Properties of suspension and concentrated products

Many food products are in the form of suspension or concentrated products. Some point during processing is important in preparation for consumer use. Food components are mixed together to form suspensions, while in other causes high-moisture liquid product are reduced in moisture content to give more highly concentrated products. Suspensions and concentrated products include variety of different particle-liquid mixtures. Product slurry is a particle-liquid mixture which is normally considered dilute in that the ratio of particles to liquid is very low. This mixture can be transported in laminar flow with a rather significant amount of settling. The particle-liquid mixture which is classified as a suspension is one in which the ratio of particles to liquid is much higher, and forms a products which cannot be easily transported in laminar flow without deposition of the particles. In other words, the transport must be conducted under turbulent conditions in order to maintain the particles in suspension. Several expressions have been proposed to allow prediction of the product properties under above condition. In the case of slurries, the proposed expression attempt to relate the slurry viscosity to the viscosity of the liquid fraction and the concentration of the particles per unit volume. Probably the most widely used expression is one proposed by Einstein:

.. (1) In which the slurry viscosity is related almost directly to particle concentration.Einstein was derived above equation utilizing several assumptions are as follows: (1) Particles are spherical in shape. (2) Particles are large in composition to the liquid molecules. (3) No slip occurs between particles and liquid (4) Particles are unchanged. (5) Turbulence is not present.

Guth & Simha and Manley & Mason developed similar expression for higher particle concentrations. According to Guth & Simha-

(2) For 20% concentration by volume.According to Manley & Mason: (3) Problem

Compare Einstein, Guth & Simha and Manley & Mason equation when predicting the viscosity of a 15% by volume slurry and a 30% by volume suspensions. The viscosity of the liquid suspended medium is 1.310-3 lbm/ft sec.Solution

(1) For 15% suspensions: From Einstein equation: Where, = 1.3 10- (1+2.5 0.15) [L = 1.3 10- lbm/ft sec. = 1.788 10- lbm/ft sec. Xc = 0.15]

From Guth & Simha equation: = L (1+ 2.5XC +14.1XC2) = 1.3 10- {1+2.5 0.15+14.1 (0.15)2} = 2.2 10- lbm/ft sec. From Manley & Mason equation: = L (1+ 2.5XC +10.05XC2) = 1.3 10- {1+2.5 0.15+10.05 (0.15)2} = 2.081 10- lbm/ft sec. (2) For 30% suspensions: From Einstein equation: = L (1+ 2.5XC) where, = 1.3 10- (1+2.5 0.3) [L = 1.3 10- lbm/ft sec. = 2.275 10- lbm/ft sec. Xc = 0.3] From Guth & Simha equation: = L (1+ 2.5XC +14.1XC2) = 1.3 10- {1+2.5 0.3+14.1 (0.15)2} = 3.925 10- lbm/ft sec. From Manley & Mason equation: = L (1+ 2.5XC +10.05XC2) = 1.3 10- {1+2.5 0.3+10.05 (0.15)2} = 3.451 10- lbm/ft sec. The consumptions indicate that differences in viscosity predicted by the equations increase as the suspension concentration increases. Properties of Granular foods and powders:

The design of the handling system for dry products requires knowledge of the properties of the products being handled.

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

Bulk density: Bulk density is defined by the following expressions

Where, M= total mass and V= total volume.

Types of bulk density

Loose bulk density: Loose bulk density is measured after placing the product in the constant volume container without variation.

Packed bulk density: Packed bulk density is measured after the sample placed in the constant volume container has been vibrated until the volume seems constant.

Bulk density value will be dependent on the particle size, characteristics and any factors which influence them. Another type of density is apparent particle density.

Apparent particle density: Apparent particle density of a dry food product is a measure of the amount of air which may be trapped with the individual particles. This parameter can be measured by utilizing a picometer in which a solvent of know density replaces air within the particles and allows computation of the apparent particle density. Two additional properties of granular food products which related to the density are: (a)Void (b)Porosity

(a) Void: The void can be defined as the ratio of the volume of space between particles to the entire volume, and can be expressed as follows:

V = B ( - Where, V = void, B = Bulk density, p = Particle density(b) Porosity: Porosity can be defined as the ratio of the air volume within the particles to the total particles volume and can be computed by the following expressions: = 1- Where, = Porosity, B = Bulk density, t = True density of air free solids making up the product. Particle size and size distribution: A very important property of granular foods or powder is the particle size and size distribution. They are related to the computation of various mean diameters of particle size and size distributions. These are: 1) The arithmetic or number mean diameter2) The surface mean diameter3) The volume surface mean diameter

The arithmetic or number mean diameter: Number mean diameter can be defined as follows: dL = Where, N = number and d= particle size.

The surface mean diameter: Surface mean diameter can be defined as follows: ds = Volume surface mean diameter: Volume surface diameter usually called sauter mean diameter, defined as follows: dv = Problem

Compare the arithmetic, surface and volume surface mean diameters for particle in a dry food product within the following distribution of sizes: NUMBER SIZE

1 40

4 30

25 20

20 15

10 10

4 5

Flow of food powders: Two specific parameters which describes the flow properties of granular foods or powders. These are: 1. Angle of repose 2. Angle of slide.1. Angle of repose: The angle of repose is defined in the following way: tan = Where, H = height of a mound of a powder which forms as it flows from a container directly above a horizontal surface. S= Circumference of the pile of powder at its base.2. Angle of slide: The angle of slide can be defined as the 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 basic and the fundamental property of powder flow is the angle internal friction () which is defined as follows: tan = Where, Share stress and n= Normal stress.On another theoretical expression which can be utilized to describe the flow rate of powder through an orifice in the bottom of a storage container is Q = Cc () []Where, Q = Flow rate of powder, D = Orifice diameter, = Angle of repose, = Bulk density, Cc = Discharge coefficient (0.5 to 0.7)PROBLEM

A dry food product with bulk density of 35 lbm/ ft3 is stored in a large storage container and is removed by gravity through a 3 inch diameter opening in the bottom of the container. The test for angle of repose gave a mound of product with 4 inch diameter and 3 inch height. Compute the rate at which the product will be released from the container.Solution

We know, Angle of repose, tan= (1) Here, S = = = 4 From equation (1) we get, tan = = 1.5 Now flow rate, Q = Cc () [] Q = 0.6 () 35 [] Q = 0.173 lbm/sec Q = 10.4 lbm/min

Flow rate = 10.40 lbm/ min. (answer)4