Manufacturing and Processing Technology in Food (Topic Notes) PDF

Manufacturing and Processing Technology in Food  Unit operations ▪ At the end of this section the student should be able to: ▪ For each Unit Operation ▪ Explain the purpose of the unit operation ▪ Give an example of where it is used in the food...

Manufacturing and Processing Technology in Food  Unit operations ▪ At the end of this section the student should be able to: ▪ For each Unit Operation ▪ Explain the purpose of the unit operation ▪ Give an example of where it is used in the food industry ▪ Describe the equipment used for the unit operation ▪ List advantages/disadvantages of the equipment (where stated). Unit operations Heating 3  Heating HEAT PROCESSING: Use of high temperatures to destroy enzymes and microorganisms that could reduce quality and/or safety of food 1. BLANCHING - A mild heat treatment that primarily destroys enzymes and reduces microbial load (does not necessarily kill pathogens), further preservation methods needed to extend shelf life. Example: Vegetables, frozen, canned  Heating 2. PASTEURIZATION - A mild heat treatment used primarily to destroy pathogenic organisms but it also destroys enzymes and reduces microbial load. Requires an addition preservation method to extend shelf life (example: refrigeration, drying).  Heating 3. COMMERCIAL STERILIZATION – A severe heat treatment that destroys pathogenic and many microorganisms that could spoil food. Extends shelf life, room temperature stable. (canned foods) 4. STERILIZATION - A very severe heat treatment that destroys all microorganisms.  METHODS OF HEAT TRANSFER 1. CONDUCTION: Heating of solids; Slow heating; Heating of fixed molecules in a row. ▪ Examples: spoon in sauce pan; Solid pack pumpkin in a can. 2. CONVECTION: Faster heating of liquids and gas; Hot liquids and gasses rise, cooler portions sink, creating a flow or current. ▪ Examples: forced air heating in houses; Canned juices. 3. RADIATION: Electromagnetic waves. Two general types: a. Heat radiation from a heat source. Flames: e.g. heating marshmallows in a fire; heating hot dogs, hamburgers on a BBQ. b. Non heat radiation Microwaves Irradiation that does not transfer heat: Gamma rays, x-rays, Ultraviolet.  Heat transfer equipment ▪ Jacketed pans ▪ Liquid is heated in a container vessel. ▪ Agitator may be present. ▪ Source of heat is steam condensing in the jacket. ▪ Heating coils immersed in liquids ▪ Used when quick heating is required, e.g. boiling of jam. ▪ Helical coil inside the pan, steam heats the coil ▪ Greater heat transfer rates than the jacketed pans  Heat transfer equipment ▪ Scraped Surface heat exchangers ▪ Used for products of higher viscosity ▪ Consists of a jacketed cylinder with an internal cylinder fitted with scraper blades. ▪ The blades rotate causing the fluid to flow through the space between the cylinders with the outer heated surface constantly scraped. ▪ Used in freezing of ice-cream and cooling of fats during margarine manufacture ▪ Plate Heat Exchanger ▪ Used for fluids with low viscosity, e.g milk ▪ Plates are clamped together, heating and cooling fluids flow between alternate plates. Heat transfer equipment Production Line for milk processing Unit operations Evaporation  Aims of the food industries today Often a raw material or a processed food contains more water than is required in the final product. When the foodstuff is a liquid, the easiest method of removing the water is to apply heat to evaporate it (process is called Evaporation). Factors that affect the rate of evaporation: ▪ Rate at which heat can be transferred to the liquid ▪ Quantity of heat required to evaporate each kg of water ▪ Maximum allowable temperature of the liquid ▪ Pressure at which evaporation takes place ▪ Changes that may occur in the foodstuff during the course of the evaporation process.  Evaporation Objective: “To concentrate a dilute solution consisting of non volatile solute and volatile solvent” In this operation, the solvent to be evaporated is generally water and concentrated solution is a product. The vapor generated usually has no value, it is condensed and discarded.  SINGLE EFFECT EVAPORATOR Single-effect evaporators: A single-effect unit usually uses steam or high-temperature hot water to heat the process liquid to its boiling point. The steam is passed through a coil or jacket and the vapors produced by the boiling liquid are drawn off and condensed. The concentrated liquid then is pumped from the bottom of the vessel. When a single evaporator is used ,the vapor from the boiling liquid is condensed and discarded. This is called single effect evaporation. It is simple but utilizes steam ineffectively. To evaporate 1 kg of water from the solution we require 1-1.3 kg of steam. In particular, single-effect evaporators are utilized for small evaporation rates, or for liquids that boil at high temperatures (high boiling-point elevation liquors).  