Food Engineering, rheology of foods, visco-elastic model, theological methods, rheological properties, food texture

The ppt is about rheology of foods, their models, methods, properties, food texture etc.
know the relevance and significance of rheology, evaluation of food products as a tool for improvements in quality of food products besides creating the consumer acceptance.

1. Welcome 1

2. UNIT I 2 FOOD ENGINEERING DE-323 (3+1)

3. 3 Rheology of foods

4. Rheology in daily life 4

5. Rheology in action How do you like the sauce to flow?? 5

6. Rheologically “pleasant” 6

7. Do we really need to study rheology?? 7

8. YES!!! For food technologists, knowledge of rheology is important for a better understanding of how process variables influence specific textural characteristics, such as pourability and mouthfeel Rheological measurements can aid in the understanding of how the viscosity and elasticity of foods are influenced by changes in composition, processing, and storage parameters 8

9. Cont… Mathematical modelling of rheological behavior permits prediction of material performance during exposure to certain processing or experimental conditions. It is particularly important in food production where most foods are forced to “flow” through processing line. Rheological measurements can provide a rapid determination of product quality and may serve as a tool for quality control  Rheological data should assist food technologists and plant engineers to design more efficient and cost- effective processes 9

10. Now…let’s define “rheology” The term comes from Greek ‘ rheos ’ meaning ‘to flow’. Rheology is defined as the mechanical properties that results in deformation and flow of material under the applied force 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. 10

11. Definitions Shear stress : Force (F) acting on area (A) to effect a movement in the liquid element between two plates. The velocity of the movement at a given force is controlled by the internal forces of the material. 11

12. Definitions Shear rate: By applying shear stress a laminar shear flow is generated between two plates. The uppermost layer moves at the maximum velocity V max , while the lower most layer remains at rest. The shear rate is defined as Where, dv =Velocity difference between adjacent velocity layers dh= Thickness differential of the flow layers 12

13. Cont… In the laminar flow the velocity difference between adjacent layers of thickness is constant ( dv =dh=constant) the differential can thus be approximated as follow 13

14. VISCOSITY The word "viscosity" derived from the Latin word " viscum " meaning mistletoe It is a measure of the internal resistance of a fluid which is being deformed by either shear stress or tensile stress The less viscous the fluid is, the greater its ease of movement (fluidity) Changes during heating, cooling, concentration 14

15. Dynamic viscosity Also called as absolute viscosity, µ Expressed in ASTM standards, as centipoise ( cP )/ Pascal second Absolute viscosity - coefficient of absolute viscosity - is a measure of internal resistance. 15

16. kinematic viscosity It is the dynamic viscosity divided by the density Unit m 2 /s, Stokes, St The kinematic viscosity is sometimes referred to as diffusivity of momentum 16

17. Classification of rheology 17

18. IDEAL ELASTIC BEHAVIOUR (HOOKEAN BODY) 18

19. Viscoelasticity  is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation 19

20. Ideal plastic behAVIOUR ( st . venant body) 20

21. 21 Ideal VISCOUS behAVIOUR (NEWTONIAN LIQUID)

22. Newtonian fluid Newtonian fluids have the straight line relationship between the shear stress and the shear rate with a zero intercept. Where, τ =  shear stress, µ = dynamic viscosity (η = µ/ρ) , - dv / dx = velocity gradient 22

23. Non-Newtonian fluids 23

24. Non-Newtonian fluid A non-Newtonian fluid is broadly defined as one for which the relationship between shear stress and shear rate is not a constant. The two most commonly used equations for characterizing non-Newtonian fluids are the power law model and Herschel- Bulkley model for fluids. Where, τ =  shear stress, K = consistency constant, γ = shear rate, n = flow behaviour index, τ 0  =  yield stress 24 τ  = K ( γ ) n   τ   =  τ 0  + K ( γ ) n  

25. Time independent fluid 25

26. fluid k n τ Examples Harschel-Bulkely >0 0<n< ∞ >0 Minced fish paste, raisin paste Newtonian >0 1 Water, fruit juice, milk, honey, vegetable oil Shear-thinning >0 0<n<1 Apple sauce, banana puree, orange juice concentrate Shear-thickening >0 1<n< ∞ 40% raw corn starch solution Bingham plastic >0 1 >0 Tooth paste, tomato paste 26

