On this article, we’ll take into account what viscosity is, how it’s measured and the purposes of viscometry within the meals and beverage {industry}.
When a pressure or stress is utilized to a fluid, it flows, leading to inner friction that resists the move. Subsequently, viscosity is outlined as the inner friction of a fluid, or in different phrases, its capability to withstand move.
Contemplating a fluid as various laminar layers, the formulation describing viscosity is easy, being represented in centipois (cP) or Pascal seconds (Pa s):
The shear stress is the pressure per space wanted to maneuver a laminar layer of fluid, whereas the shear charge is the relation of pace between two laminar layers of fluid. That is solely legitimate for Newtonian fluids, the place at a sure temperature, the viscosity would stay fixed as a operate of adjustments in shear charge (e.g., water). Nevertheless, the vast majority of fluids don’t behave this fashion, being referred to as non-Newtonian, and completely different concerns are required to measure their viscosity. For a extra in-depth understanding of those phenomena and its implications, further studying is beneficial.1
In sensible phrases, a fluid with a better viscosity would move extra slowly and seem thicker than a fluid with decrease viscosity (Determine 1). This property is very depending on the intrinsic traits of a cloth reminiscent of molecular composition, solute focus, structural group and density. Moreover, viscosity shouldn’t be perceived as an absolute worth, however quite a operate of exterior components reminiscent of temperature, shear and strain, all of which have an effect on viscosity, and thus, the technological and organoleptic properties of a meals product.
Determine 1: Movement comparability of a high-viscosity fluid (left) and a low-viscosity fluid (proper) on the identical temperature.
A viscometer, or viscosimeter, is any instrument used to measure the viscosity of a fluid.4 Some of these devices solely measure viscosity beneath one move situation, opposite to different lessons of devices which might be in a position to decide viscosity beneath various move situations, referred to as rheometers. Though viscometers are thought of much less versatile and exact, they’re cheaper, transportable, simpler to function and supply sooner determinations, making them first alternative in manufacturing and high quality management purposes.5
How does a viscometer work?
Viscosity determinations might be carried out based mostly on various strategies, the place the measuring object stays stationary and the fluid passes via it, or the fluid is stationary and the measuring object moved via it. In both case, the move situations have to be maintained on the laminar regime.6 The relative movement between the fluid and the surfaces of the measuring object causes drag, permitting viscosity to be measured. Primarily based on the relative movement and geometry of the tools, a number of varieties of viscometer, in addition to their accuracies and goal purposes might be outlined. Contemplating the completely different viscometry methodologies, there are two distinct methods to specific viscosity, absolute viscosity or kinematic viscosity. Absolutely the or dynamic viscosity is set by observing the fluid’s resistance to move beneath a managed exterior pressure, whereas the kinematic viscosity is set by observing the fluid’s move beneath the motion of gravity.1 These two methods of expressing viscosity might be interconverted by contemplating the fluid’s density, multiplying the kinematic viscosity by the density of the fluid to acquire absolutely the viscosity, or dividing absolutely the viscosity by the density to acquire the kinematic viscosity.
Widespread varieties of viscometers
– Capillary viscometer and move cups (orifice viscometers)
The working precept of those devices relies on the dedication of the time wanted for the standard quantity of fluid to cross via a capillary. The straightforward design and simple operation make them good options for routine measurements and high quality management. But, they’re restricted to low viscosity liquids with out stable particles in suspension, which restricts their utilization to drinks and liquid elements. One of the vital frequent configurations is the “U” form capillary of the Ostwald viscometer offered in Determine 2A. These devices are normally operated in a thermally-stabilized tub and after the higher bulb is stuffed with the fluid, the time wanted for the fluid to cross via the marks is recorded and multiplied by an instrument fixed to seek out out the kinematic viscosity of the fluid.2 The time it takes for the fluid to cross via the capillary tube is immediately proportional to the dynamic viscosity and inversely proportional to the density of the fluid. One other easy configuration of a capillary viscometer is the move cup or orifice sort viscometer (Determine 2B). This tools encompass a reservoir and orifice or capillary, and works by figuring out the time it takes for the fluid to cross via the orifice.7 These devices are low-cost, simple to function and versatile, nevertheless they don’t seem to be correct as they don’t guarantee fixed strain on the orifice.
