Chilled and frozen foods

Monitoring and measurement of temperature *

М. Л. Вульф, Food Standards Agency, London


The practice of measuring and storing temperature records in the food industry is not new, and in certain areas, such as canning, has been used for many years. Nevertheless, it is widely used in food refrigeration relatively recently, with the exception of setting temperature measurement equipment for refrigerators. The main reason for attention to temperature control is the possibility of food poisoning and the introduction of new legislation regarding temperature control of chilled foods where temperature deviations and the possible growth of pathogenic microorganisms can cause problems. The development of the European Community has led to the development and adoption of an agreed sanitation directive that covers changes made in individual countries. Combined with health guidelines for the production of animal products, this further emphasized the importance of risk management. Therefore, the practical use of temperature control has evolved rapidly over the past ten years, and such control has become an integral part of quality and safety management systems.

 Changes in legislation

In the United Kingdom, the Food Hygiene (Amendment) Regulations of the 1990 [1] and the Food Safety Act of the 1990 [2] have significantly changed the cooling process. According to [1], temperature control has been implemented for some types of refrigerated products used at all stages of the cooling process. Additional minor changes were made in 1991. [3]. To date, very few consumers of refrigeration units used regular temperature control (monitoring), but when they began to apply such control, they realized its merits and benefits for quality control.

The 1990 Food Safety Act gave the government additional authority to legislate in many new areas. One of the major changes to the 1990 Act is contained in the 21 section. It describes the conditions under which it is possible to defend against the charges brought under the Act. The guarantee-based protection under the 1984 Act has been replaced by “due diligence” protection. To confirm “due diligence”, companies must demonstrate that they have taken all “reasonable precautions” and “fully exercised due diligence”. Many companies have switched to improved systems of control and verification, taking into account the law of “due diligence” existing in parallel in the case law.

The Hygiene Directive 93 / 43 / EEC Directive [4] was enacted in the UK in 1995 (see [5]), and focused on HACCP risk analysis (4 directive (3 )), but no prescriptions or detailed instructions were given on sanitary requirements and methods. There is a general requirement that temporary premises and equipment for transportation should be able to maintain food at appropriate temperatures, and, if necessary, the design of premises and equipment should be able to control these temperatures.

The requirements of the Directive on temperature control are set out in a document from 1995. [6] In addition, to ensure food safety and predict the growth of microorganisms under different temperature conditions, the government was able to use a mathematical model developed over five years (MAFFMicromodel). As a result, previously adopted guidelines for temperature control have been simplified.

EU legislation has already contained requirements for the control of food products of animal origin, for example, meat, meat products, poultry, etc. To implement the single market concept after January 1993, about a dozen general directives regarding hygienic production of animal products were agreed upon. - from fresh meat to bivalve mollusks. Some of these directives were new, and the rest were re-negotiated based on directives in force in the EU. All of these directives contain specific requirements for temperature control. Work is underway to merge all the general sanitary directives into one simplified one. The only mandatory requirement for monitoring and recording temperature is based on an EU directive requiring the provision of refrigerators and vehicles for storing or transporting fresh frozen food with [7] control equipment. This requirement is also adopted in the UN Economic Commission for Europe agreement, which simplifies the transport of perishable foodstuffs between neighboring countries [8] to harmonize EU requirements for third-country vehicles.

 Systems of risk management and quality of products

When companies began to investigate and implement temperature control (monitoring) systems, it soon became clear that this made it possible to compensate for capital and labor costs. Better temperature control contributes both to improving food safety and improving their quality, and can provide economic benefits through more efficient use of energy.