27. Time dependent fluid 27

28. Importance of Rheology in food industry Mixing Flow control Settling or floating Pumping Coating Influence on unit operations 28

29. Cont… To select proper method of harvesting and sorting of raw materials To select proper ingredients to manufacture processed foods. To select proper technology/equipment to manufacture processed foods with desirable sensory and rheological properties. Helps in newer product development (e.g. dietetic ice cream, paneer , low fat mozzarella cheese etc.) In designing processing equipment, packaging machines, transportation system etc. To improve sensory quality of the products In marketing the products. 29

30. Importance of rheology in dairy industry To evaluate ingredient for potential contribution to creaminess in fat-free dairy products. To evaluate quality of cheese and applicability of cheese for various applications like suitability for pizza topping. 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. 30

31. Visco -elastic 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. 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. 31

32. Textural characteristics Properties Physical Sensory Primary Hardness Force necessary to attain a given deformation Force required to compress a substance between teeth Cohesiveness Extent to which a material can be deformed before rupture Degree to which a substance is compressed between the teeth before it breaks Springiness Rate at which a material returns to its original condition Degree to which a product returns to its original size 32

33. Cont… Secondary Fracturability /Brittleness Force with which a material fractures Force with which a sample crumbles Chewiness Energy required to masticate a food to a state ready for swallowing Time required to masticate the sample to a state ready for swallowing Gumminess Energy required to disintegrate a semisolid food to a state ready for swallowing Denseness that persists throughout mastication. 33 Properties Physical Sensory

34. Application of Rheology in the Food Industry Meat products: for measurement of toughness and compactness of meat and meat products; establishment of quality grade for marketing and export. Fruits and vegetables:  To evaluate variety of crop; for predicting the effect of storage and ripening period on process; prediction of storage and ripening period; in prediction of stage of harvesting and stage of maturing; used for sorting; measurement of\ textural variation, gives us an idea about growing practice; method of harvesting. 34

35. Jams and jellies:  helps to decide variety of blending ingredients, esp. pectin; deciding jelling quality of pectin as well as integrity of gel structure, helps in deciding ingredients. Snack foods:  To evaluate formula for dough making and paste, particularly for extrusion; for measurement and adjustment of solids content; for measurement of textural properties like crispiness, hardness, softness and other properties To decide packaging and packing material; helps in predicting shelf-life of product under given storage conditions and history of product (method of harvesting, storage conditions, pre-treatments and processing unit operations). 35

36. Confectioneries:  To evaluate the quality of raw material; to optimize the processing parameters; to decide the ingredient varieties to be used; for measuring properties like thickness of coating, chewiness, elasticity, brittleness and shelf life of product. Paste: To evaluate consistency of mixture used for measured viscometric parameters at different stages of processing; deciding the pectin retention and prediction of consistency of final products. 36

37. Bakery:  To evaluate dough consistency; to estimate floor time and rise time; effect of additives; prediction of shelf life. Dairy products:  To evaluate the effect of ingredients i.e. creaming in fat-free dairy products, fat mimic products by using micro-fluidization of whey protein concentrate, desired quality of mozzarella. 37

38. 38 Visco -elastic models

39. introduction The word viscoelastic is derived from the words " viscous " + " elastic “ A model of linear visco -elasticity by considering combinations of the linear elastic spring and the linear viscous dash-pot . These are known as rheological models or mechanical models . 39

40. The Linear Elastic Spring The constitutive equation for a material which responds as a linear elastic spring of stiffness E is shown in figure 40

41. The Linear Viscous Dash-pot By definition, the dash-pot responds with a strain rate proportional to stress 41

42. Kelvin model 42

43. Maxwell model 43

44. Four element model (burger’s model) 44

45. 45

46. Electrical analogy of mechanical models 46

47. 47

48. 48 Rheological methods

49. introduction Generally rheological properties are judged by sensory panel, it has its advantages and disadvantages depending on the person selected for judging the products. There are many instrumental methods are developed based on fundamental principle as well as experimental data. There are certain mathematical models developed by different scientist based on empirical methods, which are widely used for measurement of rheological properties of most of the food products. 49