MULTIPLE-EFFECT EVAPORATOR Multiple-effect evaporators: A multiple-effect unit consists of a series of single-effect evaporators. Vapor from the first evaporator is used as the heat source to boil liquid in the second evaporator. Boiling is accomplished by operating the second evaporator at a lower temperature than the first. The process can continue through evaporators. Increasing the evaporation per kg of steam ▪ Typical applications are caustic recovery plants in textile mills, sugar juice concentration, water recycling from distillery effluent, process evaporators, water recycling and zero liquid discharge plants for chemical, pharma, fertilizer, dye stuff, polymers, automobile, paint and other process industries. Once through (rising) Circulation  Once through evaporators: In once through operation , the feed liquor passes through the tubes only once , releases the vapor and leaves the unit as thick liquor. Evaporation is done in a single pass. The ratio of evaporation to feed is limited in single pass. These are useful for heat sensitive materials ▪ E.g Used for concentrating highly heat-sensitive materials such as orange juice, food materials etc. which require short residence times.  Circulation evaporators: In circulation evaporators a pool of liquid is held with in the equipment. Incoming feed mixes with the liquid in the pool, and the mixture passes through the tubes. Unevaporated liquid is discharged from the tubes returns to the pool , so that only part of the evaporation occurs in one pass. These are not suited for heat sensitive materials. Separations in Food Processing  Separations ▪ Separations are vital to all areas of the food processing industry. Separations usually aim to remove specific components in order to increase the added value of the products, which may be the extracted component, the residue or both. ▪ Separation processes always make use of some physical or chemical difference between the separated fractions; examples are size, shape, colour, density, solubility, electrical charge and volatility.  Separations ▪ Density ▪ Size ▪ Shape ▪ Color ▪ Charge：electrophoresis  Separations based on… ▪ Density ▪ Cream separator ▪ Clarification ( Precipitation) ▪ Size ▪ Membrane processes  Separating Mixtures Filtration: Separates components of a mixture based upon differences in particle size. Normally separating a precipitate from a solution, or particles from an air stream. Centrifugation: Separates components of a mixture based upon differences in particle density. Extraction : Separation is based upon differences in solubility in different solvents (major material). Crystallization : Separation is based upon differences in solubility of components in a mixture. Distillation : separation is based upon differences in volatility. Unit operations Filtration  Filtration  Filtration is the separation of an insoluble solid from a liquid. The solid could be the “desired” or the “undesired” material.  Together, the liquid and the insoluble suspended solids are known as slurry (suspension). During filtration, the slurry passes into a sieve-like material called a filter (or filter medium).The filter allows the liquid to pass through but not the solid. The solid material that remains on the filter is called the filter cake. The liquid that passes through the filter is called the filtrate and is collected separately.  Filtration Equipment ▪ Centrifugal filter ▪ A centrifugal filter separates the solid from a liquid in a slurry. The slurry is fed into a basket which is lined with a filter medium (cloth) within the centrifuge. The centrifuge uses rotational speed (centrifugal force) to force the liquid portion of the slurry out through the filter, and the solid portion is retained. The centrifuge is used when the solid material retained is the desired product and the filtrate is waste material.  Filtration Equipment ▪ Filter Press (Plate and Frame press) ▪ The filter press is used to remove unwanted solid material from a liquid. A filter press consists of a series of chambers containing square or rectangular filter plates supported in a frame. Once the filter chambers are loaded with slurry, the plates are forced together with hydraulic rams that generate pressures typically in the region of 100 pounds per square inch (70,000kg per m2). ▪ For comparison, a car tyre would be inflated to around 30 pounds per square inch.  