In our first lecture, I presented an overview of the course -- the scope, objectives and what you would expect to learn. Details of the scope and syllibus are presented above. It is important to appreciate that physical properties such as colour, texture, flow behaviour, surface property, phase transition (e.g., crystallization) are important parameters to consider, together with the chemical properties and reactions in food systems. I have given some examples of the applications of physical properties of food in food processing, formulation and product development. I hope you have got some ideas of the significance/importance of learning about physical properties of foods.

We started off with the topic of food rheology. I gave some background of the subject before defining the term "rheology". It is important to understand that, while the term "rheology" may not sound familiar to you, we actually encounter "rheological events" in our daily life. These include pouring tomato sauce from the bottle, pumping and dispensing liquid, spray drying of milk, scooping yoghurt from the cup, mixing/stirring, spreading peanut butter on the toast, etc.

Rheology is the study of the manner in which materials respond to applied strain and stress. All materials have rheological properties and the area is relevant in many fields of studies: geology and mining, concrete technology, soil mechanics, plastic processing, plastic processing, polymers and composites, tribology, paint flow, bioengineering, blood, interfacial rheology, structural material, electrorheology, psychorheology, cosmetics, and pressure sensitive adhesion. The focus of this class is food where understanding is critical in optimizing product development efforts, processing methodology, and final product quality. Rheology of Food Products is a significant component of the food processing industry. Detailed knowledge of rheology of ingredients is important for successful process control and systems engineering.

At the end of the course, students should be able to:
  • Explain the forces contributing to food rheology.
  • Distinguish solid, viscous and viscoelastic behaviour of foods.
  • Describe various types of fluid flow behaviour
  • Recognize the different principles behind various techniques for evaluation of rheological properties.
  • Recommend an appropriate technique for measuring the rheological properties of a specific food.
  • Determine rheological properties with selected techniques.
  • Explain the relationship between texture and microstructure.
  • Apply knowledge of food rheology to new situations.
  • Present new concepts in food rheology in a meaningful fashion.
  • Understood the significance/importance of rheological properties in food formualtion, processing, and consumer acceptance.

No textbook but several good references are available in the library:
  • Bourne, M.C. Food Texture and Viscosity: Concept and Measurement. 1982. Academic Press, New York.
  • Steffe, J.F. 1996. Rheological Methods in Food Process Engineering, second edition (second printing).Freeman Press, East Lansing, MI, USA.This book can be downloaded FREE (428 page, 1.3 MB, pdf). Download now!
  • Blanshard, J.M.V. and P. Lillford, (Eds.). Food Structure and Behaviour. 1987. Academic Press, London, Eng.
  • DeMan, J.M. Rheology and Texture in Food Quality. 1976. AVI Pub. Co., Westport CN.
  • Lawless, H.T. and H. Heymann. Sensory Evaluation of Food: Principles and Practices. 1998. Chapman & Hall, New York.
  • Lewis, M.J. (Ed.) Physical Properties of Foods and Food Processing Systems. 1987. VCH, Chichester, Eng.
  • Matz, S.A. Food Texture. 1962. AVI Pub. Co., Westport CN.
  • Moskowitz, H.R. Food Texture: Instrumental and Sensory Measurement. 1987. M. Dekker, New York.
  • Muller, H.G. An Introduction to Food Rheology. 1973. Heinemann, London, Eng.
  • Peleg, M. and E.B. Bagley. Physical Properties of Foods. 1983. AVI Pub. Co., Westport CN.
  • Rao, M.A. 1999. Rheology of fluid and semisolid foods: Principles and Applications. Aspen Publishers, Gaithersburg, MD.
  • Sherman, P. 1979. Food Texture and Rheology. Academic Press, New York.

