FOOD ENGINEERING: RHEOLOGY OF LIQUID FOODS
Diagram
Newtonian and non-Newtonian fluids under shear
Newtonian fluid
Viscosity stays the same at any shear rate. Stir it fast or slow, and its thickness does not change.
↓
Non-Newtonian fluid
Viscosity changes with shear rate. Stirring can make it thinner, thicker, or need a push before it starts to flow.
↓
The test
Plot shear stress against shear rate. A Newtonian fluid gives a straight line through zero. A non-Newtonian fluid gives a curve, or a line that does not pass through zero.
Figure 1: Constant viscosity compared with viscosity that changes with stirring. Drawn in our own style from Steffe, Rheological Methods in Food Process Engineering, Freeman Press, 1996, and Rao, Rheology of Fluid and Semisolid Foods, Springer, 2010.
Introduction
Newtonian and non-Newtonian fluids are the two classes of liquid foods, divided by how their viscosity behaves when they are stirred, pumped or poured. A Newtonian fluid keeps the same viscosity whatever the shear rate, so its resistance to flow is constant. A non-Newtonian fluid changes its viscosity as the shear rate changes, so the same liquid can feel thick at rest and thin when stirred, or the other way round.
The core difference is constancy against change: one number describes a Newtonian fluid, while a non-Newtonian fluid needs a curve or a model that shows how viscosity changes with shear. Most simple liquids are Newtonian, while most real foods, such as sauces, pastes and doughs, are non-Newtonian.
Quick comparison
Table 1: Newtonian fluids compared with non-Newtonian fluids
| Aspect | Newtonian fluid | Non-Newtonian fluid |
|---|---|---|
| Viscosity | Constant at any shear rate | Changes with shear rate |
| Shear stress against shear rate | A straight line through zero | A curve, or a line that misses zero |
| Describing it | One number: the viscosity | A model, such as the power law, with two or more numbers |
| Shear thinning example | Does not thin on stirring | Ketchup and yogurt thin when stirred or shaken |
| Shear thickening example | Does not thicken on stirring | Some starch pastes thicken when stirred fast |
| Yield stress | Flows at once, with no minimum push | May need a minimum stress before it starts to flow |
| Food examples | Water, thin milk, clear juice, cooking oil | Ketchup, mayonnaise, yogurt, tomato paste, dough |
| Pumping | Easy to predict: double the pressure, double the flow | Harder to predict: flow does not rise in simple proportion |
| Measurement | One viscosity reading is enough | Viscosity must be measured over a range of shear rates |
| Temperature effect | Viscosity falls as temperature rises | Viscosity also falls with temperature, and the shape of the curve can change too |
Scientific explanation
Definition
A Newtonian fluid is a fluid in which shear stress is directly proportional to shear rate, so viscosity is constant. A non-Newtonian fluid is a fluid in which shear stress is not directly proportional to shear rate, so the apparent viscosity changes with shear rate.
Working principle
In a Newtonian fluid the internal friction stays the same however fast the layers slide past each other. In a non-Newtonian fluid the structure of the liquid changes under shear: long molecules can line up and slide more easily, so the fluid thins, or particles can jam together, so the fluid thickens. Some fluids also have a yield stress, a minimum push needed before flow starts at all.
Apparatus
Steffe and Bourne name the tube viscometer and the rotational viscometer for measuring flow of liquid foods. The rotational viscometer shears the sample at set speeds and measures the resistance, which is how the viscosity curve of a non-Newtonian food is found.
Measurement procedure
The sample is held at a set temperature and sheared at a series of shear rates, from low to high, while the shear stress is recorded at each rate. For a Newtonian fluid one shear rate is enough, because the viscosity is the same at every rate. For a non-Newtonian fluid the full curve is needed, and the readings are fitted to a model such as the power law.
Factors affecting readings
The result depends on temperature, because viscosity falls as the liquid warms. It depends on the shear rate range used, on how long the sample has been sheared, and on the composition, such as solids content and particle size. A sample that has been stirred before the test can read differently from a rested sample.
Formulas and calculations
For a Newtonian fluid, shear stress equals viscosity times shear rate: tau = mu x gamma dot, where tau is shear stress, mu is viscosity and gamma dot is shear rate.
For a non-Newtonian fluid the power law model is used: tau = K x (gamma dot) to the power n, where K is the consistency index and n is the flow behaviour index. When n is less than 1 the fluid is shear thinning, when n is greater than 1 it is shear thickening, and when n equals 1 the fluid is Newtonian.
Worked example: a Newtonian liquid with viscosity 0.001 Pa s is sheared at 100 per second. Then tau equals 0.001 times 100, which is 0.1 Pa. If the shear rate doubles to 200 per second, tau equals 0.001 times 200, which is 0.2 Pa, so the stress doubles with the shear rate and the viscosity stays 0.001 Pa s.
Industrial applications
- Newtonian fluids: pumping water, milk and clear juices, where a single viscosity value lets engineers size pumps and pipes.
- Newtonian fluids: heat exchangers for thin liquids, where the flow is simple to predict.
- Non-Newtonian fluids: ketchup and sauce lines, where the product must flow when shaken or pumped but stay thick on the plate.
- Non-Newtonian fluids: dough and batter handling, where the material must flow under the mixer but hold its shape after.
- Non-Newtonian fluids: yogurt and cream filling, where shear thinning lets the product flow into the pack and then set.
Advantages and limitations
- Newtonian fluid, advantage: simple to describe and to predict, with one viscosity number for all flow conditions.
- Newtonian fluid, limitation: few real foods behave this way, so the simple model does not fit most sauces and pastes.
- Non-Newtonian fluid, advantage: matches real food behaviour, such as a sauce that pours when shaken and stays put when still.
- Non-Newtonian fluid, limitation: harder to measure and to predict, because viscosity must be mapped over a range of shear rates.
FAQ
Is water Newtonian?
Yes. Water keeps the same viscosity at any shear rate, at a fixed temperature.
Is ketchup Newtonian?
No. Ketchup is shear thinning: it is thick in the bottle and flows once it is shaken or stirred.
What is a yield stress?
A minimum shear stress that must be reached before the fluid starts to flow. Tomato paste and some creams show it.
What does the power law tell us?
It describes how shear stress rises with shear rate for a non-Newtonian fluid, using the consistency index K and the flow behaviour index n.
Why does this matter in a food plant?
Because pumps, pipes and heat exchangers must be sized for how the real product flows, and a non-Newtonian product does not follow the simple Newtonian rules.
References and further reading
- Fellows, P., Food Processing Technology: Principles and Practice, Woodhead Publishing (FTN book on disk).
- Steffe, J. F., Rheological Methods in Food Process Engineering, Freeman Press, 1996 (Google Books page read: Ch. 1, Introduction to Rheology).
- Rao, M. A., Rheology of Fluid and Semisolid Foods: Principles and Applications, Springer, 2010 (Google Books page read: Ch. 2, p. 27).
- Bourne, M. C., Food Texture and Viscosity: Concept and Measurement, 2nd ed., Academic Press, 2002 (Google Books page read: Ch. 6, Viscosity Measurement, p. 235).
