Food Chemistry and Fish in Nutrition ( 2+1)

7.3.5. Viscosity

Viscosity in food is often considered with flow properties. Ideal solutions maintain a constant viscosity coefficient independent of shear stress or shear rate. Viscosity can be related to the volume occupied by solute compared to the volume occupied by solvent. As a protein occupies more volume in solution it has a greater chance of interacting with other molecules and for there to be a resistance to flow.

When most protein solutions are exposed to increasing shear the viscosity coefficient decreases. This behavior is called shear thinning and solutions that exhibit this behavior are called pseudoplastic. As shear is applied interactions between protein molecules can be weakened. If this results in the breaking of a network, shear thinning will occur and the apparent viscosity will decrease with increased shear. This leads to an alignment of molecules in the direction of shear which decreases resistance to flow. When the shear is removed it is possible for interactions to occur again. If these interactions lead to an increase in apparent viscosity, the solution is said to be thixotropic. A number of protein solutions exhibit pseudoplastic and thixotropic behaviors.

Factors that can cause proteins to unfold can increase their viscosity. Factors having an important effect on viscosity include: pH, temperature, concentration, ionic strength and gelation on protein denaturation, extremes of pH can cause proteins to unfold and to increase their water binding. As the compact protein unravels its effective diameter increases and it occupies a larger percentage of the available volume.

Interactions are more probable and viscosity increases. Near the isoelectric point proteins can form aggregates more easily than when they are highly charged. If these aggregates are poorly hydrated or precipitate, the volume of the protein in solution will decrease and viscosity will be low.

 If the aggregates are highly hydrated and remain is suspension the viscosity may increase. Heating proteins can cause them to unfold. If this unfolding leads to increased interactions the viscosity will increase.