Temperature measurement It is often stated by those in the meat and refrigeration industries that 'anyone can measure a temperature. Many millions of measurements are made of both meat and environmental temperatures in the meat industry. However, in many cases the measurements made are an unreliable guide to the effectiveness of the refrigeration process. Even when the correct tem- peratures have been obtained the data are often poorly analysed and rarely acted upon If a group of people are asked to measure the temperature of a beef carcass. the number of values obtained is often the same as the number of people in the group. Few initially ask the obvious question, what is meant by the temperature of the carcass? Is it the average temperature, the ighest temperature, the lowest surface temperature, the average surface temperature.? The increase in temperature legislation and the desire of meat produc ers and retailers to maintain the organoleptic and microbiological quality of meat throughout the chilled and frozen distribution chain has cre ated an increased demand for equipment and expertise on temperature measurement The industry needs to measure temperatures accurately, reliably, mean ingfully, simply and cheaply. It needs to be able to analyse the data and respond when required. It needs the correct instrumentation and the exper- tise to collect and interpret the temperature data
14 Temperature measurement It is often stated by those in the meat and refrigeration industries that ‘anyone can measure a temperature’. Many millions of measurements are made of both meat and environmental temperatures in the meat industry. However, in many cases the measurements made are an unreliable guide to the effectiveness of the refrigeration process. Even when the correct temperatures have been obtained the data are often poorly analysed and rarely acted upon. If a group of people are asked to measure the temperature of a beef carcass, the number of values obtained is often the same as the number of people in the group. Few initially ask the obvious question, ‘what is meant by the temperature of the carcass?’ Is it the average temperature, the highest temperature, the lowest surface temperature, the average surface temperature...? The increase in temperature legislation and the desire of meat producers and retailers to maintain the organoleptic and microbiological quality of meat throughout the chilled and frozen distribution chain has created an increased demand for equipment and expertise on temperature measurement. The industry needs to measure temperatures accurately, reliably, meaningfully, simply and cheaply. It needs to be able to analyse the data and respond when required. It needs the correct instrumentation and the expertise to collect and interpret the temperature data
84 Meat refrigeration 14.1 Instrumentation The first consideration is the range of temperatures to be measured. For the meat industry, a range from -40 to +150C would cope with the tempera tures found in freezers, chillers, storage rooms, retail display cabinets and in water used for cleaning or scalding tanks in the abattoir. If they produce cooked meat products then the upper temperature may rise to 250-300C. As well as the measuring range, the range of ambient temperatures over hich the instrument will work needs to be considered The electronics of many temperature measurement instruments are designed to work to the pecified accuracy only within certain ambient temperature ranges, usually 0-40C. If temperatures in a cold store are to be measured the instrument itself may need to be kept warm until it is used. 14.1.1 Hand-held digital thermometers Purely from a cost consideration many small producers and retailers rely on spot temperature checks obtained using hand-held thermometers to produce the temperature records they require. The main tasks they carry ut with such equipment is the measurement of air temperature, between pack or product temperature, and the temperature of the meat itself. They require thermometers that are accurate, easy to use, react quickly and ar robust. Ease of use is a personal judgement and best answered by trying out a range of instruments. Most modern electronic thermometers are reli- able if handled with reasonable care. However, in general, the more robust the sensor the slower the response. There are three types of digital thermometer generally available: ther- mocouple, platinum resistance or semi-conductor(thermistor). The name refers to the type of temperature sensor used. Type T(copper-constantan) thermocouple thermometers with a wide range of interchangeable sensors are the most widely used because of their wide temperature range and rea- sonable accuracy. The accuracy of the temperature measurement of a digita thermometer will depend on how accurate the instrument and the sensor It can be seen from Table 14.1 that only thermometers based on ther mistor or platinum resistance sensors can be guaranteed to provide better than +0.5C accuracy. However, it is possible to calibrate any thermometer at known temperatures and use the calibration curve obtained to correct errors in measured values. In many cases the supplier of the instru- ment can provide a calibration curve for a particular instrument/sensor combination Sensors do not immediately in which they are positioned. Their ure the temperature of the meat or air onse rate depends on the sensor itself and the environment in which it is used. a thin sensor in a wet/solid food will respond rapidly, and a thick sensor in still air very slowly. When
