Once a time temperature profile has been resolved the entire installation can be planned based upon the plants throughput requirements. Target microbial destruction levels are usually expressed as an F0, P value or time above a specified temperature. Selection of process targets should be done in consultation with a specialist microbiologist and a specialist in taking temperature measurements in products. Sometimes temperatures are limited by specific characteristics of the packaging, for example welded seals on pouches soften significantly as the retort temperature increases and printing in lithographed cans will have an upper temperature limit.
As with measurements of product heating, consistent results from pressure measurements in containers or deflections will only be obtained if pack preparation is well controlled. Typical overpressures for different container types are: Plastic trays 2. This contrasts with saturated steam retorts where the process pressure is directly related to the chosen holding temperature. In the overpressure retorts process pressure conditions can be established that minimise pack damage.
Selection of specific retorts should also consider the sensitivity of the pack s in question to the rapid change in pressure or temperature that are inherent in some designs.
For batch retorts, new container designs may be accommodated in existing retort crates or, if necessary, alternative crate designs may be used.
The loading of containers into the retort crates should be considered at an early stage of process design because retort performance can be adversely affected if the crate loading is too dense. If the loading pattern needs to be adjusted at a later stage, to achieve acceptable retort temperature distributions, original estimates of retort throughput will be reduced.
The support and orientation of containers within crates is also important as it affects retort temperature distribution and product heating. For example, a semi- rigid tray which heats at different rates through its lid and base, will give a different heat penetration performance if the pack is heated lid up or lid down, because of the insulation effect of the headspace on heat transfer through the lid. Product appearance, as seen by the consumer when the pack is opened, may also play a part in determining the required container orientation during retorting.
Some plastic containers require specialised support racks to ensure adequate temperature distribution and pack performance. It should be noted that during retorting, plastic containers can soften and change shape, their orientation and support during processing will affect the extent and nature of this deformation. The plastic containers trays and pouches do have one potential advantage for product quality because as they are generally of thinner profile than cans they heat more rapidly so process safety requirements can be achieved with a minimum degree of overcooking.
Preliminary testing to ensure that any proposed combination of crate, container, pack arrangement, and layer divider can achieve acceptable temperature distribution performance is advisable. Purely practical considera- tions when planning the crate loading operation are the degree of automation achievable, the stability of containers, i.
Saturated steam retorts The basic operating principles of a saturated steam retort are covered in Section 2. A high number of saturated steam retorts are installed when production requires a large throughput of varying canned products. Installations may be in the order of 40 retorts, normally of the vertical type to save space. In such large installations the practice of venting significant quantities of steam to the atmosphere is problematic as the working environment becomes very hot and humid.
On a practical level the presence of steam in the retort room can be controlled by the installation of a vent manifold to carry the steam outside the building. However, careful design is required to ensure that this does not inhibit the flow of venting steam. Clearly this principle will not give uniform sterilisation unless it is ensured that the mixture of steam and air is uniform, as the presence of large pockets of air will result in under-sterilisation.
It should also be noted that as steam condenses, the air portion is left potentially forming an insulating layer around the packs to be heated. It has been suggested that the mixing could be achieved by continuous venting, but this is not used in commercial systems and is unlikely to be energy efficient.
Fan failure or damage would be regarded as critical process deviations. Retort manufacturers have addressed this issue by including a precool stage in the operating cycle. By this means the steam contents of the retort can be condensed in a relatively slow and controlled manner while the air content is increased.
Full water immersion Probably the oldest mechanism for overpressure processing is to process containers under water with an overpressure applied to the free space above the water in the retort vessel Fig.
Typical machines of the full water immersion principle are those produced by Stock and Lubeca. A means of overcoming this problem, which also adds to the energy efficiency of the system, is to have a second vessel in which the water is preheated above the desired sterilisation temperature.
When the process is started this water is then pumped or dropped under gravity into the retort vessel containing the product. Although there may be some temperature drop in the water as it is transferred this method significantly reduces the retort come-up time and increases throughput.
This approach must be used with care where jars are being processed and a large temperature differential exists between the product and the incoming water, as thermal shock can result in breakage. At the end of the holding period the water can be pumped back to the storage vessel for use on the next batch, thus saving on the energy required to heat water.
