Chilled and frozen retail display In general, display cabinets have to accommodate three types of meat and meat products: (1)chilled wrapped, (2) chilled unwrapped and (3)frozen wrapped products. 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 and meat prod- ucts ranges from a few days to many weeks and is primarily limited by microbiological considerations. Retailers of unwrapped meat and deli catessen products, for example sliced meats and pate, normally require a display life of one working day. Frozen products can remain on display for many weeks 11.1 Chilled display of wrapped meat and meat products To achieve the display life of days to weeks required for wrapped chilled meat, the product should be maintained at a temperature as close to its initial freezing point, -15C, as possible. To maintain product temperature in the range-1-0C is the stated aim of at least one manufacturer of multi- deck display cases for wrapped meat(Rolls, 1986). Growth of salmonella is prevented by temperatures below 7C. Whilst growth of Listeria mono- cytogenes is slowed by refrigeration, it still multiplies very slowly even at 1C unless the pH is below ca. 5.0. Consequently displaying meat at tem- peratures consistently in the -1-3C range would substantially improve product safety Air movement and relative humidity(rh) have little effect on the display life of a wrapped product, but the degree of temperature control
11 Chilled and frozen retail display In general, display cabinets have to accommodate three types of meat and meat products: (1) chilled wrapped, (2) chilled unwrapped and (3) frozen wrapped products. 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 and meat products ranges from a few days to many weeks and is primarily limited by microbiological considerations. Retailers of unwrapped meat and delicatessen products, for example sliced meats and paté, normally require a display life of one working day. Frozen products can remain on display for many weeks. 11.1 Chilled display of wrapped meat and meat products To achieve the display life of days to weeks required for wrapped chilled meat, the product should be maintained at a temperature as close to its initial freezing point, -1.5 °C, as possible. To maintain product temperature in the range -1–0 °C is the stated aim of at least one manufacturer of multideck display cases for wrapped meat (Brolls, 1986). Growth of Salmonella is prevented by temperatures below 7 °C. Whilst growth of Listeria monocytogenes is slowed by refrigeration, it still multiplies very slowly even at 1 °C unless the pH is below ca. 5.0. Consequently displaying meat at temperatures consistently in the -1–3 °C range would substantially improve product safety. Air movement and relative humidity (RH) have little effect on the display life of a wrapped product, but the degree of temperature control
232 Meat refrigeration can be important especially with transparent, controlled atmosphere packs During any control cycle, the cabinet temperature rises, heat enters the pack, the atmosphere inside the pack warms with consequent reduction in elative humidity and increase in the surface temperature of the product. As the surface temperature rises so does its saturation vapour pressure(a factor controlling evaporation) and more water evaporates into the sealed atmosphere of the pack. If the cabinet temperature was stabilised ther evaporation would continue until the atmosphere became saturated However, in practice the cabinet air temperature cycles and as it is reduced the wrapping film is cooled. If it reaches a temperature below the dew point of the atmosphere inside the pack, then water vapour will condense on the inner surface of the pack. This film of water can obscure the product and consequently reduce consumer appeal. As the cycling process continues the appearance of the product deteriorates. To maintain product temperatures close to 0C, the air off the coil must typically be -4'C and any ingress of humid air from within the store wil quickly cause the coil to ice up. Frequent defrosts are often required and even in a well maintained unit the cabinet temperature will then rise to 0-12C and the temperature of the product will rise by at least 3C (Brolls 1986). External factors such as the store ambient temperature, the siting of he 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 air flow 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-8.0-14.0° C with a mean of 5.3°Cand18% above9°C(Rose,1986) Other surveys(Bogh-Sprensen, 1980: Malton, 1971)have shown that tem- peratures of packs from the top of stacks were appreciably higher than hose from below owing to radiant heat pick up from store and cabinet ghting. It has also been stated that products in transparent film over wrapped packs can achieve temperatures above that of the surrounding refrigerated air owing to radiant heat trapped in the package by the green house'effect. However, specific investigations failed to demonstrate this effect(Gill, 1988) 11.1.1 Factors affecting display life The display life of wrapped meat can be affected by the diet and the treatment of the meat before display During display 4C TBARS values were lower and Huntera' values higher in pork chops from pigs fed with a high 100-200mg a-tocopherol acetate per kilogram diet than those fed with 10mgkg(Monahan et al., 1994). Lipid oxidation and colour deterioration were also faster during display of chops that had
