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This branch of physics explains why ketchup’s hard to pour

When Henry J. Heinz started selling ketchup in bottles in 1876, he chose glass to show his product was pure and uncontaminated. However, Heinz created an unintended consequence—how to get the condiment out of the small opening of the bottle?

The first thing to know about ketchup is that it actually doesn’t want to move onto your burger, says Anthony Stickland of the University of Melbourne’s School of Engineering. “If you tilt a bottle of water, the water flows out because it is a liquid. But tomato sauce prefers to be in the bottle because it is technically a solid, not a liquid.”

Like toothpaste and paint, ketchup is a “soft solid” or “yield stress fluid” that only moves when the right amount of force is applied. The sauce is actually a suspension of pulverized tomato solids in a liquid. The solids are touching each other to create a continuous network, which gives the sauce a physical strength that resists motion. You need to overcome this strength in order to get it to flow.

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Heinz scientists have figured out that the optimal flow of ketchup is roughly 0.028 miles per hour. You need to apply the right amount of force to get it to flow at this speed, which is why the tap or whack on the sauce bottle is actually necessary.

But it’s what’s happening inside ketchup that is most fascinating to Stickland and other scientists in the field of rheology. Rheology is the study of these soft solids or yield stress fluids, including many common condiments, as well as paint and industrial applications like sewage treatment and mining.

Foods often have exotic and complicated rheology in order to get the right texture, for example mayonnaise and chocolate. Concentrated suspensions do not obey Sir Isaac Newton’s Law of viscosity, published as part of Newton’s 1687 Laws of Motion, in his Principia Mathematica Philosophiae Naturalis.

The Law of viscosity states that a fluid flows at a speed proportional to the force that is applied, where the constant of proportionality is the viscosity. Because ketchup does not obey this Law, it is known as a non-Newtonian fluid.

Suspension viscosity is infinite at low forces, but once it yields to a force and starts to flow, the viscosity decreases the faster it flows. This is called “shear thinning.” At very high velocities, the suspension viscosity can be similar to water.

“Suspension rheology explains all the phenomena seen in [ketchup] bottles and provides the answers to the perennial sauce question, which can be tackled in three main steps,” says Stickland.

1. Shake

“Always start by giving the sauce a good shake,” says Stickland. “You need to overcome the yield stress to mix it, so it needs a decent oomph—briefly invoke your inner paint shaker. Remember to keep the lid on, of course.”

There is a chance the solid particles in the sauce may have segregated or settled, leaving watery sauce at the top, or some sauce in the neck of the bottle may have dried out and created a plug. In either case, you need the shake to spread these solid particles in the sauce evenly, known as re-homogenisation.

2. Invert the bottle

“If there’s not much left in the bottle, you may need a strong whack to dislodge tomato sauce from the bottom. Turn the bottle upside down (still with the lid on) and thrust downward at high speeds, accelerating both the ketchup and the bottle. Swiftly stopping the bottle should slump the sauce into the neck.”

3. Tilt and pour

“There needs to be some force to overcome the yield stress, but not too much—the last thing you want is sauce running down your hand or, in the paint analogy, paint running down the walls before it dries,” says Stickland.

“The amount of force depends on how much is left in the bottle. A full bottle will have the weight of the sauce pushing down whenever the bottle is tilted, whereas a nearly empty bottle will need some help.

“With the bottle upright, remove the lid and start to tilt the bottle, aiming at the desired location. Increase the angle until sauce begins to flow. If it does not flow once the bottle is almost upside down, there is not enough weight of sauce left to induce flow.

“This is where some sort of encouragement, like tapping, slapping, or whacking, is required. But remember this sauce is shear thinning, so it flows more easily the greater the ‘encouragement.’ This explains why sauce explosions can happen. The trick is to get the sauce flowing, but not too fast.

“You need to find the ‘sweet spot’ of force needed to move it towards your burger. Start by pointing the open end of the bottle toward your food at an angle of around 45 degrees with one hand around the bottleneck, and the other delivering gentle but firm taps on the bottom of the bottle. Increase the force of the taps until you balance the force applied with the mechanical strength of the sauce in order to get it to flow.”

Source: University of Melbourne

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