Theme
Secondary title

Author
Keywords“Food products evaluation technology using viscosity”(Measurement of the physical properties of thickening
agents and powdered food products in relation to theevaluation of dysphagia)
Naoto IZUMO
Thickening, gelling, tuning fork vibro viscometer,
static viscosity, viscosity of powders

1) Introduction
The word viscosity is not used very often day to day, however you will often
hear words which express the viscous characteristics of liquids, such as
watery or sticky. The history of measurement of viscosity, which expresses
these characteristics of liquids, is long and stretches back to the Industrial
Revolution. Oil was used to reduce friction in the axle bearings when theforce of steam engines was converted to rotary movement; documents showthat a need arose at that time to evaluate the viscosity of that oil.*1
However, this use didn’t become commonplace until the culmination of theIndustrial Revolution with the mass production of the automobile. At thispoint, precise control of engine oil flow in a combustion engine, where it is ofabsolute necessity, became widespread as an essential part of the
manufacturing process.
At first, viscosity of fluids was measured by the proportionate time taken
to pass through the small opening at the bottom of a funnel. This method
survives today with such devices as capillary type viscometers, with thetime taken to pass through an orifice of set circumference being
proportional to [viscosity ÷ density], allowing viscosity to be calculated. Thismeasurement method was the first established as a standard to measureviscosity, together with water as a standard substance for measurement,
due to its universal viscosity characteristics.
Following this development, further methods for measuring viscosity were
established, such as the rotational method of immersing a rotor in a liquid
to measure the torque required to rotate the rotor, with this torque being
proportional to viscosity; or the falling ball method of dropping a metallic
ball into a liquid with the time taken for the ball to drop also being

proportional to viscosity. These methods each had their own drawbacks andadvantages, however in general all of them used large-scale devices which
required significant amounts of liquid to be properly used. The range ofviscosities which could be measured by these devices was quite small andthey couldn’t provide stable measurement results for liquids with a low
viscosity such as water, a liquid used as a viscosity standard. They all also
had the added problem of difficulties converting their values to digital
signals.
In recent years, with the development of electronic parts using
semiconductors, measurement and control technology has progressed
rapidly. As a result, a new type of viscosity measurement was proposed
about 50 years ago which differed from the three methods described above:
the vibro viscometer, which was commercialized relatively recently.*2 In theinitial stages of selling vibro viscometers, as there had been no change inthe field of viscosity measurement for many years there was a rejection of
the proposal of a new method. However in recent years, as a result of thespecial features of vibro type viscometers earning a strong reputation, they
have now joined the ranks of officially recognized standards in viscosity
measurement, with their acceptance in the JIS Viscosity Standards
Revision of 2011.*3 Further, prior to that acknowledgement, they were also
designated as a JCSS-recognized device.*4
Viscosity measurement has grown in scope significantly in recent years
beyond its traditional realm of heavy industry, where it is used to measureoil, petroleum, etc. It is now being put to use in many fields, including in
the development of new materials such as nanocolloids, in ink for printing,adhesives or food products. Moreover, as we face the increasing problem ofan aging population, tuning fork vibro viscometers are starting to be usedin new applications relating to health, such as in the measurement ofimportant indexes like “thickening” or “gelling temperature” which arevital in countermeasures to avoid the accidental ingestion of food or
drink.*5 One reason the tuning fork vibro viscometer is chosen for researchsuch as this is there was previously no such high-sensitivity viscosity
measurement device with excellent repeatability available which could
measure so simply and clearly the subtle differences in the sensations ofliquids that humans can distinguish. Another reason given is thatalternative measurement devices can be extremely expensive.
This paper summarizes the results of tests measuring the viscosity ofcharacteristic liquids and powders used in thickening and food products,
etc., using a tuning fork vibro viscometer and tuning fork vibro rheometerwith a low frequency drive of 30Hz. Using this method, it is now possible toevaluate the basic viscous properties of materials which could not bemeasured easily until now without causing damage to the material.
2) Principles of measurement for the tuning fork vibro rheometer
The difference between a tuning fork vibro viscometer and tuning fork
vibro rheometer is whether the amplitude of the oscillators (which act as
the sensor unit) is set at a fixed value or can be changed. Here the principlesof the tuning fork vibro viscometer, which forms the basis of the sensing
method, are explained.
○ Theoretical development of tuning fork vibro type
A tuning fork vibro viscometer, similarly to an actual tuning fork, has two
oscillators, or sensor plates, that resonate in the direction parallel to theirfaces. The vibration energy needed to maintain the set amplitude of the two
oscillators is generated by electromagnetic power. In other words, a drive
force equivalent to the viscosity resistance of the liquid is exerted and as
this drive force is proportional to viscosity resistance, the viscosity value ofthe liquid can be known. For vibro-type viscometers, in principle viscosity
resistance is equal to [viscosity × density]. We call this viscosity × density
value [static viscosity]*6 to differentiate it from the kinetic viscosity value
given by capillary type viscometers and the dynamic viscosity value given
by rotational types as well.
With tuning fork vibro method, by calculating the drive force necessary toresonate the oscillators of the tuning fork and the motion equation relating
to the inertia term, viscosity term and elasticity term, it can be understoodthat the energy that drives the tuning fork oscillators is proportional toviscosity × density.