can be important especially with transparent, controlled atmosphere packs. During any control cycle, the cabinet temperature rises, heat enters the pack, the atmosphere inside the pack warms with consequent reduction in relative humidity and increase in the surface temperature of the product. As the surface temperature rises so does its saturation vapour pressure (a factor controlling evaporation) and more water evaporates into the sealed atmosphere of the pack. If the cabinet temperature was stabilised then evaporation would continue until the atmosphere became saturated. However, in practice the cabinet air temperature cycles and as it is reduced the wrapping film is cooled. If it reaches a temperature below the dew point of the atmosphere inside the pack, then water vapour will condense on the inner surface of the pack. This film of water can obscure the product and consequently reduce consumer appeal. As the cycling process continues the appearance of the product deteriorates. 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 will then rise to 10–12 °C and the temperature of the product will rise by at least 3 °C (Brolls, 1986). External factors such as the store ambient temperature, the siting 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 air flow 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 -8.0–14.0 °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 failed to demonstrate this effect (Gill, 1988). 11.1.1 Factors affecting display life The display life of wrapped meat can be affected by the diet of the animal and the treatment of the meat before display. During display for 8 days at 4 °C TBARS values were lower and Hunter ‘a’ values higher in pork chops from pigs fed with a high 100–200 mg a-tocopherol acetate per kilogram diet than those fed with 10 mg kg-1 (Monahan et al., 1994). Lipid oxidation and colour deterioration were also faster during display of chops that had 232 Meat refrigeration
Chilled and frozen retail display 233 been previously frozen and thawed before display. Overageing of meat can limit its display life. Bell et al. (1996) found that hot-boned bull beef aged at 5'C for 6 days could only be displayed for 24 h at 5C before it was unac ceptable because of its dull dark lean tissue and grey to green discoloration of the fat. Similar meat that was unaged but also stored for 70 days -1.0±0.5° could be displayed for48h Retail display characteristics of steaks from hot-boned logissimus dorsi (LD)and M. semimembranosus(SM)muscles from electrically stimulated sides were found to be similar to those from cold-boned unstimulated sides (Griffin et aL., 1992). Whole muscles from both treatments were stored for up to 21 days before cutting into steaks. The colour of the lean from meat stored for 21 days was brighter than that stored for 7 or 14 days. Lean colour, fat colour and overall appearance scores all decreased with time over the 5 day display period Previous storage will reduce the display life of meat and it is bette store meat in large pieces. Meat that had been minced before storage lost its red colour more rapidly during display than that minced immediately before display(Madden and Moss, 1987). The addition of carbon dioxide (CO2)prior to storage has a beneficial effect on colour and bacterial growth during display. Additions of 2-4g of solid CO2 per kilogram of meat resulted in growth of total viable bacterial counts similar to that of unstored con trols. However, total anaerobic levels were much higher than controls. Storing prepacked meat in a gas flushed ' bag has been advo- cated as a method of extending the chilled storage life without reducing the retail display life of the packs. Scholtz et al.(1992)packed pork loin chops in individually overwrapped Styrofoam trays which were then bulk packed in vacuum bags which were subsequently inflated with 100%CO2. After up to 21 days storage in the mother bags at 0"C the packs had a subsequent retail display life of 4 days. A retail display life of 4 days could only be attained after storage for 14 days in modified atmosphere packs or for 7 days in vacuum skin packs a display life of 6 days can be achieved in pork loin chops obtained fro resh pork loins and vacuum packed in high oxygen-barrier films(Vrana et al., 1985). The chops were displayed at 2+ 2C for cycles of 14h under an illumination of 1614 lux, followed by 10h in the dark. Under similar con- ditions chops packed in high oxygen-permeable film had a display life of 4 11.1.2 Layout of chilled cabinet a typical cabinet has a refrigeration unit behind the display ed to the chilled air from the refrigeration unit is blown by a fan and delivere relevant area by duct work behind the display area( Fig. 11. 1). After the air has been delivered to the display area it is then drawn back into the duct through a grille and is refrigerated again to continue the cycle
