Advances in the cold chain to improve food safety, food quality and the food supply chain
This chapter begins by discussing the importance of maintaining the cold chain for the microbiological safety and quality of foods. The specific stages of the cold chain (chilling freezing storage transport and retail display) are then discussed in detail. Environmental issues and advances in energy reduction are also discussed. Finally a section is included on the practical and theoretical factors that need to be considered by food producers/suppliers when specifying refrigeration systems for the food supply chain
Refrigeration stops or reduces the rate at which changes occur in food. These changes can be microbiological (i.e. growth of microorganisms), physiological (e.g. ripening, senescence and respiration), biochemical (e.g. browning reactions, lipid oxidation and pigment degradation) and/or physical (such as moisture loss). An efficient and effective cold chain is designed to provide the best conditions for slowing, or preventing, these changes for as long as is practical. Effective refrigeration produces safe food with a long high-quality shelf life.
To provide safe food products of high organoleptic quality, attention must be paid to every aspect of the cold chain from initial chilling or freezing of the raw ingredients, through storage and transport, to retail display. The cold chain consists of two distinct types of operation. In processes such as primary and secondary chilling or freezing, the aim is to change the average temperature of the food. In others, such as chilled or frozen storage, transport and retail display, the prime aim is to maintain the temperature of the food. Removing the required amount of heat from a food is a difficult, time and energy consuming operation, but critical to the operation of the cold chain. As a food moves along the cold chain it becomes increasingly difficult to control and maintain its temperature. This is because the temperatures of bulk packs of refrigerated product in large storerooms are far less sensitive to small heat inputs than single consumer packs in open display cases or in a domestic refrigerator/freezer. Failure to understand the needs of each process results in excessive weight loss, higher energy use, reduced shelf life or a deterioration in product quality.
Temperature is one of the major factors affecting microbiological growth. Microbiological growth is described in terms of the lag phase and the generation time. When a microorganism is introduced to a particular environment there is a time (the lag phase) in which no increase in numbers is apparent, followed by a period when growth occurs. The generation time is a measure of the rate of growth in the latter stage. Microorganisms have an optimum growth temperature at which a particular strain grows most rapidly, that is the lag phase and generation time are both at a minimum. They also have a maximum growth temperature above which growth no longer occurs. Above this temperature, one or more of the enzymes essential for growth are inactivated and the cell is considered to be heat injured. However, in general, unless the temperature is raised to a point substantially above the maximum growth temperature then the injury is not lethal and growth will recommence as the temperature is reduced. Attaining temperatures substantially above the maximum growth temperature are therefore critical during cooking and reheating operations.
Of most concern during storage, distribution and retail display of food is a third temperature, the minimum growth temperature for a microorganism. As the temperature of an organism is reduced below that for optimum growth, the lag phase and generation time both increase. The minimum growth temperature can be considered to be the highest temperature at which either of the growth criteria, the lag phase and the generation time, becomes infinitely long. The minimum growth temperature is not only a function of the particular organism but also the type of food or growth media that is used for the incubation. Although some pathogens can grow at 0 °C, or even slightly lower (Table 18.1), from a practical point of view the risks to food safety are considerably reduced if food is maintained below 5 °C.
|Minimum temperature (°C)|
|Pathogenic Escherichia coli strains||7–6|
|Clostridium botulinum non-proteolytic||3|
|Yersinia enterocolitica||− 2|
There are little data on the impact of the initial freezing process on the safety of foods. However, it is difficult to envisage any sensible freezing process that would result in most foods being held for substantial periods at temperatures that would support a dangerous growth of pathogens. Providing the food does not rise above − 12 °C during frozen storage and display, there are no issues of food safety with frozen storage and display.
Food may also become microbiologically unacceptable as a result of the growth of spoilage microorganisms. Their growth can produce unacceptable changes in the sensory quality of many foods and their rate of growth is also very temperature dependent. The development of off odours is usually the first sign of putrefaction and in meat it occurs when bacterial levels reach approximately 107 cm− 2 of surface area (Ingram, 1972). When bacterial levels have increased a further ten-fold slime begins to appear on the surface and meat received in this condition is usually condemned out of hand. At 0 °C beef with average initial contamination levels can be kept for at least 15 days before any off odours can be detected. Every 5 °C rise in the storage temperature above 0 °C will approximately halve the storage time that can be achieved.
Microbial safety and spoilage are not the only aspects of food quality that are temperature dependent. The rate of loss of vitamins from fruit and vegetables during storage also depends upon the storage temperature. It is of interest to note that it is not always a case of the lower the better, especially for citrus and tropical fruits. The optimum temperature for oranges is approximately 12 °C with the rate of vitamin loss increasing at temperatures warmer or colder than this value.
