Unit Operations Mixing & Size Reduction (July 2021) PDF

Unit Operations Mixing  Mixing At the heart of transforming raw ingredients into food for human consumption is the mixing operation. It occurs in innumerable instances in the food industry and is the most encountered of all process operations. Mixing is the di...

Unit Operations Mixing  Mixing At the heart of transforming raw ingredients into food for human consumption is the mixing operation. It occurs in innumerable instances in the food industry and is the most encountered of all process operations. Mixing is the dispersing of components, one throughout the other, to establish consistency. Even with the right amount of ingredients and flavours, a great recipe will not transform into good food unless the components are well-mixed. Taste, texture, colour, appearance – these are all crucial parameters intimately influenced by the mixing process.  Reasons for mixing to bring about intimate contact between different species in order for a chemical reaction to occur; and to provide a new property of the mixture which was not present in the original separate components. An example is a food mixture of a given component for nutritional purposes. mixing is brought about by agitation.  Types of Mix Perfect mixture (Figure 1) Random Mixture (Figure 2) Segregated Mixture (Figure 3)  Causes of Segregation Differences in Particle size Density Differences Differences in Shape Aeration Overblending Static Charges Core Flow Vibration  Powder Flow behaviour Funnel flow Core flow Rat hole  Powder Flow behaviour Mass Flow All powder moves First in – first out sequence Reduces segregation Hopper wall steep and smooth Outlet large enough to prevent arch formation  Mixer Equipment Various types and styles of mixing equipment are utilized within the food industry. Their use and application are determined by the phases being mixed (liquid-liquid, solid-liquid, or solid-solid) as well as physical characteristics of the end product (like viscosity and density).  Liquid Mixing The agitation of a liquid is defined as the establishment of a particular flow pattern within the liquid, usually a circulatory motion within a container. Mixing is brought about by agitation. Purposes of agitation Liquids are agitated for a number of purposes, depending on the objectives of the processing step. These purposes include: 1. Suspending solid particles. 2. Blending miscible liquids, for example, methyl alcohol and water. 3. Dispersing a gas through the liquid in the form of small bubbles. 4. Dispersing a second liquid, immiscible with the first, to form an emulsion or a suspension of fine drops. 5. Promoting heat transfer between the liquid and a coil or jacket.  Agitation Equipment Liquids are usually agitated in a reactor vessel. The vessel is rounded at the bottom to eliminate sharp corners or regions into which fluid currents would not penetrate. The liquid depth is approx. equal to the diameter of the vessel. An impeller is mounted on a shaft supported from above (sometimes from the bottom) The shaft is driven by a motor, sometimes connected directly to the shaft, but more often connected to it through a speed-reducing gearbox. The impeller creates a flow pattern in the vessel, causing the liquid to circulate through the vessel and return eventually to the impeller, and mixing takes place. Accessories such as inlet and outlet lines, coils, jackets, and wells for thermometers or other temperature-measuring devices are usually included. Baffles are often included to reduce tangential motion(swirling ).  Liquid-liquid/ liquid-solid mixing Mixing two or more liquids together Some form of agitator/stirrer is used. Most common type is the paddle or propeller stirrer. Liquids get mixed also by turbulent flow in pipes and when passing through pumps.  Liquid/liquid-solid mixing equipment For the deliberate mixing of liquids, the propeller mixer is the most common and the most satisfactory. In using propeller mixers, it is important to avoid regular flow patterns such as an even swirl round a cylindrical tank, which may accomplish very little mixing. To break up these streamline patterns, baffles are often fitted, or the propeller may be mounted asymmetrically (off centered).  Impeller types Divided into two classes – those that generate currents parallel with the axis of the impeller (axial-flow impellers) and those which generate currents in a radial direction (radial-flow impellers). Three main types- paddles, propellers and turbines. – Propeller mixer is probably the most common. 1. Paddle impeller – Used to mix low-moderate viscous liquids – Flat paddle: simplest design, turns on a vertical shaft, two-bladed and four-bladed are common, mixture moves radially, no vertical motion – If blades are pitched, some vertical motion – Normally rotate at around 20-150 rpm – Typical for mild agitation.  