Cooling is carried out with an external cooling water supply. A side effect of the double vessel water immersion retort process is that the water capacity of the storage vessel is usually matched to the requirement for a fully loaded retort.
Therefore if part loads are processed there is insufficient water. To overcome this problem the retort manufacturers supply dummy crates whose function is simply to replace the missing crates of product.
In water there is a natural tendency for convection currents to develop which will make the top of the retort hotter than the bottom, and therefore there will be different levels of product sterilisation at each position. This problem is overcome by different means by different retort manufacturers. The more sophisticated systems use an external water circulation loop on the vessel, so that water is pumped from the colder regions of the vessel through a steam injector and back to the retort.
Where possible this mixing process is combined with rotary agitation of the load, which further aids the mixing of the water. It is not uncommon to find vertical full water immersion retorts that can also be used in saturated steam mode. In these systems the agitation of the water is sometimes provided by the use of a cross-shaped spreader which directly injects steam into the retort vessel.
These are designed to give good mixing of the steam and water, for example by having two live and two dead quadrants to increase mixing. The headspace above the water can either be filled with pressurised air or steam. The use of steam has the potential advantage that process deviations due to abnormally low water level are unlikely to result in gross understerilisation.
It is generally recommended that the water level in such systems should be kept at least 10 cm above the topmost containers in the process. Some packs will have a tendency to float during full water immersion processes; where this is not desired a suitable restraint must be put in place, e. There are circumstances where this buoyancy effect is beneficial, e.
In water immersion this tendency for packs to sag is minimised. It is difficult to say whether the raining water and sprayed water systems are directly comparable in the physical mechanisms by which they transfer heat to the load. The raining water principle is simple: water is sucked from the trough at the bottom of a horizontal vessel, passed through a heat exchanger, and pumped to the top of the vessel where it is dumped at high velocity onto a sieve plate in the top quadrant of the vessel Fig.
The water is then distributed under gravity over the sieve plate and runs down through the holes onto the load below. Sprayed water systems operate in a similar manner except that the water rather than being put onto a sieve plate is put into one or more spreaders which run down the length of the vessel.
These spreaders have spray nozzles along their length which spray water into the load from the top and sometimes sides. A potential weakness of the sprayed water systems compared with raining water is the natural tendency for the water pressure to drop along the length of the spreader meaning that water coverage is less even. In practice the heat transfer rates in raining water systems seem to be comparable to those from saturated steam.
Clearly for any of the raining water or spray water systems correct maintenance of the water circulation system is critical for ensuring uniform sterilisation is achieved.
This means that when water contamination is likely due to overspill at container filling, or pack failure during the process, regular cleaning is required. The recirculation system usually includes a filter that must be routinely inspected and emptied. Ironically one of the niche markets for raining water systems is the industry canning oily fish where these types of retort effectively double as can washers. Retort technology 21 switched from steam to cold water.
In theory this means that the water should be free from microorganisms and therefore is an excellent means of preventing post process recontamination. However, post process recontamination spoilage incidents have occurred which could be attributable to leakage in the heat exchanger, dead-legs regions of low flow in the recirculation loop building up contamination or contamination of the air supply.
Crateless retorts These retorts are large vertical vessels with doors at either end. During filling the top door is opened and cans feed under gravity from a conveyor. The cans fall to the bottom of the retort and are cushioned on their way down by water. Once the retort is full the top door is closed and the water is flushed out by steam, and the process hold commences.
When the hold is finished, the load is cooled with water, drained and the bottom door is opened. The cans then fall out into a cooling canal. Batch rotary retorts Increased retort throughput can be achieved for products that undergo forced convection by using rotary batch retorts.
Such rotary agitation can be applied to any of the heating media described above, e. Rotary processes, which agitate the product inside the container, are often used in conjunction with higher process temperatures than would be used for the same product in a static process.
The higher temperature is less detrimental to product because the movement prevents overheating, particularly at the container wall. Commercial batch retorts generally use end over end agitation, as this is more suited to easy loading and simple design of the crate system.
A very important factor in the effectiveness of agitation in bringing about product heating is the size of the headspace the free space in the top of the container after filling which plays a big part in the mixing process. In rotary retorts the product within the crates must be clamped in place preventing damage from the movement during rotation.
