would be reduced. Instead, it seems likely that the yield stress and highly viscous flow behavior of the sour cream gives it a stiffer sensation.

Mashed potatoes aren’t just for eating though, a time-honored tradition is using them to mold all sorts of structures and works of art. I was curious about how moldable the two different varieties I had made were and decided to play around with them in my hands. As you can see from the videos, the sour cream seems to significantly affect the surface properties of the material, making it much stickier and a more difficult to effectively shape; the sans sour cream mash on the other hand is relatively easy to shape and roll into a ball, but its a bit more brittle and crumbly. There are thus some trade offs in which material is better for making sculptures. The brittle nature of the material without sour cream probably makes it more difficult to rectify any mistakes you would make in molding, like if the structure broke. For the material with sour cream though, it was nearly impossible for me to make nice smooth surfaces even using the back of a metal spoon.

For my art, I decided to go with a classic mashed potato volcano and I used the sour cream-laden potatoes. To my meal I added some local sambal chili paste to the volcanic crater to complete the look, while my wife preferred to have some melted butter which overflowed down the side. Both were pretty tasty and I recommend this very simple recipe for some quick, rheologically interesting food/art.