Changes in legislation also required the introduction of appropriate food safety systems. All food production is responsible for determining the critical moments of their technological processes. In some countries and in the world community, the HASCP method {Hazard Analysis and Critical Control Points, risk analysis for critical control points) has been adopted. Using this method identifies risks and process breakpoints to manage these risks. It is important here that the HACCP plan is determined for a specific product and technological process, as well as the fact that it must be constantly monitored. The HACCP Directive on Hygiene and Sanitation, which provides guidance on specific areas (six guidelines have been published by 1990), helps implement [9]. Depending on the specific product and technology, temperature control may or may not be included in the HACCP plan. There is no special requirement to keep records of temperature checks, but these records may be useful to demonstrate that legal requirements are met. It is important that temperature control is associated with other control points and is part of the overall HACCP system.

It is clear that HACCP is rarely implemented in isolation - this method is combined with quality control systems to ensure the production of safe food of uniform quality on the process equipment. There are many quality assurance systems, and the most widely used ones are based on either ISO 9000 or TQM (Total Quality Management). The ISO 9000 [10] system contains two main standards (7509001 and 9002) and various guidelines. Companies are recognized as complying with these standards (by accreditation) after their implementation. The TQM system is more related to the production culture, mobilizing all employees of the organization to achieve consistent quality and customer satisfaction, as well as to continuous improvement of production.

 Improvement of technology

The relatively cheap means of microelectronics allowed the manufacture of relatively small devices for storing large amounts of information. These devices are now widely used in combination with computerized control systems. Over the past few years, an enormous step forward has been made in computer technology and communication technology. Satellite tracking systems can track the position of the vehicle and transmit to the base general information about its refrigeration unit and engine. Storefronts can also be equipped with built-in temperature and humidity control systems to ensure the validity of bulk goods throughout the shelf life. Thus, where temperature measurement is part of a safety and quality system, new technology helps to accumulate and process data.

 Importance of monitoring (monitoring) of temperature

The requirements for temperature control in England and Wales are applied to those food products in which the probability of growth of microorganisms or the formation of toxins is high.

Such products should be stored at or below 8 ° C, but this requirement must be implemented in conjunction with other conditions specified in the General Hygiene and Sanitation Guidelines [5].

Obviously, if it is possible to exclude the ingress of pathogenic microorganisms into food products, then temperature control is necessary only to extend the shelf life of the product. However, this is rarely the case, and the approach adopted in the HACCP system is to determine the temperature at each stage of product processing, where there are risks, and the possibilities for their control. Reducing the temperature does not destroy microorganisms, but slows their growth, and therefore storage of raw materials, intermediate and finished products at low temperatures plays a role in ensuring food safety. Other important areas are appropriate operator training, the prevention of mechanical contamination, the use of appropriate fittings and equipment, the correct cleaning and washing modes, and pest control.

Refrigeration equipment is designed to work for a long time without human intervention; however, various events and in addition to breakdowns can affect the temperature regulation. It is important to control the correct frequency of defrost cycles and the loading of products into refrigerators, which is crucial for their normal operation and movement of air in them. Air temperature monitoring can show whether refrigeration equipment is working and controlled correctly, although it may be more difficult to determine the temperature of the products. In some cases, air temperature monitoring is not possible, and it is necessary to determine the temperature of a product or product model.

 Principles of temperature monitoring

System Selection

Currently, many different temperature monitoring systems are commercially available - from a simple thermometer to a fully computerized system connected to a local cooling system or even to a central control system. The choice of system depends on the amount of information that the operator needs, and the cost of obtaining this information. If the monitoring system must provide detailed information on the operation of the system connected to other regulatory systems, then it is clear that a more sophisticated and complex system is required. For a complete picture of the temperature distribution in the cooling system, it may contain many sensors. The system may also contain sensors to obtain other information — for example, on defrost cycles, compressor pressures and control valves, opening doors, and energy consumption. The system may be associated with an alarm system (and even a telephone), contain information on stocks and batch codes of the product. On the other hand, if you only need to check whether the storage temperature of the product is in a certain range (critical control point), the amount of information collected can be reduced.

