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Food Technologies: Chilling
FOOD TECHNOLOGIES
Chilling SJ James and C James, The Grimsby Institute of Further & Higher Education (GIFHE), Grimsby, UK r 2014 Elsevier Inc. All rights reserved.
Glossary Conduction, thermal Mechanism for heat transfer. The process of heat transfer through a solid material/medium in which kinetic energy is transmitted by the particles of the material from particle to particle without gross displacement of the particles. Convection, thermal Mechanism for heat transfer. The process of heat transfer through a liquid or gas by means of circulating currents caused by changes in density. Hazard A biological, chemical, or physical agent in, or condition of, food with the potential to cause an adverse health effect. Heat transfer coefficient Coefficient used in thermodynamics to calculate heat transfer, typically by convection or phase change, between a fluid and a solid. Pasteurization A form of heat treatment that kills certain vegetative bacteria and/or spoilage organisms in milk and other foods. Temperatures below 100 1C are used. Radiation, thermal Mechanism for heat transfer. Electromagnetic radiation generated by the thermal motion
of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. Refrigeration May be defined as the process of removing heat from any substance to: (1) render colder – reduce temperature, (2) change its state – for example, water to ice, (3) maintain its state – preserving foods, storing ice. Retail An operation that stores, prepares, packages, serves, or otherwise provides food directly to the consumer for preparation by the consumer for human consumption. This may be freestanding markets, sections in grocery or department stores, packaged, chilled, or frozen and/or full service. Shelf-life The period during which the product maintains its microbiological and chemical safety and sensory qualities at a specific storage temperature. It is based on identified hazards for the product, heat or other preservation treatments, packaging method, and other hurdles or inhibiting factors that may be used.
Introduction
Chilling Process
There is no strict definition of what constitutes a chilled food, although some countries provide legislative requirements for particular chilled foods. In general, a chilled food is any food in which the temperature of the food is reduced to, and maintained at, a temperature below that of the ambient temperature, but above the temperature where any of its water content will change from a liquid to a solid (i.e., begin to freeze). With fish and meat the maximum chilled shelf-life will be achieved at a temperature close to their initial freezing points. However, for some foods, such as bananas and other tropical fruit, the optimum temperature can be as high as þ 14 1C. Chilling is often preferred over other food preservation methods, such as smoking, drying, salting, or canning, because it produces no significant changes in the texture, taste, smell, or appearance of the food and maintains the original ‘fresh’ quality characteristics of the food. Temperature is the principal controlling factor for the safety of chilled foods. Temperature is particularly important in such foods in slowing, or inhibiting, the growth of pathogenic bacteria.
Chilling is a process of removing heat from a food and can only be achieved by four basic mechanisms: radiation, conduction, convection, or evaporation. Conduction requires a good physical contact between the food to be chilled and the cooling medium, and this is generally achieved only with foods that can be shaped into regular shapes, such as blocks of meat or fish. Radiation does not require any physical contact but a large temperature difference is required between the surface of the food being cooled and that of surrounding surfaces to achieve significant heat flow. In primary chilling, radiation is only important in the initial stages of the process in a system where the food is not surrounded by other products. Again, in the initial stages of the chilling of cooked food products (e.g., pies and other pastry products, meat joints, baked cakes, etc.), radiant heat loss can be substantial if the products are surrounded by cold surfaces. Evaporation from a food surface reduces yield and is not desirable in most food refrigeration operations but can be useful again in the initial cooling of cooked food products and
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Encyclopedia of Food Safety, Volume 3
doi:10.1016/B978-0-12-378612-8.00265-1
Food Technologies: Chilling
Table 1
141
Generation times for foodborne bacteria in raw meat
Bacteria
Temperature (1C)
Time (h) Lag
Salmonella spp. Clostridium perfringens Escherichia coli O157:H7, pH 5.7 E. coli O157:H7, pH 6.3 Salmonella spp. Salmonella Typhimurium Yersinia enterocolitica E. coli SF B. cereus Y. enterocolitica Listeria monocytogenes L. monocytogenes E. coli O157:H7
12.5 12 12 12 10 10 10 8.2 5 5 4 4 2
is used in the immediate postharvest cooling of many fruits and vegetables. However, as soon as the surface of the food is close to that of the cooling medium then any heat loss due to evaporation is minimal. Convection is by far the most important heat transfer mechanism employed in the majority of food chilling systems. In most cases, refrigerated air is the transfer medium; however, in some cases brine or a cryogenic gas can be used. The rate of heat removal depends on the following: 1. Surface area available for heat flow. 2. Temperature difference between the surface and the medium. 3. Surface heat transfer coefficient. Each combination of product and cooling system can be characterized by a specific surface heat transfer coefficient whose value depends principally on the thermophysical properties and velocity of the medium. Typical values range from 5 W m2 K1 for slow moving air to 500 W m2 K1 for immersion in an agitated refrigerant. Heat must also be transferred from within the food to its surface before it can be removed. In a solid food this will be via conduction; whereas in a liquid this will mainly be via convection. Most foodstuffs are poor conductors of heat and this imposes a severe limitation on attainable chilling times for either large individual items or small items cooled in bulk.
Effect of Chilling on Food Safety The principle of chilling as a preservation process is the basis that all biological systems are controlled by enzymatic reactions including those that control microorganisms and cause quality degradation and that the rate of these reactions is directly related to temperature. Reducing temperatures below the optimum growth range of a microorganism increases its generation time. The main group of microorganisms of concern in chilled foods are psychrophilic pathogens. These organisms can grow at temperatures below 0 1C, and some reportedly have an optimum growth temperature as low as 10 1C. The optimum temperature growth range of mesophiles is 25–30 1C and with
Generation
16.2 2.78 45 40
References
6.79 11.5 6.0 3.9 13.87 9.65 12.73 6.9 8.3 16.53 22.8 9.3 No growth
Mackey et al. (1980) Lund et al. (2000) Walls and Scott (1996) Walls and Scott (1996) Mackey et al. (1980) Smith (1985) Logue et al. (1998) Smith (1985) Lund et al. (2000) Logue et al. (1998); Lund et al. (2000) Lund et al. (2000) Pawar et al. (2000) Ansay et al. (1999)
many the minimum growth temperature is approximately 10 1C. Because most chilled food is kept below this temperature mesophiles are not usually of concern in chilled distribution. However, mesophilic pathogens, such as Staphylococcus aureus and Bacillus cereus, are also of concern when food handlers and producers fail to chill foods properly or when adequate temperatures are not maintained during storage and handling. In addition, some mesophilic microorganisms are psychrotrophic and can grow on refrigerated foods. Although microorganisms can grow at low temperatures, they grow more slowly as the temperature is reduced. Thus the generation time for a pseudomonad (a common form of spoilage organism) might be 1 h at 20 1C, 2.5 h at 10 1C, 5 h at 5 1C, 8 h at 2 1C, or 11 h at 0 1C. As temperatures are reduced below 10 1C, fewer strains can grow and cause spoilage. In general, food will spoil approximately four times as fast at 10 1C and twice as fast at 5 1C, as at 0 1C. In the usual temperature range used for the storage of chilled meat and meat products, 1.5 to 5 1C, there can be as much as an eight-fold increase in growth rate between the lower and upper temperatures used (Table 1). Storage of chilled meat and meat products at 1.570.5 1C would attain the maximum storage life without any surface freezing of the product. Chill temperatures also have a marked effect on the type of spoilage microflora present on food by altering the microbial community. For example, raw milk stored at temperatures close to 0 1C tends to putrefy because of the activity of pseudomonads, rather than to sour due to the activity of lactic acid bacteria. The essential characteristics of pathogenic microorganisms can be found in numerous texts. There is a certain degree of conflicting data concerning the importance of various pathogens with regard to safety of specific foods and the effect of specific temperatures or packaging atmospheres on their growth or inhibition. Pathogenic bacteria such as Salmonella spp., L. monocytogenes, Clostridium botulinum type E, psychrotrophic B. cereus, Aeromonas hydrophila, and Y. enterocolitica are of particular concern in chilled foods because they are capable of growth at low temperatures (Table 2). Many of the organisms that compete with pathogens at ambient temperatures will not grow at low temperatures, thus low temperatures may
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Food Technologies: Chilling
Table 2 Minimum and optimum growth temperatures for pathogens associated with foods
Campylobacter spp. Arcobacter spp. C. perfringens B. cereus mesophilic C. botulinum proteolytic Shigella spp. S. aureus Pathogenic E. coli strains E. coli O157:H7 Salmonella spp. B. cereus psychrotrophic Vibrio parahaemolyticus C. botulinum non-proteolytic A. hydrophila L. monocytogenes Y. enterocolitica
Minimum temperature (1C)
Optimum temperature (1C)
30–32 15 12 10–15 10 7–10 7 7 6–7 5 4–5 5–10 3 0.1 to 1.2 1 to 0 2
42–45 37 43–47 35–40 35 37 35–40 35–40 42 35–43 28–35 30–37 30 15–20 30–37 22–30
preferentially favor the growth of pathogenic organisms. Pasteurization may also favor the outgrowth of surviving spores of psychrotrophic strains of C. botulinum and B. cereus by destroying competing microflora, a particular concern in vacuum packaged pasteurized foods (‘sous vide’ products), and ready-to-eat (RTE) products. However, most will not grow, or produce toxins, below 4 1C, with the exception of L. monocytogenes, C. botulinum type E, psychrotrophic B. cereus, A. hydrophila, and Y. enterocolitica. Investigations into the effect of different storage atmospheres on pathogenic growth at low temperatures appear to show that carbon dioxide (CO2)-enriched atmosphere produce the greatest inhibitory effect on psychrotrophic pathogens (Y. enterocolitica, A. hydrophila, and L. monocytogenes). Garcı´a de Fernando et al. (1995) concluded that ‘‘at a normal meat pH (i.e., 5.5) and at a low temperature (e.g., 1 1C) the growth of psychrotrophic pathogens is stopped when the CO2 concentration is 40%.’’ However, high pH meat (Z6) and/or higher storage temperatures will support growth of such pathogens. The bacterial safety and rate of spoilage depends on the numbers and types of microorganisms initially present, the rate of growth of those microorganisms, the conditions of storage (temperature and gaseous atmosphere), and characteristics (pH, aw) of the food. Of these factors, temperature is by far the most important. In the context of food safety the safest practical chilled temperature will be the lowest that can be used without significantly affecting the quality of the food. In many cases this will be as close to the freezing point of the food as practically possible. Maintaining temperatures and the integrity of the whole of the chill chain is vital to ensure the safety and quality of chilled foods.
