Rheology of liquids in the context of powder mixing
The Rheology deals with the phenomenon of flow and deformation of bodies and masses, when forces act on them. When small quantities of liquid are to be evenly are to be uniformly distributed, one speaks of "wetting of the powder particles." Depending on the stickiness and viscosity of the liquid, such a powder mixing process can produce inhomogeneous results.
Viscosity changes on exposure to heat
Lecithins, molasses, honey, fats, oleoresins, baking extracts and vegetable oils have different flow properties. At room temperature, their viscosity may be too high to be mixed comfortably into the powder mixture. Then it is advantageous to adjust the viscosity appropriately. Since the viscosity mostly decreases with increasing temperature, it is useful to warm up the liquid before it is introduced into the mixer.
Viscosity changes with shear stress
Depending on their type (dilatant, Newtonian or structural viscous), liquids behave differently when subjected to shear stress. Shear stress is triggered, for example, when a liquid is mixed or pumped. Shear stress is also present when the fluid flows through pipes. Shear is particularly high when the liquid is sprayed.
For example, if one were to attempt to pump a dilatant fluid with a centrifugal pump or inject it into the mix with a nozzle, its viscosity would increase. This is unfavorable for the mixing process. It is then likely that many small moist granules will form in the powder. These become coated with the powder. Amino acid additions in liquid form, such as aspartic acid, glycine, methionine, tryptophan, lysine, threonine or valine, tend to behave dilatantly; so does honey. Starch suspensions are also highly dilatant.
A structurally viscous (pseudoplastic) liquid, on the other hand, reacts completely differently to shear impact. Its viscosity decreases when it is pumped, conveyed, metered or sprayed. In this case, the liquid material mixing in the powder is positively supported.
Water or edible oil, on the other hand, behave Newtonian. They largely retain their viscosity - independent of any shear effects. This also applies, for example, to aqueous solutions; but not to most suspensions.
Learn more about the wetting of powders here!
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