Tahun 2000. Part IV processing by the removal of heat

19.1.3 Cook–chill systems

Individual foods (for example sliced roast meats) or complete meals are produced by cook–chill or cook–pasteurise–chill processes (Byrne, 1986). An example is sous-vide products, which is the term commonly used to refer to foods that are vacuum packed prior to pasteurisation (although it strictly refers only to vacuum packing). These products, which include complete meals or components such as sauces, were developed for institutional catering to replace warm-holding,¹ which reduces losses in nutritional and eating quality and is less expensive. Their production is described in detail in Ghazala and Trenholm (1998) and Creed and Reeve (1998). In retail stores, sales of an increasingly wide range of cooked–chilled ready meals have rapidly expanded owing to their convenience, high quality and healthy image.

The range of chilled foods can be characterised by the class of microbial risk that they pose to consumers as follows:
Class 1 foods containing raw or uncooked ingredients, such as salad or cheese as ready-to-eat (RTE) foods (also includes chill-stable raw foods, such as meat, fish, etc.)

Class 2 products made from a mixture of cooked and low risk raw ingredients

Class 3 cooked products that are then packaged

Class 4 products that are cooked after packaging, including ready-to-eat-products- for-extended-durability (REPFEDs) having a shelf life of 40+ days (the acronym is also used to mean refrigerated-pasteurised-foods-for-extended- durability).

In the above classification, ‘cooking’ refers to a heat process that results in a minimum 6D reduction in target pathogens (see Chapters 1 and 12 for an explanation of D-values). Some Class 1 products require cooking by the consumer, whereas other cooked–chilled products may be ready to eat or eaten after a short period of re-heating. Gorris (1994) and Betts (1998) describe other methods of mild processing to improve the safety of ready-to-eat foods.

The manufacturer is only able to control the safety of these products by minimising the levels of pathogens on the incoming ingredients and by ensuring that processing and storage procedures do not introduce pathogens or allow their numbers to increase. Therefore, in addition to normal hygienic manufacturing areas, the products in Classes 1,

2 and 4 require a special ‘hygienic area’, designed to be easily cleaned to prevent
1. Where food is kept hot for long periods before consumption.
bacteria, such as Listeria spp. becoming established in it. Products in Classes 2 and 3 also require an additional ‘high-care area’, which is physically separated from other areas and is carefully designed to isolate cooked foods during preparation, assembly of meals, chilling and packaging. Such areas have specified hygiene requirements including:
 positive pressure ventilation with micro-filtered air supplied at the correct temperature and humidity

 entry and exit of staff only through changing rooms

 ‘no-touch’ washing facilities

 construction standards and materials for easy cleaning

 only fully processed foods and packaging materials admitted through hatches or air- locks

 special hygiene training for operators and fully protective clothing (including boots, hairnets, coats, etc.)

 operational procedures to limit the risk of contamination

 production stopped for cleaning and disinfection every 2 hours.

Detailed descriptions of the design and operation of facilities for cooked–chilled foods are given by Brown and Gould (1992), Rose (2000) and Anon. (1998), and Nicolai et al. (1994) describe computer aided design of cook–chill foods. Microbiological considera- tions when producing REPFEDs are described by Gorris and Peck (1998).

After preparation, cooked–chilled foods are portioned and chilled within 30 min of cooking. Chilling to 3ºC should be completed within 90 min and the food should be stored at

0–3ºC. In the cook–pasteurise–chill system, hot food is filled into a flexible container, a partial vacuum is formed to remove oxygen and the pack is heat sealed. It is then pasteurised to a minimum temperature of 80ºC for 10 min at the thermal centre, followed by immediate cooling to 3ºC. These foods have a shelf life of 2–3 weeks (Hill, 1987).


19.2 Equipment
Chilling equipment is classified by the method used to remove heat, into:
 mechanical refrigerators

 cryogenic systems.

Batch or continuous operation is possible with both types of equipment, but all should lower the temperature of the product as quickly as possible through the critical warm zone (50–10ºC) where maximum growth of micro-organisms occurs.

