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

Size Reduction Solid  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 PSD Gaussian 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 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 operation 1. In the first stages of milling PSD becomes bi-modal 2. As milling progresses PSD gradually becomes mono- modal 3. Gradually, the large particles disappear and the frequency of a certain size increases 4. 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 η c  Defined as the ratio of the increment in surface energy to the total energy imparted to the material.

Mechanical efficiency η m  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 lawRittinger’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 millA 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