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Professor Dr Abd Karim Alias Universiti Sains Malaysia

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What is…err…Rheology?

• We encounter rheology in our daily life!

• We eat breakfast, perhaps using a range of spreads for toast, or perhaps we eat yoghurt with pieces of fruit suspended in it.

• We all have squeezed toothpaste tubes, kneaded bread dough or tried to rub skin lotion on our leg.

• Rheology is simply one way of describing those sensations!!

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What is…err…Rheology?

• In particular, rheology science addresses fluid and structural properties of raw materials, intermediate products, ingredients, and final products of the food and pharmaceutical industries

• Consumers use subjective tests to ascertain the perceived texture of a product. Rheology is usefully used to objectify these perceptions

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Rheology in Daily Life

We normally take for granted a lot of things in life and these include some of the “rheological events”. The pictures show the different roles of rheology in food and nonfood products. Can you think of other examples?

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Rheology in Action

How do you like the sauce to flow? Liquid food products should be formulated to display desired rheological behaviour, e.g., easy to pour from the bottle, but flow in controlled manner and recover the viscosity upon pouring on the plate.

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More Rheology in Action

Rheology principles are operating in these pictures. Rheology aims at measuring those properties of materials that control their deformation and flow behaviour when subjected to external forces (pouring, sucking, scooping, etc.).

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Rheology “inaction”

Liquid products are formulated in such a manner to exhibit “desirable” flow property, or viscosity. What happens when the products are not correctly formulated “rheologically”?

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Consumers are informed, for instance, that products are ‘thick and creamy’ as well as ‘new and improved’, because such rheological properties are more pleasing to the eye and mouth.

Rheologically “Pleasant”

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This product is a jelly drink – it is formulated in such a manner that it can be sucked through a straw. Notice how the gel/fruit beads are suspended nicely in the “liquid” jelly.

Rheologically “Balanced”

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But…Do I Really Need to Learn Rheology?

• For food technologists, knowledge of rheology is important for a better understanding of how process variables influence specific textural characteristics, such as pourability and mouthfeel

• Rheological measurements can aid in the understanding of how the viscosity and elasticity of foods are influenced by changes in composition, processing, and storage parameters

YES!

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But…Do I Really Need to Learn Rheology?

• Mathematical modeling of rheological behavior permits prediction of material performance during exposure to certain processing or experimental conditions. It is particularly important in food production where most foods are forced to “flow” through processing line.

• Rheological measurements can provide a rapid determination of product quality and may serve as a tool for quality control

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But…Do I Really Need to Learn Rheology?

• Rheological data should assist food technologists and plant engineers to design more efficient and cost-effective processes (so it is essential that they speak the same language…rheology…that is)

Now…let’s define “rheology”

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• “Rheology” comes from Greek rheos, meaning ‘to flow’

• The Greek philosopher Heraclitus described rheology as panta rei —everything flows (if you wait long enough!)

• Rheology aims at measuring those properties of materials that control their deformation and flow behaviour when subjected to external forces

Definition of Rheology

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• The subject of rheology is concerned with the study of deformation and flow of matter

• When subjected to external forces, solids (or truly elastic materials) will deform, whereas liquids (or truly viscous materials) will flow.

Definition of Rheology

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• Contemporary rheology is more interested in the behaviour of real materials with properties intermediate between those of ideal solids and ideal liquids. These industrially important materials are called viscoelastic materials, which include almost all real materials

Definition of Rheology