Here are some interesting resources to help you learn more about rheology (general principles):
  • Malvern Instruments (manufacturer of rheometer, particle size analyzer) provides recorded version of live webcast on various topics, including the subject of rheology. First, you have to register before you can download the tutorial. Click this link to downlod a webcast entitled "A basic introduction to rheology".
  • Interesting topics on rheology are also available as part of the On Demand Training on Malvern Instruments website. Check under the topic of "Rheology". Download and listen to the presentation. Believe me, you will learn a lot!
  • TA Instrumnents is another manufacturer of rheometers and thermal analysis instruments. They have an extensive literature on rheology and thermal analysis studies in the Application Library. Login, peruse and download to your heart content...and of course, read
  • Read this article entitled "What is Rheology Anyway?", written by F.A. Morrison. He explained rheology from "phenomenogical" perspective, i.e., things that we observed or experienced everyday. I like it! You may download the pdf version of the article. Enjoy!
  • For those knowledge hungry, read more about the history and origin of rheology studies in this article entitled "The Origin of Rheology: A Short Historical Excursion": This is not compulsory reading for our course - but no harm reading and getting more knowledge folks!
  • Hey, care to learn more about rheology? Visit our best friend...Wikipedia, and read all about rheology...

OK, that's enough for now, before you get overwhelmed with literatures. I will give you more references in the next lecture. No excuses folks, there's no lack of reading materials!

In this lecture, I described the meaning of two important terms, STRESS and STRAIN. At the end of the 50 min lecture, I hope I did'nt put too much 'stress' to your brain and you did'nt get too much 'strain' (deformed brain?). Well, in the context of rheology, stress is related to force whereas strain is related to deformation (change of shape or dimension as a result of the applied force). Let's think of one extreme example: hold one egg (just hold it still) about 1 meter from the floor. Did you see anything drastic happen to the egg? Now, let it go...prrraaap...what have you done?? It's broken...well, rheologically speaking, the egg has deformed...permanently. The egg fell to the ground (floor) due to gravity force and resulted in permanent change of shape (deformation) of the egg. It's not possible to calculate the strain in this case. Why? (Well, I will answer my own question this time...because...the egg has broken up into small pieces!!). This example is rather extreme, but in the lecture I illustrate the concept using an apple (study the slides again). I hope the illustration helped you to understand and visualize the concept.

I also explained that, in order to compare the intensity of force, we have to consider the surface area (i.e., the area where the force is applied). So, when we devide the force (we give notation F) by surface area (A), we actually calculate the force per unit area -- and this give us the definition for stress. Put it another way, stress can be considered as normalised force, or 'force intensity'. Strain is simply the extent (amount) of deformation as a result of the applied stress. It is simply a dimensionless ratio of the dimension (e.g. length) of the object before and after deformation. In similar fashion, strain can be considered as a quantitative measure of the intensity of deformation.

As I stated earlier in previous lectures, rheology aims at measuring those properties of materials that control their deformation and flow behaviour when subjected to external forces. Thus, rheology is mainly concerned with the relationship between strain, stress, and time (so we use terms such as strain rate, flow rate, shear rate, etc.). Let me repeat again, just to emphasise: strain and stress are related to deformation and force, respectively. Strain accounts for the size effect on material deformation due to difference in length (or height) of specimens, whereas stress accounts for the size effect on applied force due to difference in cross-sectional area of specimens. Using strain and stress, rheologists are able to obtain true material properties independent of the sample size and geometry, and compare results for sample of different sizes and geometries. OK, clear so far?

In this lecture also, I have introduced the term 'viscosity'. Basic rheology concepts can conveniently be classified into viscous flow (typically liquid such as water or oil), elastic deformation (e.g., in dough), and viscoelasticity (most foods). This classification depend on the extent of recovery after deformation. Think of two extreme examples: stretch the spring and let go, it will recover to the original shape (provided you did not stretch it beyond the elastic limit). So we say that the spring is elastic. Now, pour water from the bottle, it wil flow readily (you can't get the water to flow back into the bottle, can you? -- so, in this case, permanent deformation, no recovery. Thus, for a 'liquid-like' material, we always talk about 'viscosity'. In a simple term, viscosity is a measure of a fluid's ability to resist motion when a shearing stress is applied. Imagine yourself pouring a tomato sauce from the bottle. Can you pour the toothpaste from the tube? You can't, you have to squeeze the tube. So we say that the tomato sauce and the toothpaste exhibit different resistance to motion (flow). They have different viscosity!