Effect of Chilling Rate Whether ‘rapid’ chilling offers any clear advantages to product safety will depend on what biological hazards (pathogens,
etc.) are present, at what numbers they are present, and whether they are on, or in, the food in question, and how ‘rapid’ the rate is in comparison with other rates. There is no definition of ‘rapid’ and ‘slow’s rates. Size of product will also have a big effect on relative rates of chilling, because conduction through the product will become the rate-limiting factor as product size increases. There are instances where excessively rapid chilling rates, or too low a chilling temperature, can cause quality problems in foods. For example, a serious defect known as ‘woolly texture’ can be produced in rapidly cooled peaches. Substantial textural problems due to a phenomenon known as ‘cold shortening’ can occur in rapidly chilled meats (particularly beef and lamb), although electrical stimulation before rapid chilling will mitigate this problem. Few countries provide legislative requirements for chilling foods that specify chilling rates. For example, European legislation requires that red meat carcasses be chilled to a maximum temperature of 7 1C and poultry carcasses to a maximum temperature of 4 1C. No time limits on achieving these times have been set. There are very little data on the effect of current commercial chilling rates and conditions on changes in bacterial numbers during the process. In most cases no change or a small reduction (0.5 to 1 log10 cfu cm2) in number of organisms on the surface has been measured. Current European legislation for foods of animal origin places importance on core meat temperatures, because microbial contamination is primarily a surface phenomenon there is an argument to be made that surface temperatures are far more important than deep temperatures. This is the basis behind the controls in the Australian Export Control (Meat and Meat Products) Orders 2005 and their Refrigeration Index (RI) model. The central idea of the RI model is to measure the performance of the chilling process until all the sites of microbiological interest are at or below 7 1C and calculate the risk of E. coli growth from temperatures measured during chilling. Although there are few legislative requirements, there are many guidelines and recommendations for chilling cooked/ pasteurized food products (Table 3), particularly RTE products that can be eaten cold. Inadequate cooling of such products has been identified as potential safety risk, as there is always the possibility that some microorganisms, particularly spores, will not be killed by the cooking/pasteurization process. Therefore most guidelines recommend that cooked products should be cooled as quickly as possible through the temperature range 63 to 5 1C or less to minimize risk of spore germination and outgrowth. There is particular concern with vacuum packaged pasteurized foods (termed by some as refrigerated processed foods of extended durability, and including ‘sous vide’ products) that conditions (low oxygen and destruction of competing microflora) may favor the outgrowth of surviving spores of psychrotrophic strains of C. botulinum and B. cereus. Consequentially lower storage temperatures (o3 1C) are recommended for these products and/or more severe heat processes. With RTE products, there have been concerns over the possible presence and outgrowth of spores of psychrotrophic B. cereus. Although many of these guidelines were initially produced specifically for cook-chill catering operations, in many countries, the producers of chilled ready meals for retail sale use them.
Food Technologies: Chilling
Table 3
143
International chilling time guidelines/recommendations for the cooling of cooked foods
Country
Chilling range (1C)
Time (h)
Chilling rate (1C min 1)
Storage temperature (1C)
References
Australia
60–21 21–5 60–20
r2 r4 r2
0.33 0.07 0.33
5
De Jong et al. (2004)
4
Canadian Food Inspection System Implementation Group (CFISIG) (2004)
20–4 60–10 65–10 70–10 80–15 (15–2) 70–3
r4 r2 r3 r2 r2 r24 r2.5
0.07 0.42 0.31 0.50 0.54
– o5 0–3 2
Codex Alimentarius Commission (1999) Evans et al. (1996) Evans et al. (1996) Evans et al. (1996)
0.45
3
60–7 7–4 80–8 70–3 60–5
r5 r24 r4 r1.5 4–6
0.18
–
Food Safety Authority of Ireland (FSAI) (2004) De Jong et al. (2004)
0.30 0.74 0.23–0.15
3 3 –
Evans et al. (1996) UK Department of Health (1989) De Jong et al. (2004)
Canada
Codex Alimentarius Denmark France Germany Ireland The Netherlands Sweden UK USA
Because prolific histamine-forming bacteria are predominately mesophilic, rapid cooling of at-risk fish species (such as tuna and mackerel) immediately after catching is recognized as the key control for reducing histamine formation in such fish, as is the maintenance of adequate temperature control during the rest of the cold-chain. Similarly prompt and rapid cooling of other seafood, particularly fish and shellfish that are eaten raw, such as oysters, is an important control for V. parahaemolyticus. The maintenance of tight temperature control during the rest of the cold-chain is also very important with V. parahaemolyticus because it is capable of growth at temperatures as low as 5 1C, and is known for rapid growth and generation times as short as 12–18 min.
Chilling Operations Chilling systems for foodstuffs will contain many, if not all, of the following unit operations:
• • • • • •
Preparatory treatment; conditioning, waxing, cooking, pasteurizing, blanching, etc. Chilling; primary or secondary. Storage. Transportation. Retail display. Domestic storage.
During the preparatory treatment there can be a range of temperature responses, from a large gain to a small decrease in the temperature of the product. During chilling there is a substantial decrease in the mean temperature of the product. Within a correctly designed coldchain there should be no significant change in mean product temperature during storage, transport, retail display, or domestic storage. From a food safety-based approach, prepacking the food before chilling will lower the risk of contamination/
cross-contamination during the chilling process, however, it will significantly reduce the rate of cooling, and this may allow the growth of any microorganisms present. Provided the cooling media (air, water, etc.) and refrigeration equipment used are kept clean, no one cooling method can be said to be intrinsically more hygienic, or safe, than any other. For example, although there has been much debate concerning the safety of chilled water-cooling versus blast air chilling of poultry carcasses, reviews of published data have found little evidence of any difference microbially between poultry cooled by either method. Similarly there appears to be no evidence of a difference between spray chilling versus blast air chilling of red meat carcasses. Although some pathogens, such as campylobacters and arcobacters, may show sensitivity to air chilling; immersion and spray chilling may result in some physical removal (washing off) of microorganisms. In addition the use of antimicrobials, such as chlorine or ozone, in chilled water immersion/spray systems has been shown in some studies to reduce microbial contamination and subsequently may improve food safety. Such substances, however, are not permitted in all countries (notably the European Union) at present. The potential for the fans used in air chilling to disseminate molds and bacteria has been identified in a number of reviews as a potential hazard, but very little work has been carried out to evaluate whether this is in fact the case. Similarly, condensation in the chiller has also been identified as a possible source of cross-contamination during chilling and storage (particularly of unpackaged foods). EU legislation regarding the chilling of foods of animal origin specifies that ‘‘during the chilling operations, there must be adequate ventilation to prevent condensation on the surface of the meat.’’ However, there appears to be little published data to support such a control. It is however prudent that chillers should be properly constructed and maintained. The design of chillers should be such as to provide for effective cleansing and disinfection.