Filtration Equipment ▪ Filter Press (Plate and Frame press) ▪ Each plate is covered by a material or membrane that acts as the initial filter when the press is in operation. As the solid filter cake builds up, the cake adds to the removal of fine particles. The solution coming through the filter, called the filtrate, will be very pure. If it is not wanted the filtrate can be drained away for safe disposal. ▪ At the end of the compression, the solid filter cake can be removed. The whole process is often computer controlled to make it automatic or semi-automatic. ▪ The filter press is generally used to clarify solutions and remove small amounts of solid Filtration Equipment  Filtration Equipment ▪ Filter Press (Plate and Frame press) Applications ▪ In the food industry it is used to remove the “lumps ” (insoluble impurities) from food products which would affect their quality. ▪ Honey ▪ Water ▪ Fruit juice ▪ Soft drinks (coke) ▪ Edible oil  Filtration Equipment Rotary Drum Vacuum Filter A rotary drum vacuum filter is a continuous filtration device in which the solids are separated by a porous filter cloth wrapped around a drum-like structure with a perforated curved surface. The drum is rotated, partially submerged, through a feed solution held in a trough and vacuum applied on the inside. The filtrate flows through the filter media to the inside of the drum and the cake accumulates on the outside. The working principle of a rotary drum filter is shown in Fig. 10.9. At any given location on the filter, the cake layer builds up as it moves through the feed. Filtration Equipment  Filtration Equipment Rotary Drum Vacuum Filter  Filtration Equipment Rotary Drum Vacuum Filter  Filtration Equipment Rotary Drum Vacuum Filter ▪ Advantages The rotary vacuum drum filter is a continuous and automatic operation, so the operating cost is low. The variation of the drum speed rotating can be used to control the cake thickness. The process can be easily modified Can produce relatively clean product by adding a showering device. ▪ Disadvantages ▪ Due to the structure, the pressure difference is limited up to 1 bar. Besides the drum, other accessories, for example, agitators and vacuum pump, are required. The discharge cake contains residual moisture. High energy consumption by vacuum pump.  Filtration - Applications ▪ Edible oil refining ▪ After extraction crude oil is filtered to remove insoluble impurities such as fragments of seeds, nuts, etc (plate and frame(P&F) or rotary drum(RD) filters) ▪ Sugar refining ▪ The juice produced by extraction from sugar cane or sugar beet contains insoluble impurities. The juice is treated with lime to produced a flocculated precipitate. Supernatant and precipitated are separated using plate and frame or rotary drum filters. ▪ When sugar crystals have grown to an appropriate size, they are separated from the juice using centrifugal filters. ▪ Beer Production ▪ During maturation a deposit of yeast and trub (sediment) forms on the bottom of the maturation tanks. The beer is recovered from this using filters (P&F; RD) ▪ Others: wine making; filtration of starch and gluten suspensions; clarification of fruit juices,  Filtration Parameters  Filtering operations are controlled by a series of process parameters. The quality of the final product is dependent on these parameters being properly controlled. Operators must control the parameters in order to produce a product of a high quality and to the correct specifications. The common parameters are listed below and will be discussed in more detail on the following screens.  Flow (Vacuum, Pressure, Centrifuge Rotational Speed)  Filter Type  Quantity of Solid in the Slurry  Depth of Cake  Filtration Parameters - Flow  For a filtration process to work efficiently the slurry must be forced into the filter medium. The faster the flow rate of the liquid through the filter medium the quicker the filtration. There are three main methods used to speed up flow in a filtration process. These are:  Pressure  Vacuum  Centrifugal Force  Filtration Parameters - Flow Pressure ▪ The slurry is pumped into the equipment at pressure. This forces the liquid through the filter. The correct pressure, which is specified in the BPR, must be used. ▪ Using too high a pressure could cause the following problems: ▪ The filter medium could be damaged or burst and will need to be replaced or repaired ▪ The filter cake could be compacted and stop the filtration process by not allowing the liquid to pass through. ▪ Using too low a pressure will slow down the filtering process as there will not be enough force for the liquid to pass through the filter medium. Filtration Parameters - Flow  Filtration Parameters - Flow Vacuum  In some types of filtration equipment the slurry is drawn into the filter medium by a vacuum system. The correct vacuum, which is specified in the BPR, must be used.  Using too strong a vacuum could cause the following problems:  The filter medium could be damaged or burst and will need to be replaced or repaired.  The filter cake could be compacted and stop the filtration process by not allowing the liquid to pass through.  Using too weak a vacuum will slow down the filtering process as there will not be enough force to suck the liquid through the filter medium Filtration Parameters - Flow  Filtration Parameters - Flow Centrifugal Force  In centrifuges the slurry is forced into the filter medium by a centrifugal or rotating force. As the basket is rotated, the slurry is forced against the filter medium.  