Does viscosity always work the same?

Viscous material flowing

A proper milkshake, yoghurt, or fruit juice are simple instances of mass-produced food items that rely heavily on their viscosities to meet consumer expectations. For example, who would like liquified ketchup on top of their burgers? Probably nobody. Good ketchup should be viscous enough to be spreadable and palatable. Although viscosity might sound like a simple and understandable word, it is, in fact, a complex concept. Consequently, viscosity measurement has emerged as an integral and necessary component of many quality control procedures in food processing.

 

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 honey. Milk is relatively thin and flows easily, while honey is thick and flows very slowly. It is defined as the rate of shear stress to shear rate and describes the internal resistance of a fluid to flow.

Within the food sector, two general analytical instruments that measure the viscosity are the Bostwick consistometer and the Brookfield Viscometer. A Bostwick consistometer measures the distance a fluid travels over a set period of time, while a Brookfield viscometer uses a torque sensor and rotational speed to measure viscosity.

 

Consistometer
Bostwick Consistometer. Photo credit: CSC Scientific.

 

viscometer
Brookfield viscometer. Photo credit: Labomat.

Monitoring viscosity on-line/in-line provides real-time analysis, enabling food technologists to control many unit operations in food processing with greater precision and confidence.

Wether you are a food scientist or not, I am sure that you have applied shear to fluids at some point at home, perhaps when preparing a porridge or a protein shake if you are a gym lover.

The real complexity appears when several scientific assumptions related to fluids are taken into consideration. These are related to the type of fluid behaviour and not to its composition, and play a huge role when working with solutions at the industrial level, either edible or not. Let’s get into it!

All fluids can be divided into Newtonian and non-Newtonian.

If a fluid follows Newtonian’s law, its viscosity remains constant, no matter the amount of shear applied for a continuous temperature. These fluids have a linear relationship between viscosity and shear stress. Some examples of this type of fluids are water, mineral oil, gasoline, or alcohol.

On the other hand, the fact that a fluid follows a non-Newtonian’s law means that the theory is the opposite of Newtonian fluids. When shear is applied to non-Newtonian fluids, the viscosity of the fluid changes. The behaviour of the fluid, though, can be described in four ways:

  • Dilatant non-Newtonian fluids: The viscosity of the fluid increases when shear is applied. Examples are quicksand, the famous Silly Putty toy, or cornflour and water. Click the following link to watch an exciting experiment that demonstrates this theory.
  • Pseudoplastic non-Newtonian fluids: It is the opposite of dilatant; the more shear applied, the less viscous it becomes. Examples are ketchup or yoghurt.
Types of viscosity
Dilatant and pseudoplastic fluids. Photo credit: Crane Engineering.

 

  • Rheopectic non-Newtonian fluids: Similar to dilatant fluids. When shear is applied, viscosity increases. The difference is that the viscosity increase is time-dependent. Examples are cream or Gypsum paste.
  • Thixotropic non-Newtonian fluids: Fluids that decrease in viscosity when shear is applied. This is a time-dependent property as well. Examples are paint, cosmetics, asphalt, or glue.
Types of viscosity
Rheopectic and thixotropic fluids. Photo credit: Crane Engineering.

In summary, the viscosity of foods is a crucial property that plays a vital role during food processing. A range of techniques is available to measure viscosity from simple, quick-to-perform tests to more comprehensive characterisation in the form of flow curves. Non-Newtonian materials such as ketchup show complex viscous behaviour and therefore require specially designed tests to obtain flow properties relevant to end-use.

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