Choice of mixer/agitator 2. Marine Propellers – Axial flow, high speed impellers for liquids of low viscosity. – Motion is highly turbulent – Blades vigorously cut or shear the liquid. – Effective for use in very large vessels, rarely exceed 18 inches in diameter regardless of vessel size. – Advantages Cheap to manufacture Easy to clean High mixing rates compared to paddles Speeds of around 1000 rpm – Disadvantages Not suitable for very viscous liquids  Choice of mixer/agitator 3. Rushton turbine – Effective over a range of viscosities. In low viscous liquids they generate strong currents throughout and destroy stagnant pockets. The priciple flow patterns are radial and tangential (swirling). – Flat bladed; resemble multi-bladed paddle agitators with short blades turning at high speed on a shaft mounted centrally in the vessel. Many designs. – Speeds of around 2000 rpm  Flow Patterns The flow pattern created by an impeller in a mixing vessel is useful in establishing whether there are stagnant or dead regions in the vessel. Impellers are divided into two classes based on the type of flow pattern generated: – Radial flow: flow perpendicular to the impeller shaft – Axial flow: flow parallel to the impeller shaft  Flow Patterns - Propellers Propellers usually exhibit axial flow. They may also produce tangential flow or vortex flow. – To minimise the vortices being created, baffles are usually fitted to the tank. Marine Propeller  Flow Patterns - Marine Propellers Baffles are a series of flat plates placed around the circumference of the tank. The purpose of the baffles are to reduce radial/tangential flow and therefore improve mixing. Each plate is about 10% of the diameter of the vessel in width. There can be four of these in any one vessel. Impart random movement to flow and thereby enhance mixing. Baffled Vessel Another way to minimise the creation of vortices in a vessel with a marine propeller installed is to mount it “off centre”. Vortex formation with/without baffles  Flow Patterns - Turbines Turbine impellers produce a strong radial flow. The flow pattern can be altered by changing the impeller geometry. – For example if turbine blades are angled to the vertical, a stronger axial flow component is produced. This could be useful for suspending particles. – Curving the turbine blades could make the agitator more suitable for viscous mixes. Flat blade or Rushton Turbine  Flow depends on Impeller type Characteristics of fluid (viscosity, density) The size and proportions of the vessel Power input Baffles – Indentations on vessel walls to generate more turbulence Prevent swirling and vortex formation Powder and Particle (solid-solid) mixers Critical Parameters Choice of blender Time of blend Fill level Blender RPM Loading Pattern Mixing of widely different quantities Simplest when quantities to be mixed are roughly in the same proportions. Where very small quantities of one component have to be blended uniformly into much larger quantities of other components, the mixing should be split into stages, keeping the ratio of the two components constant is each stage. E.g. Vitamin addition to powdered cereal. Powder and particle(solid-solid) Mixers Must displace parts of the mixture with respect to other parts. Examples: – Ribbon Blender – Double cone blender  Ribbon blenders A ribbon blender consists of a U-shaped horizontal trough and an agitator made up of inner and outer helical ribbons that are pitched to move material axially in opposite directions, as well as radially. Applications: Dry such as cake and muffin mixes, flour, bread improvers, cereals, trail mixes, snack bars, spices & herbs, tea (leaves or iced tea powders), coffee (whole or ground beans), and other beverage blends including whey protein shakes, chocolate drinks, powdered juices, energy drinks, etc. Liquids can be added through a charge hole on cover or by spray bars. Flowable slurries and pastes.  Tumble blender The tumble blender is a rotating device that commonly comes in double-cone or V- shaped configurations. Generally, tumble blenders operate at a speed of 5 to 25 revolutions per minute. Materials cascade and intermix as the vessel rotates. Mixing is very low-impact.  Dough and Paste Mixers Machines are heavy and powerful. Large power requirements therefore good mixing efficiency is important. The power is dissipated in the form of heat which may heat the product and affect its quality. Thus, some mixers may require jackets to remove as much heat as possible with cooling water.  Dough and Paste Mixers Most commonly used in the kneader – Two contra-rotating arms of special shape (sigmoid shape) which fold and shear the material across a cusp, or division, in the bottom of the mixer. – Arms rotate at different speeds, e.g.ratio of 3:2. Unit Operations Size Reduction  Size Reduction Raw materials often occur in sizes that are too large to be used, and therefore must be reduced in size. Size reduction or ‘comminuition’ is the unit operation in which the average size of solid pieces of food is reduced by the application of grinding, compression or impact forces. Benefits of Size reduction in food processing: There is an increase in the surface-area-to- volume ratio of the food which increases the rate of drying, heating or cooling and improves the efficiency and rate of extraction of liquid components (for example fruit juice or cooking oil extraction. A similar range of particle sizes allows more complete mixing of ingredients (for example dried soup and cake mixes).  