50. Tests for Measurement of Rheological Properties Fundamental tests  which measure the properties that are inherent to the material and do not depend on geometry and shape of the sample, conditions of loading or type of apparatus used Empirical tests or imitative tests  (because these imitate the chewing in mouth). e.g. properties like puncture force, extrusion energy, cutting force required, pressing/compression force required for juice extraction, etc. – where mass of sample, geometry and speed of test will decide the magnitude of parameter estimated. 50

51. Fundamental test Generally fundamental tests are applied on solid foods and these are further classified into quasi-static and dynamic tests The most important tests includes Stress-strain tests Creep tests Stress relaxation-tests Dynamic test 51

52. Stress-strain test 52

53. Creep test Load (stress) is suddenly applied and held constant, and deformation (strain) is measured as a function of time. The results are expressed in terms of time-dependent parameters E(t) and D(t) in tension and compression creep G(t) or J(t) in shear creep K(t) or B(t) in bulk creep 53

54. The rheological model to represent the creep behaviour is the 4-element Burgers model or its equivalent The change in the strain of material can be measured, when stress is removed it known as creep curve. In short we can say that creep curve shows strain as a function of time at constant stress. Visco-elastic materials can often be characterized by a modulus and relaxation time, which can be determined by an analysis of strain curve with time. 54

55. Cont… If in a creep test of a visco -elastic material, results can be presented in terms of strain Vs time , the complete curve for creep and recovery upon unloading will normally look like the curve. 55

56. Stress-relaxation test Stress relaxation test measures the stress required to maintain the level of strain as a function of time after rapidly deforming a specific volume of the material. The results are expressed in terms of time dependent modulus E(t) in tension or compression, G(t) in shear and K(t) in bulk compression 56

57. The rheological model representing stress-relaxation is the generalized Maxwell model. One of the most important visco-elastic parameters which can be obtained from a stress relaxation test is the time of relaxation . 57

58. Disadvantages of creep and stress relaxation test In order to obtain complete information about viscoelastic behaviour of a material, it is necessary to make measurements over many decades of time scales. This, in addition prolonging the experiment, may cause, particularly in the case of bio-materials, chemical and physiological change which will affect the physical behavior of the material Impossibility of having a truly instantaneous application of load or deformation at the beginning of the experiments. 58

59. Dynamic test Dynamic mechanical test, in which the specimen is deformed by a stress which varies sinusoidally with time. Since a periodic experiment at frequency is quantitatively equivalent to test at time 1/ ω it is possible to provide considerable amount of information corresponding to very short time . 59

60. Cont… Suitable for rigorous evaluation of the rheological characteristics of the materials. Allows the calculation of elastic modulus and mechanical damping over a wide range of frequencies Useful in studying the molecular structure and chemical composition of high polymers 60

61. Visco- elastometer 61

62. 62 Measurement of rheological parameters

63. Viscometers vs. rheometers?? 63

64. Instruments capable of measuring rheological properties of fluids & semi solids are classified into: Tube/Capillary type : Allow only one pass of the material through the system, time independent behavior is investigated 64

65. Tube type Liquid is forced through a tube and viscosity is determined from rate of flow, applied pressure and the geometry of the tube The relationship between shear rate and shear stress is obtained from the measurement of the pressure gradient and volumetric flow rate of the fluid through the tube. 65

66. Common assumptions Flow is steady Properties are independent of time Flow is laminar Fluid velocity has no radial or tangential components The fluid exhibits no slippage at the wall The fluid is incompressible The fluid viscosity is not influenced by pressure The measurement is conducted under isothermal condition 66

67. Consider a laminar flow of Newtonian or Non Newtonian liquid in a tube of radius R and length L ∆P is the pressure difference between the ends of the tube Basic equation for tube/capillary viscometer τ = Shear stress at the surface of the moving column of the fluid r = radius from the centre of the tube (0 to R) 67

68. 3 categories Glass capillaries U tube viscometer Cannon fenske glass capillary High pressure capillaries Pipe viscometers All three establishes a pressure difference to create flow Major difference between capillary and pipe is the difference in diameter of the tube 68

69. U-tube viscometer Also known as Ostwald viscometers , named after Wilhelm Ostwald Consists of a U-shaped glass tube held vertically in a controlled temperature bath In one arm of the U is a vertical section of precise narrow bore Above this is a bulb, with it is another bulb lower down on the other arm 69