The published temperature monitoring literature [11, 12,13,14] contains very few specific recommendations. The guidelines published by IFST [15] provide information on monitoring air temperature and are supplemented in the guidelines of the Ministry of Health [16]. These guidelines were later replaced by the Industrial Code [9]. Practical recommendations for temperature monitoring are available in the annexes to some of them (for example, the rules and regulations of retail and catering), but they are not part of them.

 What temperature should I control?

When designing a monitoring system (monitoring) and selecting the temperature measured in the cooling system, the following should be considered:

  •  the choice of controlled temperatures (air, products or their models) depends on the specific system and how it works;
  •  It is desirable to place the sensors in places where they will not be damaged during operation; If the readings are read by the operator, the sensors must be accessible;
  •  The selected temperatures must fully characterize and reflect the performance of the system, and therefore must be indirectly related to the temperature of the product.

 Monitoring (monitoring) of air temperature

For appropriate regulation and as part of HACCP, the temperature of food products should be monitored, however, the storage time of refrigerated products is relatively short, which complicates monitoring their temperature without disrupting normal commercial activities and the need to interfere with the system of experienced operators. It is easier to install the sensors outside the loaded products and connect them to the readout systems, which allow recording temperatures automatically or manually.

Cooling systems mainly work by passing cold air through the evaporator system, and then over loaded food products to remove heat from them. The movement of air is carried out with the help of fans or in some cases under the action of convection, that is, due to the greater density of cold air compared to warm. In the case of mechanical circulation, the air returns to the evaporator after passing over the products, the return air temperature being equal to or higher than the temperature of the cooled products. Local thermal effects, such as from lighting, can cause hot spots or an uneven temperature distribution and make a small portion of the loaded product warmer than return air. In general, the relationship between air and product temperatures is best established by determining the temperature difference between the cold air leaving the evaporator and the warmer air returning to the evaporator. This difference serves as a criterion for evaluating the operation of the refrigeration system and its effectiveness in keeping products cold [13], as well as the basis for monitoring (controlling) air temperature. However, to determine the ratio of air temperature and product temperature, it is necessary to perform a load test. The load test involves determining the difference in air temperature and comparing it with the product temperature for a sufficient period of time to ensure that the system is operating normally.

In closed systems, such as refrigerators and vehicles, where the only reasons for changing the mode are defrost cycles, opening doors and changing batches of goods, determining the relationship between air temperature and product temperature is easier. It is necessary to determine the warmest places in the system and monitor the temperatures of the products for some time in order to establish their relationship with the air temperatures.

The operation of open systems, such as display counters, is more dependent on environmental conditions and location. Changes in the temperature and humidity of the room, disturbances of the air curtain by drafts or movement of customers can change the temperature distribution. Under these conditions, testing under load may be more difficult.

Manufacturers of display counters perform load tests to verify the effectiveness of their products (BS EN441-5: 1996 [17]) using the specified load in the form of standardized gel blocks (tylose) (BS EN441-4: 1995 [18] in controlled ambient temperature environment with a constant air flow along the front surface of the display counter.It will be a test under load, made by the manufacturer, differ from the test under load at the place of operation, depends on how the conditions and the load correspond to the real conditions p bots display stands. Influence of the location and environment (drafts, lighting) must be verified using a variety of food products.

Alternatives to monitoring air temperature

There are situations in which monitoring air temperature is unacceptable or requires modification. In the shelves, showcases (for example, using refrigerated storage with convective cooling) after opening the doors to restore the air temperature takes considerable time [19]. Therefore, the periodic reading of the air temperature would have little meaning and would not be related to the temperatures of the stored products. In this case, it would be better to monitor the product sample or its model (equivalent). The heat flow of the sample makes it less sensitive to rapid changes in air temperature. You can also choose a food product model (simulator) with a similar heat transfer coefficient or similar thermal diffusivity with a controlled food product [20]. The use of such monitoring was important, for example, where cooling occurs due to thermal conductivity, as in the case of a cooling table used for distribution in catering, or where the air flow rate is low (counters with gravity feed).