Size Reduction Size reduction and emulsification have little or no preservative effect. They are used to improve the eating quality or suitability of foods for further processing and to increase the range of products available. In some foods size reduction may promote degradation by the release of naturally occurring enzymes from damaged tissues, or by microbial activity and oxidation at the increased area of exposed surfaces, unless other preservative treatments are employed.  Size Reduction Two main categories depending on material type Type Operation Solid Grinding and Cutting Liquid Emulsification or atomisation  Grinding and Cutting Reduce the size of solid materials by mechanical action, dividing them into smaller particles. Examples: – Milling of grains to make flour – Grinding of flour to make corn starch – Grinding of sugar – Milling of dried foods, e.g. vegetables Cutting is used to break down large pieces of food into smaller pieces suitable for further processing, e.g. preparation of meat for retail sales, preparation of processed meats and processed vegetables.  Grinders The term grinder refers to a variety of size reduction machines for intermediate duty. Some of the commercial grinders are hammer mills, impactors, rolling compression machines, attrition mills, and tumbling mills.  Hammer mill These mills all contain a high-speed rotor turning inside a cylindrical casing. Usually the shaft is horizontal. Feed dropped into the top of the casing is broken and falls out through a bottom opening. In a hammer mill, the particles are broken by sets of swing hammers pinned to a rotor disk. A particle of feed entering the grinding zone cannot escape being struck by the hammers. It shatters into pieces, which fly against a stationary anvil plate inside the casing and break into still smaller fragments. These in turn are rubbed into powder by the hammers and pushed through a grate or screen that covers the discharge opening.  Hammer mill Brittle materials are suitable for fracture by the hammer action. Fibrous materials require a cutting edge. Some hammer mill models have a cutting edge that can be used by turning the rotar by 180o. Cutting edge Hammer  Roller mill – used widely to grind flour. – Two to five porcelain or metal rollers operating at different speeds – Adjustable gap between the rolls ( as small as 20 mm) – Material is sheared as it passed through the gap Size reduction range – 200 - 1 mm particle diameter Wheat Mill Grinding Rolls Roller Mill  Ball mills In a ball mill or pebble mill, most of the reduction is done by impact as the balls or pebbles drop from near the top of the shell Limited use in food industry – Grinding food colouring materials  Cutters Generally simple Rotating knives in various arrangements Keeping knives sharp is a problem. – Bowl chopper: flat bowl in which the material revolves beneath a vertical rotating cutting knife.  Milling equipment All mills consist of three Feed basic components: Feed chute for material Mesh delivery Grinding mechanism Discharge chute Mesh Chute  General Milling Information Rate of feed is important Slow rate reduces the amount of smaller particles Fast rate could choke the mill and the powder will remain in the chamber for a longer period of time giving rise to smaller particles. Ideally rate of input should equal rate output. Cooling may be required. Emulsification  Emulsions Emulsions are stable suspensions of one liquid in another, the liquids being immiscible. Dispersion of very fine droplets of one liquid (disperse phase) through the other liquid (continuous phase). Two simple types of emulsions: Water in oil (w/o) Oil in water (o/w)  What happens when you mix oil and water? Oil and water can’t mix, so you form two layers. We call these substances ‘immiscible’ (DON’T MIX) Is there any way of forcing the two layers to mix? What is Fairy Liquid really for? To make an emulsion: You need: e.g. washing up! Oil Grease Water Hot water An emulsifier Fairy liquid! But how does it work…? What can you use EGG YOLK for? To make an emulsion: You need: e.g. mayonnaise Oil Oil Water Vinegar An emulsifier Egg yolk  Processed Foods Processed foods, including vegetable oils, may have chemicals added to them Lecithin (E322) is one example – it is an emulsifier which allows oil and water to mix, used in margarine, ice cream, salad cream etc… Additives are listed on the ingredients label of such foods, and many of these additives have E numbers to identify them… Other examples of emulsifying agents are phosphates and glycerol monostearate.  