Some foods exhibit particular quality advantages as a result of rapid cooling. In meat the pH starts to fall immediately after slaughter and protein denaturation begins. The result of this denaturation is a pink proteinaceous fluid, commonly called ‘drip’, often seen in prepackaged meat joints. The rate of denaturation is directly related to temperature and it therefore follows that the faster the chilling rate the less the drip. Investigations using pork and beef muscles have shown that rapid rates of chilling can halve the amount of drip loss (Taylor, 1972). Fish passing through rigor mortis above 17 °C are to a great extent unusable because the fillets shrink and become tough (Morrison, 1993). A relatively short delay of an hour or two before chilling can demonstrably reduce shelf life.
However, chilling has serious effects on the texture of meat if it is carried out rapidly when the meat is still in the pre-rigor condition, that is, before the meat pH has fallen below about 6.2 (Bendall, 1972). A phenomenon known as cold shortening occurs, which results in the production of very tough meat after cooking. As ‘rules of thumb’ cooling to temperatures not below 10 °C in 10 hours for beef and lamb and in 5 hours for pork can avoid cold shortening (James and James, 2002). Poultry meat is generally less prone to cold shortening. However, electrical stimulation can be utilised to enable more rapid cooling to be carried out without the occurrence of cold shortening.
The rate of sugar loss (sweetness) in freshly harvested sweet corn is very temperature dependent. After 20 hours at 30 °C almost 60% of the sweetness is lost compared with 16% at 10 °C and less than 4% at 0 °C. Prompt cooling is clearly required if this vegetable is to retain its desirable sweetness. Similarly, the ripening of fruit can be controlled by rapid cooling, the rate of ripening declining as temperature is reduced and ceasing below about 4 °C (Honikel, 1986).
For several fruits and vegetables, exposure to temperatures below a critical limit, but above the initial freezing temperature, may result in chilling injury. Typical symptoms of chilling injury are internal or external browning, superficial spots, failure to ripen, development of off flavours, and so on. Fruits and vegetables from the tropical and subtropical zones are primarily susceptible to chilling injury, however several Mediterranean products are also susceptible (IIR, 2000). The extent of damage depends on the temperature, duration of exposure and the sensitivity of the fruit or vegetable. Some commodities have high sensitivity, while others have moderate or low sensitivity. For each commodity, the critical temperature depends on the species and/or variety. In some cases, unripe fruits are more sensitive than ripe fruits.
The formation of ice crystals during freezing and frozen storage causes physical changes to the structure of foods. In most cases, these changes are perceived as reducing the quality of the thawed material. In extreme cases, such as cucumbers, freezing completely destroys the structure of the food. In meat and fish, the main result is increased drip on cutting. There is a general view that fast freezing offers some quality advantage, with ‘quick frozen’ appearing on many frozen foods in the expectation that consumers will pay more for a ‘quick frozen’ product. However, there are little data in the scientific literature to prove that, in general, the method of freezing, or the rate of freezing, has any substantial influence on the final eating quality of many frozen foods, with the possible exception of fruits, egg products, frozen deserts and products containing flour thickened sauces.
A final, but important, quality and economic advantage of temperature control is a reduction in weight loss, which results in a higher yield of saleable material. Meat, for example, has a high water content and the rate of evaporation depends on the vapour pressure at the surface. Vapour pressure increases with temperature and thus any reduction in the surface temperature will reduce the rate of evaporation. The use of very rapid chilling systems for pork carcasses has been shown to reduce weight loss by at least 1% when compared with conventional systems (James et al., 1983).
During chilling and freezing, heat can only be removed by four basic mechanisms: radiation, conduction, convection or evaporation. To achieve substantial rates of heat loss by radiation, large temperature differences are required between the surface of the product and that of the enclosure. This occurs in the initial stages of chilling or freezing of cooked or warm products. Physical contact between the product and the source of refrigeration is required in order to extract heat by conduction. Plate conduction coolers are used for quick cooling of some packaged products and highly perishable products such as fish blocks. For the majority of foods, the heat lost through evaporation of water from the surface is a minor component of the total heat loss, although it is the major component in vacuum cooling. Evaporation from the surface of a food reduces yield and is not desirable in many refrigeration operations, but can be useful in the initial cooling of unpackaged cooked food products. Convection is by far the most important heat transfer mechanism employed in the majority of food refrigeration systems.
For the majority of chilled and frozen foods, air systems are used, primarily because of their flexibility and ease of use. However, other systems can offer much faster and more controlled chilling or freezing.
From a hygiene/HACCP based approach, prepacking the food prior to chilling or freezing will lower the risk of contamination/cross-contamination during the chilling process. However, in most cases it will significantly reduce the rate of cooling and this may allow the growth of any microorganisms present. Provided that the cooling media (air, water, etc) and refrigeration equipment used are kept sufficiently clean, no one cooling method can be said to be intrinsically more hygienic than any other. For unwrapped food, a rapid cooling system allows less time for any contamination/cross-contamination to occur than slower cooling systems.
