Food Engineering

Lesson 6. RHEOLOGICAL PROPERTIES OF GRANULAR FOODS AND POWDERS

Module 1. Rheology of foods

Lesson 6

RHEOLOGICAL PROPERTIES OF GRANULAR FOODS AND POWDERS

6.1 Introduction

Dry food products make up a considerable portion of the total amount of food products available. Like fluid food products they are handled in various ways in different parts of processing plant .The design of handling system for dried products requires knowledge of the flow properties of the product being handled and transported or conveyed. The manner in which granular foods or powder may flow into or out of container is of particular concern in processing plants. In addition to the density and particle size parameters, there are specific parameters which describe the flow properties of these types of food products. Two common parameters used for this purpose are the angle of response and the angle of slide. Both of these parameters lack theoretical considerations but do serve as a means of comparing different food powders. The angle of slide is a rather simply defined parameter in which the powder is placed on a horizontal plate and the angle of the plate is changed until the powder slides from the plate. The angle from the horizontal which is required for the powder to lose its position on the plate is measured and this angle will be a function of the type of surface on which the powder is placed.

Dry food products are handled in various ways in different parts of processing plants. The design of handling system for dry products requires knowledge of the properties of the product being handled.

6.2 Density

Density is one of the basic properties of any material but in the case of granular food products, various types of densities have been defined:

6.2.4 Particle shape

All particles are not exactly of spherical shape, how far it is deviated from spherical shape is expressed by the term spherocity. The term spherocity Φ_S which is independent of particle size is used to express shape of the particle.

Φ_S = 6 Vp/ Dp Sp

Where Dp equivalent diameter of particle

Sp surface area of one particle

Vp volume of one particle

For a regular particle Φ_{S =}1

For many crushed material Φ_{S =} 0.6 to 0.7

6.2.5 Particle size and size distributions

A very important property of granular foods and powders is particle size and size distribution. One of the important factors to consider when discussing the mean diameter of a particle is the type of diameter being utilized. Mugele and Evans (1951) developed a generalized expression, which can be used to define all types of mean diameters.

This expression is

d = Σ(d^q N)

Σ (d^p N )

where:

Symbol	Name of Mean Diameter	p	q	Order
dL	Linear arithmetic	0	1	1
dS	Surface	0	2	2
dV	Volume	0	3	3
dM	Mass	0	3	3
dSD	Surface diameter	1	2	3
dVD	Volume diameter	1	3	4
dVS	Volume surface	2	3	5
dMS	Mass surface	3	4	7

For e.g. Arithmetic or number diameter is

D_L = Σ d N / N

Which is obtained when p = 0 and q = 1. Another commonly used notation is volume surface diameter usually called sauter mean diameter.

D_VS = Σ d³ N/Σ d² N

Example: Compute the arithmetic, surface diameter and volume - surface mean diameter for particles in a dry food product with the following distribution of sizes.

Numbers sizes (microns)

1 40

4 30

25 20

20 15

10 10

4 5

Solution:

Arithmetic mean diameter:

dL = Σ (d¹ N) / Σ (d⁰ N)

= 40 X 1 + 30 X 4 + 20 X 25 + 15 X 20 + 10 X 10 + 5 X 4

1 + 4 + 25 + 20 +10 +4

= 16.9 µ

Surface diameter:

d_SD = Σ (d² N) / Σ ( d¹ N)

= 40 X 1 + 30 ² X 4 + 20 ² X 25 + 15 ² X 20 + 10² X + 10 +5² X 4

40 X 1 + 30 X 4 + 20 X 25 +15 X 20 + 10 X 10 + 5 X 4

= 19.26 µ

Volume surface diameter:

d_VS = Σ (d ³ N)

Σ (d ² N)

= 40³ X 1 + 30 ³ X 4 + 20 ³ X 25 + 15 ³ X 20 + 10 ³ X 10 + 5 ³ X 4

40 ² X 1 + 30 ² X 4 + 20 ² X 25 + 15 ² X 20 + 10 ² X 10 + 5 ² X 4

= 21.6 μ