been previously frozen and thawed before display. Overageing of meat can limit its display life. Bell et al. (1996) found that hot-boned bull beef aged at 5 °C for 6 days could only be displayed for 24 h at 5 °C before it was unacceptable because of its dull dark lean tissue and grey to green discoloration of the fat. Similar meat that was unaged but also stored for 70 days at -1.0 ± 0.5 °C could be displayed for 48 h. Retail display characteristics of steaks from hot-boned logissimus dorsi (LD) and M. semimembranosus (SM) muscles from electrically stimulated sides were found to be similar to those from cold-boned unstimulated sides (Griffin et al., 1992). Whole muscles from both treatments were stored for up to 21 days before cutting into steaks. The colour of the lean from meat stored for 21 days was brighter than that stored for 7 or 14 days. Lean colour, fat colour and overall appearance scores all decreased with time over the 5 day display period. Previous storage will reduce the display life of meat and it is better to store meat in large pieces. Meat that had been minced before storage lost its red colour more rapidly during display than that minced immediately before display (Madden and Moss, 1987). The addition of carbon dioxide (CO2) prior to storage has a beneficial effect on colour and bacterial growth during display.Additions of 2–4g of solid CO2 per kilogram of meat resulted in growth of total viable bacterial counts similar to that of unstored controls. However, total anaerobic levels were much higher than controls. Storing prepacked meat in a gas flushed ‘mother’ bag has been advocated as a method of extending the chilled storage life without reducing the retail display life of the packs. Scholtz et al. (1992) packed pork loin chops in individually overwrapped Styrofoam trays which were then bulk packed in vacuum bags which were subsequently inflated with 100% CO2. After up to 21 days storage in the mother bags at 0 °C the packs had a subsequent retail display life of 4 days. A retail display life of 4 days could only be attained after storage for 14 days in modified atmosphere packs or for 7 days in vacuum skin packs. A display life of 6 days can be achieved in pork loin chops obtained from fresh pork loins and vacuum packed in high oxygen-barrier films (Vrana et al., 1985). The chops were displayed at 2 ± 2 °C for cycles of 14 h under an illumination of 1614 lux, followed by 10h in the dark. Under similar conditions chops packed in high oxygen-permeable film had a display life of 4 days. 11.1.2 Layout of chilled cabinet A typical cabinet has a refrigeration unit behind the display area. The chilled air from the refrigeration unit is blown by a fan and delivered to the relevant area by duct work behind the display area (Fig. 11.1). After the air has been delivered to the display area it is then drawn back into the duct through a grille and is refrigerated again to continue the cycle. Chilled and frozen retail display 233
234 Meat refrigeration Fig. 11.1 Multi-deck display cabinet for wrapped products The duct provides two functions: (1)to provide cold air through the holes n the rear panel and(2)to provide an air curtain at the front of the cabinet The holes in the rear panel direct chilled air over the food and the air curtain provides a thermal barrier between the chilled display area and the like perfect' cabinet would have its chilled air form a closed cycle, much like a domestic refrigerator when the door is closed. In reality warm mois air from the surrounding store entrains with chilled air from the air curtain causing a loss of chilled air from the cabinet and a gain of warm air and moisture 11.1.3 Air curtain The air curtain differs from a solid door as it provides no physical barrier between customer and product, but is similar to a door in that it does provide a thermal barrier. The air curtain is a jet of chilled air of about l ms that exits the duct at the top of the cabinet and falls down the face of the cabinet to the return grille. Owing to the fact that the temperature of the air from the air curtain is lower than the surrounding air it is denser and therefore is aided by natural convection in its downward motion. The air curtain is very sensitive and its effectiveness has other implica tions. An ineffective air curtain is likely to have the following effects Increased icing up and therefore more defrosts of the refrigeration coil