Immiscible Liquids Immiscible liquids do not mix together, e.g. oil floats on the surface of the water when mixed. If you shake oil and water together then leave them to stand, tiny droplets of oil float upwards – they join together until eventually the oil is floating on the water again ❖ This (immiscibility) is not a useful property when dealing with foods which often contain both oil and water (such as salad cream) – without a binder to hold the two together they would keep separating…  Emulsifier Emulsifiers are molecules that have two different ends: – A hydrophilic end (water-loving) that forms chemical bonds with water but not with oils – A hydrophobic end (water-hating) that forms chemical bonds with oils but not with water  Emulsifier The hydrophilic 'head' dissolves in the water and the hydrophobic 'tail' dissolves in the oil In this way, the water and oil droplets become unable to separate out – the mixture formed is called an emulsion Oil droplet But after a while, when two droplets collide, they will merge to form a larger droplet. Eventually the emulsion will separate back into two layers. The longer this takes, the more stable the emulsion was.  Emulsifier Versus Emulsion An emulsion is a mixture of oil and water An emulsifier is a specific molecule able to bind the two ends so they ‘stick together’ (i.e. the oil and water bind) E.g. Lecithin is an emulsifier which binds the emulsion of water and oil  Emulsions There are many common emulsions, including: - – Butter (water dispersed in fat) – Milk (fat dispersed in water) – Ice cream (fat in water which is then frozen) – Mayonnaise (oil in water) – Salad cream – Margarine – Moisturising lotion – Emulsion paint – Skin cream  Preparation of Emulsions The essential feature of an emulsion is the small size of the disperse phase droplets. – Shearing stresses are imposed upon the liquid to be dispersed, which break the material into fine particles. – Use a homogenizer Liquid is passed through a high pressure pump, and then discharged through a small gap or nozzle. Passing through the nozzle large shear forces are exerted on the liquid, disrupts cohesion and dispersing it into very small particles.  Preparation of emulsions Mechanical equipment for emulsification (Agitation) Mechanical stirrers – Propeller type mixers – Turbine mixers Homogenizers Ultrasonifiers 65 Mechanical stirrers 66  Turbine stirrer For drawing the material to be mixed from above. Generates axial flow in the vessel. 67  Propeller stirrers Standard stirring element. For drawing the material to be mixed from the top to the bottom. Local shearing forces. Generates axial flow in the vessel. Used at medium to high speeds. 68 Preparation of emulsions - Homogeniser 69  Homogenizer A homogenizer consists of – A pump that raises the pressure of the dispersion to a range of 500 to 5000 psi – An orifice (gap) through which the fluid impinges upon the homogenizing valve held in place on the valve seat by a strong spring. – As the pressure builds up, the spring is compressed and some of the dispersion escapes between the valve and valve seat. – At this point , the energy that has been stored in the liquid as pressure is released instantaneously and subjects the product to intense turbulence and hydraulic shear.  Ultrasonifiers Viberating blade Intlet Outlet Nozzle Ultrasonic vibrations induced in the liquid 71  Examples of emulsions in food industry - Milk Milk is an emulsion of fat in water, which is not stable and separates into skim milk and cream. – Due to density differences between the fat and the water. The fat globules rise and coalesce at the surface to form a layer of cream. – After homogenizing, this separation does not occur as the globules are much reduced in size.  Summary Questions 1. Copy and complete using the words below: emulsifier emulsion cosmetics ice mayonnaise mix separating small Oil and water do not ______ together. But if the oil droplets can be made very ______ it is possible to produce a mixture of oil and water called an ______. To keep the oil and water from ______ we can use a chemical called an ______. Important examples of food made like this include___________ and ______ cream. Emulsions are also important in paints and in ______.  Summary Questions 2. a) Salad cream is an emulsion made from vegetable oil and water. In what ways is salad cream different from both oil and water ? b) Why do we need to add an emulsifier to an emulsion like salad cream ? 3. Explain how emulsifier molecules do their job.  Summary Questions (Answers) 1. Copy and complete using the words below: emulsifier emulsion cosmetics ice mayonnaise mix separating small Oil and water do not MIX together. But if the oil droplets can Be made very SMALL it is possible to produce a mixture of oil And water called an EMULSION. To keep the oil and water from SEPARATING we can use a chemical called an EMULSIFIER. Important examples of food made like this include MAYONNAISE and ICE cream. Emulsions are also important in paints and in COSMETICS.  Summary Questions (Answers) 2. a) Salad cream is thick (viscous) and is not transparent b) To prevent the oil and water in the emulsion from separating 3. The ‘tails’ of the emulsifier molecules dissolve into the oil, leaving ‘heads’ of the molecules lining the surface of the oil droplet. These droplets then repel each other and remain spread throughout the water