FOOD ENGINEERING

Lesson 4. MEASUREMENT OF RHEOLOGOCAL PARAMETERS

Module 1. Rheology of foods

Lesson 4

MEASUREMENT OF RHEOLOGICAL PARAMETERS

4.1 Introduction

The instrumental methods that have been used to evaluate the rheological properties of food may be empirical one or fundamental ones. Empirical methods include imitative ones, the Texture Profile Analysis (TPA) method employing the Texturometer as described by Friedman. The TPA has also been performed by many workers using Instron Universal Testing Machine. In these methods, mostly food samples are compressed between two plates using an Instron testing machine or a comparable apparatus and the force is recorded as a function of the compression. Until now no standardization of these tests has been made and many different executions of that have been described. Examples of differences are: shape and size of the test piece, treatments of the plates to increase or decrease the friction between the plates and the test piece, compression rate and temperature. One or more of the following parameters are usually derived from these tests:

· Force (or stress) at a given compression

· Force at the first maximum in the force-compression curve (often designated as fracture force)

· Initial slope (or modulus) of the force-compression curve

· Compression at the first maximum in the force-compression curve (often designated as fracture compression)

· Work done until a given compression

· Height recovered after deformation

· Adhesive force during ascending motion after compression

4.2 Textural Profile from Instron

The textural characteristics of the food samples can be interpreted from their respective force-distance compression curve obtained. A generalized texture profile curve obtained from the Instron Universal Testing Machine is shown in Fig:4.1 and the following textural parameters can be interpreted form the Instron Curve:

Fig. 4.1 Generalized textural profile curve obtained from instron universal machine

(i) Hardness (Kg_f): The force necessary to attain a given deformation, i.e. the highest point of peak in the first bite curve (Fig-4.1).

Hardness= H₁ , Kg_f

(ii) Brittleness (Kg_f): Force with which the sample crumbles, crackes or shatters

Brittleness (or Fracturability) = H_2, Kg_f

(iii) Adhesiveness: It is the work necessary to overcome the attractive forces between the surfaces of the sample and the other materials with which sample comes in contact. It is negative force area for the first bite curve (Fig-1)

Adhesiveness = A₃

(iv) Cohesiveness: The extent to which a material can be deformed before it ruptures

Cohesiveness = A₂/A₁

A₁ = Area under the first bite curve before reversal of compression

A₂ = Area under the second bite curve before reversal of compression

(v) Springiness (mm): The height of sample recovers between the first and second compression, on removal of the deformation force

Springiness = S, mm

(vi) Gumminess (Kg_f): It is the energy required to masticate a sample to a state ready for swallowing a product of hardness and cohesiveness

Gumminess = Hardness x Cohesiveness x 100

(vii) Chewiness (kg-mm): It is the energy required to masticate a sample to a state ready for swallowing. It is a product of hardness, cohesiveness and springiness.

Chewiness = Hardness x Cohesiveness x Springiness

Table 4.1 Definition of textural characteristics