The duct provides two functions: (1) to provide cold air through the holes in the rear panel and (2) to provide an air curtain at the front of the cabinet. The holes in the rear panel direct chilled air over the food and the air curtain provides a thermal barrier between the chilled display area and the store. A ‘perfect’ cabinet would have its chilled air form a closed cycle, much like a domestic refrigerator when the door is closed. In reality warm moist air from the surrounding store entrains with chilled air from the air curtain causing a loss of chilled air from the cabinet and a gain of warm air and moisture. 11.1.3 Air curtain The air curtain differs from a solid door as it provides no physical barrier between customer and product, but is similar to a door in that it does provide a thermal barrier. The air curtain is a jet of chilled air of about 1ms-1 that exits the duct at the top of the cabinet and falls down the face of the cabinet to the return grille. Owing to the fact that the temperature of the air from the air curtain is lower than the surrounding air, it is denser and therefore is aided by natural convection in its downward motion. The air curtain is very sensitive and its effectiveness has other implications. An ineffective air curtain is likely to have the following effects: • Increased temperature of product, • Increased icing up and therefore more defrosts of the refrigeration coil, 234 Meat refrigeration Fig. 11.1 Multi-deck display cabinet for wrapped products
Chilled and frozen retail display 235 Increased energy consumption of the refrigeration compressor. About 60% of electricity consumed in modern supermarkets is used by display cabinets for frozen and chilled foodstuffs, Decreased temperature in the store next to the cabinet. This is described Is thecold feet effectand can lead to temperatures as low as 10C ir he centre of refrigerated aisles. The are many variables affecting the efficiency of the air curtain, for The temperature difference between the chilled air and the store air The velocity of the air curtain The thickness of the air curtain The pressures either side of the curtain Obstructions in the path of the air curtain. Some cabinets use a dual air curtain which has an extra jet of air parallel to the first jet but on the store side. This jet has the same velocity as the first jet but the air is not refrigerated as it is taken from the store and is therefore at store temperature. The idea behind the dual air curtain is that there will be little entrain- ment between the two air jets as they are travelling at the same speed Therefore there will be little heat gain through the barrier between the two air curtains. The entrainment will take place at the interface between the second curtain and the store, and because there is no temperature differ ence between this jet and the store there will be no heat infiltration One of the difficulties of dual air curtains is getting them to stay together all the way down the front of the cabinet. As the first curtain is chilled will be forced downwards due to natural convection but this will not happen to the second curtain because it is not colder than the surroundings. 11.1.4 Cabinet development Getting the air curtain to work properly is critical to the correct operation of the cabinet. Temperatures of the food simulants inside the cabinet can be monitored within specified store conditions to see if the cabinets meet the required specifications. British Standard methods of test for commer- cial refrigerated cabinets are contained in parts 1-8 of BS 6148 with part 3 covering the determination of temperature. The determinations are carried out in a controlled environment corresponding to the climatic class of inter est. Temperatures are measured in M-packages, 50 x 100 x 100 mm pack- ages of a meat simulant, positioned at defined positions in the cabinet. Set positions are 150mm from the centre line and within 150mm of one end ith additional positions for large cabinets. The standard also states 'In ddition to these M-packages, two extra M-packages shall be located within he useful net volume so that the maximum and minimum test package