Some containers are not suitable for rotary processing, as they do not perform well when clamped, e. Some products are not suitable for rotary processes, for example where the product texture is adversely affected, e. For those products that heat rapidly by natural convection, e. Current manufacturers of hydrostatic retort systems include Stork and FMC. The process applied is determined by the temperature in the process chamber which is limited by the size of the hydrostatic legs and the speed of the conveyor system.
Hydrostatic retorts have historically been used for production of cans, glass bottles and plastic bottles, though this situation is changing see Section 2. The loading mechanisms and orientation of the containers tend to differ, cans are loaded from a conveyor at right angles to the retort conveyor, while bottles are fed into pockets from the direction of travel of the retort conveyor.
Hydrostatic retorts are designed to work with a limited range of container sizes, depending upon the carrier bar diameter. Some systems have two sets of carrier bars of different diameters on either side of the conveyor chain. Those retorts using the bar mechanism can cope with variations in can heights without much difficulty, e. Retort technology 23 A small amount of product agitation is imparted during the change of direction on the conveyor of a hydrostatic retort, but this does not dramatically affect product heating.
Some hydrostatic retorts are designed with a planetary motion of the carrier bars to enable high temperature short time processing, e. Care must be taken in the design of hydrostatic retort installations and any other continuous retort to ensure that there is no possibility of the unprocessed containers jumping from the conveyors feeding the retort to those taking product away.
With hydrostatic retorts there is also the specific issue that the water used in the preheat leg should not be in direct contact with the outfeed leg because cross-contamination from fill overspill can result. In fact the machines are like a hydrostatic retort operated in a horizontal orientation.
Some of these machines have the unusual feature that during some of the conveyed distance inside the steam chamber the carrier bars become free to roll imparting extra agitation to the product. Reel and spiral retorts Despite its mechanical complexity the reel and spiral retort Fig. The principle of operation is that cans the system can only be used for cans are fed from a conveyor system, twisted onto their side, so that they roll freely and then passed through a star valve directly into a pressurised retort vessel.
Each can fits into a pocket between the points of the star, which as it turns moves the can from outside to inside the retort. Once inside the retort vessel, which is a horizontal cylinder, the cans are fed onto an internal spiral track in the inner wall of the cylinder and pushed along by blades attached to a reel rotating in the centre of the vessel.
The spiral nature of the track means that the cans move from one end of the cylinder to the other, and when they are in the bottom third of the cylinder they rotate freely on the bottom wall. Where such transfer values are used for moving cans between heating shells, the valves are fitted with a steam supply to ensure that cans in the valves are exposed to the intended process temperature and pressure conditions, especially during stoppages.
The configuration number and types of shells of reel and spiral retorts varies depending on the type of product to be processed. The process duration is determined by the length of the shells, the number of shells, and the rate of rotation of the reel process time and reel speed cannot be controlled independently.
Retort technology 25 might have a preheat shell operating a temperature well below sterilisation temperature, one sterilisation shell and a cooling shell, while a machine for cans of baked beans might have two sterilisation shells and two cooling shells to allow for the longer hold and cool periods required.
The shells are generally operated with steam or water for heating and water for cooling. Reel and spiral retorts rely on the movement of containers rotating on their sides, which limits their use to cans.
There is little flexibility in can size that can be processed, with machines being built for one can diameter and a limited range of heights. Agitation is inherent in the reel and spiral design enabling high temperature short time processes so they are not suitable for product sensitive to mechanical action, e.
For many years can rotation rates in reel and spiral retorts have been assumed but more recent electronic can rotation counters have become available. These devices can be put through the retorts to ensure that the theoretical rotation rates are being achieved. Failures in can rotation have been attributed to track wear, bowing of the reel and build up of deposits of lacquer removed from the rims of cans as they rotate.
There is evidence to suggest that changes in can specification can alter rotation rates and therefore product heating. This was because during the heating processes the internal pressures in the cans causes the ends to dome slightly. With an easy open end this doming can cause the ring pull to stick out to the extent that it can catch on the spiral tracks inside the shell, with the consequence of turning the retort into a very large can opener.