Generally we recognise that liquids could be viscous (likat), or less viscous (kurang likat). So we can say that viscosity is a property of a liquid which is a measure of its resistance to flow -- so that high viscosity liquids (e.g., honey) flow slowly and low viscosity liquids (e.g., water) flow quickly. If you recall in our last lecture, we imagine that the fluid behaves as a series of parallel layers whose velocities are proportional to their distance from the lower (stationary) plate. The differentiation of velocity with respect to the distance (dv/dy) is defined as shear rate. The dynamic viscosity (or coefficient of viscosity) is defined as the ratio of the shear stress to the shear rate.

You can see that shear stress and shear rate are two important parameters to enable us to calculate viscosity. Why do we use the term shear rate, not shear strain? (remember in previous lecture I talked about stress and strain?).

I explained that shear rate prevailing in common processes can range from 10^-6 to 10^-4 s-1 in sedimentation process, to 10^3 - 10^4 s-1 in spraying process, and from 10^5 - 10^6 s-1 in high speed coating. It is important to know the range of typical shear rate for a particular process because when we measure viscosity (for non-newtonian type fluids), we have to choose an appropriate value of shear rate to determine the viscosity. For example, if you were to spray dry a liquid product (e.g., milk), and you wanted to know how it would flow through the atomiser (flow behaviour), then you would measure the viscosity within the range of shear rate relevant to the spray drying process (10^5 - 10^6 s-1).

I have also briefly covered a few more concepts regarding Newtonian and non-Newtonian fluids. A Newtonian fluid is one in which the viscosity does not depend on the shear rate—no matter what shear is applied, the viscosity stays the same. In many applications, however, this is not the case and, as the fluid is sheared at greater rates, the viscosity will change. These types of liquids are known as non-Newtonian and there are many classifications (read more in the handout).

In addition, factors which affect viscosity have been discussed as well. One of the factor is temperature. Typically, viscosity is inversely related to temperature with some fluids showing as much as a 10- to 12-percent change in viscosity per degree Celsius. If the fluid temperature is expected to change, it should be understood what effect this will have on the fluid viscosity and the application. There are a few cases where there is a direct relationship between temperature and viscosity. Do you remember the examples I mentioned in the lecture? Other factors affecting viscosity are shear rate and pressure. Try to figure out how these factors affecting viscosity.

Lecture outline :
  • Flow behavior and flow curves
  • Definition of terms: shear thinning, shear thickening, pseudoplastic, dilatant, and more
  • Rheological models

In this lecture, I continue to bore you (I heard someone yawning, sorry about that!) by elaborating on the important rheological parameter, i.e., viscosity of fluid or liquid-like foods. It is important to recognize that different fluids flow at different rates, as I have mentioned earlier. It turn out that when we measure the fluid flow in a suitable instrument, and plot the data in the form of shear stress vs shear rate, we obtain either a straight line through the origin, or a curve. We call such a plot as a flow curve (although in the case of Newtonian fluid, it’s a straight line). A flow curve can also be presented in the form of viscosity (apparent viscosity) vs shear rate (or shear stress). This plot is better in a sense that we can see clearly how viscosity changes as a function of shear rate (i.e., when we increase or decrease the shear rate).

I repeated my explanation about the classification of fluids based on the shape of the flow curves. With Newtonian fluids, the shear rate is directly proportional to the shear stress, and the plot begins at the origin. Typical Newtonian foods are those containing compounds of low molecular weight (e.g., sugars, salt) and do not contain large concentrations of either dissolved polymers (e.g., pectins, proteins, starches, gums) or insoluble solids. Typical examples include water, sugar syrups, most honeys, most carbonated beverages, edible oils, filtered juices, and milk. Can you think of other examples?

A few terms have been introduced in the lecture. Now, you can easily get confuse with the meaning of the terms. Let me reiterated (explain again) what I have covered in the lecture (see also the glossary). In many applications, we are more concerned with fluid foods that are non-Newtonian, which means either that the shear stress-shear rate plot is not linear or that the material exhibits time-dependent rheological behavior as a result of structural changes. Hmmm…to make things complicated, flow behavior may depend only on shear rate and not on duration of shear (time independent) or may depend also on the duration of shear (time dependent).