• Increased energy consumption of the refrigeration compressor. About 60% of electricity consumed in modern supermarkets is used by display cabinets for frozen and chilled foodstuffs, • Decreased temperature in the store next to the cabinet.This is described as the ‘cold feet effect’ and can lead to temperatures as low as 10°C in the centre of refrigerated aisles. The are many variables affecting the efficiency of the air curtain, for example: • The temperature difference between the chilled air and the store air, • The velocity of the air curtain, • The thickness of the air curtain, • The pressures either side of the curtain, • Obstructions in the path of the air curtain. Some cabinets use a dual air curtain which has an extra jet of air parallel to the first jet but on the store side. This jet has the same velocity as the first jet but the air is not refrigerated as it is taken from the store and is therefore at store temperature. The idea behind the dual air curtain is that there will be little entrainment between the two air jets as they are travelling at the same speed. Therefore there will be little heat gain through the barrier between the two air curtains. The entrainment will take place at the interface between the second curtain and the store, and because there is no temperature difference between this jet and the store there will be no heat infiltration. One of the difficulties of dual air curtains is getting them to stay together all the way down the front of the cabinet. As the first curtain is chilled it will be forced downwards due to natural convection but this will not happen to the second curtain because it is not colder than the surroundings. 11.1.4 Cabinet development Getting the air curtain to work properly is critical to the correct operation of the cabinet. Temperatures of the food simulants inside the cabinet can be monitored within specified store conditions to see if the cabinets meet the required specifications. British Standard methods of test for commercial refrigerated cabinets are contained in parts 1–8 of BS 6148 with part 3 covering the determination of temperature. The determinations are carried out in a controlled environment corresponding to the climatic class of interest. Temperatures are measured in M-packages, 50 ¥ 100 ¥ 100 mm packages of a meat simulant, positioned at defined positions in the cabinet. Set positions are 150 mm from the centre line and within 150 mm of one end with additional positions for large cabinets. The standard also states ‘In addition to these M-packages, two extra M-packages shall be located within the useful net volume so that the maximum and minimum test package Chilled and frozen retail display 235
236 Meat refrigeration temperatures will be recorded. The difficulty of achieving this requirement has already been described in papers by Marriott(1992)and Gigiel and ames(1992). These two papers also clearly reveal the need for test proce- dures that will relate to the likely performance of the cabinets within the retail environment When the products do not meet the required temperatures it is often the air curtain that is to blame. The air curtain isible and so it needs to be made visible to check that it is doing what is required. Smoke is probably the most used method to view the air curtain. When smoke is blown into the air curtain it can be clearly seen. The cabinet can now be modified and its effect viewed using smoke 11.1.5 Computer modelling Developing a cabinet can be a very lengthy process. The cabinet tempera tures are not steady with time, as the cabinets coil ices up and then defrosts Any movement in front of the cabinet will have an effect on the air curtain and product temperatures. Any changes made to the cabinet may not have an immediate effect on product temperatures, therefore a number of small changes to a display cabinet can be a time-consuming and costly process Computational fluid dynamics(CFD)is becoming widely accepted as tool that can be used to aid development of display cabinets. CFD alloy the user to make changes to a computer model of the cabinet and see its effect before changing the real thing. If computing resources allow it,a number of changes can be made to a computer model relatively quickly and the best case tried on a real cabinet. mpt CFD has been used to show the effect of removing shelves from a retail display cabinet( Foster, 1995).A two-dimensional model of a chilled cabinet was used to predict the effect of removing shelves from the cabinet(Fig 11. 2). The predictions showed that the refrigeration consumption was least (570w per metre length of cabinet) when the case was fully loaded. As shelves were removed from either the top downwards or bottom upwards the energy consumption increased to a maximum of 653 Wm when all of he shelves were removed. CFD predictions of the cabinet with different configurations of shelving demonstrate that when shelves are removed, pressure differences between the cold cabinet and the store cause the curtain to bend inwards. This causes more mixing between the cold and warm air, increasing product temperature, reducing store temperatures and Increasing energy consumption 11.1.6 Store conditions One factor that can greatly effect the operation of a retail display cabinet is its positioning relative to the store's heating and ventilation system ( Foster, 1997). Because of the cold feet effect, supermarket stores are keen