Determine 2: Diagram of an Ostwald viscometer (A), a move cup (B) and tube viscometer (C).
Pressurized variations of the capillary and move cup viscometers can be utilized in an effort to preserve a relentless strain throughout the measurement and to function beneath excessive pressures, enabling their utilization for extremely viscous fluids. In such circumstances, the fluid is moved by fuel strain or by a piston that may be triggered by a weight or a driving motor.
In the identical class of capillary viscometers, tube viscometers (Determine 2C) use a horizontal tube fed by a pressurized tank to find out viscosity.2 In these devices, the strain is set at completely different positions alongside the tube, and the strain drop at a selected move charge is used to calculate the resistance to move, and thus the viscosity. Tube viscometers are particularly fitted to on-line determinations and sometimes utilized in industrial services.
– Falling piston viscometers
The falling piston viscometer makes use of a piston to displace a fluid in a cylinder or container. As soon as the cylinder is stuffed with the check fluid, the piston drops beneath the motion of gravity, displacing the fluid via the gaps between the piston and the cylinder (Determine 3), inflicting a shearing impact.8 By measuring the piston’s falling time, pattern viscosity might be decided. The flexibility to manage the shear charges, by altering the piston’s mass, makes this tools appropriate to characterize fluids concerned in excessive shear charge operations. These devices are additionally simple to function and preserve, providing an extended product life. The principle downside is that efficiency for non-homogeneous samples, reminiscent of particulate suspensions, could also be impaired.
Determine 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 via a fluid contained in a tube, as gravity acts because the driving pressure.2 The inclination might range from a vertical configuration (between 80° and 90°), permitting a free fall of the ball, thus being designated a falling ball viscometer, or inclined (between 80° and 10°), designated rolling ball. The time wanted for the ball to descend a decided distance is immediately proportional to the fluid’s viscosity (Determine 4). The variables concerned within the measurement are the fluid density, the ball density, the size and time of the ball motion and its terminal pace. These devices are nicely fitted to measuring Newtonian fluids, are simple to function and might even be simply improvised in a laboratory. The adjustment of the angle permits the tools to be tuned in line with the viscosity vary, making them versatile. Nevertheless, they’re restricted to non-particulate samples and can’t be utilized in opaque samples as a result of the ball can’t be seen.
Determine 4: Diagram of rolling ball viscometer. α signifies the angle of inclination.
– Rotational viscometers
Rotational viscometers function beneath the principal of a rotational shifting half, pushed by a motor, in touch with the check fluid. The torque on the rotating shaft is then used to find out the resistance to move of the fluid and calculate the dynamic viscosity. As a result of these devices are operated by a motor, they will operate beneath completely different shear charges. This characteristic permits them to function beneath and above the vary of gravity-based tools, thus enabling the measurement of a a lot wider vary of samples.1 Moreover, they permit viscosity measurements to be made for a pattern beneath completely different shear charges, thus facilitating the manufacturing of g viscosity curves, that are basic for non-Newtonian fluids.
The most typical variation of this sort of tools is known as the Brookfield viscometer that operates beneath the Searle precept9 (Determine 5). The motor drives a spindle contained in the fastened cup containing the check fluid. The rotational pace of the spindle causes drag and consequently a sure motor torque. The torque wanted to beat the viscous forces of the examined fluid is due to this fact a measure for its viscosity. This technique can be utilized for each Newtonian and non-Newtonian meals and it’s the most typical sort of viscometer that’s utilized in high quality management due its versatility. The tools operates at completely different rotational speeds and can be utilized with completely different spindles which have completely different geometries. These vary from discs, cylinders, cylinder-cones with completely different diameters and particular shapes together with a number of varieties of paddles and vanes, permitting the person to decide on completely different situations for various pattern varieties, reminiscent of low viscosity, excessive viscosity, particulate and gel-like fluids.
One other particular geometry of rotational viscometers are the double-gap techniques, that are specifically devised for measuring low-viscosity fluids. The cup is a double concentric cylinder, reminiscent of an elongated ring, and the spindle is formed as an inverse cup that turns within the ring-shaped hole of the cup. This configuration maximizes the floor space in touch with the fluid, thus, it is ready to detect low torque values as usually generated by low-viscosity samples.