However, this difficulty has now been largely overcome. Therefore we can expect to see a continued increase in high temperature short time processes, generally aided by rotary processing to minimise product degradation at the container surfaces. The science of in- container mixing is not as well developed as the technology, and research is under way to optimise process conditions to enhance the mixing processes taking place. This might, for example, be achieved by study of the headspace movement in products or simulants of equivalent rheological properties at the intended process temperature which can determine when the headspace is most effectively passing through the container to bring about mixing.
Results seem to indicate that for several traditional canned products heat transfer rates can be greatly improved even in comparison with rotary agitation. The mechanism for this extra efficient agitation is presumably the fact that greater turbulence is introduced into the movement of fluids in the packs compared with rotary motions. One possible drawback of the approach is that the shaking motion may damage tender food components but trials using asparagus have indicated that this is not the case.
For example, it seems that there are forces developing which will drive canned food manufacturers, especially in the catering sector, away from cans toward packs that are more space efficient in disposal, recyclable and less likely to contaminate product on opening.
It seems that the heat processable pouch is becoming the natural alternative to the catering can. For batch retort systems a change to a new pack format may mean a simple, though not cheap, change of racking system.
With batch systems there is an increasing move toward automatic retort loading and unloading which yields long-term benefits in reducing labour costs, as retort operators are commonly supported by a team of loaders. Second, automation of the loading system can be used to control the flow of product through the factory. If implemented correctly this automated control of the loading operation can be one of the mechanisms used to prevent unprocessed product bypassing the heat process, which is one of the major safety risks associated with any such operation.
In some installations this control is enhanced by the use of double doors, one at either end of the process vessel, with a wall built to prevent product getting from one room in the factory to another without going through the retort though this does not necessarily guarantee that a process is applied. However, for the continuous retort systems, especially reel and spiral retorts, changes in packaging format have been more difficult.
Stork have introduced their new Vario hydrostatic retort system which uses a cassette which allows a range of packaging formats to be processed through the same retort. These cassettes are the basic unit which is transported through the retort, within which, depending on the internal construction, any pack can be processed, e.
Most of the modern overpressure retort systems already incorporate energy and water efficiency features but there may be further developments. Retort technology 27 2. Such situations are described as process deviations and may result, for example, from failure in services, e. Historically deviations have been dealt with either during the process from tables prepared from experimental data or from experimental data generated after the problem has occurred.
More recently, computational modelling methods have been used to predict off-line the effect of time temperature deviations based on known product heat transfer characteristics. Commercial programs of this type have existed for several years, e.
It is a logical step that in order to minimise the product lost from deviations and to minimise safety risks, this kind of mathematical model should be used on-line. The application of such heat transfer models to continuous processes is more complex because the deviation will have a different impact upon containers at different points through their residence time. However, this is a likely development. Information on packaging systems can be obtained from packaging suppliers or the Metal Packaging Association.
In Heat Preserved Foods, eds J. Rees and J. Bettison, Blackie and Sons Ltd. Food Technology December Journal of Food Process Engineering 15 Journal of Food Science 6 Liquid and semi-liquid products such as milks, juices and sauces suffered from overprocessing in the traditional low temperature—long time of in-container or batch processing. Caramelised flavours, poor colour retention and a lack of a reproducible product were all problems associated with products processed by batch methods.
Improving quality whilst maintaining product safety was the main aim for those developing continuous processing approaches. The achievement of safe products by thermal processing is based upon the theory behind the destruction of microorganisms.
Products must be heated to a set temperature for a set time in order to achieve a commercially sterile product. For continuous heat processing, also called continuous flow processing, the product is thermally processed before being placed into an appropriate container, on a continuous basis through a heat exchange plant.
Heat exchange apparatus will be used for both the heating and cooling if required phase of the process. In a continuous system the foods under consideration are liquid or semi-liquid products, which may be pumped through a system, heated and cooled whilst continuously flowing down the processing line. A wide range of products are processed by this method, either as the main process to achieve a safe product as in Ultra Heat Treated or Ultra High Temperature UHT processing or as a step within a further process.
Continuous heat processing is not a new technology and several good texts exist which give background to the developments through the years in this area. The three main types of process that are suitable for continuous flow processing are, aseptic systems high and low acid , hot fill systems and pasteurisation processes.
