ERT 426 Food Engineering Semester 1 Academic Session 2017/18 Extrusion ERT 426 Food Engineering Semester 1 Academic Session 2017/18

Subtopics Introduction Extrusion Equipment Applications 2.1 Cold Extrusion 2.2 Hot Extrusion Equipment 3.1 Introduction 3.2 Single-screw Extruder 3.3 Twin-screw Extruders 3.4 Control of Extruders Applications Effect on Foods and Microorganisms ERT 426 Food Engineering

1. Introduction Extrusion is a continuous process that combines several unit operations including mixing, cooking, kneading, shearing, shaping and forming. It is used to produce a wide range of products, including breakfast cereals, snack foods, biscuits, pasta, sugar confectionery and soya-based meat analogues, as well as pet foods and fish feeds. ERT 426 Food Engineering

Introduction Examples of extruded foods

Introduction They can be used for processing at relatively low temperatures, as with pasta, or at very high temperatures with flatbreads and extruded snacks. The pressures used in extruders to control shaping, to keep water in a superheated liquid state and to increase shearing forces in certain screw types, may vary from around 15 to over 200 atmospheres. ERT 426 Food Engineering

Introduction Extrusion technology provides several advantages over traditional methods of food and feed processing, including the following: versatility for processing a variety of food products by changing a minor ingredient and/or processing conditions on the machine and/or the shape of the dies. different shapes, textures, colors, and appearances obtained by minor changes in hardware and processing conditions ERT 426 Food Engineering

Introduction The major directly expanded, extrusion-cooked breakfast cereals on the market ERT 426 Food Engineering

Introduction energy efficient processing, and often lower in cost compared to other options. availability of automation with most new extruders, which can increase productivity. improved product quality over other processes because cooking is done in a very short time and less destruction of heat sensitive ingredients occurs. easy scale-up of extrusion processes from pilot plant to commercial production. ERT 426 Food Engineering

Introduction 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 ERT 426 Food Engineering

2. Extrusion 2.1 Cold Extrusion Hot Extrusion (Cooking) In cold extrusion, the temperature of the food remains below 100 0C. 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. ERT 426 Food Engineering

2. Extrusion: 2.2 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, denaturation or re-orientation of proteins, ERT 426 Food Engineering

Extrusion: Hot Extrusion plasticisation of the material to form a fluid melt, formation of glassy states, and expansion and solidification of food structures when they emerge from the die. For many years, understanding of these interactions developed empirically, but mathematical modelling of fluid flow and heat transfer inside the extruder barrel has led to a greater understanding of the operation and control of extruders. ERT 426 Food Engineering

Extrusion: Hot Extrusion Formulation Type of starch Particle size Fat content Moisture content Minor Ingredients (emulsifiers, vitamins, sugars) Pre-Extrusion Condition Blending Pre-conditioning Extruder Design Single or Twin Screw Screw Length : Diameter ratio Die Design (number, shape & diameter of apertures) ERT 426 Food Engineering

Extrusion: 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 ERT 426 Food Engineering

Extrusion: Hot Extrusion Product Quality Size and Shape Organoleptic properties (texture / hardness / flavour / colour ) Bulk density Degree of cooking Microbiological Quality Nutritional Quality ERT 426 Food Engineering

Extrusion: Hot Extrusion 2.2.1 Properties of Ingredients The properties of the feed materials have an important influence on the conditions inside the extruder barrel and hence on the quality of the extruded product. Different types of feed material produce completely different products when the same operating conditions are used in the same extruder. ERT 426 Food Engineering

Extrusion: Hot Extrusion Properties of Ingredients This is because of differences in the type and amounts of starch, proteins, moisture and other added ingredients (e.g. oil or emulsifier), which result in different viscosities and hence different flow characteristics. Mostly, ingredients used in extrusion cooking have low moisture contents (10-40%), and they are transformed into a fluid melt by the shearing action and the high temperature and pressure in the extruder. ERT 426 Food Engineering

Extrusion: Hot Extrusion Properties of Ingredients Under these conditions, ingredients interact with each other to affect the types of transformation that take place. The physicochemical properties of the ingredients are therefore more important than in other food processes. e.g. the hardness, frictional characteristics and particle size of powders or the lubricity and plasticising power of fluids. ERT 426 Food Engineering

Extrusion: Hot Extrusion Properties of Ingredients Similarly, addition of acids to adjust the pH of the feed material causes changes to starch gelatinisation and unfolding of protein molecules. This in turn changes the viscosity and hence the structure and strength of the extruded product. ERT 426 Food Engineering

