Mechanical engineers have put an Oreo’s cream filling through a number of tests to better understand rheology and determineimage018 what happens when two wafers are twisted apart.

Over the last year or so, MIT engineers decided to object the sandwich cookie through rigorous materials tests to get to the centre of a tantalising question: Why does the cookie’s cream stick to just one wafer when twisted apart?

When you twist open an Oreo cookie to get to the creamy centre, you’re mimicking a standard test in rheology – the study of how a non-Newtonian material flows when twisted, pressed, or otherwise stressed.

In pursuit of an answer, the team subjected cookies to standard rheology tests in the lab and found that no matter the flavour or amount of stuffing, the cream at the centre of an Oreo almost always sticks to one wafer when twisted open. Only for older boxes of cookies does the cream sometimes separate more evenly between both wafers.

The researchers also measured the torque required to twist open an Oreo, and found it to be similar to the torque required to turn a doorknob and about 1/10th what’s needed to twist open a bottlecap. The cream’s failure stress — i.e. the force per area required to get the cream to flow, or deform — is twice that of cream cheese and peanut butter, and about the same magnitude as mozzarella cheese. Judging from the cream’s response to stress, the team classifies its texture as “mushy,” rather than brittle, tough, or rubbery.

So, why does the cookie’s cream glom to one side rather than splitting evenly between both? The manufacturing process may be to blame.

Videos of the manufacturing process show that they put the first wafer down, then dispense a ball of cream onto that wafer before putting the second wafer on top. Apparently that little time delay may make the cream stick better to the first wafer.

In an experiment that they would repeat for multiple cookies of various fillings and flavors, the researchers glued an Oreo to both the top and bottom plates of a rheometer and applied varying degrees of torque and angular rotation, noting the values that successfully twisted each cookie apart. They plugged the measurements into equations to calculate the cream’s viscoelasticity, or flowability. For each experiment, they also noted the cream’s “post-mortem distribution,” or where the cream ended up after twisting open.

In all, the team went through about 20 boxes of Oreos, including regular, Double Stuf, and Mega Stuf levels of filling, and regular, dark chocolate, and “golden” wafer flavors. Surprisingly, they found that no matter the amount of cream filling or flavor, the cream almost always separated onto one wafer.

While Oreo cream may not appear to possess fluid-like properties, it is considered a “yield stress fluid” — a soft solid when unperturbed that can start to flow under enough stress, the way toothpaste, frosting, certain cosmetics, and concrete do.

The team’s study isn’t simply a sweet diversion from bread-and-butter research, it’s also an opportunity to make the science of rheology accessible to others.

To that end, the researchers have designed a 3D-printable “Oreometer” – a simple device that firmly grasps an Oreo cookie and uses pennies and rubber bands to control the twisting force that progressively twists the cookie open. Instructions for the tabletop device can be found here.

In addition, the understanding gained from the properties of Oreo cream could potentially be applied to the design of other complex fluid materials.

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Materials provided by Massachusetts Institute of Technology. Original written by Jennifer Chu.

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