TEXTURE

1. TEXTURE Dr. Mohamad Yusof Maskat Fakulti Sains Teknologi Universiti Kebangsaan Malaysia 43600 Bangi, Selangor

2. Introduction • is very important physical property of foods • assessed by consumers visually (appearance) and also during eating eg. during biting, mouthfeel

3. Texture is affected by several factors : • Mechanics or rheology • Geometry • Chemistry • Thermal • Acoustic • psychology

4. can be measured through objective and sensorial methods • Both methods have advantages and disadvantages • Advantages of objective method: • Test only involves small deformations • Sensory : involves large deformations, a lot of samples.

5. Chewing involves a combination of compressive and shear forces. Less specific. • Chewing also involves increase in heat, according to body temperature. • Chewing involves external factors apart from the food eg. saliva containing enzymes, moisture. • Sensory evaluation may involves a combination of several sensory data eg. visual and not only mechanical.

6. there are several objective methods to measure texture • it depends on the type of food and the physical properties to be measured • Objective tests usually is based on the interaction between food and force • Thus, only measures the mechanical and physical aspects only

7. is an advantage of objective methods • Eg. as discussed previously • also serves as a weakness for objective methods • Eg. not comprehensive – limited to certain aspects; the eating process may be different

8. Objective measurement • involves normal or shear force • Normal force is a force which is perpendicular (90o) to the surface being acted upon • produces tension or compression • normal stress is defined as total force for each unit of surface area (F/A) • Similar to pressure (P=F/A)

9. Normal force

10. Shear force is defined as a force parallel to the surface being acted upon • shear stress is defined as total shear force for each unit of surface area (F/A)

11. Shear Force

12. For most material, external forces causes various types of internal stress • Eg. bending of a material : • resulted in a compression and tension effect • also involves shear force and shear stress

13. Membengkok bahan

14. sometimes, application of a normal force also can produce a limited shear stress. • among the commonly used tests are puncture test, shear test, compression test and others

15. Figure 1(d)

16. Application of force • Objective tests use several approaches for force application • depends on the properties to be studied • i. Constant rate of deformation or displacement • Constant probe speed • probe will interact with food at a constant rate

17. Constant rate of deformation or displacement

18. ii. Constant rate of force increment • force is applied to the sample at a constant rate of increase • difficult to achieve using UTM • usually needs a hydraulic apparatus • not commonly used for food analysis • because the force range involved is usually larger than which is commonly used for food analysis.

19. iii. Step deformation or loading • application of force all at once and immediate • difficult to achieve ideally • because of limitations in instruments • usually considered as practical step deformation

20. Step Deformation or Loading

21. iv. Sinusoidal deformation • Dynamic deformation • force is applied according to a sinusoidal pattern • at a small maximum amplitude • different force frequency can be used

22. Sinusoidal Deformation

23. Deformation measurement • involved the application of force to the sample • measures force that deforms the sample • frequently mimics the eating action • measurement is based on • 1. Maximum or minimum force • 2. Curve pattern of form

24. Puncture test • to measure product’s firmness • use a probe that pierce into the food • includes a spring to measure force • the probe is pressed onto the sample’s surface • force is applied until the probe punctures the surface and pierce into the sample

25. Puncture Test

26. Puncture test

27. maximum force is measured when the probe pierced the surface • due to the force has reached a point where the surface yields to the force, hence, termed yield point • after the sample is punctured, change in force differs according to sample type

28. Force Profile of Puncture Test

29. Compression test • for measuring hardness of samples • usually, compression onto one surface only eg. top surface • sample is placed in a container • subsequently, compressed for a certain distance eg. 80% from initial height of sample • maximum force is recorded

30. 2 type of compression test commonly used • 1. Linear compression : sample is subjected to stress in one direction only. The other surfaces is not affected. • 2. Bulk compression : sample is subjected to stress on all surfaces. Example, through hydraulic pressure.

31. Linear and bulk compression

32. Probes for linear compression test

33. Compression-extrusion test • Based on compression test • but including elements of extrusion • force is applied until the sample is forced through a series of apertures or holes • Maximum force needed is recorded • used for viscous materials eg. Gels, shortenings

35. Example of force profile for compression-extrusion test

36. Extrusion force is inversely related to the width of the probe • or space between the probe and wall of the container • width of the probe also influences the smoothness of the curve • the smaller the probe, the smoother the curve

37. Effect of probe width on the force profile of a compression-extrusion test

39. 3 point bending test • to measure the bending/flexibility or breakability of a sample • Eg. breakability of biscuits, flexibility of jellies • maximum force for flexing or breaking is recorded

40. 3 point bending test

41. Shear test • Definition of shear : • true shear : relative motion of parallel planes (satah) due to a tangential force being applied to one of the planes • cutting shear : any cutting action that divides a material • 2 commonly used shear test • Warner-Bratzler shear test • Kramer shear test

42. True and cutting shear

43. Warner-Bratzler shear test • to measure toughness of a product • probe head consist of a 'V‘ shaped plate • also have 2 additional plates on each side of the 'V‘ shaped • the 2 side plates will hold the sample until the sample is cut by the 'V‘ shaped plate • the maximum force needed to cut the sample is recorded

44. Warner-Bratzler shear test

45. Kramer shear test • Mimics the action of biting • Involves a combination ot several plates as probe • Interaction between probe and sample involves several force action: • 1. Compression • 2. Shearing • 3. Extrusion

46. Kramer shear test

47. Tensile test • involves the action of pulling and stretching food samples • not widely used in measurements of food systems • low correlation with sensory data • because during eating, the actions involved is more on compressive and shearing force

48. Tensile test

49. tensile test is also difficult to apply to foods due to the sample needed to be clamped at the end part • Samples are pulled until it breaks • it is hoped that the sample breaks normal to the direction of the force • but sometimes the sample breaks at the clamped ends due to being soft • Eg. tensile force of chewing gum, mee

50. Torsion test • torsion is produced when a pair of force rotates an object • represented by torsion force or torque • used to study certain physical attibutes of samples • not related to eating action • Eg. Farinograph, rotational viscometer