Even where the system is forcibly cooled by air, but changes in air temperature are high — for example, in small delivery vehicles and refrigerators of shop windows (counters), the results of air temperature monitoring are difficult to interpret. By increasing the response time or “damping” the sensor or measuring system, you can track the direction of change in air temperature, eliminating short-term changes. “Damping” can be achieved by increasing the heat flux through the sensor or electronically by changing the electronic reading circuit.

Realization of monitoring (monitoring) of temperature

Cold storage Small refrigerating chambers

Small cooling chambers consist of an insulated chamber, depending on the size, cooled by one or several cooling fans. The location of the cooling devices in the chamber varies, but they are usually located near the ceiling (fig. 5.1). Air circulation should be such as to ensure appropriate distribution of cold in the chamber and to exclude any hot spots or the occurrence of air layers. Almost always, the temperature recovery after opening the door or defrosting (thawing) occurs quickly, which makes the air temperature the most convenient controlled parameter. Preserving cold air can be further improved by using a curtain of plastic strips or an air curtain at the door, minimizing the access of warm air when opening the doors.

The number of sensors used to monitor the temperature of the air in the refrigerating chamber depends on its size and the number of cooling units. In tab. 5.1 indicates the minimum number of sensors depending on the volume of the chamber, and if the volume of the chamber is less than 500 м3, one sensor can be used to monitor the air temperature. It is positioned so that it controls the highest temperature.Air circulation in the refrigerating chamber

Fig. 5.1. Air circulation in the refrigerating chamber

Table 5.1. The number of sensors recommended for the refrigerating chamber

The volume of the camera, m3, more



500 2
5 000 3
20 000 4
50 000 5
85 000 6

air and therefore the warmest products in the chamber. The location in the chamber of the warmest place depends on its design, especially on the location of the refrigeration unit.

In fig. 5.2 is represented by the air temperature during 24 h large refrigerator compartment operation. The graph allows you to compare temperature changes with the most active movements of cooled products in the afternoon, evening and morning, in a period of less active load.

In this case, the differences between the readings of the wall sensors and the return air temperature are very small and may depend on the location of the sensors in the chamber. For cold rooms with a volume of less than 500 m3, you can use one sensor placed on the path of the return air of the refrigeration unit. In a closed system (such as a chamber with proper air distribution), the temperature readings for return air are approximately equal to the average temperature of the loaded product. If there is no good air distribution, it may be preferable to place one sensor at the point with the highest air temperature. This point may be in the following places:

  •  At the maximum height of the loaded products, at the maximum distance from the refrigeration unit;
  •  at a height of about two thirds of the height of the chamber, away from the door and direct air movement from the refrigeration unit;
  •  At a height of two meters from the floor, directly opposite the refrigeration unit.Record the control of the air temperature of a large refrigerating chamber (40 LLC m3)

Fig. 5.2. Record the control of the air temperature of a large refrigerating chamber (40 LLC m3)

If a cooling device is placed above the door, the vacuum created by the fan can increase the amount of air drawn into the chamber when the doors are opened. Therefore, monitoring the temperature of the return air (outgoing air flow) in this case is often unacceptable. For large chambers (storage), different sensors can be used to determine temperatures in different parts of the chamber. In addition, the placement of additional sensors at the air outlet and air intakes of one or more refrigeration units provides additional information on the operation of the refrigeration system.

Refrigerator Cabinets

Refrigerated cabinets are autonomous installations of small size with one or two doors. These cabinets can be cooled with cold air with the help of fans or by natural circulation from the built-in air cooler or cooled plate (Fig. 5.3, a, b and c). As noted above, air temperature control for refrigeration systems of this type is not as suitable as for small cold rooms.

