FOOD ENGINEERING

Lesson 10. INSTRUMENTS FOR RHEOLOGICAL MEASUREMENT

Module 1. Rheology of foods

Lesson 10

INSTRUMENTS FOR RHEOLOGICAL MEASUREMENT

10.1 Introduction

There are diverse range of instruments for measuring texture properties of dairy and food products. Some of the equipments are described as below:

(i) Wire Cutting Devices

A wire driven at a Constant speed to cut the sample is used for certain daily products. An advantage is that the sample area in Contact with the wire is constant throughout the test which minimizes the effect of friction and adhesion between the product and the test cell surfaces.

(ii) Circular Cutting Devices

The Cherry- Burrel Curd tension meter is used in the dairy industry to determine curd tension of milk and firmness of cottage cheese. A circular blade is driven at a constant speed of 2.54 cm per 7.5 sec. to cut the curd

(iii) Cone Penetrometer of Varying Dimensions

It consists of a cone of varying dimensions which is allowed to penetrate chhana, paneer,khoa or any other soft dairy product. The hardness values are read out on a mechanical linked graduated scale in terms of mm penetration.

(iv) Pea Tenderometer

It consists of a grid of shearing blades (test cell) rotated at constant speed through a second grid suspended, so that the force on the second, grid is counter balanced by a pendulum which is displayed by a pointer on a graduated scale. It is widely used by the pea industry.

(v) The Warner – Bratzler Shaler Test

A cylindrical sample usually 2.5 cm. in ilia. is placed ill a triangular hole in a thin blade of 0.25 cm thickness cut by pulling the blade through a slot and the sheer force indicated by a spring scale. It is widely used for meat products.

(vi) Kramer Shear Press

It consists of a hydraulic press where the ram speed can be selected to complete its down stroke in 15 to 100 seconds. The ram operated by a hydraulic pump drives the moving components of the texture test cell into stationary component supported by the press frame. It is based on the principle of a multi- blade shear compression cell. Because of limitation of control by ram speed, the instrument does not give precise and accurate reading of force exerted.

(vii) Instron Machine

The Instron universal machine is an instrument for measuring texture through tension and compression testing within the force range of <1N to 5kN. It is a versatile instrument for application in research, development and quality control laboratories. It comprises of a standard load frame and drive unit, a load weighing system and a microprocessor based control system. A beam carrying a load cell (moving cross head) is located between the base unit and the fixed crosshead at the top of the frame. The crosshead moving in vertical direction at a selected speed is supported and driven by two lead screws. It contains a force sensing and recording system which measures the force during the test and transmits them to a strip chart recorder. The Instron can be programmed for automatic return, cycling and relaxation test etc.

(viii) The Ottawa Texture Measuring System

This machine is similar to the Instron Machine in design and operation except that it uses a single screw as drive instead of twin screws. The Ottawa cell consists of a rectangular metal box containing 8 or 9 thin stainless steel rods. The sample is compressed by a plunger and sheared and extruded through a wire-grid. If offers operational flexibility for research and quality control laboratories. It uses modern electronic system to record force, deformation and time precisely.

(ix) General Foods Texturometer

This instrument imitates the chewing action of the mouth for mastication of foods. The chewing forces are detected by strain gauges with the help of a position transducer and displayed on an oscilloscope. This helps ascertain the force required for teeth penetration into the food, thus reflecting the food texture.

(x) Other Instruments

The curd tension, curd firmness, consistency of cheese etc. can be determined by various techniques especially milk curcio-meter, containing a star shaped knife attached to a balance. Other texture measuring device employed for milk products include ball and needle penetrometers, extenders, gelographs.