Question

In food processing, tomato ketchup exhibits which type of non-Newtonian flow behaviour?

• A. Dilatant
• B. Bingham plastic
• C. Pseudoplastic (shear-thinning)
• D. Newtonian

Hint:

"Shear-thinning" means that applying force (shaking, pumping, or stirring) thins the fluid, making it flow more easily.
Ketchup is the classic household example of a pseudoplastic fluid.

Answer 1

The Correct Option is C

Step 1: Understanding the Problem:
The question asks for the rheological classification of tomato ketchup, specifically which non-Newtonian flow behavior it demonstrates during processing and handling.

Step 2: Key Concepts and Approach:
Fluid foods are classified based on how their viscosity changes in response to an applied shear stress (force):
1. Newtonian: Viscosity remains constant regardless of the shear rate (e.g., water, clear fruit juices).
2. Non-Newtonian: Viscosity changes as shear rate changes.
3. Pseudoplastic (Shear-thinning): Viscosity decreases as the shear rate increases.
4. Dilatant (Shear-thickening): Viscosity increases as the shear rate increases.

Step 3: Detailed Explanation:

• Behavior of Ketchup: Tomato ketchup is a complex suspension of insoluble tomato solids and polymers in an aqueous phase.
When stationary inside a bottle, it has a high viscosity and resists pouring.

• Effect of Shear: Shaking or squeezing the bottle applies a shear force to the ketchup.
This force aligns the suspended particles and polymer chains parallel to the direction of flow, reducing friction.
As a result, the viscosity of the ketchup decreases, allowing it to flow easily out of the bottle.
This reversible drop in viscosity under shear is the definition of pseudoplastic (shear-thinning) behavior.

Step 4: Final Answer:
Therefore, tomato ketchup exhibits pseudoplastic (shear-thinning) behavior, matching Option (C).