Refrigerated cases with fans relatively quickly restore the temperature after opening the doors, but frequent opening of the doors, especially during periods of active use, makes it difficult to interpret any temperature readings. Monitoring (monitoring) air temperature can be more meaningful if a damped sensor is used with an interval of about 15 min located on the return air path (Fig. 5.3, a). You can dampen the sensor using a metal or plastic shell, as well as placing the sensor in water, oil, or glycerin. In fig. 5.4 shows the effect of damping the sensor by placingRefrigerators: a) a refrigerator with forced air circulation; B) a refrigerator with a refrigeration unit; C) refrigerator with cooled plates

Fig. 5.3. Refrigerators: a) a refrigerator with forced air circulation; B) a refrigerator with a refrigeration unit; C) refrigerator with cooled platesDamping effect of the air temperature sensor

Fig. 5.4. Damping effect of the air temperature sensor

it in the center of the plastic bath, while the readings are compared with the air temperature after opening the doors.

Since refrigerated cabinets cooled with cooled plates (panels) or a refrigeration unit have poor air circulation and long recovery periods after opening the doors, it is more accurate to control the temperature using food temperature or, even better, the temperature of model products.

Since food products are microbiologically unstable, temperature control of products requires the use of different products every day and can lead to losses. Permanent sensor installation requires a stable food product model. When choosing a product model, it is important that it behaves like a controlled product and is resistant to different working conditions. It is recommended to determine the thermal conductivity of a specific package or portion of the product and select a model with the appropriate characteristics, or select a model that corresponds to the product according to the thermal diffusivity [20]. In the literature, there are values ​​of thermal conductivity coefficients for various products and package sizes, as well as thermal diffusivity coefficients for a number of plastic materials [20]. To make sure that the sensor placed in the model functions normally and gives correct readings, and also that the model behaves normally, it is necessary to conduct regular checks of the system with the food product model.

Refrigerated transport

Chilled products are loaded into various vehicles - from large 40-feet (12 m) heavy-duty vehicles with autonomous refrigeration units to light-duty vehicles, in which the temperature of the pre-cooled products is maintained only by isothermal containers. Since the design of most refrigeration units is designed to maintain the temperature and not to cool the cargo, it must be pre-cooled to the appropriate temperature.

Transport with adjustable temperature

An autonomous refrigeration unit, usually receiving energy from a diesel engine (often with an additional electric motor), provides in the refrigeration chamber circulation of cold air from the evaporator in front of the car. Frequently, in car parks engaged in the carriage of various goods, vehicles with movable partitions are used, which makes it possible to simultaneously transport frozen and chilled products at different temperatures. Each compartment (chamber) is equipped with its own evaporator, which can regulate the temperature independently.

The cold air is distributed in different vehicles in various ways, but in most cases the cold air comes out from above the air cooler near the roof and returns through the base to the front of the car to the return air intake (fig. 5.5). Proper loading and positioning of the cargo in the car at an appropriate distance is crucial to ensure the correct distribution of cold air in the chamber. If the required distances are missing, the circulation can be difficult and hot spots can occur. The maximum length and width of vehicles is set by the rules, and therefore the free space for the cargo in the insulated chamber creates additional restrictions to achieve proper loading. ExistsControl of air temperature in transport with adjustable temperature

Fig. 5.5. Control of air temperature in transport with adjustable temperature

transport cooled by direct evaporation of liquid nitrogen from a reservoir on the vehicle. The advantage of such vehicles is that they are much quieter than vehicles with mechanical cooling, and temperature control in them can be better. However, during transportation, an appropriate supply of liquid nitrogen is necessary, which may limit the range and number of stops of such transport.

Due to the need to read the temperature and use of single-channel recorders on the refrigerated trucks for many years, the sensor was placed so as to measure the return air temperature. This return air characterizes the average temperature of the cargo with good air access to all its parts. A small circle of air circulation can lead to lower return air temperatures.