Extrusion: Hot Extrusion Properties of Ingredients The ingredients has been characterised according to their functional roles into: structure-forming materials; disperse-phase filling materials; plasticising or lubricating materials; soluble solids; nucleating materials; colouring materials; flavouring materials. ERT 426 Food Engineering

Extrusion: Hot Extrusion Properties of Ingredients During extrusion cooking of starch-based foods, any added water is absorbed and causes starch granules to swell and become hydrated. Smaller particles, such as flours or grits, are hydrated and cooked more rapidly than larger particles and this in turn also alters the product quality. The elevated temperature causes starch to gelatinise and form a viscous plasticised fluid melt. ERT 426 Food Engineering

Extrusion: Hot Extrusion Properties of Ingredients This in turn forms the walls of foam bubbles that contain superheated water vapour. When the material leaves the extruder die, the sudden drop in pressure causes these bubbles to expand rapidly, lose moisture by evaporation and simultaneously cool. These changes cause a rapid increase in the viscosity of the material, followed by the formation of a glassy state that cools and sets the cellular structure. ERT 426 Food Engineering

Extrusion: Hot Extrusion Properties of Ingredients Expansion of extrudate at the die of an extruder, showing bubble growth and stabilisation of the foam. ERT 426 Food Engineering

Extrusion: Hot Extrusion Properties of Ingredients The changes in starch solubility under different conditions of temperature and shear rate are monitored by measuring the water absorption index (WAI) and the water solubility index. The WAI of cereal products generally increases with the severity of processing, reaching a maximum at 180-200 0C. ERT 426 Food Engineering

The most important extruder operating parameters are: 2. Extrusion: 2.2 Hot Extrusion 2.2.2 Extruder Operating characteristics The most important extruder operating parameters are: the temperature and pressure in the barrel, the diameter of the die apertures and the shear rate. the shear rate is influenced by the internal design of the barrel, its length : diameter ratio the speed and geometry of the screw(s). ERT 426 Food Engineering

Extrusion: Hot Extrusion Extruder Operating characteristics Extrusion cooking operates continuously under steady state equilibrium conditions. The amount of heating of feed materials and the rate of heat transfer to the food determines the type and extent of physicochemical changes that take place and hence the quality of the final product. An energy balance can be used to correlate the energy used in the extruder with the energy transferred to the material. ERT 426 Food Engineering

Extrusion: Hot Extrusion Extruder Operating characteristics Pmech + Pheat = Pcool + Ploss + Pmat Where, Pmech = the mechanical power supplied by the motor (for producing frictional heat). Pheat = thermal power supplier by barrel heaters Pcool = thermal power absorbed by barrel cooling Ploss = thermal losses to the environment, Pmat = thermal power absorbed by the material ERT 426 Food Engineering

Extrusion: Hot Extrusion Extruder Operating characteristics 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. Although there are unique aspects associated with estimating power consumption for single- versus twin-screw systems, a general approach for estimating total power consumption (pt) is used. ERT 426 Food Engineering

Extrusion: Hot Extrusion Extruder Operating characteristics A general approach for estimating total power consumption (pt) is: pt = ps + VdΔP Power consumption for viscous dissipation associated with shear of the feed ingredients. Power needed to maintain flow through the barrel and die of the extrusion system. ERT 426 Food Engineering

Extrusion: Hot Extrusion Extruder Operating characteristics The power needed for viscous dissipation is expressed in terms of the Screw Power number ( Np ), as follows: where the screw speed (N), screw diameter (D), screw length (L), as well as density of extrudate (ρ) are considered. rpm m kg/m3 W ERT 426 Food Engineering

Extrusion: Hot Extrusion: Extruder Operating characteristics For extrudate with rheological properties described by the power-law model, the Screw Rotational Reynolds number is defined as follows: where K = apparent viscosity. kg/m3 m Pa.s /cP.s rpm ERT 426 Food Engineering

Extrusion: Hot Extrusion Extruder Operating characteristics The magnitude of Np is dependent on the Screw Rotational Reynolds number (NRes ). Dimensionless correlation for extruder power consumption.