temperatures will be recorded.’ The difficulty of achieving this requirement has already been described in papers by Marriott (1992) and Gigiel and James (1992). These two papers also clearly reveal the need for test procedures that will relate to the likely performance of the cabinets within the retail environment. When the products do not meet the required temperatures it is often the air curtain that is to blame. The air curtain is invisible and so it needs to be made visible to check that it is doing what is required. Smoke is probably the most used method to view the air curtain. When smoke is blown into the air curtain it can be clearly seen. The cabinet can now be modified and its effect viewed using smoke. 11.1.5 Computer modelling Developing a cabinet can be a very lengthy process. The cabinet temperatures are not steady with time, as the cabinet’s coil ices up and then defrosts. Any movement in front of the cabinet will have an effect on the air curtain and product temperatures. Any changes made to the cabinet may not have an immediate effect on product temperatures, therefore a number of small changes to a display cabinet can be a time-consuming and costly process. Computational fluid dynamics (CFD) is becoming widely accepted as a tool that can be used to aid development of display cabinets. CFD allows the user to make changes to a computer model of the cabinet and see its effect before changing the real thing. If computing resources allow it, a number of changes can be made to a computer model relatively quickly and the best case tried on a real cabinet. CFD has been used to show the effect of removing shelves from a retail display cabinet (Foster, 1995).A two-dimensional model of a chilled cabinet was used to predict the effect of removing shelves from the cabinet (Fig. 11.2). The predictions showed that the refrigeration consumption was least (570 W per metre length of cabinet) when the case was fully loaded. As shelves were removed from either the top downwards or bottom upwards, the energy consumption increased to a maximum of 653 W m-1 when all of the shelves were removed. CFD predictions of the cabinet with different configurations of shelving demonstrate that when shelves are removed, pressure differences between the cold cabinet and the store cause the air curtain to bend inwards. This causes more mixing between the cold and warm air, increasing product temperature, reducing store temperatures and increasing energy consumption. 11.1.6 Store conditions One factor that can greatly effect the operation of a retail display cabinet is its positioning relative to the store’s heating and ventilation system (Foster, 1997). Because of the cold feet effect, supermarket stores are keen 236 Meat refrigeration
Chilled and frozen retail display 237 0866 9E-8c emoved Number of shelves removed Fig 11.2 Refrigeration load per metre length of cabinet as shelves are remo from the top downwards(source: Foster, 1997) to put heat into the store near the cabinets. This has to be carefully con trolled, as fast moving air near an air curtain will disrupt it. If the air is also warm it can greatly affect the temperature of the product inside 11.2 Retail display of unwrapped meat and delicatessen products The market for delicatessen meat products in the UK was estimated to be worth ca f3 billion in 1992. The demand for delicatessen products has been influenced by a number of factors over the last few decades, ranging from demographic changes to membership of the European Union(MlC, 1992) The delicatessen market as a whole has benefited from the belief that del- icatessen products are fresh and natural, and for their convenience, all of which make them attractive to the consumer It has been recognised for many years that temperatures close to the initial freezing point(0+1.0C)are required to provide a long display life for unwrapped meat. Studies have shown that control of relative humidity over the surface of sliced meats and other delicatessen products is critical if a high quality display life is to be achieved Surveys carried out in a number of EU countries revealed retail display cabinets to be the weakest link in the chill chain(Malton, 1972: Moerman 1972: Bogh-Sorensen, 1980: Lyons and Drew, 1985). Product temperatures in Denmark(Fig. 11.3)were very similar to those measured in Sweden and the UK. Poor temperature control, either in terms of a temperature