Regarding the details of a theoretical model for a tuning fork vibro
viscometer, please refer to a separate paper.*2 This paper only explains theconclusions of related study. For the setup in Fig. 1, when the oscillators
vibrate with a frequency of f, the mechanical impedance (Rz) received by theoscillators from the liquid can be calculated as:Rz  A πfwith each of the terms on the right side of the equation representing:f:vibration frequency (Hz); A: surface area of both sides of the oscillating
sensor plates;  : viscosity of the liquid;  : density of the liquid. Here, if
we make the force exerted on the oscillators by the electromagnetic drive
unit F, and its resulting constant vibration velocity Ve it , the equation canalso be expressed as:FRz  it  A πfVe
From the equation above it can be understood that the force from theelectromagnetic drive unit is proportional to static viscosity (the product ofviscosity  and density  ). In actual measurement, the proportionality ofstatic viscosity [viscosity × density] to electrical current is used to measureviscosity, as current is adjusted to a level necessary to control the torquegenerated for the electromagnetic drive unit to maintain a constant
amplitude for the oscillators in the sample liquid.
Spring plate
Displacement sensor
Electromagnetic drive

Temperature sensor
SampleOscillator(Sensor plate)

Vibration

Fig. 1

Sensor unit configuration

Fig. 2

Photo of entire system

各振幅での粘度測定範囲とニュートン流体(水、粘度校正用標準液)測定時のずり速度(実効値)の関係
100000ニュートン流体(水、粘度校正用標準液)の振幅、粘度、ずり速度(実効値)の関係(0.07 , 25000)

10000

粘度測定範囲10000

5s(0.6 , 5000)

-1

1000

1 0 s- 1

JS200

JS2000JS14000

6 0 s- 1

振幅1.2mm100

振幅1.0mm
振幅0.8mm

8 0 s- 1

振幅0.6mm

1 0 0 s- 1

1

JS20

1000
5 0 s- 1

4 0 s- 110010イオン交換水(1.2 , 3000)
3 0 s- 1ずり速度(1/s)

粘 度 値 (m P a・ s )2 0 s- 1

2 0 0 s- 1
3 0 0 s- 1

0.1 (0.2 , 0.3)
0
0.2

10

4 0 0 s- 1
5 0 0 s- 1

8 0 0 s- 1

振幅0.4mm

1 0 0 0 s- 1

振幅0.2mm

6 0 0 s- 1

(1.2 , 0.3)
0.4

0.6
0.8
振動子の振幅値(mm)1Fig. 3 Measurement range of viscosity

振幅0.1mm
振幅0.07mm

1

1.2

0.1

1

10

100

1000

10000

100000

粘度値(mPas)

