Rheology for beginners
Part 1. Viscosity
The ketchup will not flow out of the bottle, the mayonnaise is too thin, the chocolate mousse is wonderfully creamy and the new sauce recipe does not work in the process. All this can be described and explained with rheology. Rheology is an excellent tool to describe how liquids and semifluids are flowing and is also very well suited for describing the consistency for most foods. This article explains rheology from the start and begins with viscosity.
Rheology is not difficult even if it is often judged as hard to understand or too theoretical. This is after all about phenomenon which we can see with our bare eyes in contrast to for example molecule movements or nuclear reactions, which we gladly accept happening. If we begin with viscosity, viscosity describes how viscous a liquid is. If it is easy to stir the soup, oil or cream the viscosity is low. A common way of measuring viscosity is by stirring. In a rotation viscosimeter there is one rotating cylinder and the torque required to rotate the cylinder with a fixed speed is measured. If twice as much torque is needed to rotate the cylinder twice as fast, the liquid is called Newtonian. Examples of Newtonian fluids are cooking oils, syrup and diluted solutions such as different beverages.
Most liquid foods are not Newtonian but shear thinning. This means that they become thinner, i.e. has a lower viscosity the faster it is stirred, which is every chef’s and food producer’s good fortune. If a cake mix or whipped cream would not be shear thinning, the chefs would have to be athletes in order to stir the mixture or whip the cream. If all mixtures and preparations which are pumped through different food processes would not be shear thinning, much more powerful pumps and also considerably more energy would be needed for the process. The diagram shows flow curves, explaining how the viscosity changes with shear rate for xanthan and syrup. Xanthan is a food stabiliser (E415) which is extremely shear thinning. The diagram shows that the viscosity of the xanthan solution decreases from 10 000 Pa s to 0.01 Pa s. The shear rate is expressed in reciprocal seconds, s-1, in order to be able to compare different geometries, see the formula box. The area in the diagram stretches from low shear rates found for sedimentation to high shear rates applied for rubbing or spreading. An ordinary rotational viscometer can measure shear rates within the blue area while the yellow and red areas requires more advanced methods. In the yellow area we used creep measurements (more about this in Rheology school part 2) and in the red area a conical measuring system is used as shown in the figure. The specially designed conical measuring system is self centering and thus able to perform high rotational speeds without instabilities. With an ordinary cylindrical measuring system or a cone-plate system (a and b in the figure) the fluid would be thrown out of the cup at the high rotation rates needed in the area. It is important to cover the whole area if you would like to describe stability, consistency as well as process properties.
For some fluids the viscosity varies not only with rotation speed, but often with time. If you stir such a liquid with constant speed the viscosity is decreasing the longer it is stirred, which is due to that the structure in the liquid being destroyed mechanically. If the structure, and by that the viscosity, is recovering when the liquid may rest the fluid is called thixotropic. This is often used in paints where the paint gets a lower viscosity and is then easier to work with as soon as the paint brush is put in the tin, while the paint is highly viscous and stable when resting, thus preventing the dye particle from sedimenting. Foods such as yoghurt and sour milk are also thixotropic which is very obvious when a newly opened yoghurt is stirred.
One more important parameter is the temperature. The viscosity of a fluid is decreasing with temperature, provided no other temperature dependent reaction such as melting or gelling is occurring. The decrease in viscosity is important both for the consistency experience and for the measuring accuracy. A food substance has a totally different viscosity at mouth temperature than at room temperature or as refrigerated. The change is often dramatic, for example the viscosity of syrup is decreased with 70% if the temperature is increased from 20°C to 37°C. When performing a viscosity measurement the temperature has to be kept constant, at least ±0.3°C in order to get an accuracy of ±1% in the viscosity measurement.
This is a short description of viscosity and flow properties which are relevant for foods. There are of course much more to be add but it is too much to fit in this article. If you are interested in reading more you will find suggested literature and links in the box below. The next part of Rheology for beginners deals with viscoelasticity.
The effect of stirring depends on what you stir with and the shear rate is therefore used instead of the rotational speed. The shear rate is:
where v is the fluid velocity of the liquid.
The force or torque which is transferred by stirring also depends on the geometry, this is why the stress s is used, which is the force/area and is measured in N/m2 which is the same as Pa.
The viscosity h is defined as