The decrease of viscosity with shear rate is called shear thinning and must be distinguished from a decrease in viscosity with time of shearing, which is called thixotropy. The expression shear thinning is also synonym with pseudoplastic. Shear thinning may be thought of being due to breakdown of structural units in a food due to the hydrodynamic forces generated during shear. Most non-Newtonian food exhibits shear thinning behavior, including many salad dressings and some concentrated juice.

Some materials exhibit time-dependent rheological behavior. The one commonly encountered in foods is thixotropic behavior. Thixotropy refers to a continuous decrease of apparent viscosity with time under constant steady shear, and the subsequent recovery of viscosity when the flow discontinued. This kind of behavior is the result of time dependent breakdown or buildup of structure in response to shear. How do you determine thixotropic behavior experimentally? Using an instrument called “rheometer”, the shear stress is increased to a certain maximum value and then decreased back to the starting stress. The corresponding shear rate during the “up” ramp and the “down” ramp is recorded resulting in two curves (‘up curve’ and ‘down curve’). The two curves may not lie totally superimposed on top of each other. This is an indication of thixotropy and is due to a change of structure within the sampe that is dependent upon time for which the shearing has been carried out. The area between the two curves is often called “hysterisis loop”.

Some other food materials possess a so-called “yield stress”. This is simply defined as the minimum stress required to initiate flow of the liquid. One example of a fluid with a yield stress is toothpaste (non-food). I give this example because I think everyone can relate to this example (unless if you don’t brush your teeth…). Toothpaste will not flow out of the tube under gravity, but squeezing exerts stresses exceeding the yield stress value and the toothpaste flows out of the tube. Mayonnaise and tomato sauce are example of foods that possess measurable yield stress.

Mathematical descriptions of flow curves – At some points, we have to be able to measure and express viscosity as a number. It’s probably alright to say that honey appears to be ‘more viscous’ than water — but, how viscous? Actually, once you get your flow curve, you can extract a lot of information from it. For example, you can calculate the apparent viscosity at any shear rate of interest. Mathematical description of the flow curve (also known as rheological model) will enable you to calculate viscosity and other parameters. Rheological models are mathematical equations relating shear stress to shear rate providing a "flow fingerprint" or flow profile for fluid foods. In addition, the models permit prediction of rheological behavior over a wide range of operating conditions.

For non-Newtonian fluids, Power-law model is perhaps the most popular one. In the power-law model, the consistency index k has the strange units of Pa.s^n and the power-law index is dimensionless and usually ranges from 1 for Newtonian liquids towards 0 for very non-Newtonian liquids. Power-law alone is quite sufficient to describe many non-Newtonian flows within shear rates around 1 to 1000 s^-1 or so, but above 1000 s^-1, there is usually some flattening (i.e., the data does not fitted satisfactorily above 1000 s^-1). For engineering calculations, power-law predictions are quite reasonable if limited to the medium range.

Lecture outline:
  • Interpretation of flow curve
  • Why make rheological (viscosity) measurements
  • How is viscosity measured?

To get a flow curve for a particular sample is relatively easy, if you have the right instrument, of course! So, in this lecture, I have described selected examples of measuring devices to help you to obtain an estimate of viscosity and if desired, the whole flow curve. But before that, I presented 2 examples of flow curve for real food samples and discussed the features and interpretation of the curves. In the handout (Rheology of Liquid Foods), I have given a few more examples and detailed discussion. Study the examples carefully and see whether you can make some sense of the flow curve! I should stress here again, doing the measurements and getting the data are relatively peanuts, but to make sense of the data is another question. The handout also answers the questions of how, in general, viscosity is measured; what kind of information it provides; and how the information obtained may be useful. In addition to what we have covered (or will cover in the next lecture), I have also included a discussion on in-line viscometers. These are viscometers installed directly in the processing line and collect viscosity data in “real time” during the process. Data gathered on the process stream enable poor (incorrect) viscosity to be corrected before it affects the finished product.