70. Liquid is drawn into the upper bulb by suction, then allowed to flow down through the capillary into the lower bulb Two marks (one above and one below the upper bulb) indicate a known volume The time taken for the level of the liquid to pass between these marks is proportional to the kinematic viscosity 70

71. Cannon fenske Glass Capillary Viscometer Gravity operated glass capillary Only suitable for Newtonian fluid because the shear rate varies with discharge 71

72. High pressure capillaries: Operate at high shear rate but involve a significant end pressure correction Pipe and mixer viscometers: Can handle much larger particles than cone and plate or parallel plate devices Problem associated with slip & degradation in structurally sensitive materials are minimized in mixer type 72

73. Rotational rheometers Rotational type : Instruments may be operated in constant angular velocity or oscillating mode. They investigate time dependent behavior 73

74. Cont… Hence rotational viscometers are more widely used than capillary/ tube viscometers In rotational viscometer, viscous drag of rotating body immersed in a liquid is measured as a function of speed 74

75. Use the idea that the torque required to turn an object in a fluid is a function of the viscosity of that fluid The torque required to achieve a certain rotational speed is measured and plotted A graph of shear stress (torque) against shear rate (angular velocity) yields the viscosity in a straight forward manner 75

76. Rotational type Useful in evaluating time-dependent fluid behavior because they easily allow materials to be subjected to alternate periods of shear and rest 76

77. instruments for measuring texture properties of dairy and food products Wire Cutting Devices: A wire driven at a Constant speed to cut the sample is used for certain dairy products. An advantage is that the sample area in Contact with the wire is constant throughout the test which minimizes the effect of friction and adhesion between the product and the test cell surfaces. 77

78. Circular Cutting Devices The Cherry- Burrel Curd tension meter is used in the dairy industry to determine curd tension of milk and firmness of cottage cheese. A circular blade is driven at a constant speed of 2.54 cm per 7.5 sec. to cut the curd Cone Penetrometer It consists of a cone of varying dimensions which is allowed to penetrate chhana , paneer, khoa or any other soft dairy product. The hardness values are read out on a mechanical linked graduated scale in terms of mm penetration. 78

79. Instron Machine Measuring texture through tension and compression testing within the force range of <1N to 5kN . It comprises of a standard load frame and drive unit, a load weighing system and a microprocessor based control system. A beam carrying a load cell (moving cross head) is located between the base unit and the fixed crosshead at the top of the frame. 79

80. The crosshead moving in vertical direction at a selected speed is supported and driven by two lead screws. It contains a force sensing and recording system which measures the force during the test and transmits them to a strip chart recorder. The Instron can be programmed for automatic return, cycling and relaxation test etc. 80

81. The Ottawa Texture Measuring System The Ottawa cell consists of a rectangular metal box containing 8 or 9 thin stainless steel rods. The sample is compressed by a plunger and sheared and extruded through a wire-grid. If offers operational flexibility for research and quality control laboratories. It uses modern electronic system to record force , deformation and time precisely. 81

82. 82 Measurement of food texture

83. Food rheology vs. food texture?? 83

84. It is the set of properties of a food able to be perceived by the eyes, the tact, the muscles of the mouth including sensations like harshness, smoothness, granular, etc Perceptions that tend to constitute a valuation of the physical characteristics of the food which they are perceived through the mastication and also a valuation of the chemical characteristics that are perceived through the pleasure 84 introduction

85. Texture Profile Analysis Initial step is begun at the first bite Masticatory step is perceived during chewing Residual step is perceived after the mastication and swallowing  85

86. Textural Properties of Foods Textural properties of food are those characteristics that are sensed by the feeling of touch, and are related to deformation, disintegration, and flow of food under the application of forces. These properties can be measured objectively as function of force, distance, and time. Textural properties of foods are greatly influenced by internal structure and other variables such as size, shape, moisture, and fat contents and their inter-relationships.  86

87. Properties which deals with sense of touch, kinesthetic properties and manifestation of rheological properties are called as Textural properties 87 Textural properties Examples Hardness Grains , Cashew nuts, coconut Softness Bread, creams, paste, jelly Brittleness Chips, kurkure , biscuits Firmness Cakes, fruits, meat Ripeness Fruits, cheese, butter Tenderness Almond, roti , meat Textural properties

