We all know the struggle of getting ketchup out of the bottle to enjoy on hot dogs, fries, or hamburgers. But, most of us probably haven’t given it much thought. Inevitably, the ketchup is at first impossible to get out of the bottle and then makes a huge mess as it flows everywhere. Why?
What’s Newton got to do with it?
One of the first lessons learned in food science classes are the difference between Netwonian and non-Netwonian fluids.
Think of it this way: water, milk, and oil pour as expected. The more force that is applied, the faster they flow. It can be described in a linear fashion. Sir Isaac Netwon discovered this, thus the term, Newtonian fluids.
Non-Newtonian fluids on the other hand, do not behave in a linear fashion. They may become thinner or thicker in viscosity, meaning their behavior changes as force is applied or over time (non-linear). Examples of non-Newtonian fluids include ketchup, whipped cream, mayonaisse, and more.
What is viscosity?
You are probably familiar with viscosity, whether you know the term or not. You can see differences in viscosity very easily. Think of milk versus maple syrup. Milk is relatively thin and flows easily, while maple syrup is thick and flows very slowly.
Did you know? Viscosity is the rate of shear stress to shear rate. It describes the internal resistance of a fluid to flow.
Viscosity is an important characteristic in food production. Companies measure viscosity using a Bostwick consistometer or a Brookfield Viscometer.
Viscosity is not only important for consistent product quality and consumer expectations but is also important for understanding how fluids will behave when pumped and mixed during production.
So, how does ketchup behave?
Ketchup is known as a Casson plastic, meaning it has a non-linear flow once the critical stress point is overcome. That’s why you have to hit the bottle hard to get the ketchup to flow. This force overcomes the stress point and allows the ketchup to flow very quickly.
Types of non-Netwonian Fluids
Non-Newtonian fluids may be dependent on shear rate (force) or time.
- Pseudoplastic (shear-thinning) – As force is applied, the liquid viscosity thins. Think of juice concentrates, purees, and emulsions.
- Dilitant (shear thickening) – As force is applied, the liquid viscosity thickens. Think of liquid chocolate.
- Plastics – This type of fluid is unique. Plastics do not flow until they overcome a critical stress point. Then the flow is either linear or non-linear. Think of ketchup & tomato paste (known as a Casson plastic).
- Viscoelastic – Both viscous and elastic. Once the material is deformed, it may never return to its original shape. Think of dough, cheese
- Thixotropic – Viscosity decreases over time at a constant force as the structure is degraded. Think of mayonnaise and cream.
- Rheopectic – Viscosity increases over time at a constant force as structure is formed. Think of whipping cream.
The more you know
I’m sure most of us haven’t given much thought to the physics in our food. Next time you’re enjoying some french fries, think about Newton and how ketchup doesn’t obey his rules.
Fellows, P.J. 2011. Food Processing Technology: Principles and Practice Third Ed. CRC Press. pp 37-38
Daubert C.R., Foegeding, E.A. 2009. Rheological Principles for Food Analysis. In: Food Analysis Fourth Ed. Nielsen, S.S. Springer. doi: 10.1007/978-1-4419-1478-1. pp 541-554.
What a fascinating article. I didn’t realise there were no many non-Newtonian fluids in cooking. Do you know where (English) custard fits into your schema?