The solid is retained and the liquid passes through the filter. The correct rotational speed, which is specified in the BPR, must be used.  Using too high a rotational speed could damage the filter medium. The filter cake could also be compacted and stop the filtration process by not allowing the liquid to pass through.  Using too low a rotational speed will slow down the filtering process, as there will not be enough force for the liquid to pass through the filter medium.  Filtration Parameters – Filter Type  Filters work by allowing liquid to pass through pores in the filter medium and by preventing solid particles from getting through. Filters with a small pore size prevent smaller particles from getting through. However, this also has the effect of reducing the flow rate as more material is retained.  Larger pore sizes result in increased flow rates but if they are too big, solid particles will pass through them and not be stopped. Filters with small pore sizes are blocked more easily but they can stop smaller particles. The type of filter used is therefore very important. The correct filter type is specified in the BPR and must be used.  Filtration Parameters – Depth of Cake/ Quantity of Solids  A thick filter cake can cause problems by blocking the filter medium and slowing down the process.  This can occur in two ways:  The quantity of solids in the slurry will affect the thickness of the cake. The more solids present the thicker the cake  Volume of slurry - Using too high a volume of slurry could slow the filtration process. Large volumes of slurry contain more solid material, which increases the cake size and blocks the filter. Filtration Parameters - Flow Unit Operations Centrifugation  Sedimentation vs Centrifugation Sedimentation and centrifugation Sedimentation  When a suspension is allowed to stand, the denser solids slowly settle under the influence of gravity. Sedimentation is very often used in the food industry for separating dirt and debris from incoming raw material, crystals from their mother liquor and dust or product particles from air streams. Centrifugation  A settling process that is accelerated with a centrifugal field. Comparison between filtration and centrifugation: Introduction (2/8) Feature Filtration Centrifugation Separation principal Particle size Density Employment Removal of Used when insolubles filtration is which are ineffective dilute, large and rigid Product obtained Dry cake A paste or a more concentrated suspension Expense of equipment Less More  Centrifugation ▪ Used to separate immiscible liquids and solids from liquids. ▪ Immiscible liquids: ❖If two immiscible liquids of different densities are added to the centrifuge, under the influence of centrifugal force, the more dense liquid moves towards the bowl wall while the less dense liquid will be displaced towards the centre. ❖ separation of milk into skim milk and cream. ▪ Insoluble solids from liquids ❖A liquid containing insoluble solids is fed into the centrifuge, under the influence of centrifugal force solid particles move towards the bowl wall, and if they reach the wall before being swept out by the liquid leaving the bowl at the top, they become separated from the liquid. If it does not reach the bowl wall it gets carried out by the liquid. ❖Fruit juice processing, clarifying cider and sugar syrups  Centrifugation - Equipment TUBULAR BOWL CENTRIFUGE Consists of a tall, narrow bowl rotating about a vertical axis inside a stationary casing. Suspension is usually fed through the bottom, and clarified liquid is removed from the top, flows into the stationary section and discharged through an opening in the bottom of the bowl. Suitable for feeds up to 2% solid content. Solid deposits on the bowl’s wall as a thick paste. The suspension can be fed until solid loss in the effluent becomes prohibitive. An intermittent operation.  Centrifugation - Equipment SEPARATION OF LIQUIDS BY CENTRIFUGATION  A common operation in the food and other industries. * Example: the dairy industry, in which the emulsion of milk is separated into skim milk and cream.  The Disc Stack Centrifuge http://drugtopics.mediwire.com/main/Default.aspx?P=Content&ArticleID=172950 Large particles have higher settling velocities than small particles Solids accumulate at the outer edge of the bowl Soluble material passes through with the clarified liquid  The Disc Stack Centrifuge A short, wide bowl 8 to 20 in. in diameter turns on a vertical axis. Inside the bowl and rotating with it are closely spaced “disks”, which are actually cones of sheet metal set one above the other. In operation, feed liquid enters the bowl at the bottom, flows into the channels, and upward past the disks.  Centrifugation - Equipment ▪ Process can be made CONTINUOUS by using Nozzle discharge centrifuges. ▪ Continuous discharge of solids in the form of sludge as well as the clear liquid. ▪ Many designs available. 