19.2.1 Mechanical refrigerators

Mechanical refrigerators have four basic elements: an evaporator, a compressor, a condenser and an expansion valve (Fig. 19.1). Components of refrigerators are frequently constructed from copper as the low thermal conductivity (Chapter 1, Table 1.5) allows high rates of heat transfer and high thermal efficiencies.

A refrigerant (Table 19.5) circulates between the four elements of the refrigerator, changing state from liquid to gas, and back to liquid as follows:
 In the evaporator the liquid refrigerant evaporates under reduced pressure, and in doing so absorbs latent heat of vaporisation and cools the freezing medium. This is the


Fig. 19.1 Mechanical (compression–expansion) refrigerator.

(After Patchen (1971).)

most important part of the refrigerator; the remaining equipment is used to recycle the refrigerant.

 Refrigerant vapour passes from the evaporator to the compressor where the pressure is

increased.

 The vapour then passes to the condenser where the high pressure is maintained and the vapour is condensed.

 The liquid passes through the expansion valve where the pressure is reduced to restart the refrigeration cycle.
The important properties of refrigerants are as follows:
 a low boiling point and high latent heat of vaporisation

 a dense vapour to reduce the size of the compressor

 low toxicity and non-flammable

 low miscibility with oil in the compressor

 low cost.
Ammonia has excellent heat transfer properties and is not miscible with oil, but it is toxic and flammable, and causes corrosion of copper pipes. Carbon dioxide is non- flammable and non-toxic,² making it safer for use for example on refrigerated ships, but it requires considerably higher operating pressures compared to ammonia. Halogen refrigerants (chlorofluoro-carbons or CFCs) are all non-toxic and non-flammable and have good heat transfer properties and lower costs than other refrigerants. However, their interaction with ozone in the earth’s atmosphere, and consequent contribution to global warming as ‘greenhouse gases’, has resulted in an international ban on their use as refrigerants under the Montreal Protocol. Partially halogenated CFCs (or HCFCs) are less environmentally harmful and existing HCFCs are being temporarily substituted for CFCs, but these too are to be phased out before the first decades of the new century. Newer, ozone-friendly HCFCs are being developed and are likely to become important refrigerants. These developments are described in more detail by Heap (1997). The

2. Note: CO₂ causes asphyxia in concentrations above 0.5% by volume (Section 19.2.2).

Table 19.5 Properties of refrigerants
Refrigerant Boiling Latent Toxicity Flamm- Vapour Oil

_{point heat ability density solubility}

Number Formula (ºC) at (kJ kg ¹) (kg m ³)

100 kPa
^{11 CCl}₃^{F 23.8 194.2 Low Low 1.31 Complete}

^{12 CCl}₂^F₂ <sup>29.8 163.54 Low Low 10.97 Complete

21 CHCl</sup>₂<sup>F 44.5 254.2 Low Low 1.76 Complete

22 CHClF</sup>₂ <sup>40.8 220.94 Low Low 12.81 Partial

717 NH</sup>₃ <sup>33.3 1328.48 High High 1.965 1%

744 CO</sup>₂ ^{78.5 352 Low Low 60.23 1%}

(sublimes)

main refrigerants that are now used are Freon-22 and ammonia, with the possibility of future use of propane. However, the latter two in particular are more expensive and could cause localised hazards, thus requiring additional safety precautions and training for equipment users (Heap, 2000).

The chilling medium in mechanically cooled chillers may be air, water or metal surfaces. Air chillers (for example blast chillers) use forced convection to circulate air at around 4ºC at high speed (4 m s ¹), and thus reduce the thickness of boundary films (Chapter 1) to increase the rate of heat transfer. Air-blast chillers are also used in refrigerated vehicles, but food should be adequately chilled when loaded onto the vehicle, as the refrigeration plant is only designed to hold food at the required temperature and cannot provide additional cooling of incompletely chilled food. Eutectic plate systems are another type of cooling that is used in refrigerated vehicles, especially for local distribution. Salt solutions (e.g. potassium chloride, sodium chloride or ammonium chloride) are frozen to their eutectic temperature³ (from 3 to 21ºC) and air is circulated across the plates, to absorb heat from the vehicle trailer. The plates are regenerated by re-freezing in an external freezer.