Fig. 4

Viscosity standards and shear rate
distribution○ Characteristics of tuning fork vibro method
The configuration of a tuning fork vibro viscometer’s sensor unit isdetailed in Fig.1. This method of using such a displacement sensor andcontrolling the acceleration force to keep that sensor’s output(displacement) at a constant level using electromagnetic force has the sameessential framework as the electromagnetic mass detection unit used in ananalytical balance. Also, the spring plate which provides the natural
vibration frequency also shares common elements with electromagnetic
force balances. With this technology, the resolution of the tuning fork vibro
viscometer calculated by dividing the maximum measurable viscosity of
10000mPa.s by the minimum sensitivity of 0.01mPa.s is as high as
1/1,000,000, equivalent to that of an analytical balance.
Fig.2 is a photograph of the full framework of a tuning fork vibrorheometer. From the left, the viscosity detection unit, control unit andspecial controller are displayed.
The graphs displayed in Fig. 3 and 4 are of data taken from the tuning
fork vibro rheometer and show the distribution of shear rate. With the vibro
method, only the oscillators are moving in the liquid and there is no
opposing surface which could be used to determine the shear rate. On theother hand, even under the same measurement conditions, the distance atwhich shear occurs essentially differs depending on the viscosity value ofeach different liquid. Accordingly, even under the same acceleration

conditions or with the same rotary movement, the distance for shear rate
differs depending on the viscosity of each liquid. Because of these reasons,the shear rate for the tuning fork vibro viscometer is calculated from the
known viscosity value of a viscosity standard and the shear stress value,
which is found by dividing the torque (force) needed to move the oscillatorsin the viscosity standard by the total wetted surface of the oscillators.
Incidentally, viscosity could be defined by the following equation: [shear
stress] = [viscosity] × [shear rate] and hence viscosity is a coefficient ofshear stress divided by shear rate.*7
Regarding shear rate of the tuning fork vibro method, as the oscillators
move in a back and forth motion, there is no constant value in relation to
time. Therefore, shear rate is defined by its effective (root-mean-square)value.
In addition to the technical details of the tuning fork vibro viscometer
described above, the fact that the resonance frequency is as low as 30Hz
and the basic amplitude of the oscillators is as small as 0.4mm provides thereasons why high precision viscosity measurement is now possible with theminimum of changes to the structure of the object being measured.
3) Examples of measurement with tuning fork vibro rheometer
Results of measurements of various liquids using a tuning fork vibrorheometer or viscometer are detailed below. The y-axes of the graphs below,
which should express static viscosity [viscosity × density], hypotheticallytake the density (specific gravity) of the liquid being measured as 1, so as torepresent viscosity.
3-1) Measurement of Newtonian fluids
Fig. 5 & 6 show measurement results while changing the vibrational
amplitude of the oscillators for water, a representative Newtonian fluid, andthe viscosity standard JS2000.
It is clearly shown in the graphs that both water and JS2000 exhibit the
Newtonian fluid characteristic of not changing viscosity in response to achange in vibrational amplitude.

Temperature

Temperature

Viscosity

Viscosity

Fig.5 Ion－exchanged water

Fig.6 JS2000

3-2) Measurement of thickeners
Fig. 7, 8 & 9 show the results of measurements of typical thickener
xanthane gum at different concentrations. The results show significant
non-linearity of viscosity according to concentration and data was obtained
confirming the non-Newtonian characteristics of these thickener fluids inrelation to vibrational amplitude.

Viscosity
Temperature

Fig.7 Xanthane gum 1%

Viscosity

Temperature

Fig.9 Xanthane gum 10%

Viscosity

Temperature

Fig.8 Xanthane gum 5%3-3) Measurement of powders
The jury is out on whether the viscous resistance that occurs in powders
should be called viscosity. However, using the tuning fork vibro method, it is
now possible to measure, with good repeatability, the viscous resistance of
powders which was previously difficult to quantify using other methods. Fig.
10 to 14 show measurement results for flour (both high gluten content andlow gluten content) and starch in their original powder form, as well as low
gluten flour in solutions with water.
For the viscous resistance of powders, the powder’s “bulk density” is an
influencing factor. Hence mass measurement was performed on the
powders in a fixed measurement vessel after letting the powder freely fall
into the vessel and then completely leveling them. They were thencompacted until they reached a defined volume by applying a constantpressure by vibration. The bulk density value at that time was sought andviscosity measurement was then conducted.
When shear rate increased, the static viscosity of the powders suddenly
decreased. The hysteresis relative to shear rate is almost uniform whenincreasing and when decreasing. However, only in the case of starch was
the hysteresis large. Further, when the ratio of water mixed with the low
gluten flour increased, hysteresis was confirmed relative to shear rate witha sudden drop in static viscosity simultaneously. A drop in static viscosity
was also seen at a middling shear rate.