It is not unusual for food products (or ingredients found in food products) to be chilled or frozen a number of times before they reach the consumer. For example, during industrial processing, frozen raw material is often thawed or tempered before being turned into meat-based products, for example, pies, convenience meals, burgers, or consumer portions, fillets, steaks, and so on. These consumer-sized portions are often refrozen before storage, distribution and sale.
Air systems range from the most basic, in which a fan draws air through a refrigerated coil and blows the cooled air around an insulated room, to purpose-built conveyerised blast chilling tunnels or spirals. Relatively low rates of heat transfer are attained from product surfaces in air-cooled systems. The big advantages of air systems are their cost and versatility, especially when there is a requirement to cool a variety of irregularly shaped products.
One of the principal disadvantages of air cooling systems is their tendency to dehydrate unwrapped products. A way around this problem is to saturate the air with water. Wet air cooling systems recirculate air over ice cold water so that air leaving the cooler is cold (0–1 °C) and virtually saturated with water vapour (100% relative humidity, RH). An ice-bank chiller uses a refrigeration plant with an evaporator (plate or coil) immersed in a tank of water that chills the water to 0 °C. Using off-peak electricity during times of low load and overnight, a store of ice is built up on the evaporator, which subsequently melts to maintain temperatures during times of high load.
Contact refrigeration methods are based on heat transfer by contact between products and metal surfaces, which in turn are cooled by either primary or secondary refrigerants. Contact cooling offers several advantages over air cooling, such as much better heat transfer and significant energy savings. Contact cooling systems include plate coolers, jacketed heat exchangers, belt coolers and falling film systems.
Immersion/spray systems involve dipping product into a cold liquid, or spraying a cold liquid onto the food. This produces high rates of heat transfer owing to the intimate contact between product and cooling medium. Both offer several inherent advantages over air cooling in terms of reduced dehydration and coil frosting problems (Robertson et al., 1976). Clearly if the food is unwrapped, the liquid has to be ‘food safe’. Cooling using ice or cryogenic substances are essentially immersion/spray processes. The freezing point of the cooling medium used dictates its use for chilling or freezing. Obviously any immersion/spray freezing process must employ a medium at a temperature substantially below 0 °C. This necessitates the use of non-toxic salt, sugar or alcohol solutions in water, or the use of cryogens or other refrigerants.
Chilling with crushed ice or an ice/water mixture is simple, effective and commonly used for fish cooling. Cooling is more attributable to the contact between the produce and the cold melt water percolating through it (i.e. hydrocooling) than with the ice itself. The individual fish are packed in boxes between layers of crushed ice, which extract heat from the fish and consequently melt. Ice has the advantage of being able to deliver a large amount of refrigeration in a short time as well as maintaining a very constant temperature, 0 °C to − 0.5 °C where sea water is present.
Direct spraying of liquid nitrogen onto a food product whilst it is conveyed through an insulated tunnel is one of the most commonly used methods of applying cryogens. The method of cooling is essentially similar to water-based evaporative cooling, cooling being brought about by boiling off the refrigerant, the essential difference being the temperature required for boiling. As well as using the latent heat absorbed by the boiling liquid, sensible heat is absorbed by the resulting cold gas. Owing to very low operating temperatures and high surface heat transfer coefficients between product and medium, cooling rates of cryogenic systems are often substantially higher than other refrigeration systems.
Food products that have a large surface area to volume ratio and an ability to release internal water readily are amenable to vacuum cooling. The products are placed in a vacuum chamber (typically operating at between 530 to 670 N m− 2) and the resultant evaporative cooling removes heat from the food. Evaporative cooling is quite significant, the amount of heat released through the evaporation of 1 g of water is equivalent to that released in cooling 548 g of water by 1 °C. Suitable products, such as lettuce, can be vacuum cooled in less than 1 hour. In general terms, a 5 °C reduction in product temperature is achieved for every 1% of water that is evaporated. Since vacuum cooling requires the removal of water from the product, pre-wetting is commonly applied to prevent the removal of water from the tissue of the product.
High pressure freezing and in particular ‘pressure shift’ freezing is attracting considerable scientific interest (LeBail et al., 2002). The food is cooled under high pressure to sub-zero temperatures but does not undergo a phase change and freeze until the pressure is released. Rapid nucleation results in small even ice crystals. However, studies on pork and beef have failed to show any real commercial quality advantages.
