heology is the branch of science that deals
with the flow and deformation of materi-
als. Rheological instrumentation and rheo-

logical measurements have become essential tools
in the analytical laboratories of food companies for
characterizing ingredients and final products, as
well as for predicting product performance and
consumer acceptance.

The materials under investigation can range from

low-viscosity fluids to semisolids and gels to hard,
solid-like food products. A knowledge of the rheolog-
ical and mechanical properties of various food sys-
tems is important in the design of flow processes for
quality control, in predicting storage and stability
measurements, and in understanding and designing
texture. Quality attributes such as spreadability and
creaminess are extremely important to the accep-
tance of semisolid food products by consumers.

In the case of food materials, texture is a key

quality factor. Rheological behavior is associated di-
rectly with textural qualities such as mouth feel,
taste, and shelf-life stability. As an example, rheo-
logical measurements are useful in storage stability
predictions of emulsion-based products such as
mayonnaise and salad dressings.

Importance of acceptable food rheology

Consumers use subjective tests to determine the

p e rceived quality of a food product. For example,
the determination of fruit firmness is based on the
deformation resulting from the physical pressure
applied by the hands and fingers. The toughness or
tenderness of meats is based on the effort required
for the teeth to penetrate and chew the flesh tissue.
Food companies use instruments in an attempt to
objectify these subjective customer perceptions and
acceptance criteria. Objective instrument tests for
texture also rely on deformation of the food mate-
rial. Toughness can be defined as the maximum
f o rce required to slice through a sample. Firmness
can be defined in terms of the force required to de-
form a body of material.

The quality of a food product depends strongly

on its formulation. Certain ingredients called modi-
fiers have a larger influence on the finished prod-
u c t ’s properties than others. Examples of modifiers
are stabilizers, emulsifiers, and structural agents.
The type of modifier and its desired effect dictate

A p p l i c a t i o n N o t e

JUNE 2000

Rheology of foods: New techniques, capabilities,

and instruments

Peter K.W. Herh, Steven M. Colo, Nick Roye, and Kaj Hedman

the type of rheological measuring technique used.
An antisettling agent should be tested at low shear
conditions, simulating settling in a bottle. A sample
with an antisettling agent will only be stable for a
specific period of time. Therefore, the desired shelf-
life stability must be known or determined. Stabil-
ity is almost exclusively a consumer preference.
Most food products change their appearance and
texture upon storage. Therefore, an objective, quan-
titative method for determining their stability is
needed. Rheological measurements can be used to
predict shelf-life stability once there is historical
data on a given product. Comparing a sample with
acceptable life versus an unknown will provide a
measure of shelf-life at the production stage, with-
out the need to wait the weeks or months that
could be necessary for the actual test sample to set-
tle or phase-separate.

In food products, small changes in the amount of

additive can have a dramatic effect on the final prod-
uct. The formulation of popular sports drinks can be
used as an example. To produce an acceptable lemon-
based drink, 1.5% stabilizer/emulsifier is needed in
the water/oil interface. At 1.5%, the drink is water
based. However, an increase to 1.55% changes the
product to an oil-based drink and results in a totally
undrinkable product. In addition, the oil-based prod-
uct will show a drastic increase in viscosity with time
compared to the water-based drink, which is com-
pletely stable. A simple rheological test could be the
monitoring of viscosity at low shear rates to establish
if the correct amount of stabilizer is added.

A switch in one of the raw ingredients can also

have a dramatic effect on the final product. For ex-
ample, if the Florida orange crop is not up to stan-
dards, a producer of fruit juice may have to switch
to fruit from California or Spain. Due to the fruit's
geographically different environment, the natural
pectin content will differ. A change in formulation
will be required to achieve a similar product.

Rheological instrumentation

In principle, every type of consumable food prod-

uct has some rheological characteristic. Most readily
consumable food products contain ingredients that
have a major impact on the rheology of the final
product. Food processors and raw material manufac-
turers have been aware of the importance of viscos-
ity for many years. To d a y, rheological instrumenta-
tion is considered a required analytical tool utilized
by food scientists on a daily basis. These instruments
are

M i c r o s o f t

Windows™ (Redmond, WA ) - b a s e d ,

and measurements are made quickly and easily with
the use of straightforward, user-friendly software.
The operator simply loads the sample into the in-
strument and selects the appropriate experiment,
and the instrument does the rest.