Temperature

Temperature
Viscosity

Viscosity
Fig.10 Low gluten flour(bulk density 0.49g/cm3)

Fig.11 High gluten flour(bulk density 0.56g/cm3)

Temperature
Viscosity

Fig.12 Starch(bulk density 0.70g/cｍ3)

Temperature
Viscosity

Temperature
Viscosity

Fig.13 Low gluten flour 100% + water 100% Fig.14 Low gluten flour 100% + water 200%3-4) Other viscosity measurements
In addition to thickeners and powders, measurement results for food itemswith special characteristics, etc. are summarized below.
○ Measurement of gel point
Fig.15 graphs the change in viscosity for gelatin in relation to a change intemperature. With the vertical axis forming a logarithmic scale, it was
confirmed that gel point differs according to density.
○ Measurement of the curing process of egg whites
Fig.16 shows the curing process of egg whites. Egg white consists ofseveral different proteins which will change their structure attemperatures from 60°C or 70°C. This was confirmed by changes in theirviscosity.

○ Measurement of the clouding point of non-ionic surfactant
Fig.17 shows the results of the measurement of clouding points. The
clouding point, which occurs due to structural changes in micelles, isdetected optically and measured by the temperature at that time. With thetuning fork vibro viscometer, it is also possible to capture this clouding
point as a change in viscosity. For this measurement sample, the JIS
standard mentioned above prescribes the clouding point of 35.9°C, and froma change in viscosity a clouding point of 35.4°C was confirmed.
○ Measurement example for oil
As a typical example, the temperature-viscosity curve for oil for a gasoline
engine is presented in Fig. 18. It is said that if the viscosity of engine oil
falls below 3mPa.s, burning occurs between the pistons and cylinders. It
can be conjectured from the graph that the maximum temperature the oil
can be used at is 110°C.

