Hydrocolloid systems, on the other hand, have greater water-binding properties and can generate viscosities at lower concentrations, for example 0.5 to one per cent. Typical rate of addition of starch to achieve significant viscosity of a liquid would be in the region of four to five per cent. For example, a small increase in concentration of a hydrocolloid may increase viscosity a little, but once a critical concentration is exceeded the viscosity can increase exponentially. Viscosity is also dependent on concentration and the relationship is not usually linear. The viscosity of water is low as the molecules are small. The polymer chains can also become entangled with one another, forming networks that are able to trap and immobilise water. Another striking property of these materials is that they consist of numerous chemical groups (hydroxyl groups, anionic groups etc.) along the length of the polymer chain that are water loving or hydrophilic and hence can bind water molecules. This is particularly true for the long chain polymers that are found in foods such as proteins, starches, hydrocolloids or gums, etc. Generally speaking, fluids with larger, more complex, molecules will have higher viscosities. In such liquids the time delay needed for the viscosity to change suggests that a certain amount of time is needed to re-arrange or align the structural components that results in decrease in the viscosity of the test material. Put in another way, the material shows thinning behaviour with time when it is sheared at a constant rate. Another type of viscous behaviour exhibited by fluid foods and polymer systems is thixtropy which is again shear-thinning of the material with increasing rates of shear, but is also dependent on the duration of shear. This is referred to as time independent (steady state) flow and materials showing this type of behaviour are called pseudoplastic. The viscosity of some fluids is dependent on the rate used to shear the material, a high rate of shear making the fluid thinner compared with the fluid that was sheared more slowly. If this is not observed then the liquid is non-Newtonian.

An easy way to demonstrate Newtonian behaviour is to double the shear stress during a viscosity test and this should result in doubling of the shear rate. Newtonian behaviour is displayed by simple liquids consisting of small molecules that do not interact or form any connected structure. However, it must be pointed out that long chain polymers at low concentration can also show Newtonian behaviour.

Viscosity chemistry vocabulary pdf