Aseptically packed products are processed at temperatures that will render the product commercially sterile. High acid products such as juices can be processed at pasteurisation temperatures to destroy the microorganisms that can cause the spoilage of the product; these are then rapidly cooled to reduce losses of volatiles within the product and filled into a pre-sterilised pack under sterile conditions. Low acid products will undergo the same principle, however the temperatures employed are much higher to ensure no survival of pathogenic bacteria.
Continuous flow processing systems can also be used in hot fill processes for high acid products that would otherwise lose product quality through slow cooling methods. This method allows for a much quicker throughput than a typical batch process would offer. The final heating method for this type of system is pasteurisation of low acid products that will then be cooled and held under chilled conditions e.
This processing step extends the shelf-life and ensures a safe product. The product must be chilled to maintain its safety and quality throughout the shelf-life. Shelf-lives of up to ten days can be achieved for some products.
There are two main options open to a food manufacturer considering a continuous heat process, to process by indirect method, or by direct method.
Indirect heating involves a heat transfer surface between the product and the heating media. Direct heating occurs where the product and heating media are in direct contact. Figure 3. Another established continuous heating method available is Ohmic heating. This will be discussed in Chapter There are three main types of indirect heating system: plate heat exchangers, tubular heat exchangers and scraped surface heat exchangers.
Plate heat exchangers consist of a series of plates connected on a frame. The product and heating or cooling media flow in alternate channels in thin layers to provide good heat transfer conditions Fig.
The plates are sealed by elastic sealing gaskets cemented into a perforated groove. Generally the plates are of polished stainless steel of 0. The surface of the plates is usually corrugated in order to increase the area available for heat transfer and enhance the turbulence present in the system, resulting in a high thermal efficiency. Thermal regeneration can lower energy costs substantially. The narrow gaps mean that the units are best suited to low viscosity homogenous products.
Attempts to process particulate products e. The design of the plates can vary from supplier to supplier, each having different designs to maximise process efficiency and ensure product safety. Plates can be product specific. The corrugations that are present in plates are usually of a chevron or herringbone design in order to develop a turbulent flow through the plate as it passes through the plate pack so increasing heat transfer.
As the plates are assembled, the herringbone pattern is usually alternated, with the chevrons going up on one plate and down on the next, creating the channels through which the product can flow. The design of the pattern on the plates in most cases allows for support of the whole plate pack, the plates are touching a designated point to ensure that the strength within the system is maintained but also the ease of cleanability is taken into account. The plates may also have larger spaces between, which will allow small particles to be processed in the system.
A further development in this area has been with the double-separation plate, which is designed for highest security, stopping contamination between the heating media and the processed product. As the pairs are mounted together, this forms the channels through which the product flows and these are sealed together by an elastomeric gasket.
Because of this design, if the system does suffer a gasket failure, the leak will be detected externally and action may be taken. This high level of security makes it ideal for high security operations such as in the pharmaceutical industry. The design used on the plates may also take into account the level of fouling that will occur throughout the process.
This is usually the limiting factor in production times and the longer a plant can run for, the less downtime costs there are for the process. The plates should also be designed to be cleaned in place, however, in some cases or during planned maintenance, the plates may have to be dismantled from the frame and cleaned and serviced by hand.
The application of the heat exchanger will determine the type of frame that should be used to hold the plates together. In the food industry for food applications the frame would normally be hygienically designed either being of solid stainless steel or completely clad in stainless steel. In industrial applications a mild steel frame would be sufficient for the heating of cleaning chemicals or media heating. The plates hang on the frame and are held together by a series of compression bolts which are tightened depending on plate size, thickness and number of plates.
In some cases the heat exchangers will be modular, therefore allowing for easy extension of the plate pack, or changing for new applications. The advent of the hanging frame has enabled servicing and inspection, a much easier process than with the early frames where the plates had to be removed one by one. The gaskets that are used to hold the plates together can be either cemented into place, or can be clipped into place. For ease of service, the clip on gaskets ensure that downtime is kept to a minimum whilst still ensuring that the hygienic barrier is maintained.