88. 88 Textural properties Examples Crustiness Flour, fruits, pulps, marmalades Stickiness Chewing gum, chocolates, jam, jelly. Gumminess Fruits juices, plant secretions (latex, etc.) Fibrousness Fruits, root crops Malines Cereals, pulses, dry fruits Smoothness Ice cream, pastes, flours, jelly Chewiness Chewing gum, chocolates Juiciness Fruits, jam, jelly, aloe Vera Crispiness Chips , papads, kurkure , biscuits Flakiness Extruded products Crunchiness Biscuits, chocolates

89. Probe travels from start point to the sample at pre-test speed mm/sec Probe Sample Basic principle of texture analyser 89

90. When the probe measures the trigger force the speed changes to the test speed and force, distance & time is recorded. Probe Sample Basic principle F t 90

91. Probe travels into the sample at the test speed until the test is complete Probe Sample Basic principle 91 F t

92. When test is finished, probe returns to the start at the post-test speed Probe Sample Basic principle Return to Start (Trigger) F t 92

93. TYPICAL TEST RESULT COMPRESSION/ PENETRATION TENSION / ADHESION Maximum Force Energy Maximum Force Average and Gradient Stickiness Work of Adhesion Hardness 93

94. Methods of Measuring Texture Texture Profile Analysis (TPA) Compression Puncture & Penetration Cutting and Shearing Fracture & Bending Extrusion (Forward & Backward) Tension Adhesion 94

95. Texture Profile Analysis Texture profile analysis (TPA) is an instrumental test originally developed at the General Foods Corporation Technical Centre (1963) to provide objective measurements of texture parameters, a major factor of food acceptability. Known as the “ two bite test ” Provides textural parameters which correlate well with sensory evaluation parameters FORCE TIME FIRST BITE SECOND BITE DOWN ‘COMPRESSION’ UP ‘ Decompression ’ 95

96. TPA (TEXTURE PROFILE ANALYSIS) or ‘TWO - BITE’ TEST COHESIVENESS = AREA 2/ AREA 1 GUMMINESS = HARDNESS X COHESIVENESS CHEWINESS = GUMMINESS X SPRINGINESS FORCE FIRST BITE TIME SECOND BITE Fracturability Hardness 1 DOWN UP DOWN UP Hardness 2 Area 1 Area 2 o Adhesiveness Area 3 Springiness Stringiness A B 96

97. COMPRESSION COMPRESSED SAMPLE PROBE UNCOMPRESSED SAMPLE H The sample being tested has surface area smaller than that of the probe in use PROBE 97

98. COMPRESSION Comparison of Bread Freshness by Stress Relaxation Fresh Stale 98

99. PUNCTURE and PENETRATION The sample being tested must have a larger surface area than the contact area of the probe being used Uses cylinder, conical and needle probes Causes irreversible changes in the sample Involves both compressive and shear forces

100. PENETRATION of a PEAR to Measure Ripeness Bio-Yield Point Firmness Stickiness 100

101. PUNCTURE and PENETRATION Conical probe is used for situations where the stress may vary during the use of the product e.g. Spreading Needle probes Conical probes Spherical probes etc. 101

102. Spreadability test of Margarine stickiness Maximum Shear Work of Shear 102

103. CUTTING and SHEARING Force required to cut or slice through a sample Meat products Cheese Vegetable 103

104. CUTTING and SHEARING Warner- Bratzler Blade Consist of hick nickel plated steel blade with a vee -shaped notch. Double shear action Gives more of a cutting action during test Eg . Max force related to toughness of meat 104

105. CUTTING and SHEARING Volodkevich Bite Jaws Simulate teeth when biting Final force and force at yield indicate toughness Toughness of meat Fibrousness of vegetables Sample 105

106. CUTTING and SHEARING Wire Cutter Constant contact area with sample Cutting is done by fine wire Firmness and spreadability of butter Consistency of cheese 106

107. CUTTING and SHEARING Kramer Shear Cell Cereals, Fruit, Peas, Beans Sample 107

108. FRACTURE and BENDING Three Point Bend Rig Measure fracture and break strength of...... Biscuits Chocolate Bread sticks Measure freshness of...... Vegetables 108

