Viscosity describes a fluid's internal resistance to flow. A fluid with a higher viscosity would pour slower and seem thicker than a fluid with less viscosity. When there is a change in a material’s property such as molecular weight and density, both of which affect how a liquid flows, the viscosity changes, and the quality is altered.
Liquid systems can be highly shear-sensitive and the structure can break down with the application of shear, as is the case during mixing or when pumped through pipes. There are a number of characteristic behaviors that can be observed.
Newtonian behavior 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 concentrations can also show Newtonian behavior. An easy way to demonstrate Newtonian behavior is to double the shear stress during a viscosity test and this should result in doubling of the shear rate. If this is not observed then the liquid is non-Newtonian.
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. This is referred to as time-independent (steady-state) flow and materials showing this type of behavior are called pseudoplastic. Another type of vicious behavior exhibited by fluid foods and polymer systems is thixotropy which is again shear-thinning of the material with increasing rates of shear but is also dependent on the duration of shear.
The viscosity is also dependent on concentration and the relationship is not usually linear. For example, a small increase in the concentration of a hydrocolloid may increase viscosity a little, but once a critical concentration is exceeded the viscosity can increase exponentially.
The temperature has a major effect on viscosity; the viscosity decreasing significantly with an increase in temperature. As the temperature increases the molecules in the liquid move about more, and therefore spend less time in contact with each other, thus the internal friction of the liquid decreases.
By far the most researched food in terms of viscosity is chocolate. The flow behavior of chocolates is important both during processing as well as for organoleptic reasons. Chocolates have different flow properties depending on application and products are made for enrobing and for making blocks. However, the flow behavior is complex due to the fact that a number of ingredients, namely sugar, cocoa butter, cocoa particles, and milk products, need to be finely dispersed. The viscosity plays a crucial role as it affects texture, that is, how it flows in the mouth.
The Casson method
The measurement of chocolate viscosity is highly specialized and requires a specific type of viscometer for the measurement to be performed accurately. The flow properties are usually described by the Casson flow curve and for this, it is necessary to make measurements at different rotational speeds so that shear stresses at different shear rates can be determined. The Casson method gives information about the yield stress and the plastic viscosity. Yield stress is defined as the shear stress required to initiate the flow of the chocolate and hence gives information about potential enrobing properties, whereas the plastic viscosity relates to the shear stress required to maintain a constant flow. The latter thus relates to the way the chocolate will flow in a mold or perhaps in the mouth. The viscosity test is typically carried out at 40°C and temperature control within a narrow range is important in order to perform the test accurately.
Photo by Food Photographer | Jennifer Pallian on Unsplash
Pharmaceutical R&D labs test new formulations for viscosity to quantify flow behavior properties. Viscosity affects processing decisions for mixing in manufacturing and filling containers with consumer products, as well as formulation stability during transport, storage, and consumer use.
Viscometers are used to measure the viscosity of pharmaceutical liquids and semi-solid materials like creams/ointments. Viscosity flow curves shown in the instrument display characterize the typical behavior of pharmaceutical products. As the rotational speed of the spindle increases, viscosity decreases. R&D will pick a data point on this curve and direct QC to test for that value when qualifying production batches for shipment.
There are three decisions that R&D makes in setting up the QC test method.
1. To define the viscosity range
The first is to define the viscosity range of the manufactured material. Scientific units of centipoise (cP) are typically used in North America, while other parts of the world use milli-Pascal seconds (mPa*s) as well. Water is the reference material with a viscosity of 1cP at 20ºCelsius. Cough syrups and other medicinal liquids that are swallowed are typically below 100cP. Rubbing ointments maybe around 1,000cP or higher. Thick creams can start around 10,000cP. Choice of viscometer with appropriate torque measurement range depends on knowing the material(s) that will be tested. Two basic torque ranges are used for most pharmaceutical products. “LV” applies to low viscosity materials, while “RV” is selected for “regular” or medium viscosity products.
2. To define the type of spindle and rotational speed
Once the torque range has been decided, the choice of instrument model is based on operating features needed for the measurement. R&D defines the type of spindle to use and rotational speed when measuring viscosity. Spindles in general are either cylindrical in shape or may have a disc at the bottom. Disc spindles are the most common type found in QC labs. There are other types as well, such as cone and T-bar. Other features in the choice of viscometer include built-in clock to time how long the spindle rotates before the viscosity data point is captured. If there is a temperature requirement for conditioning the sample, then the viscometer should come with built-in temperature probe for verification of sample temperature.
3. To test on multiple samples
The third and final need for QC is knowing the acceptable limits for the viscosity measurement to approve the product for shipment. R&D will do testing on multiple samples during the validation process to establish minimum and maximum cP values for the QC test. When measured viscosity is between these two values, the product passes and shipment takes place.
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