There are clear differences between the environmental conditions required for cooling, which is a heat removal/temperature reduction process and those required for storage, where the aim is to maintain a set product temperature. Three factors during storage, the storage temperature, the degree of fluctuation in the storage temperature and the type of wrapping/packaging in which the food is stored, are commonly believed to have the main influence on storage life. The storage life of most chilled foods is limited by the growth of spoilage microorganisms. The storage life of many frozen foods is limited by quality changes, primarily rancidity development in the fat of meat, for instance.
Over a million refrigerated road vehicles, 400,000 refrigerated containers and many thousands of other forms of refrigerated transport systems are used to distribute refrigerated foods throughout the world (Gac, 2002). All these transportation systems are expected to maintain the temperature of the food within close limits to ensure its optimum safety and high quality shelf life. Developments in temperature controlled transportation systems for chilled products have led to the rapid expansion of the chilled food market.
It is particularly important that the food is at the correct temperature before loading since the refrigeration systems used in most transport containers are not designed to extract heat from the load but to maintain the temperature of the load. Irrespective of the type of refrigeration equipment used, the product will not be maintained at its desired temperature during transportation unless it is surrounded by air or surfaces at or below the maximum transportation temperature. This is usually achieved by a system that circulates moving air, either forced or by gravity, around the load. Inadequate air distribution is probably the principal cause of product deterioration and loss of shelf life during transport. If products have been cooled to the correct temperature before loading and do not generate heat then they only have to be isolated from external heat ingress. Surrounding them with a blanket of cooled air achieves this purpose. Care has to be taken during loading to stop any product touching the inner surfaces of the vehicle because this would allow heat ingress by conduction during transport. In the large containers used for long distance transportation food temperatures can be kept within ± 0.5 °C of the set point. With this degree of temperature control, transportation times of 8 to 14 weeks (for vacuum packed meats stored at − 1.5 °C) can be carried out and the food retains a sufficient chilled storage life for retail display.
Products such as fruits and vegetables that produce heat by respiration, or products that have to be cooled during transit, also require circulation of air through the product. This can be achieved by directing the supply air through ducts to channels at floor level or in the floor itself. In general it is not advisable to rely on product cooling during transportation.
Recent developments in temperature control, packaging and controlled atmospheres have substantially increased the range of foods that can be transported around the world in a chilled condition. Control of the oxygen and carbon dioxide levels in shipboard containers has allowed fruit and vegetables, such as apples, pears, avocados, melons, mangoes, nectarines, blueberries and asparagus, to be shipped (typically 40 days in the container) from Australia and New Zealand to markets in the USA, Europe, Middle East and Japan (Adams, 1988). If the correct varieties are selected and rapidly cooled immediately after harvest, the product arrives in good condition and has a long subsequent shelf life. With conventional vacuum packaging it is difficult to achieve a shelf life in excess of 12 weeks with beef and 8 weeks for lamb (Gill, 1984). However, a shelf life of up to 23 weeks at − 2 °C can be achieved in cuts of lamb individually packed in evacuated bags of linear polyethylene and then placed in gas flushed foil laminate bags filled with a volume of CO2 approximately equal to that of the meat (Gill and Penney, 1986). Similar storage lives are currently being achieved with beef primals transported from Australia and South Africa to the EU.
Most International Standard Organisation (ISO) containers are either ‘refrigerated’ or ‘insulated’. The refrigerated containers have refrigeration units built into their structure. The units operate electrically, either from an external power supply on board the ship or dock or from a generator on a road vehicle. Insulated containers either utilise the plug type refrigeration units already described or may be connected directly to an air-handling system in a ship’s hold or at the docks. Close temperature control is most easily achieved in containers that are placed in insulated holds and connected to the ship’s refrigeration system. However, suitable refrigeration facilities must be available for any overland sections of the journey. When the containers are fully loaded and the cooled air is forced uniformly through the spaces between cartons, the maximum difference between delivery and return air can be less than 0.8 °C (Heap, 1986). The entire product in a container can be maintained to within ± 1.0 °C of the set point.
Refrigerated containers are easier to transport overland than the insulated types, but have to be carried on deck when shipped because of problems in operating the refrigeration units within closed holds. On board ship they are therefore subjected to much higher ambient temperatures, and consequently larger heat gains, which makes it far more difficult to control product temperatures.
Air freighting is increasingly being used for high value perishable products, such as strawberries, asparagus and live lobsters (Sharp, 1988; Stera, 1999). However, foods do not necessarily have to fall into this category to make air transportation viable, since it has been shown that ‘the intrinsic value of an item has little to do with whether or not it can benefit from air shipment, the deciding factor is not price but mark-up and profit’ (ASHRAE, 2006).