2-24 nozzles placed around the bowl. Nozzles open all the time. ▪ Slurry can contain up to 25% solids content ▪ Modification: Self opening centrifuges where the opening of nozzles can be controlled by timers or based on build up of solids. ▪ Used in brewing industry; wine making, clarifying apple juice; edible oil refining  Centrifugation -DISK Equipment CENTRIFUGE (3/14) The operation can be made continuous. Unit Operations Extraction  Extraction ▪ A liquid is used to carry out the separation process. ▪ The liquid is thoroughly mixed with either solids (solid – liquid extraction) or another liquid (liquid-liquid extraction) from which the component is to be removed and then the two streams are separated. ▪ In solid-liquid extraction, the separation of the two streams is by gravity settling. The required product can be either in the liquid which is added or the washed solid. ▪ e.g. extraction of coffee from coffee beans (product in the liquid) ▪ e.g. washing of butter with water (product is the solid, unwanted constituent is removed in the water stream. ▪ Other examples: soluble sugar is removed by water extraction from sugar cane or sugar beet.  Extraction ▪ When the extraction takes place from one liquid medium to another, the process is called liquid-liquid extraction. ▪ To separate the liquid streams, the liquids must be immiscible, such as oil and water. ▪ Applications: ▪ Oil is extracted from natural products such as peanuts, soya beans, sunflower seeds, etc ▪ Extraction of fatty acids. ▪ Factors controlling the operation: ▪ Area of contact between the streams ▪ Time of contact ▪ Properties of the material ▪ Number of contact stages employed Liquid-liquid extraction (lab scale)  Applications ▪ Edible oil extraction: oil is extracted from nuts, seeds and beans using a solvent. The most commonly used solvent is hexane. ▪ After extraction, the solution of oil in solvent is filtered, the solvent removed by vacuum evaporation followed by distillation and the solvent reused.  Applications ▪ Extraction of Sugar from Sugar Beet: Sugar is extracted from sugar beet using heated water as solvent. The beets are washed and cut into slices, known as cossettes. This increases the surface area for extraction and limits cell wall damage. ▪ The solution leaving the extractor contains about 15% of dissolved solids. This is clarified by settling and filtration, concentrated by vacuum evaporation, seeded and cooled to crystallise the sugar. The crystals are separated from the syrup by centrifugation, washed and air dried. Unit Operations Distillation  Distillation Distillation is a separation process, separating components in a mixture by making use of the fact that some components vapourise more readily than others. When vapors are produced from a mixture, they contain the components of the original mixture, but in proportions which are determined by the relative volatilities of these components. The vapor is richer in those components that are more volatile, and so a separation occurs. In fractional distillation, the vapor is condensed and then re-- evaporated when a further separation occurs.  Distillation ▪ It is difficult to prepare pure components in this way, but a degree of separation can easily be attained if the volatilities are reasonably different. ▪ Where great purity is required, successive distillations may be used. ▪ Major uses of distillation in the food industry are for concentrating essential oils, flavours and alcoholic beverages.  Distillation ▪ The temperature at which any liquid boils is known as its boiling point. Different types of liquids have their own unique boiling point. For example, the boiling point of water is 100oC and the boiling point of ethanol is 78oC. ▪ ▪ When a liquid begins to boil, the liquid at the surface becomes a vapor. In the case of water this is known as steam and the process is called evaporation. If the vapor is then cooled, it returns to a liquid in a process called condensation. ▪ Evaporation and condensation are the basic principles on which distillation and refluxing work.  Distillation ▪ Distillation is a commonly used method for removing a liquid from a reaction. First, the liquid in the reactor vessel is heated. When the temperature of the liquid reaches its boiling point, vapor is formed. This vapor leaves the reactor vessel and is transferred to a condenser, where it is cooled and converts to a liquid. The liquid is called the distillate and is collected separately in a holding vessel. ▪ Distillation can also be used to separate a mixture of two liquids that have different boiling points. The separation process depends on the difference in boiling points between the two liquids. The greater the difference, the easier it is to separate them.  