Module 1. Rheology of foods

Lesson 1


1.1 Introduction

Rheology is the science of flow and deformation of matter and describes the interrelation between force, deformation and time. It is the study of the manner in which materials respond to applied stress or strain. The term comes from Greek ‘rheos’ meaning to flow. The science of rheology is only about 76 years of age. It was founded by two scientists meeting in the late ‘20s and finding out having the same need for describing fluid flow properties. The scientists were Professor Marcus Reiner and Professor Eugene Bingham.

Sensory evaluation as a scientific discipline represents a very unique technique that harnesses human behavioral instincts of perception, learning, cognition, psychophysics and psychometric for the evaluation of foods. The textural properties of a food are that group of physical characteristics that are sensed by the feeling of touch, are related to the deformation, disintegration and flow of food under application of force. Textural characteristics are an important factor in the overall quality of many food products. Unless these quality attributes meet the standards which the consumer expects, the product will be rejected regardless of its nutritional value.

1.2 Psychrorheology

Psychrorheology of foods is the scientific study of man’s perception of texture of foods. It may be devided into two major areas (i) Qualitative psychorheology work concerns the attributes of texture to which man responds, the structure of his mental lexicon of texture descriptors and the cluster of similar meaning texture descriptors. (ii) Quantitative work may consider mathematical relations between pairs of texture descriptors, or functions relating one or several subjective textural properties. The major thrust of quantitative psychorheology has been to ascertain the class of functions relating mechanical to subjective properties and through experimentations to quantify the parameters of those functions. Now it is well established that the psychorheological models are important in texture studies.

1.3 Importance of Rheology

Study of rheological properties is important in food science due to its utility in food processing operations and sensory characteristics. It gives information about the microstructure of a food. Rheology properties are manifestation of the rate and nature of the deformation that occurs when a material is stressed. These parameters can be used to predict how the fluid will behave in a process and in determining the energy requirement for transporting the fluid from one point to another in processing plant. Rheological parameters are also useful in defining the quality attribute of food products.

1.3.1 Rheology is very important in the following area in the food industry

1. Mixing-Two or more material are blended manually or mechanically.

2. Flow Control-Flowablity of material varies from very thin to highly viscous.

3. Dispensing- Material comes out easily or with difficulty.

4. Settling/ Floating – Material with different specific gravity either settle or float depending on viscosity of the material.

5. Pumping- Liquids or semi-solids are forced through the pipe

6. Coating- Spreading of one material as thin layer over other.

7. Cleaning – Soil removal from the surface of the equipments and pipeline.

8. Control of processing parameters- velocity, magnitude of pressure drop, piping design, pumping requirement for fluid transport system, power requirement of agitation, power requirement of mixing and blending, amount of heat generated during extrusion etc.

9. Influence on unit operations – Heat transfer, Mass transfer,mixing, grinding, sedimentation, separation, filtration, evaporation and drying etc.

10. Study of rheology helps to select proper method of harvesting and sorting of raw materials

11. Study of rheology helps to select proper ingredients to manufacture processed foods.

12. Study of rheology helps to select proper technology/equipment to manufacture processed foods with desirable sensory and rheological properties.

13. Study of rheology helps in newer product development (e.g. dietetic ice cream, paneer, low fat mozzarella cheese etc.)

14. Study of rheology helps in designing processing equipment, packaging machines, transportation system etc.

15. Study of rheology helps to improve sensory quality of the products

16. Study of rheology helps in marketing the products.

1.3.2 Importance of rheological studies in dairy industry

Rheological studies of dairy products are important at a juncture when the need for modernizing the manufacturing and marketing of Traditional Indian Dairy Products (TIDP) is being emphasized in India. It helps to evaluate ingredient for potential contribution to creaminess in fat-free dairy products. Rheological studies also helps to evaluate quality of cheese and applicability of cheese for various applications like suitability for pizza topping. Further, the Bureau of Indian Standards (BIS) is actively considering the views of describing the food products based on their structure and rheology. Most fluid foods including dairy fluids like cream, ice cream mix, stirred yoghurt and liquid infant foods shows complex flow behavior at different stages of processing and it requires study of its flow behaviour for better control over the processing parameters. Viscoelastic characteristics of foods are of great importance to the manufacturer, the trade and the consumers as these properties affect 'eating quality', usage properties such as ease of cutting, spreading and melting characteristic as well as handling and packaging characteristics. Recent developments in rheological instruments hold out a definite scope for generating valuable informations on the basic rheological parameters of these products. In the context of Indian dairy industry, texture and rheology of certain solid and semi-solid dairy products such as paneer, khoa, chhana and milk sweets have been recognized to play an important role in their acceptance which has a great bearing on the success of their production in modern dairy plants.

1.4 Sensory Techniques for Evaluating Mechanical Texture Characteristics

1. Hardness: Place sample between molar teeth and bite down evenly, evaluating the force required to compress the food.

2. Cohesiveness: Place sample between molar teeth, compress and evaluate the amount of deformation before rupture.

3. Viscosity: Place spoon with sample directly in front of mouth and draw liquid from spoon over tongue by slurping, evaluating the force required to draw liquid over tongue at a steady rate.

4. Springiness: Place sample either between molar teeth (if it is solid) or between the tongue and the palate (if it is a semi-solid) and compress partially, remove force and evaluate the degree and quickness of recovery.

5. Adhesiveness: Place sample on tongue, press it against the palate and evaluate the force required to remove it with the tongue.

6. Fracturability: Place sample between molar teeth and bite down evenly until the food crumbles, cracks or shatters, evaluating the force with which the food moved away from the teeth.

7. Chewiness: Place sample in the mouth and masticate at one chew per second at a force equal to that required to penetrate a gum drop in 0.5 seconds, evaluating the number of chews required to reduce the

sample to a state ready for swallowing.

8. Gumminess: Place sample in the mouth and manipulate with the tongue against the palate, evaluating the amount of manipulation necessary before the food disintegrates.

Sensory texture profile is defined as the organoleptic analysis of the texture complex of a food in terms of its mechanical, geometrical, fat and moisture characteristics, the degree of each present, and the order in

which they appear from first bite through complete mastication. The data on these parameters is generally collected using either interval or ratio scales.

Table 1.1 Definition of textural characteristics






Force necessary to attain a given deformation

Force required to compress a substance between teeth


Extent to which a material can be deformed before rupture

Degree to which a substance is compressed between the teeth before it breaks


Rate at which a material returns to its original condition

Degree to which a product returns to its original size



Force with which a material fractures

Force with which a sample crumbles


Energy required to masticate a food to a state ready for swallowing

Time required to masticate the sample to a state ready for swallowing


Energy required to disintegrate a semisolid food to a state ready for swallowing

Denseness that persists throughout mastication.

Last modified: Friday, 9 November 2012, 4:53 AM