There was a 10–12% per year increase in the volume of perishables transported by air during in the 1990s (Stera, 1999). Although air freighting of foods offers a rapid method of serving distant markets, there are many problems because the product is unprotected by refrigeration for much of its journey. Up to 80% of the total journey time is made up of waiting on the tarmac and transport to and from the airport. During flight, the hold is normally between 15 and 20 °C. Perishable cargo is usually carried in standard containers, sometimes with an insulating lining and/or ice or dry ice but is often unprotected on aircraft pallets (Sharp, 1988). Thus it is important that the product be: (1) transported in insulated containers to reduce heat gain, (2) be precooled and held at the required temperature until loading, (3) containers should be filled to capacity and (4) a thermograph should accompany each consignment.
Overland transportation systems range from 12-m refrigerated containers for long distance road or rail movement of bulk chilled or frozen products, to small uninsulated vans supplying food to local retail outlets or even directly to the consumer. Some of the first refrigerated road and rail vehicles for chilled product were cooled by air that was circulated by free or forced systems, over large containers of ice (Ciobanu et al., 1976). Similar systems using solid carbon dioxide as the refrigerant have also been used to cool transport vehicles.
In a 1970/71 survey of vehicles in the UK used to transfer chilled meat from small abattoirs to shops, almost 70% were unrefrigerated and 20% had no insulation (Cutting and Malton, 1972). However, now the majority of current road transport vehicles for chilled foods are refrigerated using either mechanical, eutectic plates or liquid nitrogen cooling systems. Many advantages are claimed for liquid nitrogen transport systems, including minimal maintenance requirements, uniform cargo temperatures, silent operation, low capital costs, environmental acceptability, rapid temperature reduction and increased shelf life owing to the modified atmosphere (Smith, 1986). Overall costs are claimed to be comparable with mechanical systems (Smith, 1986). However, published trials on the distribution of milk have shown that the operating costs using liquid nitrogen, per 100 l of milk transported, may be 2.2 times that of mechanically refrigerated transport systems (Nieboer, 1988). The rise in supermarket home delivery services where there are requirements for mixed loads of products that may each require different storage temperatures is introducing a new complexity to local land delivery (Cairns, 1996).
The temperature of individual consumer packs, small individual items and especially thin sliced products responds very quickly to small amounts of added heat. All these products are commonly found in retail display cabinets and marketing constraints require that they have maximum visibility. Maintaining the temperature of products below set limits while they are on open display in a heated store will always be a difficult task.
Average temperatures in chill displays can vary considerably from cabinet to cabinet, with inlet and outlet values ranging from − 6.7 to + 6.0 °C, and − 0.3 to + 7.8 °C, respectively, in one survey (Lyons and Drew, 1985). The temperature performance of an individual display cabinet does not only depend on its design. Its position within a store and the way the products are positioned within the display area significantly influences product temperatures. In non-integral (remote) cabinets (i.e. those without built-in refrigeration systems) the design and performance of the store’s central refrigeration system is also critical for effective temperature control.
The desired chilled display life for wrapped meat, fish, vegetables and processed foods ranges from a few days to weeks and is primarily limited by microbiological considerations. Retailers of unwrapped fish, meat and delicatessen products normally require a display life of one working day, which is often restricted by appearance changes. Frozen food can potentially be displayed for many weeks.
Reducing energy consumption in a chilled multi-deck cabinet is substantially different from reducing it in a frozen well cabinet (James et al., 2009). Improvements have been made in insulation, fans and energy efficient lighting but only 10% of the heat load on a chilled multi-deck comes from these sources, compared with 30% on the frozen well. Research efforts are concentrating on minimising infiltration through the open front of multi-deck chill cabinets, by the optimisation of air curtains and airflows, since this is the source of 80% of the heat load. In frozen well cabinets reducing heat radiation onto the surface of the food, accounting for over 40% of the heat load, is a major challenge.
Display cabinets for delicatessen products are available with gravity or forced convection coils and the glass fronts may be nearly vertical or angled up to 20°. Sections through three of the commonest types of delicatessen cabinet are shown in Fig. 18.1. In the gravity cabinet (Fig.18.1 a) cooled air from the raised rear-mounted evaporator coil descends into the display well by natural convection and the warm air rises back to the evaporator. In the forced circulation cabinets (Fig. 18.1 b and c) air is drawn through an evaporator coil by a fan and then ducted into the rear of the display, returning to the coil after passing directly over the products (Fig. 18.1 b), or forming an air curtain (Fig. 18.1 c), via a slot in the front of the cabinet and a duct under the display shelf.