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Food Technologies: Chilling
Chilling Systems
ice/water mixture, with the carcasses being conveyed through a tank of ice/water using Archimedean screws.
Moving Air Air is by far the most widely used method of chilling food as it is economical, hygienic, and relatively noncorrosive to equipment. 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 conveyerized blast chilling tunnels or spirals. Air chilling is slow but versatile especially when there is a requirement to cool a variety of products. The cooling time of the product is reduced as the air speed is increased, thus air distribution is very important. Conveying the product through the cooling system overcomes the problem of uneven air distribution because each item is subjected to the same velocity/time profile. The refrigeration capacity and air conditions can also be varied throughout the length of the tunnel. Using higher air temperatures in the latter stages can avoid surface freezing.
Immersion/Spray As their names imply, these involve dipping product into a cold liquid, or spraying a cold liquid onto the food. When water is used as the heat transfer medium the process is often called ‘hydrocooling.’ This produces high rates of heat transfer due 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. Clearly if the food is unwrapped the liquid has to be ‘food safe.’ Any uptake of the cooling medium, whether ‘food safe’ or not, by the product may present problems both in terms of flavor changes and the requirement for periodical replacement of the medium. Hydrocooling is probably the least expensive method of achieving rapid cooling in small products. The product to be cooled is immersed in, or sprayed with, cool water, either at ambient or near 0 1C. Where permitted, the water is often treated with a mild disinfectant, such as chlorine. Practical systems vary from simple stirred or unstirred tanks to plants where the product is conveyed through agitated tanks or under banks of sprays. Hydrocooling is very effective for chilling fruit and vegetables, however, not all crops can tolerate wetting. Spraying with ambient or chilled water is also an effective method for initially cooling cooked products that can withstand wetting, i.e., hams, sausages, chubs, etc. Spray bars are either fitted in the batch cookers or in separate cooling cabinets. Spray chilling of meat carcasses is widely practiced in the USA and used for poultry in Europe. 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 melted water percolating through it than with the ice itself. 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 to 0.5 1C (where sea water is present). Ice may also be used for cooling fruit and vegetables, however, as with hydrocooling, ice is only applicable to vegetable and fruit produce that can tolerate wetting, such as watercress, broccoli, and some other brassicas. Large poultry carcasses, such as turkeys, are also often initially cooled by immersion in an
Vacuum Solid products having a large surface area to volume ratio and an ability to readily release internal water are amenable to vacuum cooling. The products are placed in a vacuum chamber (typically operating at between 530 and 670 N m2) 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 1C. Suitable products, such as lettuce, can be vacuum cooled in less than 1 h. In general terms, a 5 1C reduction in product temperature is achieved for every 1% of water that is evaporated. Because vacuum cooling requires the removal of water from the product prewetting is commonly applied to prevent the removal of water from the tissue of the product.
Plate Heat Exchangers In a plate cooling system product is essentially pressed between hollow (horizontal or vertical) metal plates containing a circulating refrigerant. Cooling rate depends on product thickness, good contact, and the conductivity of the product. Thus the need for regularly shaped products with large flat surfaces is a major hindrance. Air spaces in packaging and fouling of the plates can have a significant effect on cooling time.
Jacketed Heat Exchangers Batch coolers for liquid foods can range in capacity from 100 to 10 000 l with the foodstuff usually contained in a stainless steel vessel. The cooling medium may circulate through the jacket of the vessel, through a coil immersed in the liquid, or both. Most vessels are provided with agitators to improve the rate of convective heat transfer and stop temperature stratification. One common method used to decrease cooling times of liquid products in a closed vessel is to apply a vacuum to produce evaporative cooling. Temperature stratification is a problem in unstirred vats whilst in agitated vessels the design and operation of stirrers is critical if breakdown of delicate solid product is to be avoided.
Belt Heat Exchangers Belt systems consist of an endless steel belt (approximately 1 mm in thickness), the underside of which is cooled either directly with water, brine, or glycol sprays or by sliding over a stationary cold surface. Because only one side of the product is in contact with the cooling surface relatively thin products are required, such as hamburgers, fish fillets, or liquid and semiliquid products such as pure´es and sauces. The main advantages of belt systems are: (1) continuous processing, (2) easy continuous cleaning and sanitation, (3) reduced evaporative losses, in comparison with air systems, and (4) the possibility of operating with several temperature zones.
Food Technologies: Chilling Continuous Heat Exchangers The majority of liquid foodstuffs require cooling after a heat processing operation such as cooking of sauces and soups, and pasteurization or sterilization of fruit juices, milk, and other dairy products. Milk is also cooled at the point of collection to maintain its quality. Unpasteurized ‘freshly squeezed’ fruit juices are cooled immediately after production. Fermented beverages are often cooled during primary and secondary fermentation, and before storage. Multiplate coolers are extensively used for liquid foods. They have the highest available heat transfer surface, lowest material requirements, maximum efficiency (up to 90% heat recovery in counter current mode) and are very flexible in operation and easy to clean. Scraped surface heat exchangers can have advantages in the cooling of very viscous liquid foods and where surface fouling is a potential problem.
Cryogenic Cryogenic cooling uses refrigerants, such as liquid nitrogen or solid carbon dioxide, directly. 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. Avoiding surface freezing of the product is the main problem in using cryogens for chilling. Forced gas cooling is the only method that can be employed when surface freezing has to be avoided. However, the system tends to be inefficient.
Chilled Storage Most unwrapped meat, poultry, fish, fruit and vegetables, and all types of wrapped foods are stored in large rooms with circulating air. To minimize weight loss and appearance changes associated with desiccation air movement around the unwrapped product should be the minimum required to maintain a constant temperature. With wrapped products low air velocities are also desirable to minimize energy consumption. However, many storage rooms are designed and constructed with little regard to air distribution and localized velocities over products. Horizontal throw refrigeration coils are often mounted in the free space above the racks or rails of product and no attempt is made to distribute the air around the products. Using a false ceiling or other forms of ducting to distribute the air throughout the storage room can substantially reduce variations in velocity and temperature. Controlled atmosphere storage rooms were developed for specialized fruit stores. In addition to the normal temperature control plant these stores also include special gas-tight seals to maintain an atmosphere that is normally lower in oxygen and higher in nitrogen and carbon dioxide than air. An additional plant is required to control the carbon dioxide concentration, generate nitrogen, and consume oxygen. The optimum atmosphere must be determined experimentally for the specific product being stored. There is growing use of controlled atmosphere and modified atmosphere retail packs to extend the chilled storage and display life of red meats, poultry, fish, and vegetables. Because the packs tend to be large and insulate the products efficient precooling before packaging is especially
145
important if product quality is to be maintained. Provided that temperatures during chilled storage are sufficient to prevent or inhibit the growth of any pathogens present on the food in question, in general the food will spoil before there is significant growth in pathogens would be a cause of concern. The safe storage life of chilled foods depends on the control of all the preceding factors, but must be validated by challenge testing and/or modeling for each product and process under defined storage conditions. However, from a practical point of view, the lowest storage temperature that can be used without affecting the quality of the food will be the safest.
Transportation Chilled foods are transported around the world and locally via a range of transportation systems. 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. It is particularly important that the food is at the correct temperature before loading because 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. In the large containers used for long distance transportation food temperatures can be kept within 70.5 1C of the set point. Control of the oxygen and carbon dioxide levels in shipboard containers has allowed fruits 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. Even longer shelf-lives (over 20 weeks) can now be achieved for meats, particularly beef and lamb. Air-freighting is increasingly being used for high value perishable products, such as strawberries, asparagus, and live lobsters. Although air-freighting of foods offers a rapid method of serving distant markets, there are many problems because the product is usually 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 the flight the hold is normally between 15 and 20 1C. Perishable cargo is usually carried in standard containers, sometimes with an insulating lining and/or dry ice but is often unprotected on aircraft pallets. 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. 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 overland delivery.