Extrusion: Hot Extrusion: Extruder Operating characteristics The drag flow rate (Vd) for the extruder screw can be estimated as follows: 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. rpm m ERT 426 Food Engineering

Extrusion: Hot Extrusion Extruder Operating characteristics If the extruder barrel is not heated, the total energy is frictional heat generated by power from the motor. ERT 426 Food Engineering

Extrusion: Hot Extrusion Extruder Operating characteristics A simplified model for the operation of an extruder, developed by Harper (1981), assumes that the temperature of the food is constant, fluid flow is Newtonian & laminar, there is no slippage of food at the barrel wall & no leakage of food between the screw & the barrel. With these assumptions, the volumetric flow rate (V) through a single-screw extruder can be calculated using: V = G1 N Fd + G2 µ x ΔP/L x Fp ERT 426 Food Engineering

Extrusion: Hot Extrusion: Extruder Operating characteristics V = G1 N Fd + G2 µ x ΔP/L x Fp where V (m3 s-1) = volumetric flow rate in the metering section, N (rpm) = screw speed, µ(N s m-2) = viscosity of the fluid in the metering section, P (Pa) = pressure increase in the barrel, G1 & G2 = constants that depend on screw & barrel geometry, respectively, L (m) = length of extruder channel Fd & Fp = shape factors for flow due to drag & pressure respectively. ERT 426 Food Engineering

Extrusion: Hot Extrusion: Extruder Operating characteristics To model the flow behavior within the extruder, it is assumed that the flow is incompressible, steady, laminar and fully developed. In addition, the rectangular cross-section channel formed between the screw flights and the barrel surface is assumed as an infinite plate sliding across the channel (Harper, 1981). The volumetric flow rate of the Newtonian fluid in the channel cross-section: ERT 426 Food Engineering

Extrusion: Hot Extrusion: Extruder Operating characteristics where, Symbol Parameter [Unit] V Volumetric flow rate, [m3/s] µ Viscosity, [Pa.s] ∆P Pressure change, [Pa] µwall Wall velocity, [m/s] H Height, [m] W Width, [m] L Length, [m] ERT 426 Food Engineering

Extrusion: Hot Extrusion: Extruder Operating characteristics It should be noted that the above equations are based on simple models that do not take account of leakage of food between the flights and the barrel, changes in temperature or the effects of non-Newtonian fluids. In practice, modelling is very complex because changes in non-Newtonian fluids are significantly more complicated. ERT 426 Food Engineering

3. Equipment: 3.1 Introduction The selection of an appropriate extruder for a particular application should take account of: the nature of the ingredients, the type of product, its required bulk density, physical & sensory properties, the expected production rate. The basic design difference is between single- or twin-screw extruders. ERT 426 Food Engineering

Equipment: Introduction All extruders should convey the ingredients along the barrel and prevent them spinning with the screw. Twin-screw extruders act like positive displacement pumps, but single-screw extruders require design features that ensure material does not slip and rotate with the screw, which would prevent it moving along the barrel. ERT 426 Food Engineering

Equipment: Introduction These features include: grooves in the barrel, interrupted flights (spaces intentionally left between the flights on the screw), or restrictions to product flow (known variously as `throttle rings', `kneading discs', `steam locks' or `shearlocks') or shearing bolts that protrude into the barrel, each of which assists in conveying material along the barrel. ERT 426 Food Engineering

Equipment: Introduction In wet extrusion cooking, steam or water may be injected through hollow shearing bolts. There are therefore a large number of design options for extruders, including smooth or grooved barrels, `dry' (i.e. without addition of steam or water) or `wet' operation, the size, number, pitch and diameter of the flights on the screw(s), continuous or interrupted flights, and the number and position of shearing bolts or throttle rings. ERT 426 Food Engineering

Equipment: Introduction Interchangeable dies have different shaped holes, such as: round holes to produce rods, square holes for bars, or slots to produce sheets; or they may have more complex patterns for specially shaped three-dimensional products. Some products require the extruder die to be heated to maintain the viscosity and degree of expansion, whereas others require the die to be cooled to reduce the amount of expansion. ERT 426 Food Engineering

Equipment: Introduction Extruders may also be fitted with a special die to continuously inject a filling into an outer shell. This is known as `co-extrusion' and is used, for example, to produce filled confectionery. Die pressures vary from 500±2000 kPa for low-viscosity products to 17 000 kPa for expanded snackfoods. Rotary knives, fitted to the outside of the die, cut the extruded product into required lengths. ERT 426 Food Engineering

Equipment: Introduction Table 1: Advantages and limitations of different types of extruders ERT 426 Food Engineering

Equipment: Introduction Table 1: Advantages and limitations of different types of extruders Frequent product changeovers Addition of a high level of fresh meat in the product (up to 35%) ERT 426 Food Engineering Products made with low density powder