Changes in product appearance are normally the criteria that limit the display life of unwrapped foods with the consumer selecting newly loaded product in preference to that displayed for some time. Deterioration in appearance has been related to degree of dehydration in red meat (Table 18.2) and is likely to similarly occur in other foods.
|Evaporative loss (g cm− 2)||Change in appearance|
|Up to 0.01||Red, attractive and still wet; may lose some brightness|
|0.015–0.025||Surface becoming drier; still attractive but darker|
|0.025–0.035||Distinct obvious darkening; becoming dry and leathery|
Apart from any relationship to appearance, weight loss is of considerable importance in its own right. A small survey carried out in the 1980s found average relative humidity ranged from 41–73% and air velocity from 0.1–0.67 m s− 1 in delicatessen cabinets. The lowest rate of weight loss was measured in a cabinet of the type shown in Fig 18.1 c which achieved mean conditions over the products of 0.4 °C, 0.14 m s− 1 and 65% RH (James and Swain, 1986). The same study showed that relative humidity was more important than the air temperature or velocity. Reducing the relative humidity (RH) from 95% to 40% increased weight loss over a 6 hour display period by a factor of between 14 and 18. In further work, a model developed to predict the rate of weight loss from unwrapped meat under the range of environmental conditions found in chilled retail displays showed that it was governed by the mean value of the conditions (James et al., 1988). Fluctuations in temperature or relative humidity had little effect on weight loss and any apparent effect was caused by changes in the mean conditions.
There is a conflict between the need to make the display attractive and convenient to increase sales appeal and the optimum display conditions for the product. High lighting levels increase the heat load and the consequent temperature rise dehumidifies the refrigerated air. The introduction of humidification systems can significantly improve display life (Brown et al., 2005).
To achieve the display life of days to weeks required for wrapped chilled foods, the product should be maintained at a temperature as close to its initial freezing point as possible to prevent microbial spoilage. In some cases, for example particular cheeses, dairy products and tropical fruits, quality problems may limit the minimum temperature that can be used, but for the majority of meat, fish and processed foods, the range − 1 to 0 °C is desirable.
Air movement and relative humidity have little effect on the display life of a wrapped product, but the degree of temperature control can be important especially for transparent, controlled atmosphere packs. Large temperature cycles will cause water loss from the product and this water vapour will condense on the inner surface of the pack and consequently reduce consumer appeal.
Although cabinets of the type described for delicatessen products can be used for wrapped foods, most are sold from multi-deck cabinets with single or twin air curtain systems. Twin air curtains tend to provide more constant product temperatures but are more expensive. It is important that the front edges of the cabinet shelves do not project through the air curtain since the refrigerated air will then be diverted out of the cabinet. On the other hand if narrow shelves are used, the curtain may collapse and ambient air can be drawn into the display well.
To maintain product temperatures close to 0 °C, the air off the coil must typically be − 4 °C and any ingress of humid air from within the store will quickly cause the coil to ice up. Frequent defrosts are often required and even in a well-maintained unit the cabinet temperature may rise to 10–12 °C and the product by at least 3 °C (Brolls, 1986). External factors such as the store ambient temperature, the position of the cabinet and poor pretreatment and placement of products substantially affect cabinet performance. Warm and humid ambient air, and loading with insufficiently cooled products, can also overload the refrigeration system. Even if the food is at its correct temperature, uneven loading or too much product can disturb the airflow patterns and destroy the insulating layer of cooled air surrounding the product.
An in-store survey of 299 prepackaged meat products in chilled retail displays found product temperatures in the range − 8.0 to 14.0 °C, with a mean of 5.3 °C and 18% above 9 °C (Rose, 1986). Other surveys (Bøgh-Sørensen, 1980; Malton, 1971) have shown that temperatures of packs from the top of stacks were appreciably higher than those from below owing to radiant heat pick up from store and cabinet lighting. It has also been stated that products in transparent film overwrapped packs can achieve temperatures above that of the surrounding refrigerated air owing to radiant heat trapped in the package by the ‘greenhouse’ effect. However, specific investigations have failed to demonstrate this effect (Gill, 1988).
Frozen foods are always packaged before being displayed and in the majority of cases the packaging obscures, and protects, the food on display. If packed in transparent film the surface of many frozen foods will discolour rapidly when illuminated.
Traditionally open-well cabinets were used to display frozen products, but increasingly multi-deck cabinets are used because of their increased display space and sales appeal. The rate of heat gain in a multi-deck cabinet, and consequently the energy consumption, is much higher than in a well cabinet. Owing to the increased costs of energy, multi-deck cabinets are now appearing on the market with double glazed doors that have to be opened to access the food on display.
The dominant types of refrigerant used in the food industry in the last 60 years have belonged to a group of chemicals known as halogenated hydrocarbons. Members of this group, which includes the chlorofluorocarbons (CFCs) and the hydrochlorofluorocarbons (HCFCs), have excellent properties, such as low toxicity, compatibility with lubricants, high stability, good thermodynamic performance and relatively low cost, making them excellent refrigerants for industrial, commercial and domestic use. However, their high chemical stability leads to environmental problems when they are released and rise into the stratosphere. Scientific evidence clearly shows that emissions of CFCs have been damaging the ozone layer and contributing significantly to global warming. With the removal of CFCs from aerosols, foam blowing and solvents, the largest single application sector in the world is refrigeration, which accounts for almost 30% of total consumption.