Retail chill cabinets use chilled air which circulates by natural convection. The cost of chill storage is high and to reduce costs, large stores may have a centralised plant to circulate refrigerant to all cabinets. The heat generated by the condenser (Fig. 19.1) can also be used for in-store heating. Computer control of multiple cabinets detects excessive rises in temperature and warns of any requirement for emergency repairs or planned maintenance (Cambell-Platt, 1987). Other energy-saving devices include night blinds or glass doors on the front of cabinets to trap cold air. Details of the design and operation of refrigerated retail display cabinets, chilled distribution vehicles and cold stores are given by Heap (2000) (also Section 19.3).
Other methods of cooling

Foods with a large surface area (for example lettuce) are washed and vacuum cooled. The food is placed in a large vacuum chamber and the pressure is reduced to approximately

0.5 kPa. Cooling takes place as moisture evaporates from the surface (a reduction of approximately 5ºC for each reduction of 1% in moisture content). Direct immersion in chilled water (hydrocooling) is used to remove field heat from fruit and vegetables, and cheese is often cooled by direct immersion in refrigerated brine. Recirculated chilled water is also used in plate heat exchangers (Chapter 11, Fig. 11.4) to cool liquid foods
3. Where the water and salt form a single phase.
after pasteurisation. Liquid and semi-solid foods (for example butter and margarine (Chapter 4)) are cooled by contact with refrigerated, or water-chilled metal surfaces in scraped-surface heat exchangers (also Chapters 11, 12 and 21).

19.2.2 Cryogenic chilling

A cryogen is a refrigerant that changes phase by absorbing latent heat to cool the food. Cryogenic chillers use solid carbon dioxide, liquid carbon dioxide or liquid nitrogen. Solid carbon dioxide removes latent heat of sublimation (352 kJ kg ¹ at 78ºC), and liquid cryogens remove latent heat of vaporisation (358 kJ kg ¹ at 196ºC for liquid nitrogen; liquid carbon dioxide has a similar latent heat to the solid). The gas also absorbs sensible heat as it warms from 78ºC (CO₂) or from 196ºC (liquid nitrogen) to give a

total refrigerant effect of 565 kJ kg ¹ and 690 kJ kg ¹ respectively.

The advantages of carbon dioxide include:

 a higher boiling and sublimation point than nitrogen, and therefore a less severe effect on the food

 most of enthalpy (heat capacity) arises from the conversion of solid or liquid to gas.

Only 13% of the enthalpy from liquid carbon dioxide and 15% from the solid is contained in the gas itself. This compares with 52% in nitrogen gas (that is, approximately half of the refrigerant effect of liquid nitrogen arises from sensible heat absorbed by the gas). Carbon dioxide does not therefore require gas handling equipment to extract most of the heat capacity, whereas liquid nitrogen does. The main limitation of carbon dioxide, and to a lesser extent nitrogen, is its ability to cause asphyxia. There is therefore a maximum safe limit for operators of 0.5% CO₂ by volume and excess carbon dioxide is removed from the processing area by an exhaust system to ensure operator safety, which incurs additional setup costs. Other hazards associated with liquefied gases include cold burns, frostbite and hypothermia after exposure to intense cold.

