The concepts of deformation, strain, stress, and equilibrium are often taught with an abstract body in 3D space, colloquially referred to as a “continuum potato”. But have you ever thought the mechanical properties of a potato?
I certainly don’t need to introduce the potato: it’s a staple of all our diets and truly one of the most versatile foods. But have you ever thought about its mechanical properties – e.g. how much force does it take to squish it and how does that change with boiling or long term storage? Well you’re in luck. Today I’ll be talking about some scientific studies done on the humble potato, including ones from NASA and the American Journal of Potato Research (yes, that’s an actual journal).
Is potato.
Let’s start simple: how strong is a potato? Well it depends, in tension or compression? Experiments have shown that raw potatoes generally fail in a brittle manner (like your ceramic mug) and their strength varies in tension and compression. A particularly fascinating study which tested I-shaped potato coupons found they were actually stronger in tension than compression, failing at a mean fracture strength of 0.45 MPa in tension vs 0.26 MPa in compression. [1]. This is odd for brittle materials as they are generally stronger in compression, but the finding may be explained by the higher Poisson ratio of potato tissue. Regardless, these fracture strengths mean potatoes are ~800-1000x less strong as aluminum and steel. What about stiffness, you ask? Well similar mechanical testing of raw potatoes (this time with cylindrical specimens) has found a Young’s modulus of around 2 MPa, almost 1e5x less stiff than steel [2].
But how does storage of potatoes (e.g. in a fridge) affect these mechanical properties? Well turns out storage can cause both the Young’s modulus and Poisson ratio to decrease, as the potato loses moisture content over time [2]. Interestingly this lower moisture content also decreases the thermal conductivity of a potato [3]:
Comparison between experimental and predicted
values of thermal conductivity of potato at 50 C [3]
And what about during cooking? When a potato is boiled, it actually undergoes a phase change that alters the structure of the potato tissue and gelatinizes it – causing it to become soft and sticky. This phase change drastically changes the mechanical properties of the potato, including its Young’s modulus as evident from these stress-strain curves [4]:
Compressive stress-strain curves for cylindrical potato specimens [4]
Another important property is a potato’s impact resistance. This property is actually useful for the farming industry, as poor handling of potatoes can cause bruises and blackening of tissue, ultimately leading to food waste and a loss of sales. One estimate puts the cost to farmers from bruising of potatoes between $10-50k annually [5]. So how impact resistant are potatoes? Well, one study found that they can survive a drop height of about 25 cm before significant bruising [5]. They also found that colder potatoes are more susceptible to damage, and adding cushioning around the potatoes helps decrease the bruising.
That’s not all…potato research remains a small but active field today. Recently someone proposed a model for the combined anisotropic and plastic deformation of a potato [6], with the ultimate goal of predicting bruise volume and helping farmers decrease food waste. They even validated their model with compression experiments, as seen in the picture below. Other studies have compared the properties of various potato species and even looked at the time-dependent deformation (i.e. creep) of potato tissue.
Compression testing of potato tissue [6]
So to summarize, the humble potato shows some interesting mechanical, thermal, and chemical properties. It has even been the subject of some interesting research, with many studies focused on understanding bruising and reducing food waste. And many of these studies have indeed modeled the potato as a continuum solid – so maybe the name “continuum potato” is a good name after all.
References/further reading:
[1] “Fracture strengths of potato tissue under compression and tension at two rates of loading,” 1995.
[2] “Physico-mechanical properties of potato tubers during cold storage,” 2009.
[3] “Thermal conductivity of potato as a function of moisture content,” 1992.
[4] “Structure, processing, and properties of potatoes,” NASA 1992.
[5] “Potato impact damage thresholds,” 1997.
[6] “A phenomenological model for the inelastic stress–strain response of a potato tuber,” 2020.
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