109. FRACTURE and BENDING Fracture Hardness Three Point Bend of a Crisp Product 109

110. EXTRUSION (FORWARD) 110

111. EXTRUSION (BACKWARD) 111

112. EXTRUSION (BACKWARD) Viscous liquids and sauces Gels Processed fruit and vegetables 112

113. TENSION Sample SPAGHETTI RIG Measure Tensile Strength of : Noodles Human Hair Thread Packaging Seals Sausages Films Chewing gum sticks 113

114. TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data Force Time o Fracturability Definition The force at which there is the first significant break in the curve (originally called the brittleness) NB - Not always present

115. TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data Force Time o Hardness 1 Hardness 2

116. Force Time o TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data Hardness Hardness 2 Definition The maximum force during the first cycle of compression. Is also known as the “firmness ”.

117. TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data Force Time o A B Area 1 Area 2

118. Force Time o TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data A B Definition The ratio of the positive force area during the second cycle of compression to that of the first cycle.

119. Force Time o TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data A B

120. Force Time o TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data Stringiness Note Defined as the distance that the product is extended during de- compression before separating from the probe. Not an original TPA parameter and is not included in the TPA analysis macro at this point.

121. Force Time o TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data Springiness Definition The height that the food recovers during the time that elapses between the end of the first cycle and the start of the second cycle.

122. Force Time o TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data Springiness

123. Force Time o TPA (TEXTURE PROFILE ANALYSIS) Analysis of the data Definition The negative area for the first compression cycle - representing the work needed to overcome the attractive forces between the surfaces of the probe and the food. Work of Adhesion

124. Down Up Down Up Force Time A 1 A 2 A 5 Definition Ratio of the first UP ( decompression) stroke to the first DOWN ( compression) stroke 124

125. Textural Characteristics   Hardness: the force (kg) necessary to attain a given deformation. Cohesiveness: the strength of the internal bonds making up the body of the product. Elasticity: the rate at which a deformed material goes back to its undeformed condition after the applied force is removed. Adhesiveness: the work necessary to overcome the attractive force between the surface of the food and the other surface of contact (e.g., tongue, teeth, etc.)  125

126. Brittleness: the force with which the material fractures and was related to the primary parameters of hardness and cohesiveness . Chewiness: the energy required to masticate a solid food product to a state ready for swallowing and was related to hardness, cohesiveness, and elasticity. Gumminess: the force required to disintegrate a semisolid food to a state ready for swallowing and was related to hardness and cohesiveness.  126

127. Applications Gels Dairy products Biscuits, Bread & Dough Confectionery Meat, Fish & Poultry Fruit & Vegetables Cereals Cosmetics Packaging Snack Foods Pharmaceuticals .... and many more 127

128. 128 Rheological properties of foods

129. Rheological properties of granular food and powders The physical properties of granular materials and powders have direct influence on the transport of food within a food processing operation. The design of the handling system for dry products requires knowledge of the properties of the product being handled. 129

130. The basic relationships used to predict or measure the properties of these types of food product will be presented and described. Bulk Density : The magnitude of the bulk density will vary with the extent of packing within the food particle structure. One approach to measurement includes Loose Bulk Density Packed Bulk Density 130

131. Two additional properties of granular products which relates to density are void (v) and Interparticle Porosity. The void can be defined as the ratio of the volume of space between particles to the volume of solids. Porosity (ψ) can be defined as the ratio of the air volume within the particles to the total particle volume 131

132. PROPERTIES OF SOLID FOODS Solid foods are generally characterized in terms of stress - strain relationship. The classification of solid foods is even more hazy than that of fluid foods. There are two major groups: elastic and non elastic . 132

133. Elastic Solids Hookean or linear elasticity Young’s modulus (E) or elongation modulus. shear modulus (G) or rigidity . Bulk modulus (K) 133

134. Non-Hookean or non-linear elasticity 134 Cont…

135. Cont… Non-Elastic Solids 135

136. Visco-elastic Foods The reaction of a viscoelastic body to stress (or strain) consists partly of a viscous component and partly an elastic one. Since stress and strain are time-dependent, the response of the material is rate dependent. 136

137. 04-12-2011 137 Thank you

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