3. Equipment: 3.2 Single-screw extruders 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 ERT 426 Food Engineering

Equipment: Single-screw extruders Usually, in wet extrusion the feed material is premixed and preconditioned with steam or water to reduce wear on the components and to improve the product quality. The length : diameter ratio of the barrel is between 2:1 and 25:1. The pitch and diameter of the screw segments, the number of flights and the clearance between the flights and the barrel can each be changed to alter the performance of the extruder. ERT 426 Food Engineering

Equipment: Single-screw extruders The screw speed is one of the main factors that influences performance; it controls the residence time of the product, the amount of frictional heat generated, heat transfer rates and the shearing forces on the product. Typical screw speeds are 150±600 rpm, depending on the application. ERT 426 Food Engineering

Equipment: Single-screw extruders Compression is achieved in the extruder barrel by back pressure, created by the die and by: increasing the diameter of the screw and decreasing the screw pitch; using a tapered barrel with a constant or decreasing screw pitch; or placing restrictions to product flow in the barrel. ERT 426 Food Engineering

Equipment: Single-screw extruders Single-screw extruders have different degrees of shearing action on the food. High-shear extruders have high speeds and shallow flights to create high pressures and temperatures that are needed to make ready-to-eat breakfast cereals and expanded snackfoods. Medium-shear extruders are used to make breadings, texturised proteins and semi-moist pet foods. Low-shear extruders have deep flights and low speeds to create low pressures for forming pasta, meat products or gums. ERT 426 Food Engineering

Equipment: Single-screw extruders Operating data for different types of single-screw extruders ERT 426 Food Engineering

Equipment: Single-screw extruders Changes in temperature and pressure in a high-shear, single-screw cooking extruder for expanded food products. ERT 426 Food Engineering

Equipment: Single-screw extruders In dry extrusion cooking, much of the energy from the motor generates friction that rapidly heats the food. Throttle rings increase the pressure in the barrel, shearing and heating (up to ~160 0C). Additional heating can be achieved using a steam-jacketed barrel, a steam-heated screw, or electric induction heating elements around the barrel. An important use of dry extruders is to prepare oilseeds for oil extraction. ERT 426 Food Engineering

Equipment: Single-screw extruders Extrusion prior to pressing increases the throughput of an oil expeller and releases antioxidants in oilseeds, thus stabilising the oil. The process produces high-quality oil, similar to a refined, deodorised product, together with an oilcake, containing 50% protein and 90% inactivation of trypsin inhibitors, that is suitable for human consumption. ERT 426 Food Engineering

3. Equipment: 3.3 Twin-screw extruders Twin-screw extruders are grouped according to the direction of rotation of the screws (counter-rotating or co-rotating) and the degree to which they intermesh. Non-intermeshing screws act like two single-screw extruders, whereas intermeshing screws produce a positive displacement pumping action to move product through the extruder. The screws rotate within a `figure of 8' shaped bore in the barrel. ERT 426 Food Engineering

Equipment: Twin-screw extruders Screw length : diameter ratios are between 10:1 and 25:1. Twin-screw extruders can handle a wide range of ingredients and produce different products. This is achieved by changing the degree of intermeshing of the screws, the number of flights or the angle of the pitch of the screws, or fitting kneading discs to increase the shearing action. ERT 426 Food Engineering

Equipment: Twin-screw extruders Typical twin-screw extruders Kneading elements of a co-rotating twin-screw extruder showing dough mixing: movement of material ERT 426 Food Engineering BBLee@UniMAP

Equipment: Twin-screw extruders In twin-screw extruder cookers, the spacing between the flights can be adjusted so that large spaces initially convey the material to the cooking section and then smaller spaces compress the plasticised mass before extrusion through an interchangeable die. Co-rotating intermeshing screws, which are self-wiping (the flights of one screw sweep food from the adjacent screw) are most commonly found in food processing applications. ERT 426 Food Engineering

Equipment: Twin-screw extruders Where the barrel is split at the end, products can be directed into two or more channels and different colourants or flavourings can be introduced just before the die to produce two-colour products. Examples of products from twin-screw machines include co-extruded/filled snackfoods, food gums and jellies, pasta products, TVP, three-dimensional snackfood and confectionery products, marshmallows, cornflakes, chocolate-filled snacks, biscuits and instant rice or noodles. ERT 426 Food Engineering

Equipment: Twin-screw extruders 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. ERT 426 Food Engineering

3. Equipment: 3.4 Control of extruders A control system for extrusion should encompass the entire process, including: formulation of ingredients, preconditioning, extrusion and post-extrusion processing operations (e.g. drying, coating, frying, packaging) in order to obtain the required quality products. ERT 426 Food Engineering