Retail Display 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
Food Technologies: Chilling
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. The required display life and consequent environmental conditions for wrapped chilled products differ from those for unwrapped products. The desired chilled display life for wrapped meat, fish, vegetables, and processed foods ranges from a few days to many weeks and is primarily limited by microbiological considerations. Retailers of unwrapped fish, meat, and delicatessen products, for example, sliced meats, pate, cheese, and prepared salads, normally require a display life of one working day. The introduction of humidification systems can significantly improve display life of unwrapped products. Average temperatures in chilled retail display cabinets can be varied considerably from cabinet to cabinet, with inlet and outlet values ranging from 6.7 to þ 6.0 1C, and 0.3 to þ 7.8 1C, respectively, in one survey. The temperature performance of an individual display cabinet not only depends on its design but also its position within a store and the way the products are positioned within the display area significantly influence product temperatures. 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 instore survey of 299 prepackaged meat products in chilled retail displays found product temperatures in the range 8.0 to 14.0 1C, with a mean of 5.3 1C and 18% above 9 1C. Other surveys have shown that temperatures of packs from the top of stacks were appreciably higher than those from below due to radiant heat pickup 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 due to radiant heat trapped in the package by the
7.4
Mean temperature (°C)
Recommended refrigerator temperatures are in general below 8 1C throughout the world, with many countries (including the UK) recommending below 5 1C. The numerous surveys on the domestic storage of refrigerated foods show remarkable similarities in consumer attitudes and handling of chilled foods and the performance of their refrigerators. Perhaps even more remarkable is that despite numerous recommendations on handling and storage temperat‘ures, consumer use and the performance of refrigerators remain remarkably unchanged throughout the world over the past 30 years. Numerous surveys (Figure 1) show that mean temperatures range between 5 and 7 1C, with 50–70% of domestic refrigerators operating at temperatures above 5 1C. It is clear that many refrigerators throughout the world are running at higher than recommended temperatures. Because even these recommended temperatures are higher than the 0–1 1C that is usually the recommended temperature range for storing fish and seafood, meat, and many chilled products the current situation is even more detrimental to maintaining the high quality life of chilled foods. Although there have been many surveys of refrigerators, how fridge temperatures, cleanliness, and consumer practices (e.g., over filling with warm products, use for cooling hot cooked foods) impact on consumer health remains to be fully assessed. The few studies that have assessed these factors do not appear to have demonstrated clear links between incidences of food poisoning and operating temperatures of cleanliness.
7.4
7
6
7
6.6
6.5
7 6
Domestic Handling
6.3
5.9 5.4
5
4.9
5
5
5.2
New Zealand 07
8
‘greenhouse’ effect. However, specific investigations have failed to demonstrate this effect. The display life of chilled foods depends on the control of all the preceding factors, but must be validated by challenge testing and/or modeling for each product and process under defined chill display conditions. It must be recognized that the integrity of the whole of the chill chain is vital to ensure the safety and quality of chilled foods.
UK 06
146
4
4 3 2 1
UK 09
Greece 05
Ireland 05
Sweden 04
France 02
USA 99
UK 98
UK 97
Netherlands 97
New Zealand 97
North Ireland 92
UK 91
UK 90
0
Figure 1 Reported mean temperatures recorded in surveys of domestic refrigerators throughout the world since 1990.
Food Technologies: Chilling
Temperature Measurement and Monitoring Temperature measurement and monitoring is an integral part of any food cold-chain management system; as well as being, in many areas of the cold-chain, a legislative requirement. Monitoring the cold-chain requires detailed information on food product temperatures. Temperature monitoring includes both measurement and recording. Temperature measurement can be achieved using a variety of instrumentation such as bimetal style thermometers, thermistors, thermocouples, infrared thermometers, etc. Typically, in the food industry, temperature measurement is achieved using calibrated thermocouples and data loggers. Owing to the variety of available equipment, manufacturers and suppliers are best positioned to give advice to the food business on the choice of temperature measurement equipment for specific purposes and food products. Advice can also be found in numerous International and National recommendations and guidelines for chilled foods. One possible aid in the future may be the widespread use of time temperature indicators (TTI) or integrators throughout the cold-chain. TTIs are simple devices that are capable of reporting a visual and straightforward summary of either the temperature (indicators) or time-temperature exposure history (integrators) of the product. Indicators show that a product has exceeded, positively or negatively, a given temperature Whereas Integrators monitor both time and temperature during a given period and show the cumulative effect of temperature fluctuations during the history of the product.
Conclusions The safety of a chilled food depends on the numbers and types of microorganisms initially present, the rate of growth of those microorganisms, the conditions of storage (temperature and gaseous atmosphere), and characteristics (pH, aw) of the food. Of these factors, temperature is by far the most important. In the context of food safety, the safest practical chilled temperature will be the lowest temperature that can be used without significantly affecting the quality of the food. In many cases this will be as close to the freezing point of the food as practically possible, i.e., between 0 and 1 1C. Maintaining temperatures and the integrity of the whole of the chill chain is vital to ensure the safety, and quality, of chilled foods. In general, after initial chilling, as a chilled product moves along the cold-chain it becomes increasingly difficult to control and maintain its temperature. Temperatures of bulk packs of chilled 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, etc. Even when temperature controlled dispatch bays are used there is a slight heat pickup during loading. In bulk transportation the resulting temperature rise is small and the
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vehicle’s refrigeration system rapidly returns the product to the required temperature. Larger problems exist in local multidrop 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 due to the retailers’ need to display as much product as possible in a way that is easily assessable to the consumer. Increasing energy costs may be the key factor that persuades retailers to reduce consumer access, i.e., install doors on chilled retail display cabinets, and hence improve temperature control. At present domestic storage of chilled foods would appear to be the weakest link in the entire chill chain. However, despite all the many published surveys on refrigerator temperatures carried out in the past 30 years, how fridge temperatures, cleanliness, and consumer practices (e.g., over filling with warm products, use for cooling hot cooked foods) impact on consumer health remains to be fully assessed.
See also: Foodborne Diseases: Overview of Biological Hazards and Foodborne Diseases
References Ansay SE, Darling KA, and Kaspar CW (1999) Survival of Escherichia coli O157: H7 in ground-beef patties during storage at 2, 2, 15 and then 2 1C, and 20 1C. Journal of Food Protection 62: 1243–1247. Canadian Food Inspection System Implementation Group (CFISIG) (2004) Food Retail and Food Services Code. Amended edn., Ottawa, ON: Health Canada. Codex Alimentarius Commission (1999) Code of Hygienic Practice for Refrigerated Packaged Foods with Extended Shelf Life. Food and Agriculture Organization of the United Nations. CAC/RCP 46. Rome: FAO. De Jong AEI, Rombouts FM, and Beumer RR (2004) Effect of cooling on Clostridium perfringens in pea soup. Journal of Food Protection 67: 352–356. Evans J, Russell S, and James SJ (1996) Chilling of recipe dish meals to meet cook-chill guidelines. International Journal of Refrigeration 19: 79–86. Food Safety Authority of Ireland (FSAI) (2004) Guidance Note No. 15: Cook-Chill Systems in the Food Service Sector. Dublin, Ireland: Food Safety Authority of Ireland. Garcı´a de Fernando GD, Nychas GJE, Peck MW, and Ordı´n˜ez JA (1995) Growth/ survival of psychrotrophic pathogen on meat packaged under modified atmospheres. International Journal of Food Microbiology 28: 221–231. Logue CM, Sheridan JJ, McDowell DA, Blair IS, and Harrington D (1998) A study of the growth of plasmid bearing and plasmid cured strains of antibiotic resistant Yersinia enterocolitica serotype O:3 on refrigerated beef, pork and lamb. Food Microbiology 15: 603–615. Lund BM, Baird-Parker TC, and Gould GW (eds.) (2000) The Microbiological Safety and Quality of Food. Gaithersburg: Aspen Publishers Inc. Mackey BM, Roberts TA, Mansfield J, and Farkas G (1980) Growth of Salmonella on chilled meat. Journal of Hygiene (Cambridge) 85: 115–124. Pawar DD, Malik SVS, Bhilegaonkar KN, and Barbuddhe SB (2000) Effect of nisin and its combination with sodium chloride on the survival of Listeria monocytogenes added to raw buffalo meat mince. Meat Science 56: 215–219. Smith MG (1985) The generation time, lag time, and minimum temperature of growth of coliform organisms on meat, and the implications for codes of practice in abattoirs. Journal of Hygiene (Cambridge) 94: 289–300. UK Department of Health (1989) Chilled and Frozen, Guidelines on Cook-Chill and Cook-Freeze Catering Systems. London: UK Department of Health. Walls I and Scott VN (1996) Validation of predictive mathematical models describing the growth of Escherichia coli O157:H7 in raw ground beef. Journal of Food Protection 59: 1331–1335.
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Relevant Websites http://www.ecff.net/ European Chilled Food Federation. http://www.fao.org/ Food and Agriculture Organization of the United Nations.
http://www.iifiir.org/ International Institute of Refrigeration. http://www.chilledfood.org/ UK Chilled Food Association.
