PREPARED BY: DR. RAHIMAH OTHMAN FOOD ENGINEERING (ERT 426) SEMESTER 1 ACADEMIC SESSION 2016/17

Transcription

1 1 PREPARED BY: DR. RAHIMAH OTHMAN FOOD ENGINEERING (ERT 426) SEMESTER 1 ACADEMIC SESSION 2016/17

2 SUBTOPICS 2 1. Introduction 2. Basic Principles of Extrusion 3. Extrusion System 3.1 Cold Extrusion 3.2 Hot Extrusion 3.3 Single-screw Extruder 3.4 Twin-screw Extruders

3 3 OUTCOMES ABLE TO Discuss and explain basic principles of extrusion. Differentiate the extrusion systems such as cold extrusion, extrusion cooking, singlescrew and twin-screw extruders. Estimate the proper dimension and processing parameter for a single screw extruder.

4 1. INTRODUCTION 4 WHAT IS EXTRUSION? Extrusion is a process that converts raw material into a product with desired shape and form by forcing the material through a small opening using pressure. It involves a series of unit operations such as mixing, cooking, kneading, shearing, shaping and forming. It is USED to produce a wide range of products, including breakfast cereals, pasta, pet foods, snacks, and meat products.

5 EXAMPLE: 5 EXTRUSION TECHNOLOGY

6 EXAMPLE: EXTRUDED FOODS 6

7 1. INTRODUCTION 7 All extrusion systems contain FIVE (5) key components: 1. Primary feed system consisting of a container and delivery system for the primary ingredients involved in the process. 2. Pump to move all ingredients through the steps associated with the extrusion process. 3. Reaction vessel where key actions such as mixing, kneading, shearing, heating and cooling occur 4. Secondary feed system for adding secondary ingredients or energy as needed to achieve the desired product characteristics 5. Exit assembly designed to restrict flow and contribute to the shaping and forming of the final product (usually referred to

8 THE MAJOR DIRECTLY EXPANDED, EXTRUSION- COOKED BREAKFAST CEREALS ON THE MARKET 8

9 2. BASIC PRINCIPLES OF 9 EXTRUSION Extrusion involves a combination of transport processes, including flow of materials within the system, thermal energy transfer to and within the material, and mass transfer to and within the material during extrusion. Various types of food ingredients processed by extrusion referred as EXTRUDATES. Most of food extrudates are highly non- Newtonian ; the apparent viscosity decreases with increasing rate of shear.

10 2. BASIC PRINCIPLES OF 10 Absolute value of the pressure drop across the channel length (L) EXTRUSION Volumetric flow rate of Newtonian fluid Channel width (m) Velocity (m/s) Channel height (m) V PWH 12L 3 u wall 2 HW Volumetric flow rate of non-newtonian fluid Flow behaviour index Consistency coefficient V (4 n) WHu 10 wall 1 (1 2n) WH 4K 3 u wall H 1n P L

11 11 3. EXTRUSION SYSTEM EXTRUSION Cold Extrusion Hot Extrusion (Cooking) In COLD EXTRUSION, the temperature of the food remains below C. It is used to mix and shape foods without significant cooking or distortion of the food. The extruder has a deep-flighted screw, which operates at a low speed in a smooth barrel, to knead and extrude the material with little friction.

12 3. EXTRUSION SYSTEM Cold Extrusion Environmental friendly process because it does not produce process effluents and has no water treatment costs. A cold extrusion operation. Filled products using co-extrusion

13 3. EXTRUSION SYSTEM 13 HOT EXTRUSION Extrusion cooking involves the simultaneous mixing, kneading and heating of ingredients, and results in a large number of complex changes to foods. These include: hydration, gelation and shearing of starches and proteins melting of fats 3.2 Hot Extrusion denaturation or re-orientation of proteins plasticisation of the material to form a fluid melt formation of glassy states expansion and solidification of food structures when they emerge from the die.

