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Texture Analysis

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Introduction

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

Texture Measurement

Texture measurement may be by

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


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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

Texture profiling

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.

Initial Effects:

Residual effects:

Sensory methods, properly conducted give reliable results but there are problems which include;


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Instrumental Methods

These try and replicate some of the results of sensory methods. There are three main approaches


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Fundamental Methods

Types of force

Illustration of a tensile force pair
Tensile forces tend to stretch the material

Illustration of a compressive force pair
Compressive forces tend to squash the material

Illustration of a shear force pair
Shear forces tend to distort or displace the material

Elasticity

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

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
units: dimensionless

Stress-strain curves

If a graph of stress vs. strain is plotted a graph similar to that below may be produced

Typical stress-strain graph
Typical stress-strain curve

The key points on the curve are

  1. The elastic limit
  2. The yield point
  3. The Point of maximum stress (or failure)
  4. The point of fracture

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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

  1. Fracturability: The ease with which the material will break.
  2. Hardness: The force required to compress the material by a given amount
  3. Cohesiveness: The strength of the internal bonds in the sample
  4. Adhesiveness: The energy required to overcome attractive forces between the food and any surface it is in contact with.
  5. Springiness: The elastic recovery that occurs when the compressive force is removed.
  6. Gumminess: The energy required to break down a semi-solid food ready for swallowing
  7. 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)

Example of instrumental texture profile curve
Texture profile curve

Texture profile parameters are determined from:


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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

Modified equation for stress  Modified equation for 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

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