Until recently R12, R22 and R502 were the three most common refrigerants used in the food industry. R12 and R502 have significant ozone depletion potential (ODP) and global warning potential (GWP) and R22, although smaller, is still dangerous in the long term. Consequently, as a result of international agreements, pure CFCs (e.g. R12, R502) have been completely banned. Pure HCFCs (mainly R22) are banned in new industrial plant and are soon to be phased out completely. HCFC blends and HFC blends originally introduced as CFC replacements are covered by F-Gas regulations that limit leak rates.
Chemical companies are making large investments in terms of both time and money in developing new refrigerants that have reduced or negligible environmental effects. Other researchers are looking at the many non-CFC alternatives, including ammonia, propane, butane, carbon dioxide, water and air that have been used in the past.
Ammonia is increasingly a common refrigerant in large industrial food cooling and storage plants. It is a cheap, efficient refrigerant whose pungent odour aids leak detection well before toxic exposure or flammable concentrations are reached. The renewed interest in this refrigerant has led to the development of compact, low charge (i.e. small amounts of ammonia) systems that significantly reduce the possible hazards in the event of leakage. It is expected that ammonia will meet increasing use in large industrial food refrigeration systems. Carbon dioxide is being advocated for retail display cabinets and hydrocarbons, particularly propane and butane or mixtures of both, for domestic refrigerators.
As well as the direct affect of refrigerants on the environment, energy efficiency is increasingly of concern to the food industry. Worldwide it is estimated that 40% of all food requires refrigeration and 15% of the electricity consumed worldwide is used for refrigeration (Mattarolo, 1990). In the UK, 11% of electricity is consumed by the food industry (DBERR, 2005). However, detailed estimates of what proportion of this is used for refrigeration processes are less clear and often contradictory (James et al., 2009). Using the best available data, the energy saving potential in the top five refrigeration operations (retail, catering, transport, storage and primary chilling), in terms of the potential to reduce energy consumed, lies between 4300 and 8500 GWh/year in the UK (James et al., 2009).
It is clear that maintenance of food refrigeration systems will reduce energy consumption (James et al., 2009). Repairing door seals and door curtains, ensuring that doors can be closed and cleaning condensers produce significant reductions in energy consumption. In large cold storage sites, it has been shown that energy can be substantially reduced if door protection is improved, pedestrian doors fitted, liquid pressure amplification pumps fitted, defrosts optimised, suction liquid heat exchangers fitted and other minor issues corrected (James et al., 2009).
In the retail environment, the majority of the refrigeration energy is consumed in chilled and frozen retail display cabinets (James et al., 2009). Laboratory trials have revealed large, up to six-fold, differences in the energy consumption of frozen food display cabinets in similar display areas. In chilled retail display, which accounts for a larger share of the market, similar large differences, up to five-fold, were measured. A substantial energy saving can therefore be achieved by simply informing and encouraging retailers to replace energy inefficient cabinets by the best currently available.
New/alternative refrigeration systems/cycles, such as trigeneration, air cycle, sorption–adsorption systems, thermoelectric, Stirling cycle, thermoacoustic and magnetic refrigeration, have the potential to save energy in the future if applied to food refrigeration (Tassou et al., 2009). However, none appear to be likely to produce a step change reduction in refrigeration energy consumption within the food industry within the next decade.
In the author’s experience, the poor performance of new refrigeration systems used to maintain the cold chain can often be chased back to a poor, non-existent, or ambiguous process specification. In older systems it is often due to a change in use that was not considered in the original specification. There are three stages in obtaining a refrigeration system that works:
The first task in designing a system is the preparation of a clear specification by the user of how the facility will be used at present and in the foreseeable future. In preparing this specification, the user should consult all parties concerned. These may be officials enforcing legislation, customers, other departments within the company and engineering consultants or contractors, but the ultimate decisions taken in forming this specification are the users alone.
The process specification must include, as a minimum, data on the food(s) to be refrigerated, in terms of size, shape and throughput. The maximum capacity must be catered for and the refrigeration system should also be specified to operate adequately and economically at all other throughputs. The range of temperature requirements for each product must also be clearly stated. If it is intended to minimise loss, it is useful to quantify at an early stage how much extra money can be spent to save a given amount of weight. All the information collected so far, and the decisions taken, will be on existing production. Another question that needs to be asked is, ‘will there be any changes in the use of the chiller in the future?’
The refrigeration system chiller, freezer, storeroom and so on is one operation in a sequence of operations. It influences the whole production process and interacts with it. An idea must be obtained of how the system will be loaded, unloaded and cleaned, and these operations must always be intimately involved with those of the rest of the production process.