It is recommended to equip long refrigerators (especially without the distribution of cold air through the ducts near the ceiling of the chamber) with a second sensor located closer to the back of the machine (see fig. 5.5). Adding a second sensor is not enough to give a complete and accurate picture of the temperature distribution in the chamber, but by measuring the temperature of the cold air leaving the evaporator with this sensor, you can get a more complete picture of the circulation of cold air in the chamber. The second sensor serves to monitor the operation of the measuring system and complicates falsification. With this sensor, you can verify that the evaporator and the fan are functioning normally, and the cold air reaches the back of the refrigerator. This sensor provides a base temperature for measuring the return air temperature and makes it easier to record when the cooling unit is turned off or when there is insufficient cooling load. In addition, using this sensor is easier to prevent freezing of the cargo. Comparing the temperature difference between the rear sensor and the return air sensor with a normal difference can also reveal poor air distribution in the chamber.

The recording frequency for electronic devices depends on the duration of the flight. The maximum recommended interval for flights of up to 8 hours is 15 minutes. For longer flights, longer intervals can be used. Other information may be needed, such as defrost cycles, door opening and load data. It is important that the driver be aware of the occurrence of any problems with the temperature of the load. Temperature readings are often visible to the driver in the rearview mirror, and in some cases the reading is present as a mirror image. It is obvious that the attention of the driver should be fully directed to the road, and it is better that a special alarm system be installed warning the driver about various violations.

In fig. 5.6, and, illustrating the monitoring of temperature in a car equipped with two sensors, you can see the effect of opening the doors. Figure 5.6, b shows how to carefully interpret the recording air temperature. The system works fine before loading the camera. From this point on, the return air sensor gives acceptable readings, but somewhat longer cycles. At the same time, a sensor in the back of the chamber induces a rise in temperature, which indicates that the load restricts the flow of cold air. This causes the cold air to move from the evaporator in a small circle and, therefore, longerMonitoring the temperature in the car: a) record normal

Fig. 5.6. Monitoring the temperature in the car: a) record normal

Air temperature; B) recording the air temperature in a poorly refrigerated refrigerator with a chilled food product (published with the permission of Cold Chain Instruments)

Thermal overload cycles. Immediately after the load is shifted by the driver to resume air flow to the back of the camera, the temperature drops. This problem would not be obvious if there is only a sensor in the return air flow.

Monitoring in vehicles with moving partitions requires more sensors in order to record the temperature in each compartment. This can be achieved in several ways. The easiest is to control the air flow of each refrigeration unit. Another option is to mount more sensors on the roof of the chamber in order to make it possible to monitor temperatures in the compartments, regardless of the position of the partition, in addition to measuring the return air temperature. Another solution is to use small temperature recorders, the position of which can be changed taking into account the location of the partitions.

In refrigerated vessels cooled with liquid nitrogen, the sensors should be installed so as to detect any temperature gradients occurring in the chamber. Forced air circulation should eliminate gradients. If fans are not used, sensors should be placed above and below the load.

Small means of delivery

Many small cars carrying refrigerated food products are equipped with refrigeration units powered by a car engine or transmission. This means that cooling is not possible when the car is not moving. Achievements of refrigeration technology made it possible to re-equip cars with a volume of less than 3 m3 with effective refrigeration units operating from a car battery.

The quality of temperature control depends on the number and duration of opening doors in the preparation and delivery of orders. A typical downtown delivery system can lead to the fact that 40% of working time doors are open, which can make temperature control very difficult, and also make it unacceptable to use air temperature control. Strengthening over the door the backstage of plastic strips can help reduce the flow of warm air into the interior with the doors open. However, information can be obtained if the air temperature sensors are damped by hanging them in bottles of liquid such as oil or glycerin. A large variation in temperature records is eliminated, and the direction of change in the total temperature in the chamber is monitored. An example of using this control method is shown in Fig. 5.7.