The clips usually work by having two prongs which sit in the gap between two plates; in this way, in combination with the hanging plates, any changes in gasket can be carried out in situ. Plate heat exchangers were traditionally used for pasteurisation processes and have been adapted to withstand the higher temperatures and pressures required for UHT processes.
The main difficulty with plate heat exchangers was their tendency to foul followed by inefficient cleaning in place. A build-up of such debris in the streams in the system may ultimately lead to the product being understerilised leading to product spoilage or an unsterile product. The manufacturers of such systems are designing the plates in such a way as to make them suitable for cleaning in place.
Producers using these systems should have planned preventative maintenance schemes in place to ensure that there is scheduled servicing and cleaning of the machines before this situation occurs. One of the main advantages of plate heat exchangers is in the regeneration of energy used in the system. Product will pass through three sections in a plate heat exchanger. The first section will be a regeneration section where the incoming product will be heated by the outgoing hot product.
The final section that the product will pass through is the regeneration zone, this time as the outgoing hot product giving up its energy to the incoming product and so reducing the amount of cooling capability required by the system.
Preventative maintenance should take into account plate check for pinholes to ensure that there is no possibility for cross-contamination. To try and reduce the possibility of contamination the regeneration system should be run so that the pressure on the sterilised product side is higher than that on the unsterilised side.
In tubular heat exchangers product is pumped through a tube or multiple tubes, which are fixed inside a larger tube. In the space between the two tubes, heating or cooling media is pumped in counterflow to the product, maximising heat exchange efficiency. The mechanical strength of these tubes allows them to operate at high temperatures and pressures. Turbulence is achieved in the tubes by the velocity of the product and also by the presence of a corrugated surface to improve heat transfer efficiency.
The amount of corrugation can be varied for compatibility between product and plant. A more angled corrugation can introduce turbulence without high velocity in low viscosity fluids such as water, juices and dairy products. Smoother corrugations which have more gradual angles and can impart a twist or turning motion, offer gentler handling of particulate and higher viscosity products.
The more angled corrugations fill with such products and can reduce the efficiency of the exchanger. The simplest design for tubular heat exchangers is the monotube, basically a tube held within a tube.
The product flows through the central tube and is surrounded by the outer tube, which contains the heating or cooling media. This design is the most frequently used system for processing particulate products as there are few problems with the particulate matter blocking the tubes and so causing processing problems and pressure build-up.
As the heating media surround the product, this type of system allows for very gentle heating of particulate products. A more complex design, the concentric tubular heat exchanger is generally a single pass shell and tube exchanger with the product flowing through the gap between two heating or cooling media channels.
The tubes tend to be smooth reducing the pressure drop that can occur when processing viscous products. Continuous heat processing 35 Fig. Concentric tubes have a single channel design where product flows through a tube which is surrounded by a second jacketed tube containing the heating or cooling media. Through the centre of the product tube is a further tube which also has the heating or cooling media flowing through it.
In this way the product is surrounded by the media thus giving two heat transfer surfaces allowing for a more efficient heat transfer.
As there is generally only one tube for the product to flow down this makes the plant easier to clean and to sterilise. The gap for the product flow can be designed depending on the application, giving wider gaps for products containing particulates.
A third design for tubular heat exchangers is the multitube system Fig. This can be anything from two to several parallel tubes, through which the product flows, surrounded by a casing which contains the heating media allowing the heating media to be between and around each tube.
The tubes can be corrugated or smooth depending on the level of turbulence and heat transfer efficiency required. The models tend to be modular in that the heat exchangers can be put in series depending on the level of heat energy required to achieve the processing temperature. A variation on the multitube is the multichannel, consisting of several tubes in tubes allowing the heating media to flow either side of the product channel. This type of set-up allows for a very large heat transfer surface and therefore high thermal efficiency.
This design is based on narrow channels and is ideal for low viscosity products such as fruit juices. There are several advantages for the processor using tubular heat exchangers. Designs are available to produce a wide product range. They are able to produce particulate product up to 12 mm and maintain the particle integrity and quality throughout the process.
One of the main advantages though, is in the very simple designs, which cut down on maintenance costs and downtime. One of the disadvantages with tubular heat exchangers is the tendency to form thermal cracks, due to the changes in temperature that occur in this type of process having hot product on one side of the tube and cold product or media on the other.