14.1 Instrumentation The first consideration is the range of temperatures to be measured. For the meat industry, a range from -40 to +150 °C would cope with the temperatures found in freezers, chillers, storage rooms, retail display cabinets and in water used for cleaning or scalding tanks in the abattoir. If they produce cooked meat products then the upper temperature may rise to 250–300 °C. As well as the measuring range, the range of ambient temperatures over which the instrument will work needs to be considered. The electronics of many temperature measurement instruments are designed to work to the specified accuracy only within certain ambient temperature ranges, usually 0–40 °C. If temperatures in a cold store are to be measured the instrument itself may need to be kept warm until it is used. 14.1.1 Hand-held digital thermometers Purely from a cost consideration many small producers and retailers rely on spot temperature checks obtained using hand-held thermometers to produce the temperature records they require. The main tasks they carry out with such equipment is the measurement of air temperature, between pack or product temperature, and the temperature of the meat itself. They require thermometers that are accurate, easy to use, react quickly and are robust. Ease of use is a personal judgement and best answered by trying out a range of instruments. Most modern electronic thermometers are reliable if handled with reasonable care. However, in general, the more robust the sensor the slower the response. There are three types of digital thermometer generally available: thermocouple, platinum resistance or semi-conductor (thermistor). The name refers to the type of temperature sensor used. Type T (copper–constantan) thermocouple thermometers with a wide range of interchangeable sensors are the most widely used because of their wide temperature range and reasonable accuracy.The accuracy of the temperature measurement of a digital thermometer will depend on how accurate the instrument and the sensor are. It can be seen from Table 14.1 that only thermometers based on thermistor or platinum resistance sensors can be guaranteed to provide better than ±0.5 °C accuracy. However, it is possible to calibrate any thermometer at known temperatures and use the calibration curve obtained to correct errors in measured values. In many cases the supplier of the instrument can provide a calibration curve for a particular instrument/sensor combination. Sensors do not immediately measure the temperature of the meat or air in which they are positioned. Their response rate depends on the sensor itself and the environment in which it is used. A thin sensor in a wet/solid food will respond rapidly, and a thick sensor in still air very slowly. When 284 Meat refrigeration
Temperature measurement 285 Table 14.1 Accuracy of digital instrument, temperature sensor and overal temperature accuracy of combined thermometer Instrument(°C) Sensor(C)Both(°C) Type k thermocouple Tvpe T thermocouple latinum ±0.4 hermit 0.3 Table 14.2 Response times(s)of sensors in air ensor Air condition Still M Bare thermocouple Bare thermistor Shrouded thermocouple Shrouded thermistor 260 Shrouded platinu 365 sensors are shrouded to improve their robustness their response times increase substantially (Table 14.2) Most manufacturers supply a range of cased sensors(probes)normally made of stainless steel suitable for most applications. These include blunt ended probes for general purpose use, needle-point or hypodermic probes for inserting into solid or semi-solid food, probes with spring-loaded ends for measuring surface temperatures or very robust probes that can be screwed or hammered into frozen meat Probe length is not important for measuring air and liquid temperatures. However, to check the deep leg temperature of a side of beef the probe needs to be at least 15 cm long to measure the temperature at the deepest 14.1.2 Temperature recorders There may often be a requirement to measure the values of temperatures at many different positions at the same time. The simplest solution to this problem is often to attach a multipole switch to a digital thermometer A number of temperature sensors can then be connected to the switch and their temperatures monitored in succession. This procedure is com- monly used in central plant rooms where an operator can routinely look at and record the temperatures at many different locations. A hand-held digital thermometer will provide information on one temperature at one
sensors are shrouded to improve their robustness their response times increase substantially (Table 14.2). Most manufacturers supply a range of cased sensors (probes) normally made of stainless steel suitable for most applications. These include bluntended probes for general purpose use, needle-point or hypodermic probes for inserting into solid or semi-solid food, probes with spring-loaded ends for measuring surface temperatures or very robust probes that can be screwed or hammered into frozen meat. Probe length is not important for measuring air and liquid temperatures. However, to check the deep leg temperature of a side of beef the probe needs to be at least 15 cm long to measure the temperature at the deepest point. 14.1.2 Temperature recorders There may often be a requirement to measure the values of temperatures at many different positions at the same time. The simplest solution to this problem is often to attach a multipole switch to a digital thermometer. A number of temperature sensors can then be connected to the switch and their temperatures monitored in succession. This procedure is commonly used in central plant rooms where an operator can routinely look at and record the temperatures at many different locations. A hand-held digital thermometer will provide information on one temperature at one Temperature measurement 285 Table 14.1 Accuracy of digital instrument, temperature sensor and overall temperature accuracy of combined thermometer Instrument (°C) Sensor (°C) Both (°C) Type K thermocouple ±0.3 ±1.5 ±1.8 Type T thermocouple ±0.3 ±0.5 ±0.8 Platinum ±0.2 ±0.2 ±0.4 Thermistor ±0.2 ±0.1 ±0.3 Table 14.2 Response times (s) of sensors in air Sensor Air condition Still Moving Bare thermocouple 20 5 Bare thermistor 45 20 Shrouded thermocouple 150 40 Shrouded thermistor 260 50 Shrouded platinum 365 65