In cars with eutectic accumulation plates (of the “Zeroor” type) or boxes with thermal insulation, usually a food product model or a real product is used to control the temperature on the flight. The positioning of the sensor should reflect the condition of the load as best as possible. Temperatures can be read by a person, but sensors can also be connected to a recorder or a recording system.Recording the air temperature in a small car for delivering food

Fig. 5.7. Recording the air temperature in a small car for delivering food

 Shop counters

Chilled products are mainly laid out in open windows. In some cases, use of the shelves, showcases; from the point of view of monitoring, they can be viewed as refrigerated storage cabinets (see the section “Refrigerated Cabinets” above). Open windows can be divided into two main groups - multi-storey open windows (like “multi-deck”) and self-service windows.

Multi-storey shop windows

The fan draws air from the front lattice of the cabinet and, passing through the evaporator, cools. To cool products, cold air comes out in the back of the shelves and from the upper grill to form an air curtain in front of the shelves (fig. 5.8, a). A number of improvements have been made in the design of shop windows, among which is the reduction of heat gain from indoor lighting and the stabilization of the air curtain due to an improved design or the addition of a second curtain. The simplicity of temperature control in multi-storey windows is determined by their design and operation. In principle, the indicator of the efficiency of the display case is the difference in temperature between the air returning from the shelves and the air entering the shelves. Sensor placement or temperature reading is performed at the top of the air curtain grille (air outlet) and the return air grille (return air) (Fig. 5.8, a).

If a typical picture of temperature changes can be associated with the temperature of the product on the shelves, then monitoring (control) of air temperature can be used in daily work. If other factors influence the air temperature change (for example, excessive absorption of infrared radiation) or it is impossible to establish a relationship between the air temperature and the product temperature, it may be necessary to measure the temperature of the product or its model.

In fig. 5.9 shows two options for changes in air temperature. In fig. 5.9, and regular cyclical changes in air temperature are visible, and in Fig. 5.9, b temperature is much more stable (except for the defrost cycle time). In both cases, determining the relationship between the range of changes in air temperature and the highest product temperatures makes it possible to effectively monitor air temperature.

 Self-service display cabinets

In this group there are many different showcases for laying out meat, fish, delicacies, confectionery, cakes, cheeses and ready-to-eat products. In many cases, the product is cooled with cold air from a refrigeration unit, but sometimes, especially in catering, the food product is cooled by contact with a cooling plate (cold table), compartment or crushed ice. The effect of infrared radiation from lighting or sunlight may be more pronounced in the case of self-service windows and significantly affect the temperature of food products.Monitoring of air temperature in storefronts for retail sale: a) multi-storey showcase; B) showcase for distribution

Fig. 5.8. Monitoring of air temperature in storefronts for retail sale: a) multi-storey showcase; B) showcase for distribution

In fig. 5.8, b shows a typical self-service showcase for the retail sale of delicatessen products with a cold air supply fan. Air from the rear grill is supplied to the product and returned through the front grill. In the case of gravity feed windows, where air enters the rear grille and exits at the bottom shelf, there is no grill for return air. The air velocities in the self-service windows are small to reduce dehydration of the products in the window; this makes it even more difficult to measure air temperature. The position of the sensors or manual measurement of the air temperature is also shown in Fig. 5.8, b. For daily measurements of air temperature, it is necessary that a connection be established between product and air temperatures.Temperature monitoring records for two different display cases (published with the permission of Bristol University)

Fig. 5.9. Temperature monitoring records for two different display cases (published with the permission of Bristol University)

In many cases, it is easier to control the temperature of the showcase by the temperature of the products or their models. The temperature in the front of the showcase usually characterizes the warmest places and, therefore, the warmest products in the showcase. Air temperature monitoring is not suitable for windows that are cooled due to thermal conductivity (in compartments or when cooled with crushed ice). In this case, direct measurements should be made of the temperature of the products for which, like for all measurements of this kind, it is necessary to use a clean, well-disinfected probe.

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