14 3. EXTRUSION SYSTEM 14 Formulation Type of starch Particle size Fat content Moisture content Minor Ingredients (emulsifiers, vitamins, sugars) 3.2 Hot Extrusion Pre-Extrusion Condition Blending Pre-conditioning Extruder Design Single or Twin Screw Screw Length : Diameter ratio Die Design (number, shape & diameter of apertures)

15 3. EXTRUSION SYSTEM Hot Extrusion Process Conditions Feed rate Screw speed Temperature profile in barrel Pressure profile in barrel Material residence in each section of barrel Water / Steam injection rate Rotating knife speed at die Post Extrusion Operations Addition of flavours and colours Drying / Roasting / Frying

16 3. EXTRUSION SYSTEM Hot Extrusion Product Quality Size and Shape Organoleptic properties (texture / hardness / flavour / colour ) Bulk density Degree of cooking Microbiological Quality Nutritional Quality

17 3. EXTRUSION SYSTEM Hot Extrusion Properties of Ingredients: Expansion of extrudate at the die of an extruder, showing bubble growth and stabilisation of the foam.

18 3. EXTRUSION SYSTEM Extruder Operating characteristics: The most important extruder OPERATING PARAMETERS are: 1) the temperature and pressure in the barrel, 2) the diameter of the die apertures and the shear rate. the shear rate is influenced by the internal design of the barrel, 1) its length : diameter ratio 3.2 Hot Extrusion 2) the speed and geometry of the screw(s). The power requirement for operating an extrusion system is a key design factor. Power consumption is a complex function of properties of the material being extruded, extruder design, extruder motor type and extrusion conditions. 18

19 3. EXTRUSION SYSTEM 19 Where, 3.2 Hot Extrusion Extruder Operating characteristics: P mech + P heat = P cool + P loss + P mat P mech = the mechanical power supplied by the motor (for producing frictional heat). P heat = thermal power supplier by barrel heaters P cool = thermal power absorbed by barrel cooling P loss = thermal losses to the environment, P mat = thermal power absorbed by the material

20 3. EXTRUSION SYSTEM 3.2 Hot Extrusion 20 A general approach for estimating total power consumption, p t (W) is: p t = p s + V d ΔP Drug flow rate (m 3 /s) Power consumption for viscous dissipation associated with shear of the feed ingredients (W). Power needed to maintain flow through the barrel and die of the extrusion system (Pa).

21 where the screw speed (N), screw diameter (D), screw length (L), as well as density of extrudate (ρ) are considered. 3. EXTRUSION SYSTEM The power needed for viscous dissipation is expressed in terms of the Screw Power number ( N p ), as follows: N 3.2 Hot Extrusion Extruder Operating characteristics: ρn p s P 3 4 D L W rpm 21 kg/m 3 m

22 3. EXTRUSION SYSTEM For extrudate with rheological properties described by the power-law model, the Screw Rotational Reynolds number, N Res is defined as follows: m 2n DN H n kg/m 3 N 3.2 Hot Extrusion Extruder Operating characteristics: Re s 2n K rpm 22 m Pa.s /cp.s where K = apparent viscosity.

23 3. EXTRUSION SYSTEM 3.2 Hot Extrusion 23 The magnitude of N p is dependent on the Screw Rotational Reynolds number (N Res ). Dimensionless correlation for extruder power consumption.

24 3. EXTRUSION SYSTEM 3.2 Hot Extrusion 24 The drag flow rate (V d ) for the extruder screw can be estimated as follows: rpm NDWH V d 2 Extruded products may be characterised by the specific mechanical energy (SME), which is the ratio of the energy supplied and the flow of extruded material. m SME kj / kg Total Energy Mass flow rate

25 Table 1: Advantages and limitations of different 25 types of extruders 3.3 Single-screw VS 3.4 Twin-screw Extruder

26 Table 1: Advantages and limitations of different types of extruders 3.3 Single-screw VS 3.4 Twin-screw Extruder 26 Frequent product changeovers Addition of a high level of fresh meat in the product (up to 35%) Products made with low density powder

27 3.3 Single-screw extruders 27 Single-screw extruder is the most widely used design for straightforward cooking and forming applications, when the flexibility of a twin-screw machine is not needed. Single-screw extruder

28 3.3 Single-screw extruders 28 Operating data for different types of single-screw extruders

29 Single-screw extruders Changes in temperature and pressure in a highshear, single-screw cooking extruder for expanded food products.

30 3.4 Twin-screw extruders 30 Some products, including sticky caramels and other sweets cannot be made using single-screw extruders, & others, including pet foods that contain up to 30% fresh meat, or ultra-fine & high-fat aquatic feeds, have substantially higher quality using twin-screw machines.

31 3.4 Twin-screw extruders 31 Typical twinscrew extruders Kneading elements of a corotating twin-screw extruder showing dough mixing: movement of material