Size Reduction

Source : Berk Zeki, Food process Engineering and Technology, Academic Press, Elsevier 2009

Size ReductionSolid Cutting Chopping Grinding Milling

Liquid or semi solid Mashing Atomizing Homogenazing

Some important applications

Milling of cereal grains to obtain flour Fine grinding (refining) of chocolate mass Flaking of soybeans prior to solvent

extraction Cutting of vegetables and fruits to

desired shapes (cubes, strips, slices…) Fine mashing of baby food Homogenization of milk and cream.

Purpose of Size Reduction Accelerating heat and mass transfer. Facilitating separation of different parts of a

material Obtaining a desirable product texture Facilitating mixing and dispersion Portion control (slicing cold-cuts, bread, cakes) Obtaining pieces and particles of defined

shapes In addition, size reduction of food at the

moment of consumption (mastication) has a decisive effect on the perception of food quality (Jalabert-Malbos at al., 2007).

Defining the size of a single particle

Particle size distribution Particle size distribution refers to the proportion

of particles within a certain size range in a population of particles.

The importance of particle size distribution in connection with food quality and processing is obvious ( Servais et al., 2002 ).

Methods for the determination of particle size distribution include sifting, microscopic examination (usually coupled with automatic image analysis), laser diffraction techniques and others.

Sieve analysis is a simple technique, commonly used for the determination of PSD and for the quantitative evaluation of the ‘ fineness ’ of powders.

Mathematical models of PSDGaussian or Normal Distribution model

This model may well fit size distribution in unprocessed agricultural produce consisting of discrete units (e.g. fruit), but it is not very useful for representing PSD in food powders, emulsions or sprays.

Mathematical models of PSD The Log-Normal Distribution model

The log-normal distribution fits fairly well the PSD of liquid sprays and powders produced by spray-drying,

The Gaudin-Schuhmann function

Mathematical models of PSD The Rosin-Rammler (Weibull) function

The Rosin-Rammler model is fairly accurate in representing PSD of materials obtained by size reduction.

Example

Microfiltration Retentate

Ultrafiltration Retentate

Diameter of Microfiltration retentate product was 1353.7 nm, and Ultrafiltration retentate was 316,1

nm

11

Transmission Electron Microscopy Analysis

100 nm

Microfiltration Retentate

Ultrafiltration Retentate

Vitamin D encapsulated with Casein micelle(Semo et al., 2006)

Size Reduction of Solids, Basic Principles Compression and shear are the two types of force

involved in size reduction of solids. The degree of size uniformity in the product is an

important issue. PSD is determined periodically during the operation1. In the first stages of milling PSD becomes bi-

modal2. As milling progresses PSD gradually becomes

mono-modal3. Gradually, the large particles disappear and the

frequency of a certain size increases4. After prolonged milling a certain ‘ final ’ PSD is

reached.

Energy consumption As a rule, size reduction operations are heavy in

energy consumption size reduction may result in considerable increase

in the temperature of the treated material. Temperature rise as a result of size reduction may

be an important technological issue, particularly with heat-sensitive products, thermoplastic substances and materials with high fat content.

When necessary, this problem is addressed by air- or water-cooling of the machine or using cryogenics such as liquid nitrogen (cryo-milling).

Crushing efficiency ηc

Defined as the ratio of the increment in surface energy to the total energy imparted to the material.

Mechanical efficiency ηm

defined as the ratio of the energy transferred to the material to the total energy consumption W of the device, per unit mass of material treated.

K comprises the surface tension and the two efficiencies. Assuming that these values are constant, K is a constant. Rittinger’s equation is said to fit better fine milling while Kick’s expression describes better coarse grinding

Kick’s law Rittinger’s equation

Size Reduction of Solids, Equipment and Methods the following factors must be considered in selection of

Machine for size reduction : 1. Structure, composition and mechanical properties of the

material to be processed 2. The desired PSD and particle form of the product to be

obtained 3. Desired rate of throughput 4. Control of product overheating 5. Inertness of the surfaces in contact with the food 6. Sanitary design, ease of cleaning 7. Ease of maintenance 8. Environmental factors (noise, vibration, dust, explosion

hazard) 9. Capital and operating cost (e.g. energy consumption,

wear resistance etc.).

Size reduction equipment types

Main action is impact Main action is pressure Main action is attrition Main action is shearing.

Impact mills

Pressure mills

Four Roller mill A pair of roll

Attrition mills Attrition mill with fl at grinding surface

Attrition mill with conical grinding surface

Disc Mills

Colloid Mill

Ball Mill

Dry

Wet

Cube Cutter

Silent Cutter

Bowl Mixer Cutter

Meat grinder

Computer-aided water jet cutting