Distillation ▪ A normal distillation cannot separate two liquids perfectly. There will always be a mixture of the two liquids in any fraction that is collected in the storage vessel. ▪ ▪ A special type of distillation, called a fractional distillation, is the best method to use to achieve the purest fractions possible. A piece of equipment called a fractionating column is added to the distillation apparatus. This device helps to purify the fractions. The bigger the fractionating column, the more effective the separation process.  Simple Distillation ▪ If we wish to remove a single liquid, such as a solvent, from a vessel, then the Operation is called ‘Simple’ Distillation (or, where solvent is involved, ‘Solvent Recovery’). This is the process that is utilised in “batch distillation” ▪ A given volume of liquid (reaction mix) is heated to boiling and the vapour formed is condensed and routed to a Receiving Vessel. The composition of the liquid remaining in the still and the vapour collected change with time. A flow diagram for a typical “batch distillation” is shown on next slide. ▪ Used in some Whiskey Distilleries. 72  Distillation Equipment (Batch) – pot stills ▪ In batch operation, the feed to the column is introduced batch-wise. ▪ the column is charged with a 'batch' and then the distillation process is carried out. ▪ When the desired task is achieved, a next batch of feed is introduced. ▪ A pot still involves only one condensation process, whereas other types of distillation equipment have multiple stages which result in higher purification of the more volatile component namely the alcohol. Used in the manufacture of good quality whiskey Made from arsenic-free copper, have a swan neck fitted to the pot (shape: often called onion stills) Condenser is a heat exchanger also made of copper. Heat is applied to the pot by steam passing through coils or a jacket.  Distillation - Continuous Figure: Typical industrial distillation unit with a single feed and two product stream  Distillation - Continuous Basic Operation and Terminology ▪ The liquid mixture that is to be processed is known as the feed and this is introduced usually somewhere near the middle of the column to a tray known as the feed tray. ▪ The feed flows down the column where it is collected at the bottom in the reboiler. ▪ Heat is supplied to the reboiler to generate vapour. ❖ The source of heat input can be any suitable fluid, although in most chemical plants this is normally steam. ❖ The vapour raised in the reboiler is re-introduced into the unit at the bottom of the column. ❖ The liquid removed from the reboiler is known as the bottoms product or simply, bottoms.  Distillation - Continuous Basic Operation and Terminology ▪ The vapour moves up the column, and as it exits the top of the unit, it is cooled by a condenser. ▪ The condensed liquid is stored in a holding vessel known as the reflux drum. ▪ Some of this liquid is recycled back to the top of the column and this is called the reflux. ▪ The condensed liquid that is removed from the system is known as the distillate or top product. ▪ Thus, there are internal flows of vapour and liquid within the column as well as external flows of feeds and product streams, into and out of the column.  Distillation Equipment - Continuous Basic Distillation Equipment and Operation Distillation columns are made up of several components, each of which is used either to transfer heat energy or enhance material transfer. A typical distillation contains several major components: ▪ A vertical shell where the separation of liquid components is carried out ▪ Column internals such as trays/plates and/or packings which are used to enhance component separations ▪ A reboiler to provide the necessary vaporisation for the distillation process ▪ A condenser to cool and condense the vapour leaving the top of the column ▪ A reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column. Link: http://www.gea- pen.nl/gpenl/cmsresources.nsf/filenames/P06e_Destillationstechnik_09.pdf/$file/P06e_Destillationstechnik_09.pdf  Distillation-Continuous ▪ The type of column internals: ❖Tray column – ❖trays of various designs are used to hold up the liquid to provide better contact between vapour and liquid and hence better separation. ❖Packed column – ❖instead of trays, 'packings' are used to enhance contact between vapour and liquid.  Distillation Equipment ▪ Tray arrangement within column and type  Distillation Equipment Packed column ▪ Instead of trays, packings are used to enhance contact between vapour and liquid.  Distillation Equipment Column Reboilers ▪ There are a number of designs of reboilers. ▪ They can be regarded as heat-exchangers that are required to transfer enough energy to bring the liquid at the bottom of the column to boiling point.  Vacuum Distillation ▪ Distillation under reduced Pressure ▪ In some cases, it is undesirable to bring the mixture to the normal boiling temperature of the distillate. For example, the boiling point might be very high, or a heat-sensitive material might also be present in the reaction mixture. In such instances, it is possible to make the mixture boil at a lower temperature by connecting the system to a Vacuum Pump and putting it under a partial vacuum. ▪ One has to be careful in these cases because boiling can become violent and somewhat unpredictable at times, a situation that could result in ‘carryover’ (i.e. liquid rather than vapour coming across).  