Food products are complex mixtures of different

ingredients where individual ingredients are mixed
together to produce a finished product. In many
cases, the individual ingredients consist of mixtures

A knowledge of the rheological
and mechanical properties of
various food systems is important
in the design of flow processes for
quality control, in predicting
storage and stability
measurements, and in
understanding and designing
texture.

Figure 1

STRESSTECH research rheometer.

Figure 2

Strain sweep on mayonnaise.

Figure 3

Frequency sweep on mayonnaise.

AMERICAN LABORATORY

of solid as well as fluid components. Most times,
they are not homogeneous, and the properties vary
throughout the sample. Tr a d i t i o n a l l y, single-point
viscosity tests have been performed using empirical
techniques. These simple viscosity experiments ex-
press the complex rheological response of a sample
into a single parameter, and are not adequate in
characterizing and/or providing insight into the
quality of food materials. The ingredients used to-
day are numerous and expensive, and, as a result,
the cost for controlling these ingredients is high.
Detailed knowledge and an objective, reproducible,
multipoint measurement capable of decomposing
the rheological behavior into individual compo-
nents are necessary. The STRESSTECH rheometer
(

ATS RheoSystems,

Bordentown, NJ, and

R E O -

LOGICA Instruments AB,

Lund, Sweden) used in

this study provides all of the required instrument
f eature s

an d

capabilit ies.

The rheometer is a research-grade analytical in-

strument capable of measuring viscous, elastic, and
viscoelastic properties of liquids, semisolids, and
solids (F i g u re 1). The instrument was developed for
use by the serious rheologist, and provides a very
broad measurement range, spanning low-viscosity
samples such as fruit juice to more viscous products
such as creams and salad dressings, semisolids and
gels through hard cheeses and solid-state samples.

The rheometer incorporates the following fea-

tures: wide torque, shear stress, temperature, shear
rate, and frequency range; true Microsoft Windows-
based operational software; patented Differential
Pr essure Quantitative Normal Force; patented
Sealed Cell measuring system for testing samples

above their boiling point; automatic gap setting; re-
mote diagnostics capability via modem; and auto-
matic inertia compensation. In addition, all

AT S

R h e o S y s t e m s

rheometers are designed on a modu-

lar platform allowing easy upgradability. A wide
range of accessories satisfy the most demanding ap-
plications with ease of operation.

User-selectable and quantitative, controlled axial
normal force sample loading

Although transient steady shear and periodic dy-

namic oscillatory experiments provide information
on the rheological properties of food products, they
do not completely characterize the system. Con-
cerning food samples, or any complex, two-phase
system, the rheology is dependent on the sample’s
deformation history, loading conditions, and the ax-
ial normal force applied during a measurement. For
example, stress/strain sweeps were performed on a
c o m m e rcial mayonnaise product. The results of a
mayonnaise sample run immediately after loading
into the rheometer, and a new sample 300 sec after
loading into the rheometer, are shown in F i g u re 2,
where shear moduli (G

′

and G

′ ′

) are plotted as a

function of shear stress. The sample run without the
300-sec rest period exhibits lower properties since
the internal structure did not have sufficient time to
rebuild prior to testing. To dissipate this residual
loading history, it was determined that a 300-sec de-
lay time was required after a controlled normal forc e
loading of 1 Newton. The results indicate that both
the linear viscoelastic region and the crossover point
of G

′

and G

′ ′

are affected by the loading condition.

This phenomenological shear history and loading
effect require that all structured samples be run un-
der user-selectable controlled normal force condi-
tions. In addition, all samples must be provided a fi-
nite time for their respective internal structure to
rebuild after loading.