Now, let’s talk about viscosity measurement (I won’t describe details of the instruments here – please read the handout). The science of the measurement of viscosity is known as viscometry. I will discuss this from different perspective, just to add value to our lecture. Viscosity measurements will provide some viscosity values to serve as an index of level of processing or product quality. Question: Is it really necessary to quantify viscosity? – the answer is YES – ask the engineer, or people (technologists) involve in product development. Although qualitative observations can be quite useful for describing flow properties, to those working in the production and controlling such properties, a more quantitative approach is necessary.
A frequent reason for making viscosity measurement can be found in the area of quality control where raw materials must be consistent from batch to batch. Many food ingredients come in the form of liquid or semi-solid forms: glucose syrups, honey, fruit puree, vegetable oils, caramel, cream, and more (think, think!). For this purpose, flow behavior is an indirect measure of product consistency and quality. Another reason for making flow behavior studies is that a direct assessment of processibility can be obtained. For example, a high viscosity liquid requires more power to pump than a low viscosity one. Knowing its rheological behavior, therefore, is useful when designing pumping and piping systems.

Talking about viscosity measurements, actually there are “three schools of thought” out there (tiga pendekatan yang berbeza) (ref. More solutions to sticky problems – Brookfield). If you ask me, well, actually there is no “right” one – each school of thought has its merits at certain times. OK, lets have a look…The first approach of viscosity measurement is very pragmatic – this is a person who “loves” the Brookfield viscometer that generates numbers and tell him/her something useful about a product or process (we can find many of them in the industry!). This person has little or no concern about rheological theory and doesn’t bother about shear stress and shear rate!! Quality control and plant production applications are typical example of this category (tell me if you encounter this ‘practical’ person during your industrial training).

The second school of thought involves a more theoretical approach. Those adhering to this school know that some types of Brookfield viscometers will not directly defined shear rates and absolute viscosities for non-Newtonian fluids. However, these people often find that they can develop correlations of ‘dial viscosity’ with important product or process parameters. Many people follow this school of thought.

The third school of thought is…well, like me…quite academic in nature. These people require that all measurement parameters, particularly shear rate and shear stress, be defined and known. They need equipment with defined geometries and accurate temperature control. Examples are some Brookfield viscometers, rheometers (Haake, TA Instruments, Bohlin, and more). With this equipment, the shear rate is defined and accurate absolute viscosity is obtained directly.

So, how is viscosity measured? Basically, there are empirical methods (viscosity is expressed in arbitrary units such as time or distance) and fundamental (absolute) methods (viscosity is calculated from shear rate-shear stress data). In many industrial applications, it is necessary to have instruments that make measurements which are rapid, low cost, simple to carry out, and reproducible, rather than give absolute fundamental data. Thus, simple empirical instruments (e.g., Bostwick consistometer, penetrometer, falling ball, and more) are often used in quality assurance laboratories, rather than the more sophisticated and expensive instruments (such as controlled stress rheometer in our lab) used in research and development. Unfortunately, it is difficult to analyze the data from these devices using fundamental rheological concepts because it is difficult to define the stresses and strains involved. Nevertheless, these devices are extremely useful when rapid empirical information is more important than fundamental understanding.

As I described above, the suitability of a given instrument will depend largely upon the nature of the food and the purpose for which the data are being obtained. For example, instruments operating at a single shear rate are suitable only for foods known to be Newtonian. Examples of such instruments are glass capillary viscometer operated under the force of gravity and a rolling ball viscometer. Read the handout for more discussion on this aspect. I have also described selected examples of measuring devices from both categories in the handout.

Instruments available today are so easy to use that anyone can pick up a viscometer and get good data without extensive training. Many manufacturers offer portable viscometers which can be carried out to the plant floor, stuck into a vessel or vat, and the viscosity read from the display. In the quality control laboratory, portable viscometers can be used with software which can reduce the most complicated test protocol into just a few strokes. Most software programs allow the user to store several different protocols, so that the same instrument can be used to measure a wide variety of materials on a routibe basis.
OK, this is my parting words. A viscometer cannot tell you what consumer preference will be for a product, and it is not going to improve flavour . However, it can help you to objectively measure product consistency, and help make sure that if you have a winning product today, it will stay that way tomorrow!

For the topic on viscosity measurement, I would recommend the followings for further readings:

Chapter 4 (Viscosity) in Physical Properties of Foods and Food Processing Systems, M.J. Lewis [this chapter also covers other aspects of rheology such as types of flow behavior, rheological models, and more].