Chilling SJ James and C James, The Grimsby Institute of Further & Higher Education (GIFHE), Grimsby, UK r 2014 Elsevier Inc. All rights reserved.
Glossary Conduction, thermal Mechanism for heat transfer. The process of heat transfer through a solid material/medium in which kinetic energy is transmitted by the particles of the material from particle to particle without gross displacement of the particles. Convection, thermal Mechanism for heat transfer. The process of heat transfer through a liquid or gas by means of circulating currents caused by changes in density. Hazard A biological, chemical, or physical agent in, or condition of, food with the potential to cause an adverse health effect. Heat transfer coefficient Coefficient used in thermodynamics to calculate heat transfer, typically by convection or phase change, between a fluid and a solid. Pasteurization A form of heat treatment that kills certain vegetative bacteria and/or spoilage organisms in milk and other foods. Temperatures below 100 1C are used. Radiation, thermal Mechanism for heat transfer. Electromagnetic radiation generated by the thermal motion
of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. Refrigeration May be defined as the process of removing heat from any substance to: (1) render colder – reduce temperature, (2) change its state – for example, water to ice, (3) maintain its state – preserving foods, storing ice. Retail An operation that stores, prepares, packages, serves, or otherwise provides food directly to the consumer for preparation by the consumer for human consumption. This may be freestanding markets, sections in grocery or department stores, packaged, chilled, or frozen and/or full service. Shelf-life The period during which the product maintains its microbiological and chemical safety and sensory qualities at a specific storage temperature. It is based on identified hazards for the product, heat or other preservation treatments, packaging method, and other hurdles or inhibiting factors that may be used.
Introduction
Chilling Process
There is no strict definition of what constitutes a chilled food, although some countries provide legislative requirements for particular chilled foods. In general, a chilled food is any food in which the temperature of the food is reduced to, and maintained at, a temperature below that of the ambient temperature, but above the temperature where any of its water content will change from a liquid to a solid (i.e., begin to freeze). With fish and meat the maximum chilled shelf-life will be achieved at a temperature close to their initial freezing points. However, for some foods, such as bananas and other tropical fruit, the optimum temperature can be as high as þ 14 1C. Chilling is often preferred over other food preservation methods, such as smoking, drying, salting, or canning, because it produces no significant changes in the texture, taste, smell, or appearance of the food and maintains the original ‘fresh’ quality characteristics of the food. Temperature is the principal controlling factor for the safety of chilled foods. Temperature is particularly important in such foods in slowing, or inhibiting, the growth of pathogenic bacteria.
Chilling is a process of removing heat from a food and can only be achieved by four basic mechanisms: radiation, conduction, convection, or evaporation. Conduction requires a good physical contact between the food to be chilled and the cooling medium, and this is generally achieved only with foods that can be shaped into regular shapes, such as blocks of meat or fish. Radiation does not require any physical contact but a large temperature difference is required between the surface of the food being cooled and that of surrounding surfaces to achieve significant heat flow. In primary chilling, radiation is only important in the initial stages of the process in a system where the food is not surrounded by other products. Again, in the initial stages of the chilling of cooked food products (e.g., pies and other pastry products, meat joints, baked cakes, etc.), radiant heat loss can be substantial if the products are surrounded by cold surfaces. Evaporation from a food surface reduces yield and is not desirable in most food refrigeration operations but can be useful again in the initial cooling of cooked food products and
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doi:10.1016/B978-0-12-378612-8.00265-1
Food Technologies: Chilling
Table 1
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Generation times for foodborne bacteria in raw meat
Bacteria
Temperature (1C)
Time (h) Lag
Salmonella spp. Clostridium perfringens Escherichia coli O157:H7, pH 5.7 E. coli O157:H7, pH 6.3 Salmonella spp. Salmonella Typhimurium Yersinia enterocolitica E. coli SF B. cereus Y. enterocolitica Listeria monocytogenes L. monocytogenes E. coli O157:H7
12.5 12 12 12 10 10 10 8.2 5 5 4 4 2
is used in the immediate postharvest cooling of many fruits and vegetables. However, as soon as the surface of the food is close to that of the cooling medium then any heat loss due to evaporation is minimal. Convection is by far the most important heat transfer mechanism employed in the majority of food chilling systems. In most cases, refrigerated air is the transfer medium; however, in some cases brine or a cryogenic gas can be used. The rate of heat removal depends on the following: 1. Surface area available for heat flow. 2. Temperature difference between the surface and the medium. 3. Surface heat transfer coefficient. Each combination of product and cooling system can be characterized by a specific surface heat transfer coefficient whose value depends principally on the thermophysical properties and velocity of the medium. Typical values range from 5 W m2 K1 for slow moving air to 500 W m2 K1 for immersion in an agitated refrigerant. Heat must also be transferred from within the food to its surface before it can be removed. In a solid food this will be via conduction; whereas in a liquid this will mainly be via convection. Most foodstuffs are poor conductors of heat and this imposes a severe limitation on attainable chilling times for either large individual items or small items cooled in bulk.
Effect of Chilling on Food Safety The principle of chilling as a preservation process is the basis that all biological systems are controlled by enzymatic reactions including those that control microorganisms and cause quality degradation and that the rate of these reactions is directly related to temperature. Reducing temperatures below the optimum growth range of a microorganism increases its generation time. The main group of microorganisms of concern in chilled foods are psychrophilic pathogens. These organisms can grow at temperatures below 0 1C, and some reportedly have an optimum growth temperature as low as 10 1C. The optimum temperature growth range of mesophiles is 25–30 1C and with
Generation
16.2 2.78 45 40
References
6.79 11.5 6.0 3.9 13.87 9.65 12.73 6.9 8.3 16.53 22.8 9.3 No growth
Mackey et al. (1980) Lund et al. (2000) Walls and Scott (1996) Walls and Scott (1996) Mackey et al. (1980) Smith (1985) Logue et al. (1998) Smith (1985) Lund et al. (2000) Logue et al. (1998); Lund et al. (2000) Lund et al. (2000) Pawar et al. (2000) Ansay et al. (1999)
many the minimum growth temperature is approximately 10 1C. Because most chilled food is kept below this temperature mesophiles are not usually of concern in chilled distribution. However, mesophilic pathogens, such as Staphylococcus aureus and Bacillus cereus, are also of concern when food handlers and producers fail to chill foods properly or when adequate temperatures are not maintained during storage and handling. In addition, some mesophilic microorganisms are psychrotrophic and can grow on refrigerated foods. Although microorganisms can grow at low temperatures, they grow more slowly as the temperature is reduced. Thus the generation time for a pseudomonad (a common form of spoilage organism) might be 1 h at 20 1C, 2.5 h at 10 1C, 5 h at 5 1C, 8 h at 2 1C, or 11 h at 0 1C. As temperatures are reduced below 10 1C, fewer strains can grow and cause spoilage. In general, food will spoil approximately four times as fast at 10 1C and twice as fast at 5 1C, as at 0 1C. In the usual temperature range used for the storage of chilled meat and meat products, 1.5 to 5 1C, there can be as much as an eight-fold increase in growth rate between the lower and upper temperatures used (Table 1). Storage of chilled meat and meat products at 1.570.5 1C would attain the maximum storage life without any surface freezing of the product. Chill temperatures also have a marked effect on the type of spoilage microflora present on food by altering the microbial community. For example, raw milk stored at temperatures close to 0 1C tends to putrefy because of the activity of pseudomonads, rather than to sour due to the activity of lactic acid bacteria. The essential characteristics of pathogenic microorganisms can be found in numerous texts. There is a certain degree of conflicting data concerning the importance of various pathogens with regard to safety of specific foods and the effect of specific temperatures or packaging atmospheres on their growth or inhibition. Pathogenic bacteria such as Salmonella spp., L. monocytogenes, Clostridium botulinum type E, psychrotrophic B. cereus, Aeromonas hydrophila, and Y. enterocolitica are of particular concern in chilled foods because they are capable of growth at low temperatures (Table 2). Many of the organisms that compete with pathogens at ambient temperatures will not grow at low temperatures, thus low temperatures may
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Table 2 Minimum and optimum growth temperatures for pathogens associated with foods
Campylobacter spp. Arcobacter spp. C. perfringens B. cereus mesophilic C. botulinum proteolytic Shigella spp. S. aureus Pathogenic E. coli strains E. coli O157:H7 Salmonella spp. B. cereus psychrotrophic Vibrio parahaemolyticus C. botulinum non-proteolytic A. hydrophila L. monocytogenes Y. enterocolitica
Minimum temperature (1C)
Optimum temperature (1C)
30–32 15 12 10–15 10 7–10 7 7 6–7 5 4–5 5–10 3 0.1 to 1.2 1 to 0 2
42–45 37 43–47 35–40 35 37 35–40 35–40 42 35–43 28–35 30–37 30 15–20 30–37 22–30
preferentially favor the growth of pathogenic organisms. Pasteurization may also favor the outgrowth of surviving spores of psychrotrophic strains of C. botulinum and B. cereus by destroying competing microflora, a particular concern in vacuum packaged pasteurized foods (‘sous vide’ products), and ready-to-eat (RTE) products. However, most will not grow, or produce toxins, below 4 1C, with the exception of L. monocytogenes, C. botulinum type E, psychrotrophic B. cereus, A. hydrophila, and Y. enterocolitica. Investigations into the effect of different storage atmospheres on pathogenic growth at low temperatures appear to show that carbon dioxide (CO2)-enriched atmosphere produce the greatest inhibitory effect on psychrotrophic pathogens (Y. enterocolitica, A. hydrophila, and L. monocytogenes). Garcı´a de Fernando et al. (1995) concluded that ‘‘at a normal meat pH (i.e., 5.5) and at a low temperature (e.g., 1 1C) the growth of psychrotrophic pathogens is stopped when the CO2 concentration is 40%.’’ However, high pH meat (Z6) and/or higher storage temperatures will support growth of such pathogens. The bacterial safety and rate of spoilage depends on the numbers and types of microorganisms initially present, the rate of growth of those microorganisms, the conditions of storage (temperature and gaseous atmosphere), and characteristics (pH, aw) of the food. Of these factors, temperature is by far the most important. In the context of food safety the safest practical chilled temperature will be the lowest that can be used without significantly affecting the quality of the food. In many cases this will be as close to the freezing point of the food as practically possible. Maintaining temperatures and the integrity of the whole of the chill chain is vital to ensure the safety and quality of chilled foods.