粘 度 （m P a･s）

硬 化 過 程5% solution

凝 固 （非 流 動 性 ）凝 固 過 程 （流 動 性 ）蛋 白 質 の凝 固 開 始

2.5% solution

温 度 （℃ ）Fig.15 Gelatin

Fig.16 Egg whiteFig.17 Non-ionic surfactant

Fig.18 Oil for gasoline engine

4) Conclusion and considerations○ Evaluation results for thickeners
For recent thickening agents, a strong thickening effect is being demanded
for a small amount of additive. This could be considered an appropriate
development from an economic perspective, and in fact, when there was achange in the amount of thickener, the change in viscosity value wasconfirmed to be more than a one digit shift per 9% change in the proportionof thickener. This points to the extreme importance of volume managementof thickeners when added to food products in the course of nursing care.
Additionally, it has also been understood that for thickeners, the change inviscosity in relation to change in shear rate is small, as is the change inviscosity in relation to change in the speed the food product is drunk.
These special characteristics of thickeners can be considered a valuable
capability at the place of care. Specifically speaking, the capability of theviscous resistance linearly changing relative to a change in shear rate
suitably matches a person’s actual physical sensations of viscosity.
Accordingly, it can be surmised that for thickeners for the prevention of
accidental ingestion or aspiration, not only is the capability to increase
viscosity essential, but so too is having non-Newtonian characteristics as
small as possible.
○ Viscosity evaluation results for powdered food products
We presently cannot say that there is an established evaluation method
for the special viscous characteristics of powders which are used as foodproducts. That is, existing evaluation methods of the physical properties ofpowders are not represented as SI units (International System of Units).
We could say this is clear from the fact that the data taken from testingmachines for powders express the measurements in units particular to that
testing machine.
In our measurements using the RV-10000A Tuning Fork Vibro Rheometer,
the viscous resistance values for various powders for use as food products
were obtained. The instrument was calibrated before the measurements
were conducted by the viscosity values of Newtonian fluids which can act asviscosity standards. Because of this, these measurements can perhaps bethought of as the first measurement and evaluation results of the viscosityof powders using an SI unit.
The viscous resistance figures taken from vibro viscometers are expressedby [viscosity × density] in principle. Supposing that the viscous resistanceof powders was expressed by viscosity and density (bulk density), thisexperiment can be judged to deliver precise measurement of the viscosity of
powders by first seeking their bulk density.
○ Shear rate
Nearly all naturally-occurring liquids are Newtonian fluids with aconstant viscosity value in relation to shear rate. This means that theviscous resistance proportional to shear rate is obtainable. However,approximately 80% of the liquids which are synthesized are said to be
non-Newtonian fluids, in which the viscous resistance is notproportional to shear rate. For these non-Newtonian fluids, as theamount of shear rate applied to the liquid differs depending on theviscosity measurement method, discrepancies in measurement results
occur according to the measurement method used.
While the following is merely personal opinion, there are several points
that should be given consideration with regards to shear rate. First of all,an assumption of the logic stated above is that when the internal andexternal walls of a container filled with a liquid is set into relative motion,a uniform shear rate is applied to the liquid enclosed by the internal andexternal walls, produced by the difference in rotation with the internaland external walls. However, for non-Newtonian fluids, it is not proventhat the difference in rotation in a structure with internal and external
walls, or top and bottom surfaces, exerts a uniform shear rate on theliquid which has been poured inside.
In other words, the shear rate applied to non-Newtonian fluids adheresto the principles of uniform shear rate applied to Newtonian fluids, and if
it was the case that a uniform shear rate was not applied to a
non-Newtonian fluid, this would not hold true.
The “shear rate” determined with existing rotational viscometers orrheometers is determined from the above assumption of uniform shear
rate, and the geometrical shape and relative motion between rotating
bodies in relation to Newtonian fluids. However, with non-Newtonian
fluids, the fact that the propagation range of the “shear rate” differs from
Newtonian fluids under the same conditions can be thought of as inviting
non-linearity in viscosity values in relation to change in “shear rate”.
This makes it difficult not to doubt the adequacy of current viscosity
assessment methods where a constant shear rate is assumed based on aconstant rotation.
This can also be deduced from the fact that the validity of thephenomenon of a large change in the viscosity value of the material fromonly a slight difference in shear rate is not proven as a change in thephysical properties of the material.
In the present experiment with thickeners, the changes in viscosity
due to change in shear rate were comparatively small, which leads us toinfer that until the shear rate becomes significantly high, a change in thebonding force from molecular chains or crosslinks will not ensue. Fromthese inferences, it could be considered that the non-linearity of
non-Newtonian fluids is caused by differences in the speed of theequipment, which generates shear rates. Such differences will result inchanges in the reach of shear force and the distribution of shear rate,causing apparent changes in viscosity values.
With regards to these problems, the developer and supplier of viscosity
evaluation devices should have a duty to users to provide a lot more
information. Manufacturers, including A&D, need to increase theirefforts to provide data and evidence to support such information.

References*1“History of flow measurement”, Yutaka Ogawa (Oval Ltd.), Measurement
Technology, September 2004

*2

“Revised Viscosity”, Hiro Kawada, Japanese Measurement Management Society
Periodical, 1958

*2

“Vibro Viscometer Prototype”, Eiichi Fukada, Japanese Polymer Society Journal
Vol.6 [38]*2“Enhancement of the Vibroplate Viscometer” Material Testing Volume 8, Issue
67, Eiichi Fukada et al.

*3“Viscosity Measurement Methods for Liquids”, JIS Z8803:2011, May 20, 2011(revised)*4

JCSS for Viscosity: JCT20601 Application guidelines for technologicalrequirements (viscosity)*5“Design of Controlled Release Liquid Oral Medication to Contribute toCompliance Improvement For Geriatric Care”, Hokkaido Medical University
Department of Pharmaceutical Science, Kunihiko Ito, Non-prescription Drugs(2010), Foundation for Self Medication Promotion Symposium*5“Subjective Views on Thickening Adjustment between Branches of Medicine”,
Noriko Suzuki & Tota Inoue, Mie Breathing & Swallowing Rehabilitation Clinic,
20th Annual Meeting of the Japanese Society of Dysphagia Rehabilitation,
September 6, 2014

*6

“Static Viscosity and Vibro Viscometers”, Naoto Izumo & Atsushi Koiwai, A&D
Company Limited, APMF2006 hosted by The Society of Instrument and Control
Engineers, October 2, 2006

*7

“Study of shear rates relating to the measurement of ‘static viscosity’”, Naoto
Izumo, Masahiro Kanno, et al., 30th Sensing Forum hosted by The Society of
Instrument and Control Engineers, August 2013