There is often a conflict of interest in the usage of a chiller or freezer. In practice, a chiller/freezer can often be used as a marshalling yard for sorting orders and as a place for storing product not sold. If it is intended that either of these operations are to take place in the chiller/freezer, the design must be made much more flexible in order to cover the conditions needed in a marshalling area or a refrigerated store. In the case of a batch or semi-continuous operation, holding areas may be required at the beginning and end of the process in order to even out flows of material from adjacent processes. The time available for the process will be in part dictated by the space that is available; a slow process will take more space than a fast process, for a given throughput.
Other refrigeration loads, in addition to that caused by the input of heat from the product, also need to be specified. Many of these, such as infiltration through openings, the use of lights, machinery and people working in the refrigerated space, are all under the control of the user and must be specified so that the heat load emitted by them can be incorporated in the final design. Ideally, all the loads should then be summed together on a time basis to produce a load profile. If the refrigeration process is to be incorporated with all other processes within a plant, in order to achieve an economic solution, then the load profile is important. The ambient design conditions must be specified. This means that the temperatures adjacent to the refrigerated equipment and the temperatures of the ambient to which heat will ultimately be rejected. In stand-alone refrigerated processes this will often be the wet and dry bulb temperatures of the outside air. If the process is to be integrated with heat reclamation, the temperature of the heat sinks must be specified. Finally, the defrost regime should also be specified. There are times in any process where it is critical that coil defrosting and its accompanying temperature rise does not take place and that the coil is cleared of frost before commencing the specified part of the process.
Although it is common practice throughout the food industry to leave much of this specification to refrigeration contractors or engineering specialists, the end user should specify all the above requirements. The refrigeration contractors or engineering specialists are in a position to give good advice about this. However, since all the above are outside their control, the end user, with their knowledge of how well they can control their overall process, should always take the final decision.
The aim of drawing up an engineering specification is to turn the user requirements into a specification that any refrigeration engineer can then use to design a system. The first step in this process is iterative. First, a full range of time, temperature and air velocity options must be assembled for each cooling specification covering the complete range of each product. Each must then be evaluated against the user requirements. If they are not a fit, then another option is selected and the process repeated. If there are no more options available there are only two alternatives: either standards must be lowered (recognising in doing so that cooling specifications will not be met) or the factory operation must be altered.
A full engineering specification will typically include the environmental conditions within the refrigerated enclosure, air temperature, air velocity and humidity; the way the air will move within the refrigerated enclosure; the size of the equipment; the refrigeration load profile; the ambient design conditions and the defrost requirements. The final phase of the engineering specification should be drawing up a schedule for testing the engineering specification prior to handing over the equipment. This test will be in engineering and not product terms.
The specification produced should be the document that forms the basis for quotations and finally the contract between the user and his contractor and must be stated in terms that are objectively measurable once the chiller/freezer is completed. Arguments often ensue between contractors and their clients from an unclear, ambiguous or unenforceable specification. Such lack of clarity is often expensive to all parties and should be avoided.
In general, after initial chilling or freezing, as a chilled or frozen food product moves along the cold chain it becomes increasingly difficult to control and maintain its temperature. Temperatures of bulk packs of chilled or frozen product in large storerooms are far less sensitive to small heat inputs than single consumer packs in transport or open display cases.
If primary and secondary cooling operations are efficiently carried out then the food will be reduced below its required temperature before it is placed in storage. In this situation the cold store’s refrigeration system is only required to extract extraneous heat that enters through the walls, door openings, and so on.
Even when temperature controlled dispatch bays are used, there is a slight heat pick up during loading. In bulk transportation the resulting temperature rise is small and the vehicle’s refrigeration system rapidly returns the product to the required temperature. Larger problems exist in local multi-drop distribution to individual stores. There is a large heat input every time the doors are opened and product unloaded, small packs rapidly rise in temperature and the vehicle often lacks the refrigeration capacity or time to recool the food.
Temperature control during retail display is often poor owing to the retailers need to display as much product as possible in a way that is very assessable to the consumer. Increasing energy costs may be the key factor that persuades retailers to reduce consumer access and hence improve temperature control.
In recent years, energy conservation requirements have caused an increased interest in the possibility of using more efficient storage temperatures than have been used to date. Researchers, such as Jul (1982), have questioned the wisdom of storage below − 20 °C and have asked whether there is any real economic advantage in very low temperature preservation. There is a growing realisation that storage lives of several foods can be less dependent on temperature than previously thought. Since research has shown that many food products, such as red meats, often produce non-linear time–temperature curves, there is probably an optimum storage temperature for a particular food product. Improved packing and preservation of products can also increase storage life and may allow higher storage temperatures to be used.
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