Consid ering dynamic oscillation frequency

sweeps, the viscoelastic properties are also depen-
dent on the residual loading history, as shown in
F i g u re 3. To overcome the sample-dependent load-
ing criteria, the rheometers are available with
Patented Differential Pressure Normal Force capa-
b i l i t y. With this highly sensitive, robust, and accu-
rate axial loading and measurement sensor, con-
trolled normal force load ing and quantitative
measurements of normal force, first normal stress
difference, and normal stress coefficient can be
made as a function of time, temperature, rate, and
stress. In addition, the rheology of any material can
be determined independent of its sensitivity to
loading conditions.

Sensory evaluation methods for liquid foods

Systematic analysis of texture is very important

for food product development. Texture is a key
quality factor in food. One of the most important
textural terms obtained is the analysis of thickness.
Consumers usually associate changes in thickness
with changes in viscous behavior of food materials.
To develop predictive correlation between thickness
and rheological properties of foods, it is necessary
to understand the deformation process in the
mouth. The flow properties of liquids can be di-

Figure 4

Steady shear rate sweep on maple syrup.

Figure 5

Steady shear rate sweep with first normal stress on honey.

Figure 6

Steady shear rate sweep and first normal stress on syrup.

Figure 7

Steady shear rate sweep and first normal stress on chocolate pudding.

vided into two main groups: 1) Newtonian, in
which a sample's viscosity is independent of ap-
plied shear, and 2) non-Newtonian, in which a sam-
ple's viscosity is dependent on applied shear. F i g u re
4 shows these two types of flow properties on pan-
cake syrup. The results indicate the regular syrup is
predominately Newtonian, while the lite syrup
shows non-Newtonian flow properties.

Food industries are especially concerned with

variations in taste with changes in flow behavior
and viscosity. In general, it is known that increases
in solution viscosity substantially decrease taste in-
t e n s i t y. Also, it has been shown that increases in
non-Newtonian flow decrease taste intensity. These
rheological properties would allow the systemic de-
velopment of food products designed for desired
texture and taste interactions. Viscosity–taste inter-
actions are predicted by assuming that the rate of
diffusion of the tasting agent to the surface of the
tongue is the controlling parameter responsible for
the intensity of a tasting reaction.

Quantitative normal force measurements

During mixing or agitation, a viscoelastic fluid

will climb the impeller shaft in a phenomenon
known as the Weissenberg effect. This can be ob-
s e rved in the home during the mixing of cake or
chocolate brownie batter.

If a fluid is Newtonian, the viscosity is a constant

and equal to the Newtonian viscosity, and the first
and second normal stress differences are zero. How-
ever, viscoelastic fluids simultaneously exhibit both
fluid-like, viscous, and solid-like, elastic, behavior
and strong normal force response. Manifestation of
this behavior, due to a high elastic component, can
create difficult problems in processing and engi-
neering design.

Utilizing the patented Differential Pressure Nor-

mal Force Sensor, measurements of normal forc e ,
first normal stress difference, and normal force co-
efficient can be made as a function of shear rate as
shown in F i g u re 5 for a commercial honey product.
F i g u res 6 and 7 illustrate steady shear viscosity (

)

and first normal stress difference (N1) for lite syrup
and chocolate pudding, respectively. Obtaining ac-
curate data for food materials is complicated by var-
ious factors such as the presence of a yield stress,
time-dependent and shear-dependent behavior,
and chemical reactions occurring during processing
(e.g., hydration, protein denaturation, and starc h
gelatinization). This new measurement capability
will create significant advances in the utility of nor-
mal stress data by the food industry.

Rheological properties of gelling systems above
their boiling point

Food manufacturers and processors relying on

the functional properties of aqueous gelling agents
are well aware that processing conditions such as
time, temperature, and amount of shear during the
heating can alter the final viscoelastic properties
and thickening power of the resulting gelled net-
work, as well as its gelling ability. To date, acquiring
fundamental information on the viscoelastic prop-

JUNE 2000

erties of aqueous gels has been difficult due to the
experimental requirement of making small ampli-
tude dynamic oscillation measurements on low-vis-
cosity aqueous solutions above their boiling point.
Although rheological characterization of these sys-
tems at elevated temperatures is extremely impor-
tant to researchers in industry, until now there has
not been a viable method available to produce data
on reaction kinetics and rheological properties of
aqueous solutions during gelatin.