to put heat into the store near the cabinets. This has to be carefully controlled, as fast moving air near an air curtain will disrupt it. If the air is also warm it can greatly affect the temperature of the product inside. 11.2 Retail display of unwrapped meat and delicatessen products The market for delicatessen meat products in the UK was estimated to be worth ca. £3 billion in 1992. The demand for delicatessen products has been influenced by a number of factors over the last few decades, ranging from demographic changes to membership of the European Union (MLC, 1992). The delicatessen market as a whole has benefited from the belief that delicatessen products are fresh and natural, and for their convenience, all of which make them attractive to the consumer. It has been recognised for many years that temperatures close to the initial freezing point (0 ± 1.0 °C) are required to provide a long display life for unwrapped meat. Studies have shown that control of relative humidity over the surface of sliced meats and other delicatessen products is critical if a high quality display life is to be achieved. Surveys carried out in a number of EU countries revealed retail display cabinets to be the weakest link in the chill chain (Malton, 1972; Moerman, 1972; Bøgh-Sørensen, 1980; Lyons and Drew, 1985). Product temperatures in Denmark (Fig. 11.3) were very similar to those measured in Sweden and the UK. Poor temperature control, either in terms of a temperature Chilled and frozen retail display 237 70 68 66 64 62 60 58 56 54 52 50 012345 well removed Number of shelves removed Energy consumption per metre length (w) Fig. 11.2 Refrigeration load per metre length of cabinet as shelves are removed from the top downwards (source: Foster, 1997)
238 Meat refrigeration 0.50.51.52.53.54.55.56.57.58.59510.511.512.513.5 Fig. 11.3 Product temperatures in chilled display cabinets in Denmark(source Bogh-Sprensen, 1980 dient within a cabinet or due to fluctuations in temperature, is one of the problems when using retail display cabinets (James and Swain, 1986) Many practical problems associated with retail display of meat and meat products arise from failure to ensure that display cabinets are suitable for the product 11.2.1 Types of cabinet Considerable quantities of chilled unwrapped meat and sliced delicatesse products are now sold from refrigerated display cabinets of one type or nother. Display cabinets for delicatessen products are available with gravity or forced convection coils and the glass fronts may be nearly verti cal or angled up to 20 degrees. Sections through three of the commonest types of delicatessen cabinet are shown in Fig. 11.4. In the gravity cabinet (Fig. 11.4a), 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. 11. 4b and c) air is drawn through an evaporator coil by a fan. It is then ducted into the rear of the display, returning to the coil after passing directly over the prod ucts (Fig. 11.4b), or forming an air curtain(Fig. 11.4c), via a slot in the front of the cabinet and a duct under the display shelf (James, 1996) 11.2.2 Appearance changes Changes in appearance are normally the criteria that limit display of unwrapped products rather than microbiological considerations. Deterio-
gradient within a cabinet or due to fluctuations in temperature, is one of the problems when using retail display cabinets (James and Swain, 1986). Many practical problems associated with retail display of meat and meat products arise from failure to ensure that display cabinets are suitable for the product. 11.2.1 Types of cabinet Considerable quantities of chilled unwrapped meat and sliced delicatessen products are now sold from refrigerated display cabinets of one type or another. 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 degrees. Sections through three of the commonest types of delicatessen cabinet are shown in Fig. 11.4. In the gravity cabinet (Fig. 11.4a), 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. 11.4b and c), air is drawn through an evaporator coil by a fan. It is then ducted into the rear of the display, returning to the coil after passing directly over the products (Fig. 11.4b), or forming an air curtain (Fig. 11.4c), via a slot in the front of the cabinet and a duct under the display shelf (James, 1996). 11.2.2 Appearance changes Changes in appearance are normally the criteria that limit display of unwrapped products rather than microbiological considerations. Deterio- 238 Meat refrigeration Temperature (°C) 200 150 100 50 0 –0.5 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 Number of products Fig. 11.3 Product temperatures in chilled display cabinets in Denmark (source: Bøgh-Sørensen, 1980)