Other Distillations ▪ Vacuum Distillation ▪ reduce pressure to boil off solvent at lower temperature ▪ Azeotropic Distillation ▪ co-distillation of two compounds if their b.p. close  Distillation - Azeotropes ▪ An azeotrope is a liquid mixture which when vaporised, produces the same composition as the liquid. ▪ Ethanol and water form an azeotrope of 95% at 78.2oC. (95.6% ethanol in purity). It cannot be purified further by distillation. ▪ BP pure ethanol: 78.30C  Distillation - Applications ▪ Manufacture of Whisky ▪ Irish whiskey – 3 pot stills used ▪ Brandy distilled from fermented juice ▪ Rum distilled from fermented juice of sugar cane ▪ Manufacture of spirits ▪ Recovery of solvents from oil after extraction ▪ Extraction of essential oils from leaves and seeds (steam distillation) Unit Operations Crystallisation  Crystallisation ▪ Many foods and food ingredients consist of, or contain crystals. ▪ Crystallisation has two purposes in food processing: 1. The separation of a solid material from a liquid in order to obtain either a pure solid e.g. salt or sugar, or a purified liquid. 2. The production of crystals within a food such as in butter, chocolate or ice-cream. ▪ Important to control the process so that the optimum yield of crystals of the required purity, size and shape are obtained.  Crystallisation ▪ Is a process of formation of solid crystals from a uniform solution. It is also a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. ▪ Crystallisation from solution is a two step process: 1. Phase separation or “birth” of new crystals (nucleation) 2.Growth of these crystals to larger sizes (crystal growth) ▪ Nucleation is the step where the solute molecules dispersed in the solvent start to gather into clusters (nuclei). Clusters become stable when they reach a critical size ▪ Temperature; supersaturation ▪ During nucleation the atoms arrange themselves in a manner that defines the crystal structure (term which refers to the internal arrangement of the atoms).  Crystallisation ▪ The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. ▪ Nucleation and growth continue to occur simultaneously while the supersaturation (ss) exists. ▪ Depending upon the conditions, either nucleation or growth may be predominant over the other, and as a result, crystals with different sizes and shapes are obtained.  Methods of Crystallisation: Supersaturation ▪ Supersaturation: liquid (solvent) contains more dissolved solids (solute) than can ordinarily be accommodated at that temperature ▪ Can be achieved by several methods: ▪ Cooling ▪ Evaporation ▪ Solvent addition ▪ Precipitant Addition  Methods of Crystallisation: Cooling Method ▪ Concentrated solution gradually cooled below saturation temperature (50-60°C) to generate a supersaturated state ▪ Yields well defined micron-sized crystals  Methods of Crystallisation: Seeding method ▪ Sometimes crystals can be difficult to form even if the correct conditions exist. ▪ Seed crystals are used in these situations to induce crystallisation. ▪ Seed crystals are pure crystals normally obtained from a previous batch of the same product. The seed crystal is added to the solution and will help start the crystallisation process and help other crystals to form around it.  Crystallisation: Applications ▪ Production of Sugar ▪ Major operation in sugar manufacture. Beet or cane sugar consist of sucrose, different grades of product require uniform crystals of different sizes. Supersaturation is obtained by evaporation (limited to below 85oC as sugar carmelises above this temp. and above 55oC due to high viscosities below this temp.) The supersaturated solution is seeded with very fine sugar crystals and the “massecuite” (syrup/crystal mixture) is evaporated with further syrup addition. When the correct crystal size is achieved, the crystals are removed. ▪ Production of salt ❖Salt is not temperature dependent, so forms crystals more easily than sugar.  Unit operations Membrane Separations  Membrane Separations ▪ Filtration is a pressure separation process that uses membranes to separate components in a liquid solution based on their size and includes: ▪ Microfiltration ▪ Ultrafiltration ▪ Nanofiltration ▪ Reverse osmosis. ▪ Used to separate constituents of foods, where the foods are in solution (e.g components of milk – fats from proteins, etc) Membrane Separations Na+ Hemoglobin Pseudomonas Starch (0,4 nm) (7 nm) Diminuta (10000 nm) (280 nm) H2 O Glucose Influenza Staphylococc (0,2 nm) (1 nm) Virus us (100 nm) (1000 nm) Salts and Microfiltration low molecular weight Ultrafiltration compounds Cells, bacteria Nanofiltration and Virus and Emulsions Vitamins proteins and polymers Reverse and sugars colloids Osmosis 0.1 1 10 100 1000 10000 Pore diameter (nm) Name of the membrane process as a function of the particle size.  