Viscosity plays a decisive role in food science and the food industry. This parameter is crucial in quality control and process design, affecting operations such as stirring and homogenization, pumping and packaging among others.1 It is also a fundamental factor affecting food consumption, being characteristic of food texture,2 playing an important role in flavor perception and even having health and safety implications in processes such as deglutition and digestion.3
In this article, we will consider what viscosity is, how it is measured and the applications of viscometry in the food and beverage industry.
When a force or stress is applied to a fluid, it flows, resulting in internal friction that resists the flow. Therefore, viscosity is defined as the internal friction of a fluid, or in other words, its capacity to resist flow.
Considering a fluid as a number of laminar layers, the formula describing viscosity is straightforward, being represented in centipois (cP) or Pascal seconds (Pa s):
The shear stress is the force per area needed to move a laminar layer of fluid, whereas the shear rate is the relation of speed between two laminar layers of fluid. This is only valid for Newtonian fluids, where at a certain temperature, the viscosity would remain constant as a function of changes in shear rate (e.g., water). However, the majority of fluids do not behave this way, being called non-Newtonian, and different considerations are required to measure their viscosity. For a more in-depth understanding of these phenomena and its implications, additional reading is recommended.1
In practical terms, a fluid with a higher viscosity would flow more slowly and appear thicker than a fluid with lower viscosity (Figure 1). This property is highly dependent on the intrinsic characteristics of a material such as molecular composition, solute concentration, structural organization and density. Furthermore, viscosity should not be perceived as an absolute value, but rather a function of external factors such as temperature, shear and pressure, all of which affect viscosity, and thus, the technological and organoleptic properties of a food product.
Figure 1: Flow comparison of a high-viscosity fluid (left) and a low-viscosity fluid (right) at the same temperature.
What is a viscometer?
A viscometer, or viscosimeter, is any instrument used to measure the viscosity of a fluid.4 These types of instruments only measure viscosity under one flow condition, contrary to other classes of instruments that are able to determine viscosity under varying flow conditions, called rheometers. Although viscometers are considered less versatile and precise, they are cheaper, portable, easier to operate and provide faster determinations, making them first choice in production and quality control applications.5
How does a viscometer work?
Viscosity determinations can be performed based on a number of methods, where the measuring object stays stationary and the fluid passes through it, or the fluid is stationary and the measuring object moved through it. In either case, the flow conditions must be maintained on the laminar regime.6 The relative motion between the fluid and the surfaces of the measuring object causes drag, allowing viscosity to be measured. Based on the relative motion and geometry of the equipment, several types of viscometer, as well as their accuracies and target applications can be defined. Considering the different viscometry methodologies, there are two distinct ways to express viscosity, absolute viscosity or kinematic viscosity. The absolute or dynamic viscosity is determined by observing the fluid’s resistance to flow under a controlled external force, while the kinematic viscosity is determined by observing the fluid’s flow under the action of gravity.1 These two ways of expressing viscosity can be interconverted by considering the fluid’s density, multiplying the kinematic viscosity by the density of the fluid to obtain the absolute viscosity, or dividing the absolute viscosity by the density to obtain the kinematic viscosity.
Common types of viscometers
– Capillary viscometer and flow cups (orifice viscometers)
The working principle of these instruments is based on the determination of the time needed for a standard volume of fluid to pass through a capillary. The simple design and straightforward operation make them good alternatives for routine measurements and quality control. Yet, they are limited to low viscosity liquids without solid particles in suspension, which restricts their utilization to beverages and liquid ingredients. One of the most common configurations is the “U” shape capillary of the Ostwald viscometer presented in Figure 2A. These instruments are usually operated in a thermally-stabilized bath and after the upper bulb is filled with the fluid, the time needed for the fluid to pass through the marks is recorded and multiplied by an instrument constant to find out the kinematic viscosity of the fluid.2 The time it takes for the fluid to pass through the capillary tube is directly proportional to the dynamic viscosity and inversely proportional to the density of the fluid. Another simple configuration of a capillary viscometer is the flow cup or orifice type viscometer (Figure 2B). This equipment consist of a reservoir and orifice or capillary, and works by determining the time it takes for the fluid to pass through the orifice.7 These instruments are cheap, easy to operate and versatile, however they are not accurate as they do not ensure constant pressure on the orifice.
Figure 2: Diagram of an Ostwald viscometer (A), a flow cup (B) and tube viscometer (C).
Pressurized versions of the capillary and flow cup viscometers can be used in order to maintain a constant pressure during the measurement and to operate under high pressures, enabling their utilization for highly viscous fluids. In such cases, the fluid is moved by gas pressure or by a piston that can be triggered by a weight or a driving motor.
In the same category of capillary viscometers, tube viscometers (Figure 2C) use a horizontal tube fed by a pressurized tank to determine viscosity.2 In these instruments, the pressure is determined at different positions along the tube, and the pressure drop at a specific flow rate is used to calculate the resistance to flow, and thus the viscosity. Tube viscometers are especially suited for on-line determinations and often used in industrial facilities.
