Do You Know Why Oreo Biscuit Cream Sticks To One Wafer When Twisted Apart? ‘Oreometer’ Can Answer

MIT researchers have designed a 3D-printable "Oreometer", a simple device that firmly grasps an Oreo cookie. It uses pennies and rubber bands to control the twisting force that twists the cookie open.

New Delhi: Have you ever wondered why an Oreo cookie's cream sticks to just one wafer when twisted apart? Engineers at Massachusetts Institute of Technology (MIT) in the United States have just found the reason. 

People do not realise that when they twist open an Oreo to get to the creamy centre, they are mimicking a standard test in rheology, which is the study of how a non-Newtonian material flows when twisted, pressed, or stressed. A non-Newtonian fluid is one which changes its viscosity when the forces on it change, and other flow properties may also be affected. 

The engineers have subjected the sandwich cookie to rigorous materials tests to know why the Oreo's cream sticks to just one wafer when twisted apart.

What Is “Oreometer”?

The researchers have designed a 3D-printable "Oreometer", a simple device that firmly grasps an Oreo cookie. It uses pennies and rubber bands to control the twisting force that progressively twists the cookie open. 

The study, titled "On Oreology, the fracture and flow of 'milk's favourite cookie'", was published on April 20 in Kitchen Flows, a special issue of the journal Physics of Fluids. 

In a statement issued by MIT, Max Fan, one of the study authors, said there is the fascinating problem of trying to get the cream to distribute evenly between the two wafers, which "turns out to be really hard."

How Did The Researchers Conduct The Study?

In order to get an answer to the question, the researchers subjected the cookies to standard rheology tests in the laboratory and found that no matter the flavour or amount of stuffing, the cream at the centre of an Oreo always sticks to one wafer when twisted open, the only exception being older boxes of cookies. 

According to the study, the scientists measured the torque needed to twist open an Oreo. Torque is the twisting force that tends to cause rotation of an object. In other words, torque is the tendency of a force to rotate the object to which it is applied. 

The engineers found the torque required to twist open an Oreo to be similar to the torque required to turn a doorknob and about one-tenth of what is needed to twist open a bottle cap. 

The cream's failure stress is twice that of cream cheese and peanut butter, and about the same magnitude as mozzarella cheese. Failure stress is the force per area required to get the cream to flow or deform. 

After analysing the cream's response to stress, the researchers found its texture to be "mushy", rather than brittle, tough, or rubbery. 

Why Does The Cookie's Cream Stick To One Side Rather Than Splitting Evenly?

According to Crystal Owens, the lead author of the study, videos of the manufacturing process show that they put the first wafer down, then dispense a ball of cream onto the wafer before putting the second wafer on top. She said that apparently, that little time delay may make the cream stick better to the first wafer.

The study is not simply a sweet diversion from bread-and-butter research, the statement said. The research is also an opportunity to make the science of rheology accessible to others. 

The scientists developed the "Oreometer" to measure the rheology of Oreos and other sandwich cookies and snacks. 

What Does A Standard Rheology Test Look Like?

In a standard test in rheology, a fluid, slurry, or other flowable material is placed onto the base of an instrument known as a rheometer. A parallel plate above the base can be lowered onto the test material, following which the plate is twisted. Sensors are used to track the applied rotation and torque.

Owens regularly uses a laboratory rheometer to test fluid materials such as 3D-printable inks. She noted that rheometers have a striking similarity with sandwich cookies. 

How Are Sandwich Cookies Similar To Rheometers?

Owens wrote in the study that scientifically, sandwich cookies present a paradigmatic model of parallel plate rheometry in which a fluid sample is held between the two parallel plates. In the case of sandwich cookies, the cream is the fluid sample and the wafers are the parallel plates. When the wafers are counter-rotated, the cream deforms, flows, and ultimately fractures.

This leads to the separation of the cookie into two pieces, Owens explained.

Oreo Cream Is A Soft Fluid Which Can Start Flowing Under Stress

Oreo cream is considered a "yield stress fluid", which is a soft solid when unperturbed, and can start to flow under enough stress, the way toothpaste, frosting, certain cosmetics, and concrete do. Yield stress is the force per unit area that must be applied to a sample before it starts to flow. 

According to the statement, physicists had first reported in a 2016 Princeton University study that twisting Oreos by hand causes the cream to come off on one wafer.

The MIT engineers glued an Oreo to both the top and bottom plates of a rheometer, and applied varying degrees of torque and angular rotation, which is the amount of rotation. The scientists noted the values that successfully twisted each cookie apart. 

They put the values into equations to calculate the cream's viscoelasticity, or flowability. Viscoelasticity refers to the tendency of a material to act like both a solid and a fluid. 

For each experiment, the researchers also noted the cream's "post-mortem distribution", or where the cream ended up after twisting open. 

Different Types Of Oreos Including Dark Chocolate & Golden Wafer Flavours Analysed

The scientists went through about 20 boxes of Oreos, including regular, Double Stuf, and Mega Stuf levels of filling, and regular, dark chocolate, and "golden" wafer flavours. The team was surprised to find that no matter the amount of cream filling or flavour, the cream almost always separated onto one wafer.

Owens said that the researchers had expected an effect based on size. They thought if there was more cream between layers, it should be easier to deform. But that is not actually the case, she said.

Cookie’s Position In A Box Determines Which Side The Cream Tends To Stick

The scientists mapped each cookie's result to its original position in the box. They observed that the cream tended to stick to the inward-facing wafer. This means that cookies on the left side of the box twisted such that the cream ended up on the right wafer. On the other hand, cookies on the right side separated with cream mostly on the left wafer. 

According to the researchers, the box distribution may be a result of post-manufacturing environmental effects, such as heating or jostling. These effects may cause the cream to peel slightly away from the outer wafers, even before twisting.

The authors noted that the understanding gained from the properties of Oreo cream could be applied to the design of other complex fluid materials.

Owens said that if the inside of Oreo wafers were more textured, the cream might grip better onto both sides and split more evenly when twisted. 

She concluded that as the cookies are now, the team found there is no trick to twisting that would split the cream evenly.