What happens at the membrane? product (represented in green) for example: large milk proteins, fats permeate for example: salts, small milk proteins membrane / filter Process Flow Membrane Separations Different types of tubular modules.  Membrane Processes ▪ Uses membranes with varying pore sizes to separate on the basis of size and shape ▪ Reverse osmosis ▪ Uses membranes with the smallest pore and is used to separate water from other solutes ▪ Requires a high pressure pump ▪ Ultra filtration ▪ Uses membranes with larger pores and will retain proteins, lipids and colloidal salts while allowing smaller molecules to pass through to the permeate phase ▪ Requires a low pressure pump ▪ Microfiltration ▪ Pores less than 0.1 microns are used to separate fat from proteins and to reduce microorganisms from fluid food systems ▪ Requires a low pressure pump  Membrane Separations - Applications ▪ Reverse Osmosis: ▪ Concentrating fluids, by removal of water. ▪ E.g. Milk processing: used to concentrate full cream milk up a factor of 2-3 times. ▪ Concentration of tomatoe juice ▪ Nanofiltration: ▪ Used to partially reduce calcium and other salts in milk and whey, while retaining lactose. ❖(Whey is the watery portion of milk remaining after milk coagulation and removal of the curd).  Membrane Separations - Applications ▪ Ultrafiltration ▪ Milk products ❖Concentrate cheese whey, so that high protein powders can be produced which contain the functional properties of the proteins. ▪ Microfiltration ▪ Used to separate particles suspended in liquid media. ▪ Wine industry: clarification and biological stabilisation of wine musts and unprocessed wine. ▪ Bacterial removal from whole milk  Case studies Sugar Production  Sugar cane process The sugar process is divided ▪ 1 Entry or transportation of the sugar cane ▪ 2 Milling ▪ 3 Clarification ▪ 4 Evaporation ▪ 5 Crystallization ▪ 6 Separation ▪ 7 Refining ▪ 8 Drying ▪ 9 Storage  Entry or transportation of the sugar cane Sugar is obtained from the cane at mills located near centers of production. The cane first goes through a washer, then is cut into small pieces by revolving knives. After this step the small pieces are shredded. The shredder is a large powerful hammermill that shreds the cane into a fibrous material. The cells in the cane stalk containing the sugar juice are ruptured but no juice is extracted at this stage. After this preparation, the juice from the sugar cane can be extracted.  Milling Train The shredded cane is fed through a series of crushing mills to extract the sugar rich juice, which is then pumped away for further processing. The remaining fibre is called bagasse. The crushers consist of two large grooved rollers mounted horizontally, and then one above of the others. On the upper roller heavy hydraulic pressure is maintained. Clarification and evaporation  Clarification The limed juice enters a gravitational settling tank: a clarifier. The juice travels through the clarifier at a very low superficial velocity so that the solids settle out and clear juice exits. The mud from the clarifier still contains valuable sugar so it is filtered on rotary vacuum filters where the residual juice is extracted and the mud can be washed before discharge, producing a sweet water. The juice and the sweet water are returned to process.  Evaporation The clarified juice is concentrated to syrup by boiling off excess water in a series of connected vessels. Under automatically controlled conditions in the evaporator station, each subsequent vessel operates under decreasing pressure with the last one being under almost a total vacuum. After this step the syrup is ready to go to the high-vacuum boiling pans.  Crystallization Concentration of the syrup from the evaporator is continued in vacuum pans. Very small seed crystals are introduced to the concentrating syrup and these begin to grow in size. When the crystals reach the required size, the mixture of crystals and syrup is discharged from the pan.  Separation The sugar crystals are separated from the syrup in centrifugal machines that have an action similar to a spin- dryer. After leaving the centrifugals, the moist raw sugar is tumble dried in a stream of air and transferred to bulk storage bins. The separated syrup is reboiled and further sugar is crystallized. After three boilings no further sugar can be economically removed. The residual sugar is called molasses.  Sugar refining The purpose of the refinery is to remove impurities from sugar crystals. The refinery accepts raw sugar as its feed material. The sugar is dissolved (melted) and the colour is removed by various clarification processes. The final refining steps include melting the brown or raw sugar, decoloring by passing through carbon filters, recrystallizing in vacuum boiling pans, and drying by centrifuging. Refining Process