– Falling piston viscometers
The falling piston viscometer uses a piston to displace a fluid in a cylinder or container. Once the cylinder is filled with the test fluid, the piston drops under the action of gravity, displacing the fluid through the gaps between the piston and the cylinder (Figure 3), causing a shearing effect.8 By measuring the piston’s falling time, sample viscosity can be determined. The ability to control the shear rates, by changing the piston’s mass, makes this equipment suitable to characterize fluids involved in high shear rate operations. These instruments are also easy to operate and maintain, offering a long product life. The main drawback is that performance for non-homogeneous samples, such as particulate suspensions, may be impaired.
Figure 3: Diagram of a falling piston viscometer.
– Falling ball viscometers/rolling ball viscometers
A falling ball or a rolling ball viscometer measures the rolling/falling time of a ball through a fluid contained in a tube, as gravity acts as the driving force.2 The inclination may vary from a vertical configuration (between 80° and 90°), allowing a free fall of the ball, thus being designated a falling ball viscometer, or inclined (between 80° and 10°), designated rolling ball. The time needed for the ball to descend a determined distance is directly proportional to the fluid’s viscosity (Figure 4). The variables involved in the measurement are the fluid density, the ball density, the length and time of the ball movement and its terminal speed. These instruments are well suited for measuring Newtonian fluids, are easy to operate and can even be easily improvised in a laboratory. The adjustment of the angle allows the equipment to be tuned according to the viscosity range, making them versatile. However, they are limited to non-particulate samples and cannot be used in opaque samples because the ball cannot be seen.
Figure 4: Diagram of rolling ball viscometer. α indicates the angle of inclination.
– Rotational viscometers
Rotational viscometers operate under the principal of a rotational moving part, driven by a motor, in contact with the test fluid. The torque on the rotating shaft is then used to determine the resistance to flow of the fluid and calculate the dynamic viscosity. Because these instruments are operated by a motor, they can function under different shear rates. This feature allows them to operate below and above the range of gravity-based equipment, thus enabling the measurement of a much wider range of samples.1 Furthermore, they allow viscosity measurements to be made for a sample under different shear rates, thus facilitating the production of g viscosity curves, which are fundamental for non-Newtonian fluids.
The most common variation of this type of equipment is called the Brookfield viscometer that operates under the Searle principle9 (Figure 5). The motor drives a spindle inside the fixed cup containing the test fluid. The rotational speed of the spindle causes drag and consequently a certain motor torque. The torque needed to overcome the viscous forces of the tested fluid is therefore a measure for its viscosity. This system can be used for both Newtonian and non-Newtonian foods and it is the most common type of viscometer that is used in quality control due its versatility. The equipment operates at different rotational speeds and can be used with different spindles that have different geometries. These range from discs, cylinders, cylinder-cones with different diameters and special shapes including several types of paddles and vanes, allowing the user to choose different conditions for different sample types, such as low viscosity, high viscosity, particulate and gel-like fluids.
Another specific geometry of rotational viscometers are the double-gap systems, which are specially devised for measuring low-viscosity fluids. The cup is a double concentric cylinder, such as an elongated ring, and the spindle is shaped as an inverse cup that turns in the ring-shaped gap of the cup. This configuration maximizes the surface area in contact with the fluid, thus, it is able to detect low torque values as typically generated by low-viscosity samples.
Figure 5: Diagram of the Brookfield viscometer (far left) and different spindle geometries, from left to right, disc, cylinder-cone, paddle and vane geometries.
A similar principle can be used in parallel plate and cone plate geometries. In these kinds of geometries, a plate or a cone rotates over a sample placed on a fixed plate (Figure 6). These systems operate with narrow gaps between the moving geometry and the fixed plate, therefore only a small amount of sample is required. However, the small gap limits the technique to non-particulate samples and by being open on the side, the sample can escape under higher rotational speeds, by the action of turbulent flow and centrifugal forces. On the other hand, the wedge-shaped gap caused by the cone results in a constant shear rate being applied over the entire gap, yielding advantages for the measurement of absolute viscosity values.2
– Vibrational viscometers and oscillation viscometers
Vibrational viscometers operate by measuring the damping of an oscillating resonator immersed in the test fluid, which vibrates like a tuning fork (Figure 7A). An electromagnetic drive controls the vibration of the sensor plates, and the energy involved in the resonance can be determined by measuring the energy used to maintain the vibration at a constant amplitude, as the damping imposed by the fluid is directly proportional to its viscosity. Another alternative is to determine the decay time of the oscillation once the power input is cut, as the decay is also directly proportional to the fluid’s viscosity.10
Oscillation viscometers use a single probe (a sphere, rod or plate) vibrating in contact with the test fluid (Figure 7B). A vibrating surface in contact with the test fluid imposes shear stress due to the surface movement, so the viscosity can be determined by measuring the power necessary to maintain a constant amplitude of oscillation.10
Vibrational and oscillation viscometers yield advantages such as high sensitivity on a wide range of viscosities (from low viscosity liquids to semisolid or clogging fluids), have no moving parts and are easy to maintain. This characteristic makes these types of equipment the gold standard for in-line measurements in pipelines and tanks.