Effect of Chilling Rate Whether ‘rapid’ chilling offers any clear advantages to product safety will depend on what biological hazards (pathogens,
etc.) are present, at what numbers they are present, and whether they are on, or in, the food in question, and how ‘rapid’ the rate is in comparison with other rates. There is no definition of ‘rapid’ and ‘slow’s rates. Size of product will also have a big effect on relative rates of chilling, because conduction through the product will become the rate-limiting factor as product size increases. There are instances where excessively rapid chilling rates, or too low a chilling temperature, can cause quality problems in foods. For example, a serious defect known as ‘woolly texture’ can be produced in rapidly cooled peaches. Substantial textural problems due to a phenomenon known as ‘cold shortening’ can occur in rapidly chilled meats (particularly beef and lamb), although electrical stimulation before rapid chilling will mitigate this problem. Few countries provide legislative requirements for chilling foods that specify chilling rates. For example, European legislation requires that red meat carcasses be chilled to a maximum temperature of 7 1C and poultry carcasses to a maximum temperature of 4 1C. No time limits on achieving these times have been set. There are very little data on the effect of current commercial chilling rates and conditions on changes in bacterial numbers during the process. In most cases no change or a small reduction (0.5 to 1 log10 cfu cm2) in number of organisms on the surface has been measured. Current European legislation for foods of animal origin places importance on core meat temperatures, because microbial contamination is primarily a surface phenomenon there is an argument to be made that surface temperatures are far more important than deep temperatures. This is the basis behind the controls in the Australian Export Control (Meat and Meat Products) Orders 2005 and their Refrigeration Index (RI) model. The central idea of the RI model is to measure the performance of the chilling process until all the sites of microbiological interest are at or below 7 1C and calculate the risk of E. coli growth from temperatures measured during chilling. Although there are few legislative requirements, there are many guidelines and recommendations for chilling cooked/ pasteurized food products (Table 3), particularly RTE products that can be eaten cold. Inadequate cooling of such products has been identified as potential safety risk, as there is always the possibility that some microorganisms, particularly spores, will not be killed by the cooking/pasteurization process. Therefore most guidelines recommend that cooked products should be cooled as quickly as possible through the temperature range 63 to 5 1C or less to minimize risk of spore germination and outgrowth. There is particular concern with vacuum packaged pasteurized foods (termed by some as refrigerated processed foods of extended durability, and including ‘sous vide’ products) that conditions (low oxygen and destruction of competing microflora) may favor the outgrowth of surviving spores of psychrotrophic strains of C. botulinum and B. cereus. Consequentially lower storage temperatures (o3 1C) are recommended for these products and/or more severe heat processes. With RTE products, there have been concerns over the possible presence and outgrowth of spores of psychrotrophic B. cereus. Although many of these guidelines were initially produced specifically for cook-chill catering operations, in many countries, the producers of chilled ready meals for retail sale use them.
Food Technologies: Chilling
Table 3
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International chilling time guidelines/recommendations for the cooling of cooked foods
Country
Chilling range (1C)
Time (h)
Chilling rate (1C min 1)
Storage temperature (1C)
References
Australia
60–21 21–5 60–20
r2 r4 r2
0.33 0.07 0.33
5
De Jong et al. (2004)
4
Canadian Food Inspection System Implementation Group (CFISIG) (2004)
20–4 60–10 65–10 70–10 80–15 (15–2) 70–3
r4 r2 r3 r2 r2 r24 r2.5
0.07 0.42 0.31 0.50 0.54
– o5 0–3 2
Codex Alimentarius Commission (1999) Evans et al. (1996) Evans et al. (1996) Evans et al. (1996)
0.45
3
60–7 7–4 80–8 70–3 60–5
r5 r24 r4 r1.5 4–6
0.18
–
Food Safety Authority of Ireland (FSAI) (2004) De Jong et al. (2004)
0.30 0.74 0.23–0.15
3 3 –
Evans et al. (1996) UK Department of Health (1989) De Jong et al. (2004)
Canada
Codex Alimentarius Denmark France Germany Ireland The Netherlands Sweden UK USA
Because prolific histamine-forming bacteria are predominately mesophilic, rapid cooling of at-risk fish species (such as tuna and mackerel) immediately after catching is recognized as the key control for reducing histamine formation in such fish, as is the maintenance of adequate temperature control during the rest of the cold-chain. Similarly prompt and rapid cooling of other seafood, particularly fish and shellfish that are eaten raw, such as oysters, is an important control for V. parahaemolyticus. The maintenance of tight temperature control during the rest of the cold-chain is also very important with V. parahaemolyticus because it is capable of growth at temperatures as low as 5 1C, and is known for rapid growth and generation times as short as 12–18 min.
Chilling Operations Chilling systems for foodstuffs will contain many, if not all, of the following unit operations:
• • • • • •
Preparatory treatment; conditioning, waxing, cooking, pasteurizing, blanching, etc. Chilling; primary or secondary. Storage. Transportation. Retail display. Domestic storage.
During the preparatory treatment there can be a range of temperature responses, from a large gain to a small decrease in the temperature of the product. During chilling there is a substantial decrease in the mean temperature of the product. Within a correctly designed coldchain there should be no significant change in mean product temperature during storage, transport, retail display, or domestic storage. From a food safety-based approach, prepacking the food before chilling will lower the risk of contamination/
cross-contamination during the chilling process, however, it will significantly reduce the rate of cooling, and this may allow the growth of any microorganisms present. Provided the cooling media (air, water, etc.) and refrigeration equipment used are kept clean, no one cooling method can be said to be intrinsically more hygienic, or safe, than any other. For example, although there has been much debate concerning the safety of chilled water-cooling versus blast air chilling of poultry carcasses, reviews of published data have found little evidence of any difference microbially between poultry cooled by either method. Similarly there appears to be no evidence of a difference between spray chilling versus blast air chilling of red meat carcasses. Although some pathogens, such as campylobacters and arcobacters, may show sensitivity to air chilling; immersion and spray chilling may result in some physical removal (washing off) of microorganisms. In addition the use of antimicrobials, such as chlorine or ozone, in chilled water immersion/spray systems has been shown in some studies to reduce microbial contamination and subsequently may improve food safety. Such substances, however, are not permitted in all countries (notably the European Union) at present. The potential for the fans used in air chilling to disseminate molds and bacteria has been identified in a number of reviews as a potential hazard, but very little work has been carried out to evaluate whether this is in fact the case. Similarly, condensation in the chiller has also been identified as a possible source of cross-contamination during chilling and storage (particularly of unpackaged foods). EU legislation regarding the chilling of foods of animal origin specifies that ‘‘during the chilling operations, there must be adequate ventilation to prevent condensation on the surface of the meat.’’ However, there appears to be little published data to support such a control. It is however prudent that chillers should be properly constructed and maintained. The design of chillers should be such as to provide for effective cleansing and disinfection.