Rao, M.A. (1977). Measurement of flow properties of fluid foods – developments, limitations, and interpretation of phenomena. Journal of Texture Studies, 8, 257-282.

If you want to learn more from the experts, download and view the presentation available on Demand Training at Malvern Instruments website. These 3 topics are recommended (under Rheology section): (1) Do I need a rheometer or viscometer? (2) Basic sample handling techniques with a rotational viscometer (3) Viscometry: recognizing measurement artifacts.

Here are some interesting resources to learn more about viscosity:
  • Read more about shear rate and viscosity in this very interesting article entitled "Dealing with shear", published in Food Product Design (online magazine).
  • Viscosity, viscosity....aarggh...here is an interesting article entitled "What is viscosity" from Brookfield. Read all about viscosity and you don't have to come to my lecture anymore! And while you are on this page, download this excellent article entitled "More solutions to sticky problems". It also includes a guide to rheological terms, Newtonian or non-Newtonian, thixotropy, shear stress, shear rate, yield, and more. And it gives practical advice on solving problems. Also, explore this website because there is more than meet the eye. Happy reading!*
  • Virtual Experiment on Viscosity -- Although we don't have practical class, we can still do virtual experiment. Try this interactive exeriment to see how viscosity varies from liquid to liquid and how temperature affects viscosity. The animation demonstrate the rate of an object falling through the liquid and the time taken to reach the bottom is related to the viscosity of the liquid. Try with honey and water!
  • FoodViscosity.com -- This wonderful website is hosted by Brookfield Engineering Laboratories, Inc., a world leader in viscometers. This website provides a place to gain technical information and share solutions on viscosity and texture related issues. FoodViscosity.com features discussions with industry leaders, on-line technical seminars, interviews with rheology experts, articles, technical information, education, training, etc. specifically geared toward the food viscosity and texture marketplace. I especially like to listen to interview with the rheology experts (audio streaming, very fast!). So, what are you waiting for, sign up now...*
  • What is Yield Stress and Why Does it Matter? -- The presence of a significant yield stress will impart various qualities to a fluid that may or may not be desirable. Read more about this important phenomenon.
  • I mentioned about inline viscosity measurement above and also in my handout. If you are interested to learn more about the applications of this in-line viscometers in actual food processing, here are two examples: (i) chocolate processing (ii) tomato processing. *
  • Using Viscosity Profiles and Rheological Models to Benchmark Your Products-- You may be wondering why, when and how we use the rheological models such as Power-law, Herschel-Bulkley, etc. This article explains the utility of rheological models in characterizing the food products
  • Making Use Of Models: The Power Law (or Ostwald) Rheological Model - A brief article explaining the application of the Power law model.

Lecture outline
· Definition of viscoelasticity
· Definition of Deborah number
· Viscoelastic parameters

Before I summarise this lecture, let me revise some of the important points we have covered so far. Basically, I have defined basic terms such as stress and strain, shear stress and shear rates, etc. In addition, I talked about flow behavior of fluids, classifications of flow behavior, and the measurements and interpretation of flow curves. One of the important parameter we have discussed is viscosity. It should be clear to you now that we have been discussing about liquid foods, or foods that appear like a liquid – or a material that can flow. So, for a liquid, we measure viscosity. We know, however, some foods are solid, such as hard cheese, hard candy, or even an ice cube. What about foods such as jelly, bread, cakes, dough, noodle, tofu, soft cheese, etc.? These are materials which have some solid-like and some liquid-like properties – or we call them “viscoelastic materials”.

Viscoelastic materials are both elastic (having solid-like) and viscous (having liquid-like). Their rheological properties depend on the relative degrees of elasticity and viscosity and are also dependent on the time-scale of the deformation. Now, you have to understand the concept of “time-scale of deformation”. In the lecture, I gave water as an example –- water can behave like a solid or like a liquid –- depending on the time-scale of deformation. Imagine you are sitting next to the swimming pool and just dive in slowly….aahh…nice ha! Now, try to dive from the top of the Penang bridge (please don’t do this!), the water now will feel like a hard rock solid when you hit it!! Why? In the first instance, the time-scale of deformation is slow (long – equivalent to low frequency), and in the second instance the time-scale of deformation is fast (short – equivalent to high frequency).