Solid carbon dioxide can be used in the form of ‘dry-ice’ pellets, or liquid carbon dioxide can be injected into air to produce fine particles of solid carbon dioxide ‘snow’, which rapidly sublime to gas. Both types are deposited onto, or mixed with, food in combo bins, trays, cartons or on conveyors. A small excess of snow or pellets continues the cooling during transportation or storage prior to further processing. If products are despatched immediately in insulated containers or vehicles, this type of chilling is able to replace on- site cold stores and thus saves space and labour costs. Snow is replacing dry-ice pellets because it is cheaper and does not have the problems of handling, storage and operator safety associated with dry ice. For example, in older meat processing operations, dry-ice pellets were layered with minced meat as it was filled into containers. However, lack of uniformity in distribution of pellets resulted in some meat becoming frozen and some remaining above 5ºC, which permitted bacterial growth and resulted in variable product temperatures for subsequent processing. More recently the use of snow horns to distribute a fine layer of snow over minced meat as it is loaded into combo bins has eliminated these problems and resulted in rapid uniform cooling to 3–4ºC. A recent advance in the use of carbon dioxide snow for chilled and frozen distribution of foods is described in Chapter 21.

Liquid nitrogen is used in both freezing (Chapter 21) and chilling operations. For batch chilling, typically 90–200 kg of food is loaded into an insulated stainless steel cabinet, containing centrifugal fans and a liquid nitrogen injector. The liquid nitrogen vaporises immediately and the fans distribute the cold gas around the cabinet to achieve a uniform reduction in product temperature. The chiller has a number of pre-programmed
time/temperature cycles which are microprocessor controlled. A food probe monitors the temperature of the product and the control system changes the temperature inside the cabinet as the food cools, thus allowing the same pre-programmed cycle to be used irrespective of the temperature of the incoming food. As with other types of batch equipment, it is highly flexible in operation and it is therefore suitable for low production volumes or where a large number of speciality products are produced.

For continuous chilling, food is passed on a variable speed conveyor to an inclined, insulated, cylindrical barrel having a diameter of 80–120 cm and length 4–10 m depending on the capacity. The barrel rotates slowly and internal flights lift the food and tumble it through the cold nitrogen gas. The temperature and gas flow rate are controlled by a microprocessor and the tumbling action prevents food pieces sticking together, to produce a free-flowing product. It is used to chill diced meat or vegetables at up to

3t h ¹. Controlled temperature liquid nitrogen tumblers are used to improve the texture

and binding capacity of mechanically formed meat products. The gentle tumbling action in a partial vacuum, cooled by nitrogen gas to 2ºC, solubilises proteins in poultry meat, which increases their binding capacity and water holding capacity, thus improving later forming and coating operations.

An alternative design is a screw conveyor inside a 2.5 m long stainless steel housing, fitted with liquid carbon dioxide injection nozzles. Foods such as minced beef, sauce mixes, mashed potato and diced vegetables are chilled rapidly as they are conveyed through the chiller at up to 1 t h ¹. It is used to firm foods before portioning or forming operations or to remove heat from previous processing stages.

Other applications of cryogenic cooling include sausage manufacture, where carbon dioxide snow removes the heat generated during size reduction and mixing (Chapter 4) and cryogenic grinding where the cryogen reduces dust levels, prevents dust explosions and improves the throughput of mills. In spice milling, cryogens also prevent the loss of aromatic compounds. In the production of multi-layer chilled foods (for example trifles and other desserts) the first layer of product is filled and the surface is hardened with carbon dioxide. The next layer can then be added immediately, without waiting for the first layer to set, and thus permit continuous and more rapid processing. Other applications include cooling and case-hardening of hot bakery products and chilling flour to obtain accurate and consistent flour temperatures for dough preparation.

19.3 Chill storage
Once a product has been chilled, the temperature must be maintained by refrigerated storage. Chill stores are normally cooled by circulation of cold air produced by mechanical refrigeration units, and foods may be stored on pallets, racks, or in the case of carcass meats, hung from hooks. Transport of foods into and out of stores may be done manually using pallet trucks, by forklift truck or by computer-controlled robotic trucks (Chapters 2 and 26). Materials that are used for the construction of refrigerated storerooms are described by Brennan et al. (1990).

