Oreo analysis offers a lesson in rheology and fluid flow behavior

April 25, 2022 -- When you twist open an Oreo sandwich cookie, you're actually imitating a test of basic rheology that has implications for understanding and improving the texture of different food products. Lessons from analyzing the popular cookie could also be used to optimize foams, cosmetics, biological gels, screen printing, and viscoelastic inks used in 3D printing processes, according to research published on April 19 in AIP Physics of Fluids.

In the study, investigators from the Massachusetts Institute of Technology (MIT), led by Crystal Owens, a doctoral candidate in the department of mechanical engineering, characterized the flow and fracture of Oreos. In doing so, they found that cream, which failure mechanics characterizes as mushy in rheological texture, tends to stick to one side of the cookie.

Laboratory rheometers are typically used to measure torque. However, because such a device wasn't accessible, the investigators built a 3D-printed torsion testing device that was powered by rubber bands and coins, and designed for Oreos and similarly sized round objects. The device -- which they called an Oreometer -- requires no power or electronics.

In the Oreometer, a sandwich cookie was placed between two clamps. Rubber bands on the clamps adjusted torque on the wafers. Pennies were added to a chamber on one side of the device, causing the clamp to rotate, separating the cookie.

After the filling failed and the cookie broke apart, they visually quantified the amount of cream on each wafer. They found that the cream almost always came off one side of the cookie sandwich.

The authors also examined how milk, cookie flavor, amount of filling, and rotation rate would affect the final distribution of cream in the cookie. When they were dipped in milk, the cookies crumbled after about 60 seconds. Flavor and filling seemed to have little effect on cookie mechanics. Breaking the cookies apart cleanly depended on the rotation rate.

The investigators theorized that the cream may stick consistently to one side of the cookie because of the way the cookies are manufactured and oriented when they are packaged.

Understanding the flow behavior of food ingredients and products has widespread impact on economies of production, public health, and our individual diets, the engineers wrote. Oreos are not commonly considered to be a fluid. But Oreo cream is a member of the class of flowable soft solids known as yield stress fluids. These fluids act as soft solids when undisturbed and only flow under a sufficiently large applied stress.

Yield stress fluids include a variety of food materials, such as cookie dough, frosting, ice cream, peanut butter, guacamole, and ketchup. They also include foams, cosmetics, biological gels, screen printing and robocasting inks, drilling muds, concrete, and mortar.

Sandwich cookie separation is just one aspect of fluid failure behavior, which is a broader topic within rheology. The study of failure of such amorphous colloidal soft matter, and particularly, stress overshoot phenomena and related shear banding that occur during strain-controlled failure, represent an active area of research within soft glassy rheology in an effort to understand the underlying physics.

Moreover, cookies present an "enriching" abundance of topics for scientific study. For example, edge fracture is a persistent challenge for parallel plate rheometry wherein normal stress differences build up by applying torsional shearing strain, causing the fluid meniscus to deform and propagate a radial crack near the outer edge, as may happen to initiate the failure of Oreo cookies, the researchers theorized.

This effect has been harnessed to rapidly pinch off threads of viscoelastic inks during 3D printing processes, which otherwise would lead to stringing defects during printing. In addition, complex failure mechanisms can occur with torsional failure of thin disks, especially after such edge fracture surfaces have formed.

The researchers would like to encourage home-based studies that might contribute new discoveries to their exercise in rheology, analyzing how matter flows. The engineers indicate that other experiments could be performed involving temperature control, using the Oreometer on cookies quickly withdrawn from the fridge, microwaved slightly, or dipped immediately beforehand in milk.

The work at MIT may also help us understand other torsional events in the kitchen, such as braided breads, mixing dough, and even the ideal opening kinematics of stubborn jam jar lids.

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