Equipment: Control of extruders There are four main controlled variables for the operation of an extruder: 1. specific mechanical energy; 2. die melt temperature; 3. die pressure; 4. flow rate through the die. These variables are maintained at predetermined values by controlling the ingredient feedrate, the screw speed, water injection rate, temperature profile of the barrel and the speed of the rotary knife on the die. ERT 426 Food Engineering

4. Application: 4.1 Confectionery products Product uniformity is high, no after-drying is required, & there is a rapid start-up & shutdown. Hard-boiled sweets are produced from granulated sugar and corn syrup. The temperature in the extruder is raised to 165 0C to produce a homogeneous, decrystallised mass. Acids, flavours and colour are added to the sugar mass, and the moisture content is reduced to 2% as the product emerges from the die into a vacuum chamber. ERT 426 Food Engineering

Application: Confectionery products It is then fed to stamping or forming machines to produce the required shape. Compared with traditional methods which use boiling pans energy consumption in an extruder operating at 1000 kg h-1 is lower (551 compared with 971 kJ per kg of sugar mass), and steam consumption is also lower (0.193 compared with 0.485 kg per kg of sugar mass) ERT 426 Food Engineering

4. Application: 4.2 Cereal products (Breakfast cereals) In traditional cornflake manufacture, large maize kernels (grits) were needed, as the size of the individual grit determined the size of the final cornflake. Grits were then pressure cooked, dried, tempered to ensure a uniform moisture distribution, flaked, toasted and sprayed with a vitamin solution. The total processing time exceeded 5 h. ERT 426 Food Engineering

Application: Cereal products Dough pellets are now produced in a low-pressure extruder from any size of maize grit. The size of the pellets determines the size of the cornflakes. They are then flaked, toasted and sprayed as before. ERT 426 Food Engineering

Application: Cereal products The advantages of extrusion cooking for cereal products are: reductions in raw material costs (19.4%), energy consumption (>90%), capital expenditure (44%) and labour costs (14.8%). rapid processing to produce cornflakes within minutes of start-up; close control over the size and quality of the final product; flexibility to change the product specification easily. ERT 426 Food Engineering

4. Application: 4.3 Protein based foods Extrusion cooking destroys the enzymes present in soybeans, including a urease that reduces the shelf-life, a lipoxidase that causes off-flavours by oxidation of soya oil and also a trypsin inhibitor that reduces protein digestibility. This improves the acceptability, digestibility and shelf-life of the product. ERT 426 Food Engineering

5. Effect on foods and Microorganisms: 5.1 Sensory characteristics The High Temperature Short Time (HTST) conditions in extrusion cooking produce short residence times, so cooked flavours are not produced and there are only minor changes to the natural colour and flavour of foods. In some products, flavour may be produced by Maillard reactions and there has been some research adding specific amino acids as flavour precursors to generate flavours. ERT 426 Food Engineering

Effect on foods and Microorganisms: Sensory characteristics The extent of changes to the starch fluid melt produces the wide range of product textures that can be achieved by extrusion cooking. The texture depends on the size distribution of air cells in the starch matrix, the cell wall thickness & the nature of the glassy state that starch polymers form on cooling when the product emerges from the die. ERT 426 Food Engineering

5. Effect on foods and Microorganisms: 5.2 Nutritional value Reducing sugars are lost due to Maillard reactions during extrusion at high temperatures and low moisture contents. Extruded soybean products contain lower levels of flatulence-inducing oligosaccharides, including stachyose and raffinose, than unprocessed soy flours. Extrusion denatures proteins, which reduces the activity of naturally occurring enzymes. ERT 426 Food Engineering

Effect on foods and Microorganisms: Nutritional value The HTST conditions in extrusion cooking and rapid cooling as the product emerges from the die, cause relatively small losses of most vitamins and essential amino acids. Lipids may form starch-lipid complexes during extrusion, but these do not affect the nutritive value of the foods. ERT 426 Food Engineering

5. Effect on foods and Microorganisms: 5.3 Effect on Microorganisms. Most extrusion-cooked products are microbiologically safe because of both the low water activity of extruded products and the HTST heat treatment (>130 0C for a few seconds) that destroys vegetative cells. The conditions under which spores are destroyed by extrusion-cooking are not well understood. ERT 426 Food Engineering

Effect on foods and Microorganisms: Effect on Microorganisms. It is reported that if shear forces can be optimally combined with thermal forces, an acceptable sterility could be achieved at low temperatures that maximises food quality while minimising process energy requirements. Microbiological safety is a greater concern when extruded products are made from animal by-products that may have high concentrations of pathogens and/or extruded products that have higher moisture contents. ERT 426 Food Engineering