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Chilling Systems
ice/water mixture, with the carcasses being conveyed through a tank of ice/water using Archimedean screws.
Moving Air Air is by far the most widely used method of chilling food as it is economical, hygienic, and relatively noncorrosive to equipment. 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 conveyerized blast chilling tunnels or spirals. Air chilling is slow but versatile especially when there is a requirement to cool a variety of products. The cooling time of the product is reduced as the air speed is increased, thus air distribution is very important. Conveying the product through the cooling system overcomes the problem of uneven air distribution because each item is subjected to the same velocity/time profile. The refrigeration capacity and air conditions can also be varied throughout the length of the tunnel. Using higher air temperatures in the latter stages can avoid surface freezing.
Immersion/Spray As their names imply, these involve dipping product into a cold liquid, or spraying a cold liquid onto the food. When water is used as the heat transfer medium the process is often called ‘hydrocooling.’ This produces high rates of heat transfer due 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. Clearly if the food is unwrapped the liquid has to be ‘food safe.’ Any uptake of the cooling medium, whether ‘food safe’ or not, by the product may present problems both in terms of flavor changes and the requirement for periodical replacement of the medium. Hydrocooling is probably the least expensive method of achieving rapid cooling in small products. The product to be cooled is immersed in, or sprayed with, cool water, either at ambient or near 0 1C. Where permitted, the water is often treated with a mild disinfectant, such as chlorine. Practical systems vary from simple stirred or unstirred tanks to plants where the product is conveyed through agitated tanks or under banks of sprays. Hydrocooling is very effective for chilling fruit and vegetables, however, not all crops can tolerate wetting. Spraying with ambient or chilled water is also an effective method for initially cooling cooked products that can withstand wetting, i.e., hams, sausages, chubs, etc. Spray bars are either fitted in the batch cookers or in separate cooling cabinets. Spray chilling of meat carcasses is widely practiced in the USA and used for poultry in Europe. 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 melted water percolating through it than with the ice itself. 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 to 0.5 1C (where sea water is present). Ice may also be used for cooling fruit and vegetables, however, as with hydrocooling, ice is only applicable to vegetable and fruit produce that can tolerate wetting, such as watercress, broccoli, and some other brassicas. Large poultry carcasses, such as turkeys, are also often initially cooled by immersion in an
Vacuum Solid products having a large surface area to volume ratio and an ability to readily release internal water are amenable to vacuum cooling. The products are placed in a vacuum chamber (typically operating at between 530 and 670 N m2) 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 1C. Suitable products, such as lettuce, can be vacuum cooled in less than 1 h. In general terms, a 5 1C reduction in product temperature is achieved for every 1% of water that is evaporated. Because vacuum cooling requires the removal of water from the product prewetting is commonly applied to prevent the removal of water from the tissue of the product.
Plate Heat Exchangers In a plate cooling system product is essentially pressed between hollow (horizontal or vertical) metal plates containing a circulating refrigerant. Cooling rate depends on product thickness, good contact, and the conductivity of the product. Thus the need for regularly shaped products with large flat surfaces is a major hindrance. Air spaces in packaging and fouling of the plates can have a significant effect on cooling time.
Jacketed Heat Exchangers Batch coolers for liquid foods can range in capacity from 100 to 10 000 l with the foodstuff usually contained in a stainless steel vessel. The cooling medium may circulate through the jacket of the vessel, through a coil immersed in the liquid, or both. Most vessels are provided with agitators to improve the rate of convective heat transfer and stop temperature stratification. One common method used to decrease cooling times of liquid products in a closed vessel is to apply a vacuum to produce evaporative cooling. Temperature stratification is a problem in unstirred vats whilst in agitated vessels the design and operation of stirrers is critical if breakdown of delicate solid product is to be avoided.
Belt Heat Exchangers Belt systems consist of an endless steel belt (approximately 1 mm in thickness), the underside of which is cooled either directly with water, brine, or glycol sprays or by sliding over a stationary cold surface. Because only one side of the product is in contact with the cooling surface relatively thin products are required, such as hamburgers, fish fillets, or liquid and semiliquid products such as pure´es and sauces. The main advantages of belt systems are: (1) continuous processing, (2) easy continuous cleaning and sanitation, (3) reduced evaporative losses, in comparison with air systems, and (4) the possibility of operating with several temperature zones.
Food Technologies: Chilling Continuous Heat Exchangers The majority of liquid foodstuffs require cooling after a heat processing operation such as cooking of sauces and soups, and pasteurization or sterilization of fruit juices, milk, and other dairy products. Milk is also cooled at the point of collection to maintain its quality. Unpasteurized ‘freshly squeezed’ fruit juices are cooled immediately after production. Fermented beverages are often cooled during primary and secondary fermentation, and before storage. Multiplate coolers are extensively used for liquid foods. They have the highest available heat transfer surface, lowest material requirements, maximum efficiency (up to 90% heat recovery in counter current mode) and are very flexible in operation and easy to clean. Scraped surface heat exchangers can have advantages in the cooling of very viscous liquid foods and where surface fouling is a potential problem.
Cryogenic Cryogenic cooling uses refrigerants, such as liquid nitrogen or solid carbon dioxide, directly. 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. Avoiding surface freezing of the product is the main problem in using cryogens for chilling. Forced gas cooling is the only method that can be employed when surface freezing has to be avoided. However, the system tends to be inefficient.
Chilled Storage Most unwrapped meat, poultry, fish, fruit and vegetables, and all types of wrapped foods are stored in large rooms with circulating air. To minimize weight loss and appearance changes associated with desiccation air movement around the unwrapped product should be the minimum required to maintain a constant temperature. With wrapped products low air velocities are also desirable to minimize energy consumption. However, many storage rooms are designed and constructed with little regard to air distribution and localized velocities over products. Horizontal throw refrigeration coils are often mounted in the free space above the racks or rails of product and no attempt is made to distribute the air around the products. Using a false ceiling or other forms of ducting to distribute the air throughout the storage room can substantially reduce variations in velocity and temperature. Controlled atmosphere storage rooms were developed for specialized fruit stores. In addition to the normal temperature control plant these stores also include special gas-tight seals to maintain an atmosphere that is normally lower in oxygen and higher in nitrogen and carbon dioxide than air. An additional plant is required to control the carbon dioxide concentration, generate nitrogen, and consume oxygen. The optimum atmosphere must be determined experimentally for the specific product being stored. There is growing use of controlled atmosphere and modified atmosphere retail packs to extend the chilled storage and display life of red meats, poultry, fish, and vegetables. Because the packs tend to be large and insulate the products efficient precooling before packaging is especially
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important if product quality is to be maintained. Provided that temperatures during chilled storage are sufficient to prevent or inhibit the growth of any pathogens present on the food in question, in general the food will spoil before there is significant growth in pathogens would be a cause of concern. The safe storage life of chilled foods depends on the control of all the preceding factors, but must be validated by challenge testing and/or modeling for each product and process under defined storage conditions. However, from a practical point of view, the lowest storage temperature that can be used without affecting the quality of the food will be the safest.
Transportation Chilled foods are transported around the world and locally via a range of transportation systems. 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. It is particularly important that the food is at the correct temperature before loading because 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. In the large containers used for long distance transportation food temperatures can be kept within 70.5 1C of the set point. Control of the oxygen and carbon dioxide levels in shipboard containers has allowed fruits 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. Even longer shelf-lives (over 20 weeks) can now be achieved for meats, particularly beef and lamb. Air-freighting is increasingly being used for high value perishable products, such as strawberries, asparagus, and live lobsters. Although air-freighting of foods offers a rapid method of serving distant markets, there are many problems because the product is usually 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 the flight the hold is normally between 15 and 20 1C. Perishable cargo is usually carried in standard containers, sometimes with an insulating lining and/or dry ice but is often unprotected on aircraft pallets. 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. 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 overland delivery.
