Size Reduction

1. Size Reduction Source : BerkZeki, Food process Engineering and Technology, Academic Press, Elsevier 2009

2. Size Reduction Solid • Cutting • Chopping • Grinding • Milling Liquid or semi solid • Mashing • Atomizing • Homogenazing

3. 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.

4. 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) hasa decisive effect on the perception of food quality (Jalabert-Malbos at al., 2007).

5. Defining the size of a single particle

6. Particle size distribution • Particle size distribution refers to the proportion of particles within a certain sizerange in a population of particles. • The importance of particle size distribution inconnection with food quality and processing is obvious ( Servais et al., 2002 ). • Methods for the determination of particle size distribution include sifting, microscopicexamination (usually coupled with automatic image analysis), laser diffraction techniques and others. • Sieve analysis is a simple technique, commonly used for thedetermination of PSD and for the quantitative evaluation of the ‘ fineness ’ of powders.

7. Mathematical models of PSD Gaussian or Normal Distribution model This model may well fit size distribution in unprocessedagricultural produce consisting of discrete units (e.g. fruit), but it is not very usefulfor representing PSD in food powders, emulsions or sprays.

8. Mathematical models of PSD The Log-Normal Distribution model The log-normal distribution fits fairly well the PSD of liquid sprays andpowders produced by spray-drying, The Gaudin-Schuhmann function

9. 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.

10. 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 Vitamin D encapsulated with Casein micelle (Semo et al., 2006) Microfiltration Retentate Ultrafiltration Retentate

12. 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 • In the first stages of milling PSD becomes bi-modal • As milling progresses PSD gradually becomes mono-modal • Gradually, the large particles disappear and the frequency of a certain size increases • After prolonged milling a certain ‘ final ’ PSD is reached.

13. 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, particularlywith heat-sensitive products, thermoplastic substances and materials with high fat content. • When necessary, this problem is addressed by air- or water-cooling of themachine or using cryogenics such as liquid nitrogen (cryo-milling).

14. Crushing efficiency ηc • Defined as the ratio of the increment in surface energy tothe total energy imparted to the material.

15. Mechanical efficiency ηm • defined as the ratio of theenergy transferred to the material to the total energy consumption W of the device,per unit mass of material treated. Kick’s law Rittinger’s equation K comprises the surface tension and the two efficiencies. Assuming that these valuesare constant, K is a constant. Rittinger’s equation is said to fit better fine milling while Kick’s expression describes better coarse grinding

16. 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 thematerial to be processed 2. The desired PSD and particle form of the product to beobtained 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. Environmentalfactors (noise, vibration, dust, explosion hazard) 9. Capital and operating cost (e.g. energy consumption, wear resistance etc.).

17. Size reduction equipment types • Main action is impact • Main action is pressure • Main action is attrition • Main action is shearing.

18. Impact mills

19. Pressure mills Four Roller mill A pair of roll

20. Attrition mills • Attrition mill with fl at grinding surface

21. Attrition mill with conical grinding surface

22. Disc Mills

23. Colloid Mill

24. Ball Mill Dry Wet

25. Cube Cutter

26. Silent Cutter

27. Bowl Mixer Cutter

28. Meat grinder

29. Computer-aided water jet cutting