The scaling of time in rheology is achieved by means of the ‘Deborah number’, De. This is defined as De = t/T, where T is a characteristic time of the material (unique for each material) and T is a characteristic time of the deformation process. High De (De >> 1) corresponds to solid-like behavior and low De (De << 1) to liquid-like behavior. A material can, therefore, appear solid-like either because it has a very long t or because the deformation process we are using to study it is very fast. In my example above, diving slowly into the pool involve slow deformation of the water (long T), therefore resulting in small De (water behave like a liquid). Diving from the top of Penang bridge, however, involve fast deformation of the water (very small T), because you hit the water so fast. In this case, De value is high and the water behaves like a hard solid (and did you know that water, in the form of high pressure water jet, is used as a cutting tool -- cutting foods, glass, and even metal!). [note: t in both cases is constant].

Viscoelastic properties are measured by using a rheometer. The test is known as small-amplitude oscillatory tests (SAOS). SAOS measurements enable quantification of elastic and viscous components of a material simultaneously. Basically, a small oscillation (sinusoidal) stress (or strain) is applied on the sample and the respective deformation and the phase relationship between viscous and elastic components is measured. This is non-destructive test, i.e., the structure of the material is not destroyed during the test. The stress and the strain is very small, just sufficient to measure the viscoelastic properties of the material. Usually, the stress or the strain applied is within the so-called “linear viscoelastic region”.

SAOS measurements allow determination of viscoelastic parameters: storage modulus, G’ (pronounced G-prime) and loss modulus, G” (pronounced G-double prime). G’ is simply a measure of the elastic component and G” is a measure of the viscous component of the material. In other words, G’ and G” are parameters representing the relative degrees of elastic and viscous behavior of viscoelastic materials. Therefore, a sample with a larger G’ component will behave elastically (solid-like), while a material with a higher G” will be more viscous (liquid-like).
Another parameter from SAOS test is phase angle. Viscoelastic materials have a phase angle between 0 and 90°. A material with a phase angle approaching 90° will be dominated by viscous behavior; likewise, a sample with a phase angle closer to 0° will behave more elastically.

Oopss…are you following me up to this point? OK, I know, this is slightly more complicated….just slightly. As usual, as you read more and more, you will understand more. Measurement of viscoelastic properties of food materials can be considered an advanced rheological method (and an expensive one, too…a rheometer can cost at least quarter a million, or more!!). It is meant for R&D rather than for routine QC/QA. But this may change in future, when the rheometer is more affordable, perhaps.
LECTURE 8 - Food Texture Measurement

Texture is such an important physical attribute of food. In fact, when assessing the overall organoleptic quality of a food, texture must be considered together with flavor and appearance. Given the importance of texture as one of the quality attribute, I take the challenge again to put together this lecture online.

Lecture outline
  • Importance of texture in food quality perception by consumers
  • Advantages of instrumental texture analysis
  • Definition of food texture
  • Types of instrumental texture test
  • Instrumental Texture Profile Analysis
  • Examples of common empirical test

As I have explained in the class, a large determinant of the quality of a food is its texture , i.e., what sensation does the food impart to the nerves and muscles in the mouth as the food is bitten, chewed and swallowed. It is this critical importance of food texture to optimal food quality that warrants studies of food texture and an appropriate method to measure the textural attributes of foods. How do we define the term “texture” in the context of food quality? Briefly, texture is a sensory attribute, perceived by the senses of touch, sight and hearing. Well, in that case, the only direct method of measuring texture is by means of one or more of these senses, agree? Hmm…actually sensory evaluation by using trained sensory panels is one of the methods that can be used to study food texture. Sensory evaluation of texture is not covered in this lecture. Here, we will discuss about instrumental method of measuring texture.

A more general definition of texture is that “it is the composite of all physical characteristics sensed by the feeling of touch that are related to deformation under an applied force and are measured objectively in terms of force, distance, and time” (Bourne, 1982). This concept is the basis for most instrumental method to study texture. Compared with sensory panels, which are costly and time consuming, instrumental methods can save time, reduce costs, and provide more consistent, objective results. However, since it is difficult for machines to imitate biting and chewing, the need for sensory panels as a correlative test method will continue for the foreseeable future.