Retail Display 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
Food Technologies: Chilling
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. The required display life and consequent environmental conditions for wrapped chilled products differ from those for unwrapped products. The desired chilled display life for wrapped meat, fish, vegetables, and processed foods ranges from a few days to many weeks and is primarily limited by microbiological considerations. Retailers of unwrapped fish, meat, and delicatessen products, for example, sliced meats, pate, cheese, and prepared salads, normally require a display life of one working day. The introduction of humidification systems can significantly improve display life of unwrapped products. Average temperatures in chilled retail display cabinets can be varied considerably from cabinet to cabinet, with inlet and outlet values ranging from 6.7 to þ 6.0 1C, and 0.3 to þ 7.8 1C, respectively, in one survey. The temperature performance of an individual display cabinet not only depends on its design but also its position within a store and the way the products are positioned within the display area significantly influence product temperatures. 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 instore survey of 299 prepackaged meat products in chilled retail displays found product temperatures in the range 8.0 to 14.0 1C, with a mean of 5.3 1C and 18% above 9 1C. Other surveys have shown that temperatures of packs from the top of stacks were appreciably higher than those from below due to radiant heat pickup 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 due to radiant heat trapped in the package by the
7.4
Mean temperature (°C)
Recommended refrigerator temperatures are in general below 8 1C throughout the world, with many countries (including the UK) recommending below 5 1C. The numerous surveys on the domestic storage of refrigerated foods show remarkable similarities in consumer attitudes and handling of chilled foods and the performance of their refrigerators. Perhaps even more remarkable is that despite numerous recommendations on handling and storage temperat‘ures, consumer use and the performance of refrigerators remain remarkably unchanged throughout the world over the past 30 years. Numerous surveys (Figure 1) show that mean temperatures range between 5 and 7 1C, with 50–70% of domestic refrigerators operating at temperatures above 5 1C. It is clear that many refrigerators throughout the world are running at higher than recommended temperatures. Because even these recommended temperatures are higher than the 0–1 1C that is usually the recommended temperature range for storing fish and seafood, meat, and many chilled products the current situation is even more detrimental to maintaining the high quality life of chilled foods. Although there have been many surveys of refrigerators, how fridge temperatures, cleanliness, and consumer practices (e.g., over filling with warm products, use for cooling hot cooked foods) impact on consumer health remains to be fully assessed. The few studies that have assessed these factors do not appear to have demonstrated clear links between incidences of food poisoning and operating temperatures of cleanliness.
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7
6
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Domestic Handling
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5
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5
5
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8
‘greenhouse’ effect. However, specific investigations have failed to demonstrate this effect. The display life of chilled foods depends on the control of all the preceding factors, but must be validated by challenge testing and/or modeling for each product and process under defined chill display conditions. It must be recognized that the integrity of the whole of the chill chain is vital to ensure the safety and quality of chilled foods.
UK 06
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4
4 3 2 1
UK 09
Greece 05
Ireland 05
Sweden 04
France 02
USA 99
UK 98
UK 97
Netherlands 97
New Zealand 97
North Ireland 92
UK 91
UK 90
0
Figure 1 Reported mean temperatures recorded in surveys of domestic refrigerators throughout the world since 1990.
Food Technologies: Chilling
Temperature Measurement and Monitoring Temperature measurement and monitoring is an integral part of any food cold-chain management system; as well as being, in many areas of the cold-chain, a legislative requirement. Monitoring the cold-chain requires detailed information on food product temperatures. Temperature monitoring includes both measurement and recording. Temperature measurement can be achieved using a variety of instrumentation such as bimetal style thermometers, thermistors, thermocouples, infrared thermometers, etc. Typically, in the food industry, temperature measurement is achieved using calibrated thermocouples and data loggers. Owing to the variety of available equipment, manufacturers and suppliers are best positioned to give advice to the food business on the choice of temperature measurement equipment for specific purposes and food products. Advice can also be found in numerous International and National recommendations and guidelines for chilled foods. One possible aid in the future may be the widespread use of time temperature indicators (TTI) or integrators throughout the cold-chain. TTIs are simple devices that are capable of reporting a visual and straightforward summary of either the temperature (indicators) or time-temperature exposure history (integrators) of the product. Indicators show that a product has exceeded, positively or negatively, a given temperature Whereas Integrators monitor both time and temperature during a given period and show the cumulative effect of temperature fluctuations during the history of the product.
Conclusions The safety of a chilled food depends on the numbers and types of microorganisms initially present, the rate of growth of those microorganisms, the conditions of storage (temperature and gaseous atmosphere), and characteristics (pH, aw) of the food. Of these factors, temperature is by far the most important. In the context of food safety, the safest practical chilled temperature will be the lowest temperature that can be used without significantly affecting the quality of the food. In many cases this will be as close to the freezing point of the food as practically possible, i.e., between 0 and 1 1C. Maintaining temperatures and the integrity of the whole of the chill chain is vital to ensure the safety, and quality, of chilled foods. In general, after initial chilling, as a chilled product moves along the cold-chain it becomes increasingly difficult to control and maintain its temperature. Temperatures of bulk packs of chilled 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, etc. Even when temperature controlled dispatch bays are used there is a slight heat pickup during loading. In bulk transportation the resulting temperature rise is small and the
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vehicle’s refrigeration system rapidly returns the product to the required temperature. Larger problems exist in local multidrop 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 due to the retailers’ need to display as much product as possible in a way that is easily assessable to the consumer. Increasing energy costs may be the key factor that persuades retailers to reduce consumer access, i.e., install doors on chilled retail display cabinets, and hence improve temperature control. At present domestic storage of chilled foods would appear to be the weakest link in the entire chill chain. However, despite all the many published surveys on refrigerator temperatures carried out in the past 30 years, how fridge temperatures, cleanliness, and consumer practices (e.g., over filling with warm products, use for cooling hot cooked foods) impact on consumer health remains to be fully assessed.
See also: Foodborne Diseases: Overview of Biological Hazards and Foodborne Diseases
References Ansay SE, Darling KA, and Kaspar CW (1999) Survival of Escherichia coli O157: H7 in ground-beef patties during storage at 2, 2, 15 and then 2 1C, and 20 1C. Journal of Food Protection 62: 1243–1247. Canadian Food Inspection System Implementation Group (CFISIG) (2004) Food Retail and Food Services Code. Amended edn., Ottawa, ON: Health Canada. Codex Alimentarius Commission (1999) Code of Hygienic Practice for Refrigerated Packaged Foods with Extended Shelf Life. Food and Agriculture Organization of the United Nations. CAC/RCP 46. Rome: FAO. De Jong AEI, Rombouts FM, and Beumer RR (2004) Effect of cooling on Clostridium perfringens in pea soup. Journal of Food Protection 67: 352–356. Evans J, Russell S, and James SJ (1996) Chilling of recipe dish meals to meet cook-chill guidelines. International Journal of Refrigeration 19: 79–86. Food Safety Authority of Ireland (FSAI) (2004) Guidance Note No. 15: Cook-Chill Systems in the Food Service Sector. Dublin, Ireland: Food Safety Authority of Ireland. Garcı´a de Fernando GD, Nychas GJE, Peck MW, and Ordı´n˜ez JA (1995) Growth/ survival of psychrotrophic pathogen on meat packaged under modified atmospheres. International Journal of Food Microbiology 28: 221–231. Logue CM, Sheridan JJ, McDowell DA, Blair IS, and Harrington D (1998) A study of the growth of plasmid bearing and plasmid cured strains of antibiotic resistant Yersinia enterocolitica serotype O:3 on refrigerated beef, pork and lamb. Food Microbiology 15: 603–615. Lund BM, Baird-Parker TC, and Gould GW (eds.) (2000) The Microbiological Safety and Quality of Food. Gaithersburg: Aspen Publishers Inc. Mackey BM, Roberts TA, Mansfield J, and Farkas G (1980) Growth of Salmonella on chilled meat. Journal of Hygiene (Cambridge) 85: 115–124. Pawar DD, Malik SVS, Bhilegaonkar KN, and Barbuddhe SB (2000) Effect of nisin and its combination with sodium chloride on the survival of Listeria monocytogenes added to raw buffalo meat mince. Meat Science 56: 215–219. Smith MG (1985) The generation time, lag time, and minimum temperature of growth of coliform organisms on meat, and the implications for codes of practice in abattoirs. Journal of Hygiene (Cambridge) 94: 289–300. UK Department of Health (1989) Chilled and Frozen, Guidelines on Cook-Chill and Cook-Freeze Catering Systems. London: UK Department of Health. Walls I and Scott VN (1996) Validation of predictive mathematical models describing the growth of Escherichia coli O157:H7 in raw ground beef. Journal of Food Protection 59: 1331–1335.
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Food Technologies: Chilling
Relevant Websites http://www.ecff.net/ European Chilled Food Federation. http://www.fao.org/ Food and Agriculture Organization of the United Nations.
http://www.iifiir.org/ International Institute of Refrigeration. http://www.chilledfood.org/ UK Chilled Food Association.