19.3.1 Control of storage conditions

The importance of maintaining temperatures below 5ºC to meet safety, quality and legal requirements for high-risk products is described in Section 19.1. Fresh products may also require control of the relative humidity in a storeroom, and in some cases control over the

composition of the storage atmosphere (Chapter 20). In all stores it is important to maintain an adequate circulation of air using fans, to control the temperature, relative humidity or atmospheric composition. Foods are therefore stacked in ways that enable air to circulate freely around all sides. This is particularly important for respiring foods, to remove heat generated by respiration (Section 19.1.1) or for foods, such as cheese, in which flavour development takes place during storage. Adequate air circulation is also important when high storage humidities are used for fresh fruits and vegetables (Table

19.2) as there is an increased risk of spoilage by mould growth if ‘deadspots’ permit localised increases in humidity. In some situations, a lower relative humidity may be used, with some product wilting accepted as a compromise for reduced microbial spoilage.

Temperature monitoring

Temperature monitoring is an integral part of quality management and product safety management throughout the production and distribution chain. Improvements to micro- electronics over the last ten years has enabled the development of monitoring devices that can both store large amounts of data and integrate this into computerised management systems (Chapter 2). Woolfe (2000) lists the specifications of commonly used data loggers. These are connected to temperature sensors which measure either air temperatures or product temperatures to give a representative picture of the way in which the refrigeration system is functioning.

There are three main types of sensor that are used commercially: thermocouples, platinum resistance thermometers and semiconductor (thermistors). Thermocouples are a pair of dissimilar metals joined together at one end. The most widely used are Type K (nickel-chromium and nickel-aluminium), or Type T (copper and copper-nickel). The advantages over other sensors are lower cost, rapid response time and very wide range of temperature measurement ( 184ºC–1600ºC). Thermistors change resistance with temperature and have a higher accuracy than thermocouples, but they have a much narrower range ( 40ºC–140ºC). Platinum resistance thermometers are accurate and have a temperature range from 270ºC–850ºC, but their response time is slower and they are more expensive than other sensors. Sensors are usually connected to either a chart recorder or an electronic digital display, which may also be able to store data and sound an alarm if the temperature exceeds a pre-set limit. Further details of sensors are given in Chapter 2.

Monitoring air temperatures is more straightforward than product temperature monitoring and does not involve damage to the product or package. It is widely used to monitor chill stores, refrigerated vehicles and display cabinets, and Woolfe (2000) describes in detail the positioning of temperature sensors in these types of equipment. However, it is necessary to establish the relationship between air temperature and product temperature in a particular installation. Air is continuously recirculated through the refrigeration unit and storeroom. Cold air is warmed by the product, by lights in a store, by vehicles or by doors opening or operators entering. The temperature of the returning air is therefore likely to be the same as the product temperature or slightly higher. By comparing this to the temperature of the air leaving the evaporator in the refrigeration unit to find the temperature differential, it is possible to measure the performance of the refrigeration system and its effectiveness in keeping the food cold. To relate air temperature to product temperature it is necessary to conduct a ‘load test’, which involves examining the differential in air temperatures over a length of time and comparing it with the product temperature under normal working conditions.

Where a store, cabinet or vehicle is not opened for long periods, the only changes in temperature come from defrost cycles and intermittent door opening, and the relationship between product and air temperature is relatively simple. However, the operation of open retail display cabinets is more sensitive to variations in room temperature or humidity, the actions of customers and staff in handling foods, and lighting to display products. The temperature distribution in the cabinet can therefore change and load testing becomes more difficult. In such situations there is likely to be substantial variations in air temperature, but the mass of the food remains at a more constant temperature, and air temperature measurement has little meaning. To overcome this problem the food temperature can be measured or the air temperature sensor can be electronically ‘damped’ to respond more slowly and eliminate short-term fluctuations.

In addition to temperature sensors, the temperature of chilled foods can be monitored by

temperature- or time-temperature indicators, which use physico-chemical changes to display
 the current temperature

 crossing of a threshold temperature

 integration of the temperature and the time that a food has been exposed to a particular temperature.
These devices are based on either melting point temperature, enzyme reaction, polymerisation, electrochemical corrosion or liquid crystals (Woolfe, 2000). They are described in more detail in their application to frozen foods (Chapter 21), and are now also finding greater use in the chill chain (Van Loey et al., 1998).