A common principle used in instrumental texture measurement is to cause a probe to come into contact with the sample of food. The sample is deformed and the extent of the deformation and the resistance offered by the sample is noted and used as an index of the texture of the food. Various types of instruments or devices have been developed and generally, the instrument can fall into one of three categories: empirical methods, imitative methods, and fundamental methods. All three of these approaches have their merits and specific applications. Which one is chosen should depend on the problem being faced and the questions being asked. Two of the most popular machines for texture testing of food are Instron Universal Testing Machine and Texture Analyzer (Stable Micro System).

When we talk about instrumental food texture measurement, Texture Profile Analysis (TPA) is perhaps one of the most popular one (TPA is classified as an imitative test). I’m not about to give a history lesson here, but….hmmm…perhaps very briefly. The scientists in General Foods Corporation (USA) were the forerunner in this area. It all began with the introduction of General Foods Texturometer, designed to simulate mastication by means of a mechanical chewing device. The instrument operates by partially compressing the sample twice (imitating the first two bites taken of a food – you don’t just swallow the food, right?). Several parameters are obtained from the resulting force-time plots (hardness, fracturability, cohesiveness, and chewiness), and these have been found to correlate highly with sensory ratings. The values of the various parameters make up what is known now as instrumental texture profile analysis (and the rest is history…).

Finally, in this lecture I gave a few examples of typical empirical tests based on compression, puncture/penetration, cutting & shearing, extrusion, etc. In the online lecture, I explained these tests briefly with some examples of results from actual food sample. Check out the additional resources listed below to learn more about food texture measurements.


The following articles are strongly recommended if you want to learn more about Texture Profile Analysis:
  1. Bourne, M.C. (1978). Texture profile analysis. Food Technology, 32(7), 62-66.
  2. Breene,W.M. (1975). Application of texture profile analysis to instrumental food texture evaluation. Journal of Texture Studies, 6, 53-82.
  3. Pons, M. & Fiszman, S.M. (1996). Instrumental texture profile analysis with particular reference to gelled system. Journal of Texture Studies, 27, 597-624.
  4. Szczesniak, A.S. (1963). Classification of textural characteristics. Journal of Food Science, 28, 385-389.

Here are some interesting resources I found from internet on the subject of food texture and associated topics. Enjoy!
  • Practical definitions of standard TPA terms – self-explanatory – also, you will find a list of references on food texture. In addition, if you are using TA.XT2 Texture Analyzer (Stable Micro System) to do your TPA, procedures to calculate the parameters are also given. *
  • Benefits of Food Texture Analysis – this article is from Instron website. You may also view or download some of the literatures provided for further study. *
  • Food Testing Overview – an overview of various probes/fixtures for different types of testings and food samples (also from Instron).
  • Standard Testing Procedures for Baked Products -- This is a collection of procedures for testing the texture of common bakery products with the TA.XT2 Texture Analyzer. These procedures are used at the American Institute of Baking's Experimental Bakery Lab in Manhattan, Kansas.
  • Making sensory tests instrumental -- Correlating sensory data with instrumental testing helps designers optimize the eating pleasure to make products more successful. This article, from Food Product Design website, explains the various facets of sensory and instrumental texture evaluation.
  • Evaluating gel strength -- In the designing of a new product, once a gel system has been selected the next step is to test a large number of samples to obtain data points which can, in turn, be used to develop specifications for the gel system. Both the strength of the gel and how it affects the final product should be measured. This article, from Food Product Design website, describes the common methods (mainly empirical) to test gel strength. *
  • The sensory perception of texture and mouthfeel -- Texture and mouthfeel are fundamental sensory properties of foods and beverages. The formulation of specific textures and mouthfeels has been very challenging for product developers and manufacturers. This is due to the limited understanding of the physiology of texture and mouthfeel perception, and of consumer preferences for tactile and kinesthetic characteristics. This article provides an update on the latest advances in the understanding of oral texture and mouthfeel perception. Download the article from the link below. *