To overcome this a floating end design may be used; this allows the internal tube bundle to move slightly within the outer shell, as they are not welded together as in other designs. The floating end configuration also allows for changes in the tube configuration, allowing monotubes to be replaced by multitubes if a multipurpose system is required e.
Tubular heat exchangers can also suffer very high pressure drops in the system due to the long pipe lengths used in the systems and this can lead to practical processing problems and issues with recontamination.
When the product is cooled the viscosity will rise again and cause a large pressure build-up in the system. Tubular heat exchangers can also suffer from fouling and burn on. As the tubes tend to be long, the processor does not have the ability to open and inspect the plant after processing or cleaning so any fouling problems that can occur must be understood and strictly monitored.
The basic design consists of a large tube in a tube similar to the simple monotubes with the heating or cooling media on the outer shell. The central processing tube contains a shaft which is connected to a motor and is supported by bearings at either end. The shaft has blades attached, which are designed to scrape the heating surface of the tube as the motor activates the rotation of the shaft.
This design is ideal for viscous products as the rotation causes turbulence within the heating chamber, so increasing the heat transfer into the product and second, the blades scraping on the heating surface reduce the build-up of fouling that can occur with such products. The shell of the heating tube can be chrome plated nickel due to the high thermal conductivity that it offers , stainless steel, bimetallic or chromed stainless steel, depending on the application for which it is to be used.
The shell is usually of a standard diameter and manufacturers offer a range of central shafts or rotors for a specific set of conditions to optimise processing.
There are chapters on temperature and pressure measurement, validation of heat processes, modelling and simulation of thermal processes, and the measurement and control of changes in a food during thermal processing.
The final part of the book looks at emerging thermal technologies which becoming more widely used in the food industry. There are chapters on radio frequency heating, microwave processing, infrared heating, instant and high-heat infusion, and ohmic heating A final chapter considers how thermal processing may be combined with high pressure processing in producing safe, minimally-processed food products. Thermal technologies in food processing provides food manufacturers and researchers with an authoritative review of thermal processing and food quality.
Part 1 Conventional technologies: Retort technology; Continuous heat processing — indirect and direct. Part 2 Measurement and control: Pressure measurement; Temperature measurement; Validation of heat processes: Temperature distribution testing, heat penetration testing, microbiological spore methods, biochemical time and temperature indicators; Modelling and simulation of thermal processes; Modelling particular thermal technologies: Continuous heating and cooling processes, ohmic and microwave heating; Thermal processing and food quality: The Maillard reaction.
Part 3 New thermal technologies: Radio frequency; Microwave processing; Infrared heating; Instant and high-heat infusion; Ohmic heating; Combined high pressure thermal treatment of foods; Use of integrated kinetic information in process design and optimization. Decareau, R. Pasteurization and Sterilization. Microwaves in the Food Processing Industry. Orlando: Academic. Fellows, P. Food Processing Technology: Principles and Practice , 2nd edn. Cambridge: Woodhead Publishing. Itoh, K.
Drying of vegetable by far infrared radiation. Shokuhin Kikai Souchi , 23 , 45— Ishi, T. Possibility of far infrared radiation to food processing. Food Processing Shokuhin To Kaihatu , 20 , 28— Kimura, Y. The Food Industry Shokuhin Kogyo , 42 , 32— Kinn, T. Basic theory and limitations of high frequency heating equipment.
Kino, T. Application of far-infrared heating on roasting of coffee beans. The Food Industry Shokuhin Kogyo , 42 , 29— Kiyohira, K. Rice cookie baking machines and chikuwa baking machines. The Food Industry Shokuhin Kogyo , 42 , 52— Kutsunai, T. Heating equipment for oyster by far infrared radiation. Japan Food Science , 27 , 53— Leadley, C.
Novel commercial preservation methods. In Tucker, G. Oxford: Blackwell Publishing, pp. Mans, J. Electrifying progress in aseptic technology - food processing - food plant ideas. Metaxas, A. Industrial Microwave Heating. London: Polu Pelegrinus Ltd. Moyer, J. The blanching of vegetables by electronics. Nakamura, A. Far-infrared heating and heating process in bread baking.
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