Rheology and the Consistency of Ketchup

Everyone is familiar with ketchup, the tomato sauce which is often eaten with fries, sausages, hamburgers and pasta dishes. Besides the taste, the flow behavior of ketchup is also important for both consumers and manufacturers.

A classic ketchup mainly consists of tomatoes, vinegar, salt, and a number of different herbs. During the manufacturing process the ketchup should be easy to mix and easy to fill into bottles. Consumers want ketchup which flows easily out of the bottle but then stays on top of the hot french fries, i.e. does not run off the food or the plate.

It is the yield point which is responsible for the behavior of ketchup when we squeeze the bottle and ketchup flows out. It is assumed that the ketchup is in a solid state until the yield point is reached and then it begins to flow. As soon as the pressure in the bottle drops, the ketchup returns to its previous solid state and remains in the bottle.

To simulate the flow behavior and regeneration behavior after squeezing out the ketchup a rheological test called a step test is used. This test has three phases. The first test phase describes the behavior at rest. The second phase shows the structural decomposition and the third phase describes the structural regeneration.

Step test as a rotational test in all three measurement -intervals
Fig. 1: Step test as a rotational test in all three measurement intervals

Depending on requirements the step test can be carried out with a ball-bearing rheometer as a rotational test with set shear rate or with a more sensitive air-bearing rheometer carried out as an oscillatory test with set deformation and angular frequency.

In rotational tests a constant and very low shear load is set in the first test interval (behavior at rest) and third interval (regeneration). In the second interval (shear load) a constant high shear load is applied in order to simulate the shearing process which occurs when the ketchup is squeezed out of the bottle or filled into the bottle.

Fig. 2: Comparison of the structural regeneration of two types of ketchup (rotation)
Fig. 2: Comparison of the structural regeneration of two types of ketchup (rotation)

After shear load, Ketchup 1 clearly shows a complete structural regeneration quicker than Ketchup 2, despite Ketchup 1 having a lower viscosity in the second phase. Ketchup 2 shows a considerably slower structural regeneration, which is not yet complete even after the end of the measurement duration of 600 s.

In oscillatory tests the first and third phases have a constant set angular frequency and a consistent deformation within the linear-viscoelastic range. The second interval is carried out with rotation at a constant shear rate in order to simulate the shearing process.

Strukturaufbau Oszillation
Fig. 3: Comparison of the structural regeneration of two types of ketchup (oscillation)

Despite its high viscosity at rest, Ketchup 1 has a lower viscosity under shearing (pouring out of the bottle) than Ketchup 2. The structural regeneration of Ketchup 1 takes longer than the time required by Ketchup 2. This means that Ketchup 1 has a longer time to flow which results in a layer thickness which is thinner than that of Ketchup 2.

The step test in rotation or oscillation used on a quickly adjusting MCR rheometer with a highly dynamic measuring drive provides the capability to determine the time-dependent structural regeneration of ketchup. This is quickest and easiest on one of the two newly developed rheometers: MCR 72 and MCR 92. Both models provide the highest accuracy combined with simple operation.

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