A measuring system designed specifically for the

measurement of viscoelastic rheological properties
(G

′

, G

′ ′

, tan delta) of solutions above their boiling

point is the Sealed Cell shown in Figure 8.

The patented Sealed Cell measuring system used

in conjunction with the STRESSTECH rheometer al-
lows measurements under pressure with full dy-
namic oscillation and viscometric capability. The
cell employs a noncontacting, air-bearing seal
rather than standard “O” rings. The air-bearing seal
is effectively frictionless, and permits dynamic os-
c i l l a t o ry testing throughout the frequency range of
the instrument. Aqueous samples along with sol-
vent-based systems can be measured above their
boiling point.

The gelatin behavior of an aqueous system has

been studied. First, a dynamic oscillatory tempera-
ture scan was performed from 30 to 120 °C at a
heating rate of 5 °C/min and a frequency of 0.2 Hz
(F i g u re 9). The results indicate the sample possesses
a low viscosity of 70 mPa s at room temperature,
and gelatin starts at a temperature around 63 °C, as
shown by the increase in viscoelastic properties.
The reaction continues as a function of temperature
and time, and the end result is a thick, gel-like con-
sistency sample by 120 °C. Of particular interest are
the results above 100 °C, where the data indicate
that the properties of the gelled system are stable.
The data integrity is maintained well above the
boiling point of water. No other device can obtain
dynamic oscillatory results, especially on low-vis-
cosity samples, above the sample's boiling point
due to the mechanical friction limitation of the
cell’s seals and bearings.

Rheometer system setup

STRESSTECH is a modular research rheometer

with a wide range of measuring systems and acces-
sories. Measuring systems are available as concen-

tric cylinders, cone/plate, parallel plate, double con-
centric cylinders, sealed/pressure cells, and dy-
namic mechanical analysis (DMA) of rods, bars
fibers, and films. Special measuring systems for low-
volume, high-shear rates, and high sensitivity are
also offered. The measuring geometries can be
made in stainless steel, titanium, polycarbonate, or
any user-defined material. The instrument is sup-
plied standard with a patented Differential Pressure
Quantitative Normal Force Sensor for reproducible
sample loading history, thermal expansion mea-
surements, and quantitative normal stress measure-
ments. The diffusion air bearing has a low inertia
with high axial and radial mechanical stiffness.

The rheometer is operated with a separate power

supply unit that should be left on continuously.
This reduces start-up times and makes it possible for
the instrument processor to maintain values as gap
and other user-defined settings.

STRESSTECH HR, a high-resolution version of the

instrument, allows measurements at microradian
displacement and extremely low applied torque.

Figure 10

DYNALYSER complete rheological characteri -

zation system.

Figure 9

Gelation profile of an aqueous system above 100 °C.

Figure 8

Patented Sealed Cell measuring system.

APPLICATION NOTE

continued

JUNE 2000

tions for unique testing requirements, and can be
reset to default values using default buttons. An ex-
ample is the Oscillation Frequency Step measuring
program, where stresses, delay times, integration
periods, and sample sizes could be set individually
for all frequencies. Another example is the zooming
function, which is presented both in the Vi s c o m e-
t ry Stress Step and the Oscillation Frequency Step,
allowing any number of steps and increments to be
selected. The instrument also performs controlled
strain and constant shear rate measurements, and
has automatic gap adjustments and thermal expan-
sion compensation using the Differential Pressure
Normal Force Sensor. The system enhances mea-
surement reproducibility since the sample loading
history is reproduced identically each time.

Rheometers for all user levels and applications

D Y N A LYSER advanced research rheometers (

AT S

R h e o S y s t e m s / REOLOGICA Instruments AB

) (F i g -

u re 10) are modular research level rheometer systems
designed specifically to address the challenging and
diverse testing needs and requirements of the serious
rheologist. The instrument’s capability and perfor-
mance result from a design and development effort
focused exclusively on input and recommendations
from rheometer users. The rheometer is designed for
testing any rheological significant material, includ-
ing thermoplastics, thermosets, elastomers, semi-
solids, and/or fluids systems.