Determine 5: Diagram of the Brookfield viscometer (far left) and completely different spindle geometries, from left to proper, disc, cylinder-cone, paddle and vane geometries.
The same precept can be utilized in parallel plate and cone plate geometries. In these sorts of geometries, a plate or a cone rotates over a pattern positioned on a hard and fast plate (Determine 6). These techniques function with slender gaps between the shifting geometry and the fastened plate, due to this fact solely a small quantity of pattern is required. Nevertheless, the small hole limits the approach to non-particulate samples and by being open on the facet, the pattern can escape beneath increased rotational speeds, by the motion of turbulent move and centrifugal forces. Alternatively, the wedge-shaped hole brought on by the cone ends in a relentless shear charge being utilized over the complete hole, yielding benefits for the measurement of absolute viscosity values.2
Determine 6: Diagram of parallel plate (left) and cone plate (proper) geometries.
– Vibrational viscometers and oscillation viscometers
Vibrational viscometers function by measuring the damping of an oscillating resonator immersed within the check fluid, which vibrates like a tuning fork (Determine 7A). An electromagnetic drive controls the vibration of the sensor plates, and the power concerned within the resonance might be decided by measuring the power used to keep up the vibration at a relentless amplitude, because the damping imposed by the fluid is immediately proportional to its viscosity. One other various is to find out the decay time of the oscillation as soon as the facility enter is minimize, because the decay can also be immediately proportional to the fluid’s viscosity.10
Oscillation viscometers use a single probe (a sphere, rod or plate) vibrating in touch with the check fluid (Determine 7B). A vibrating floor in touch with the check fluid imposes shear stress as a result of floor motion, so the viscosity might be decided by measuring the facility vital to keep up a relentless amplitude of oscillation.10
Vibrational and oscillation viscometers yield benefits reminiscent of excessive sensitivity on a variety of viscosities (from low viscosity liquids to semisolid or clogging fluids), don’t have any shifting elements and are simple to keep up. This attribute makes these kinds of tools the gold commonplace for in-line measurements in pipelines and tanks.
Determine 7: Diagram of vibrational (A) and oscillating (B) viscometers.
– Different sort of devices
There are additionally imitative viscometers that depend on empirical rules to imitate the move of non-Newtonian fluid meals beneath operational situations. This easy equipment usually gives a one-point measurement. Though they’ve restricted sensitivity and working vary, they are often helpful in high quality management and tough measurements. An excellent instance of this class of apparatus is the Bostwick Consistometer (Determine 8), comprising a pattern compartment (a) with a gate operated by a spring set off (b). As soon as the gate is opened, the check fluid flows over a ramp (c) sitting at a selected angle. After a longtime time, the size travelled by the flowing pattern is recorded, and is proportional to the product’s viscosity. This tools is simple to function, strong and gives quick and constant dedication, being significantly fitted to sauces, pastes, jams and all sort of extremely viscous and particulate-containing meals.
Determine 8: Diagram of the Bostwick Consistometer.
As beforehand mentioned, viscometers are normally easy-to-operate devices, present quick outcomes and might even be utilized in on-line measurements. Nevertheless, in an effort to measure viscosity accurately, a sequence of things have to be thought of, specifically:
- temperature variations
- part separation
- non-uniform samples as a result of presence of enormous particles
Subsequently, in line with the specificities of the product being measured, probably the most acceptable sort of instrument have to be chosen.11
Within the case of course of management devices, they’re put in inside the processing plant set up (e.g., tanks or pipes) in an effort to acquire real-time information of the method. Tube sort, vibrational or oscillation sort viscometers are ideally used for these kinds of purposes.12 Relating to high quality management measurements, the place samples are taken on a random foundation to validate the processing situations and the standard requirements of a product, rotational, “U” tube viscometers or Bostwick Consistometers are sometimes used. These devices are the first alternative in these purposes as they’re versatile, can present measurements for a number of varieties of samples and processing situations (e.g., huge viscosity vary and operational temperature) and might rapidly present the mandatory information in industrial situations. If wanted, they are often cleaned rapidly and tailored to different operational situations, permitting the measurement of a number of samples with completely different specificities in a brief interval. It’s this sort of versatility and fast response that yields the best benefit of viscometers towards probably the most exact devices reminiscent of rheometers.13
Makes use of of viscometry within the meals and beverage {industry}
Many components might affect the standard and stability of meals, and viscosity is unquestionably one of many facets receiving extra consideration throughout the manufacturing course of. Relating to meals manufacturing, viscosity is a key parameter in optimizing move situations, dimensioning pumps, pipelines, power consumption, lowering waste and within the management of the ultimate product’s high quality.1,14
Viscosity can also be very related within the formulation phases of meals merchandise. Let’s take the instance of bakery merchandise, during which various phases (i.e., flour, water, fats, air) are blended to a secure dispersion. In an effort to preserve that dispersion, keep away from part separation and retain air throughout mixing and baking, the system should maintain enough viscosity. The identical precept applies for the preparation of foams, emulsions and colloidal suspensions reminiscent of in sauces, soups, custards and so forth.