Figure 7: Diagram of vibrational (A) and oscillating (B) viscometers.
– Other type of instruments
There are also imitative viscometers that rely on empirical principles to mimic the flow of non-Newtonian fluid foods under operational conditions. This simple apparatus typically provides a one-point measurement. Although they have limited sensitivity and operating range, they can be useful in quality control and rough measurements. A good example of this class of equipment is the Bostwick Consistometer (Figure 8), comprising a sample compartment (a) with a gate operated by a spring trigger (b). Once the gate is opened, the test fluid flows over a ramp (c) sitting at a specific angle. After an established time, the length travelled by the flowing sample is recorded, and is proportional to the product’s viscosity. This equipment is easy to operate, robust and provides fast and consistent determination, being particularly suited for sauces, pastes, jams and all kind of highly viscous and particulate-containing food.
Figure 8: Diagram of the Bostwick Consistometer.
How to measure viscosity
As previously discussed, viscometers are usually easy-to-operate instruments, provide fast results and can even be used in on-line measurements. However, in order to measure viscosity correctly, a series of factors must be considered, namely:
- temperature variations
- phase separation
- non-uniform samples due to the presence of large particles
Therefore, according to the specificities of the product being measured, the most appropriate type of instrument must be selected.11
In the case of process control instruments, they are installed within the processing plant installation (e.g., tanks or pipes) in order to obtain real-time data of the process. Tube type, vibrational or oscillation type viscometers are preferably used for these types of applications.12 Regarding quality control measurements, where samples are taken on a random basis to validate the processing conditions and the quality standards of a product, rotational, “U” tube viscometers or Bostwick Consistometers are often used. These instruments are the primary choice in these applications as they are versatile, can provide measurements for several types of samples and processing conditions (e.g., wide viscosity range and operational temperature) and can quickly provide the necessary data in industrial conditions. If needed, they can be cleaned quickly and adapted to other operational conditions, allowing the measurement of several samples with different specificities in a short period. It is this kind of versatility and quick response that yields the greatest advantage of viscometers against the most precise instruments such as rheometers.13
Uses of viscometry in the food and beverage industry
Many factors may influence the quality and stability of foods, and viscosity is definitely one of the aspects receiving more attention during the manufacturing process. Regarding food production, viscosity is a key parameter in optimizing flow conditions, dimensioning pumps, pipelines, energy consumption, reducing waste and in the control of the final product’s quality.1,14
Viscosity is also very relevant in the formulation stages of food products. Let’s take the example of bakery products, in which a number of phases (i.e., flour, water, fat, air) are mixed to a stable dispersion. In order to maintain that dispersion, avoid phase separation and retain air during mixing and baking, the system must hold sufficient viscosity. The same principle applies for the preparation of foams, emulsions and colloidal suspensions such as in sauces, soups, custards and so on.
Quality control is another area where viscosity determinations are often implemented and are fundamental for the food industry. They can be used to control the quality of the ingredients, such as syrups, oils and liquid egg among others. This not only validates their suitability to be used on the processes, but also allows analysts to predict deviations and perform corrections on the formulations. Regarding the final product control, viscosity determinations are standard methods to validate the product’s quality and stability and ensure they meet technical specifications.
But the importance of viscosity determinations in the food industry reaches far beyond the classical production models and it is also vital in assisting the ongoing food revolution. Fueled by environmental, ethical and societal changes, production and consumption patterns are being redefined. New foods, such as meat analogues, aim to reduce or shift protein consumption from animal-based to plant-based sources. The development of meat alternatives and meat analogues is dependent on innovative techniques such as 3D printing or high moisture extrusion, using protein-based ingredients as well as gums and other polymers. The goal of these processes is to create the right texture and consistency to emulate the meat structure. Viscosity was found to be a key factor, defining the design of equipment, processes and being a determinant factor in the final properties of the developed foods.15,16, 17
The design of new foods also follows the trend of providing additional safety and health benefits. Food products developed for specific pathologies are now a reality, and among the most common cases are formulations targeting deglutition control in baby foods or dysphagia patients.18 It has been reported that a rational design based on the control of viscosity at specific shear rates reduced the risk of asphyxiation.19 Another action area is the relationship between the stomach-emptying rate and satiety, digestion and glycemic index. In fact, it was established that the viscosity of food plays a decisive role in gastric-emptying, and therefore the design of tuned viscosity foods may bring important implications in public health, potentially helping to control diabetes and obesity.20
Therefore, it can be concluded that the determination of the viscosity in foods and beverages and their ingredients is crucial in quality and process control, and also impacts the consumption, sensory experience, safety and health of the consumers. The techniques used to measure viscosity are often simple and quick, but careful selection among the various test equipment and conditions is fundamental to obtain quality measurements. The selection will be determined by the type of food product and application, and is influenced by the temperature, sample homogeneity, phase changes and type of flow.
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