VISCOTECH (

AT S R h e o S y s t e m s / R E O L O G I C A

Instruments AB

) is an entry-level research rheome-

ter system that is fully upgradeable to a STRESS-
TECH unit as the user’s needs and requirements dic-
tate. DSR QC is a dynamic shear rheometer (DSR)
designed specifically for routine viscoelastic mea-
surements in the QC laboratory.

All the rheometers described here are produced

according to ISO 9001 and are tested to operate ac-
cording to the electromagnetic compatibility rules
within the European Community. The instruments
are tested to be labeled with the CE-mark.

Conclusion

This article reviewed the important rheological

characteristics of several different food products,
and results generated with a STRESSTECH rheome-
ter were presented. In addition, a detailed interpre-
tation of data and correlation of the rheological re-
sponse with the physical/chemical properties of
different semisolid food products were detailed. The
rheological characterization of foods provides im-
portant information for engineers and food scien-
tists to improve and optimize their products and
manufacturing processes. To d a y, most formulators
count on rheological measurements to develop cus-
tomer-favored products with a competitive edge in
the marketplace. A reliable research-level rheometer
and a thorough understanding of food rheology is
now a necessity for food companies.

Temperature control cells are offered that use cir-

culating fluid, Joule Thomson effect, and cry o g e n-
ics covering the range from –180 to 500 °C. All mea-
suring geometries are supported, i.e., cone/plate/
parallel plate, concentric cylinder, and solids in tor-
sion and tension. Several high-pressure cells with
an upper range of 5800 psig are available.

Electronic unit

The instrument electronics are contained within

the mechanical unit and the instrument is built
around a dedicated, high-speed 32-bit central proc-
essing unit (CPU). This consolidation enhances per-
formance and versatility due to electrical connec-
tions on the motherboard bus, rather than through
cables to a separate electronics cabinet. In addition,
use of valuable bench space is kept to a minimum.
The motor control is based on digital rather than
analog drive technology. The unit comes with a
built-in diagnostic system and quick diagnostic ser-
vice port for service engineers. Also included is a
modem port for remote control operation and fault
diagnostics for service. The electronics power sup-
ply is designed to operate on a line voltage between
180 and 260 V or 90 and 140 V and an operating
frequency between 47 and 63 Hz.

Software package

RheoExplorer 5.0 software (

AT S R h e o S y s t e m s

)

is based on the Windows operating platform and
runs under Windows NT 98/2000. The standard
software package is a true multitasking interface
with selectable user levels, thus providing many ad-
vantages to the food scientist. It is designed to pro-
vide flexibility for configuring and using the rheol-
ogy system. The computer is not de dicated to
simply running the instrument and is available for
other use when making measurements. The com-
puter can be used for printing previous results, writ-
ing a report, or performing measurements with an-
other instrument.

The software enables a normal PC to be used as

the interface to allow the user to control the instru-
ment, and then collect and analyze the resulting
data. Viscometry, oscillation under stress and strain
control, creep and recovery, constant rate, yield
stress, fast oscillation, process control, and project
(multiexperiment linking), time-temperature super-
position, and spectrum transformation packages al-
low the sample to be analyzed via different rheolog-
ical procedures. Powerful data analysis capability
allows model fitting, graph and table customiza-
tion, and cut/paste operation to all other Windows-
based software.

The software includes possibilities to link user-

designed methods including instrument setup and
zero gapping using the Project Software. The dia-
logue windows have many storable, editable func-

The rheological characterization
of foods provides important
information for engineers and
food scientists to improve and
optimize their products and
manufacturing processes.

Mr. Herh is Applications Manager, and Mr. Colo is Presi -

dent,

ATS RheoSystems,

52 Georgetown Rd., Bord e n -

town, NJ 08505, U.S.A.; tel.: 609-298-2522; fax: 609-

298-2795; e-mail: i n f o @ a t s rheosystems.com; home

page: www.atsrheosystems.com. Mr. Roye is Western Re -

gion Manager,

ATS RheoSystems,

North Hollywood, CA,

U.S.A.; Dr. Hedman is VP, Software Development,

R E O -

LOGICA Instruments AB,

Lund, Sweden.

APPLICATION NOTE

continued