High quality management is one other space the place viscosity determinations are sometimes carried out and are basic for the meals {industry}. They can be utilized to manage the standard of the elements, reminiscent of syrups, oils and liquid egg amongst others. This not solely validates their suitability for use on the processes, but in addition permits analysts to foretell deviations and carry out corrections on the formulations. Relating to the ultimate product management, viscosity determinations are commonplace strategies to validate the product’s high quality and stability and guarantee they meet technical specs.
However the significance of viscosity determinations within the meals {industry} reaches far past the classical manufacturing fashions and additionally it is important in aiding the continuing meals revolution. Fueled by environmental, moral and societal adjustments, manufacturing and consumption patterns are being redefined. New meals, reminiscent of meat analogues, goal to cut back or shift protein consumption from animal-based to plant-based sources. The event of meat options and meat analogues relies on progressive strategies reminiscent of 3D printing or excessive moisture extrusion, utilizing protein-based elements in addition to gums and different polymers. The purpose of those processes is to create the suitable texture and consistency to emulate the meat construction. Viscosity was discovered to be a key issue, defining the design of apparatus, processes and being a determinant issue within the last properties of the developed meals.15,16, 17
The design of recent meals additionally follows the development of offering further security and well being advantages. Meals merchandise developed for particular pathologies at the moment are a actuality, and among the many commonest circumstances are formulations concentrating on deglutition management in child meals or dysphagia sufferers.18 It has been reported {that a} rational design based mostly on the management of viscosity at particular shear charges diminished the danger of asphyxiation.19 One other motion space is the connection between the stomach-emptying charge and satiety, digestion and glycemic index. The truth is, it was established that the viscosity of meals performs a decisive function in gastric-emptying, and due to this fact the design of tuned viscosity meals might deliver essential implications in public well being, doubtlessly serving to to manage diabetes and weight problems.20
Subsequently, it may be concluded that the dedication of the viscosity in meals and drinks and their elements is essential in high quality and course of management, and in addition impacts the consumption, sensory expertise, security and well being of the shoppers. The strategies used to measure viscosity are sometimes easy and fast, however cautious choice among the many varied check tools and situations is prime to acquire high quality measurements. The choice will probably be decided by the kind of meals product and software, and is influenced by the temperature, pattern homogeneity, part adjustments and sort of move.
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13. Rao MA. Rheology of Fluid, Semisolid, and Stable Meals. New York, NY: Springer US; 2014. doi:10.1007/978-1-4614-9230-6
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17. Dekkers BL, Growth RM, van der Goot AJ. Structuring processes for meat analogues. Traits Meals Sci Technol. 2018;81(Might):25-36. doi:10.1016/j.tifs.2018.08.011
18. Frazier J, Chestnut AH, Jackson A, Barbon CEA, Steele CM, Pickler L. Understanding the viscosity of liquids utilized in toddler dysphagia administration. Dysphagia. 2016;31(5):672-679. doi:10.1007/s00455-016-9726-6
19. Sukkar SG, Maggi N, Travalca Cupillo B, Ruggiero C. Optimizing texture modified meals for oro-pharyngeal dysphagia: A troublesome however doable goal? Entrance Nutr. 2018;5. doi:10.3389/fnut.2018.00068
20. Jin Y, Wilde PJ, Hou Y, Wang Y, Han J, Liu W. An evolving view on meals viscosity regulating gastric emptying. Crit Rev Meals Sci Nutr. 2022:1-17. doi:10.1080/10408398.2021.2024132
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