TEXTURE Dr Mohamad Yusof Maskat Fakulti Sains Teknologi

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

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

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

• 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.

– 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.

• 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

• 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

Objective measurement • involves normal or shear force • Normal force is a force which is perpendicular (90 o) 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)

Normal force

• 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)

Shear Force

• 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

Membengkok bahan

• 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

Figure 1(d)

Application of force • Objective tests use several approaches 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

Constant rate of deformation or displacement

• 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.

• 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

Step Deformation or Loading

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

Sinusoidal Deformation

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

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

Puncture Test

Puncture test

• 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

Force Profile of Puncture Test

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

• 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.

Linear and bulk compression

Probes for linear compression test

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

Example of force profile for compression-extrusion test

• 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

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

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

3 point bending test

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

True and cutting shear

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

Warner-Bratzler shear test

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

Kramer shear test

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

Tensile test

• 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

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

Torsion force

Texture profile analysis (TPA) • measurement of texture attributes by mimicking the act of chewing • the probe will interact with the food 2 times • change of force and time will be recorded • from the force-time curve, texture attributes of the sample can be determined/ calculated

TPA force profile Distance 1 Distance 2

• among the attributes that can be determined are: • i. fracturability – force needed to produce the first significant fracture during the first bite • ii. hardness – largest force produced during the first bite

• iii. cohesiveness – ratio of area under the positive curve between the first and second bite (Area 2/Area 1) • iv. adhesiveness – work needed to pull out the probe from the sample after the first bite (Area 3) • v. springiness – ration of the biting distance of the second bite with the first bite (Distance 2/ Distance 1)

• vi. gumminess – hardness x cohesiveness • vii. chewiness – gumminess x springiness

Universal Testing Machine (UTM) • Instrument commonly used to analyse texture • Brands : Instron (USA), Shimadzu (Jepun), TA (UK) • Measures the change of force • Now connected to computer • can be attached with various probes

Single column UTM

twin column UTM

Shimadzu AGS-J

Shimadzu AGS-J Software

Linear measuring instruments • • Measures linear distances distance of food movements or distance of probe movements relates the linear ditance with physical attributes • sometimes, more practical

Bostwick consistometer • used to determine the consistency of semiliquid or viscous foods eg. sauces, ketchup, puree • a trapdoor will hold the sample and prevent it from flowing • the trapdoor is immediately opened • Sample will flow along the container

• the distance covered by the sample in 30 sec will indicate the consistency of the sample • the more viscous the sample, the shorter the distance travelled in 30 sec. • but also will be influenced by other factors • Eg. density, surface tension, wettability

Bostwick consistometer

Ridgelimeter • Measures rigidity of gels eg. pectin gel • Samples are placed on the instrument • the probe end with a micrometer attached is lowered until it touches the sample • the distance recorded by the micrometer is recorded • after 2 minutes, the measurement is repeated • the difference in distance is shown as %

Ridgelimeter

Haughmeter • used to measure the quality of eggs • the egg is broken and placed on a piece of glass • the Haughmeter is positioned on the egg white at 10 mm from the egg yellow • Micrometer is rotated until its end touches the egg white • the more the distance traveled by the probe, shows a lower quality

Haughmeter

USDA consistometer • developed by United States Department of Agriculture (USDA) • Samples are prepared according to a certain dimension eg. height - 3. 25 inches and diameter - 3 inches • Samples are placed on a sheet with several concentric circles drawn and divided into quarters • after 1 minute, distance of flow is measured in all 4 quarters and averaged

USDA consistometer

Hilker-Guthrie Drop • Measures the consistency of semi-solid samples eg cream, yoghurt • an aluminum tube is dropped onto the sample • a scale on the aluminum tube allows the determination of distance penetrated by the tube into the sample • the more viscous the sample is, the lesser the penetration will be

Hilker-Guthrie drop

Volume measuring instruments • the sample is placed in a measuring box • an amount of seeds enough to fill the measuring box when it is empty will be used • the seeds will be poured into the measuring box containing the sample until the box is full

• the volume of the balance of the seeds not used to fill the box is measured eg. graduated cylinder • volume of sample = volume of balance of seeds not used to fill the measuring box

Bread loaf measuring instrument