- Sensory Analysis of Texture
- Instrumental Methods
- Fundamental Methods
- Instrumental Profile Analysis
- Food Texture Properties
Texture is essentially the effect of physical and chemical properties on our senses of sight, hearing and touch and is an important attribute in our perception of food. If texture is not right, we do not like the food - even if the flavour is OK. e.g. we do not like
- soggy crisps and biscuits or
- tough and over chewy meat
Texture measurement may be by
- Sensory Analysis
- Instrumental Analysis
Sensory methods permit more complex attributes to be evaluated and cost more to administer.
Instrumental methods, on the other hand, are reliable within their limitations and are more economical to use
Sensory Analysis of Texture
Sensory measurement of texture is based on texture profiling
Texture profiling requires a trained panel who produce ratings based on three phases of chewing the food
- Initial effect: perception on first bite.
- Masticatory: perception on chewing
- Residual: changes perceived during chewing and swallowing
The initial and masticatory effects are subdivided into mechanical, geometrical and other - essentially moisture and oiliness. The mechanical effects are further subdivided and are rated on a numeric scale.
- Rate of breakdown
- Type of breakdown
- Moisture absorption
- Mouth coating
Sensory methods, properly conducted give reliable results but there are problems which include;
- Training and maintaining panels
- Sophisticated analytical tools are needed
These try and replicate some of the results of sensory methods. There are three main approaches
- Fundamental methods: based on measurement of physical properties such as stress/strain and rheology
- Imitative methods: aim to simulate chewing mechanically
- Emprical methods: tend to use specific devices to give specific measurements which have been found to have practical value
Types of force
Tensile forces tend to stretch the material
Compressive forces tend to squash the material
Shear forces tend to distort or displace the material
An elastic material is one which returns to its original shape and size when any deforming force is removed
If A graph of load vs extension is a straight line, the material is said to obey Hooke's law.
Stress and strain
The extension resulting from the application of a force is a property of
- The dimensions of the material,
- The physical properties of the material
Stress and strain are quantities which are related to force and extension in such a way as to incorporate the material dimensions
Stress is defined as force per unit area
Strain is defined as extension per unit length
Tensile stress is defined as σ = F/A
units = N m-2 or Pa
Tensile strain is defined as ε = h/Δh
If a graph of stress vs. strain is plotted a graph similar to that below may be produced
Typical stress-strain curve
The key points on the curve are
- The elastic limit
- The yield point
- The Point of maximum stress (or failure)
- The point of fracture
Instrumental Profile Analysis
This technique was developed in the 1960's with the aim of reflecting sensory attributes of texture. It involves a "two bite" compression test which simulates first two chews on the food and the output is a curve of force vs time.
From the texture profile curve, seven parameters are determined
- Fracturability: The ease with which the material will break.
- Hardness: The force required to compress the material by a given amount
- Cohesiveness: The strength of the internal bonds in the sample
- Adhesiveness: The energy required to overcome attractive forces between the food and any surface it is in contact with.
- Springiness: The elastic recovery that occurs when the compressive force is removed.
- Gumminess: The energy required to break down a semi-solid food ready for swallowing
- Chewiness: The energy required to chew a solid food into a state ready for swallowing.
The properties of Gumminess and Chewiness are mutually exclusive
Not all the other properties will be found in a given food sample.
One other parameter has subsequently been added; The Modulus of Deformability.
This is defined as the initial slope of the force-deformation curve before the first break in the curve (i.e. before fracture of the sample)
Texture profile curve
Texture profile parameters are determined from:
- Fracturability = F1
- Hardness = F2
- Cohesiveness = A2/A1
- Adhesiveness = (based on) A3
- Springiness = D1
- Gumminess = hardness x cohesiveness = F2 x A2/A1
- Chewiness = hardness x cohesiveness x springiness = F2 x A2/A1 x D1
- Modulus of deformability (based on) slope, S1
Food Texture Properties
Strictly speaking the data from the curve should be either expressed as a force deformation curve or turned into a stress strain curve. For foods, however, Relatively large deformations mean large changes in area.
For this reason, alternative definitions of stress and strain have been developed
Alternative definitions of stress and strain
These are sometimes referred to as "True stress" ((T) and "True strain" ((T) or "Hencky' strain". TPA data based on this gives more reproducible data for comparing different foods since the stress and strain relationships are less dependant on experimental conditions
Produced by Geoff Walker
Last modified 03-Dec-2004