Chilled and frozen retail displa Fig. 11.4 Three types of retail display cabinet for unwrapped products. sliced beef topside after display for bb orative weight loss and appearance of Table 11.1 Relationship between eva vaporative loss(gcm-) Change in appearance 0.0150.025 ing drier, still attractive but darker 0.0250.035 Is darkening, becoming dry and leathery Dry, blackening 0.05-0.10 Black Source: James and Swain. 1986. ration in the appearance of unwrapped meats has been related to the degree of dehydration(Table 11.1), which makes the product unattractive to consumers(James and Swain, 1986). Weight loss on its own cannot only be a measure of performance but also has important economic considera tions to the retailers. In the UK, the direct cost of evaporative weight loss in excessEs pped products in chilled display cabinets was estimated to be 11.2.3 Effects of environmental conditions The rate of dehydration is a function of the temperature, velocity and rel- ative humidity of the air passing over the surface of the food James and Swain(1986) found that changes in relative humidity had a substantial effect with a reduction from 95 to 40% causing increasing weight loss over a 6h display period by a factor of between 14 and 18(Fig. 11.5). The effect of air velocity on weight loss was compounded by that of relative
ration in the appearance of unwrapped meats has been related to the degree of dehydration (Table 11.1), which makes the product unattractive to consumers (James and Swain, 1986). Weight loss on its own cannot only be a measure of performance but also has important economic considerations to the retailers. In the UK, the direct cost of evaporative weight loss from unwrapped products in chilled display cabinets was estimated to be in excess of 6.25 m euros per annum (James and Swain, 1986). 11.2.3 Effects of environmental conditions The rate of dehydration is a function of the temperature, velocity and relative humidity of the air passing over the surface of the food. James and Swain (1986) found that changes in relative humidity had a substantial effect with a reduction from 95 to 40% causing increasing weight loss over a 6 h display period by a factor of between 14 and 18 (Fig. 11.5). The effect of air velocity on weight loss was compounded by that of relative Chilled and frozen retail display 239 Fig. 11.4 Three types of retail display cabinet for unwrapped products. Table 11.1 Relationship between evaporative weight loss and appearance of sliced beef topside after display for 6 h 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 0.05 Dry, blackening 0.05–0.10 Black Source: James and Swain, 1986
240 Meat refrigeration Chamber Cond tions Cabinet Cond itions r33 3 ABCD23 8P4℃E 0.07 0.04 婴003 0.02 0.01 Relative humidity (% Fig 11.5 Mean weight loss of samples of corned beef after display for 6h at dif- ferent relative humidities, air velocities and temperatures(source: James and Swain 1986) humidity. Raising the air velocity from 0. 1ms to 0.5ms had little effect on the weight loss at 95%RH, however, the magnitude of the effe increased as relative humidity decreased producing maximum changes at 40%RH. When changing the temperature from 2 to 6C the effect on the weight loss was far smaller than the changes in relative humidity or ai In their mathematical prediction of weight loss, Fulton et al. (1987)and James et aL.(1988a and b) showed that fluctuations in temperature or rela- tive humidity had little effect on weight loss. The weight loss under fluctu Iting conditions was identical to that experienced under the mean of the fluctuations. Evans and Russell (1994a, b)also showed that relative humidity was the main factor controlling weight loss in the display life of delicatessen prod ucts. At a relative humidity of 40% the effect of surface drying became apparent after ca 100 min At 85% RH the products could be displayed for
humidity. Raising the air velocity from 0.1 m s-1 to 0.5 ms-1 had little effect on the weight loss at 95% RH, however, the magnitude of the effect increased as relative humidity decreased producing maximum changes at 40% RH. When changing the temperature from 2 to 6 °C the effect on the weight loss was far smaller than the changes in relative humidity or air velocity. In their mathematical prediction of weight loss, Fulton et al. (1987) and James et al. (1988a and b) showed that fluctuations in temperature or relative humidity had little effect on weight loss. The weight loss under fluctuating conditions was identical to that experienced under the mean of the fluctuations. Evans and Russell (1994a, b) also showed that relative humidity was the main factor controlling weight loss in the display life of delicatessen products. At a relative humidity of 40% the effect of surface drying became apparent after ca. 100 min. At 85% RH the products could be displayed for 240 Meat refrigeration 6 °C, 0.5m/s 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 40 60 80 100 Relative humidity (%) Weight loss (g cm–2 ) 2 *2' *3' B B' 3 D' D A A' 1' C C' Chamber Conditions 2 °C, 0.1m/s 6 °C, 0.1m/s 2 °C, 0.3m/s 2 °C, 0.5m/s 6 °C, 0.3m/s Cabinet Conditions A & A' 4.4 °C, 0.32m/s B & B' 9.6 °C, 0.15m/s C & C' 2.3 °C, 0.25m/s D & D' 2.9 °C, 0.67m/s 1 & 1' 0.4 °C, 0.14m/s 2 & 2' 2.5 °C, 0.10m/s 3 & 3' 2.6 °C, 0.48m/s Fig. 11.5 Mean weight loss of samples of corned beef after display for 6 h at different relative humidities, air velocities and temperatures (source: James and Swain, 1986)
