Unit Operations in Food Processing: Size Reduction

Size Reduction in Food Processing

30.1 Introduction

  • Definition: Size reduction refers to processes that break down solid materials into smaller units without changing their chemical properties.

  • Application: Used in multiple industries:

  • Breaking crude ore to desired sizes.

  • Converting synthetic chemicals into powder.

  • Cutting plastic sheets into various pieces.

  • Purpose:

  • Separation of unwanted materials through mechanical methods.

  • Allows for a reduction ratio to be established: (average size of feed) / (average size of products).

  • Measurement Techniques:

  • Screening and sieving to evaluate particle size distribution.

  • Size Reduction Ratios:

  • Coarse crushing: < 8:1.

  • Fine grinding: > 100:1.

30.2 Principles of Comminution

  • Definition: Comminution is the general term for size reduction. Common comminution equipment includes crushers and grinders.

  • Ideal Equipment Characteristics:

  1. High capacity.

  2. Low power input per product unit.

  3. Producing product of uniform or desired size.

  4. Easy operation.

  • Performance Comparison:

  • Ideal vs Actual equipment:

    • Actual equipment usually yields a mix of particle sizes.

30.3 Energy and Power Requirements in Comminution

  • Energy Expenditure: Major cost factors in crushing and grinding operations.

  • Mechanics:

  • Feed particles undergo distortion from energy application, creating stress energy.

  • Energy Calculation:

  • Rittinger’s Law, Kick’s Law, and Bond’s Law provide mathematical models for estimating energy requirements:

    • Rittinger’s Law: Energy proportional to new surface area produced.

    • Kick’s Law: Energy proportional to size reduction ratio.

    • Bond’s Law: Proportional to a specific work index for different material types.

30.4 Size Reduction Equipment

  • Types of Equipment:

  1. Crushers: Coarse and fine crushing.

  2. Grinders: Intermediate and fine grinding.

  3. Ultrafine Grinders: For very fine particle sizes.

  4. Cutting Machines: For cutting feed to specified sizes.

30.4.1 Crushers

  • Function: Break large solid materials into smaller lumps.

  • Types:

  1. Jaw crushers

  2. Gyratory crushers

  3. Crushing rolls

30.4.1.1 Jaw Crushers

  • Mechanism: Feed between two jaws; one fixed, one movable (reciprocating).

  • Operations: 250-400 jaw opens and closes per minute.

30.4.1.2 Gyratory Crushers

  • Design: Circular jaws; material consistently crushed.

  • Mechanism: Conical head gyrates, crushing materials continuously.

  • Capacity Dependence: Jaw setting, feed’s impact strength, and speed of gyration.

30.4.1.3 Crushing Rolls

  • Types:

  • Smooth-roll Crushers: Rotate at same speed, providing compression.

  • Tooth-roll Crushers: Break apart using teeth and compression.

30.4.2 Grinders

  • Purpose: Reduce crushed material to powder.

  • Types:

  • Hammer mills

  • Rolling-compression mills

  • Attrition mills.

30.4.2.1 Hammer Mills and Impactors

  • Mechanism: High-speed rotor with swinging hammers breaking feed material.

30.4.2.2 Rolling-Compression Machines

  • Mechanism: Materials caught between a roller and a case, utilized in bowl and roller mills.

30.4.2.3 Attrition Mills

  • Mechanism: Soft solids are rubbed between grooved flat faces of rotating disks.

30.4.3 Ultrafine Grinders

  • Definition: Used for fine powders, producing particles of 1 to 20 µm.

  • Types:

  • Classifying hammer mills

  • Fluid energy mills

  • Agitated mills.

30.4.4 Cutting Machines

  • Function: Cut objects that are too resilient to break through compression, impact, or attrition.

  • Example: Rotary knife cutters, granulators.

Points of Interest

  • Power Requirements: Vary widely depending on the machine type and material properties.

  • Application in Food Processing: Critical for ensuring desired product dimensions in food engineering, influencing texture, presentation, and processing efficiency.