86 Meat refrigeration time. However, in many cases there is a need to measure temperatures over a long time period. In these situations a temperature recorder is required Historically, the temperature history of a point has been obtained usin temperature sensor connected to a moving chart. In its simplest form this is a stylus on the end of a bimetallic strip that bends in response to tem perature changes and scratches a continuous trace on a carbon chart moved by a clockwork motor. More sophisticated devices use electrical tempera ture sensors attached to a small chart recorder. The recorders can be driven from batteries or direct from the mains and the chart can be circular or rec angular and mounted on a drum or on continuous rolls. Typically instru ments will provide a continuous trace for up to a week, but some specially developed for long distance shipboard transportation of meat can operate for 6-8 weeks Increasingly, solid state electronic devices are being used to obtain the temperature history of a point. In most solid state devices the output from an integral electronic sensor is measured at set time intervals, converted to a temperature measurement and stored in a computer memory chip. In a small number of devices the interval between recordings can be adjusted and recordings started using buttons or switches on the instrument and the temperatures examined on an in-built display. a development is the use of small printers that can either be used to print out the temperatures as they are measured or attached after data collection is finished and the whole temperature history printed out. However, with the majority of instruments a small computer is required to set up start times, logging intervals and so on and recover the temperature recordin With many systems it is difficult to look at the temperature while it is being recorded or even check that the required information has been obtained before leaving the recording site. Some of the newest instruments are totally encapsulated in waterproof plastic and can be placed in direct contact with solid or even within liquid foods. Solid state devices can be very small, effectively tamper proof, and the value of the temperature at set times easily obtained, but the requirement for an associated processing facility substantially increases their cost. In many cases moving chart instruments may still provide the most economic and convenient solution to a monitoring problem. If precise temperature values at certain times are not required, then a quick examination of the chart may be sufficient to show that the temperature of the display cabinet, store room or transport vehicle has kept within the prescribed limits. However, obtaining tempera ure values from a small chart can be time consuming and inaccurate. In some situations the actual or relative position of the sensing points is important, whilst in others the position of maximum or minimum temper ature is required. There are few, if any, commercial sources of multi-point temperature probes and most have to be specially constructed. The sensors are attached to basic probes constructed from composite fibre or wood of
time. However, in many cases there is a need to measure temperatures over a long time period. In these situations a temperature recorder is required. Historically, the temperature history of a point has been obtained using a temperature sensor connected to a moving chart. In its simplest form this is a stylus on the end of a bimetallic strip that bends in response to temperature changes and scratches a continuous trace on a carbon chart moved by a clockwork motor. More sophisticated devices use electrical temperature sensors attached to a small chart recorder. The recorders can be driven from batteries or direct from the mains and the chart can be circular or rectangular and mounted on a drum or on continuous rolls. Typically instruments will provide a continuous trace for up to a week, but some specially developed for long distance shipboard transportation of meat can operate for 6–8 weeks. Increasingly, solid state electronic devices are being used to obtain the temperature history of a point. In most solid state devices the output from an integral electronic sensor is measured at set time intervals, converted to a temperature measurement and stored in a computer memory chip. In a small number of devices the interval between recordings can be adjusted and recordings started using buttons or switches on the instrument and the temperatures examined on an in-built display. A development is the use of small printers that can either be used to print out the temperatures as they are measured or attached after data collection is finished and the whole temperature history printed out. However, with the majority of instruments a small computer is required to set up start times, logging intervals and so on and recover the temperature recordings. With many systems it is difficult to look at the temperature while it is being recorded or even check that the required information has been obtained before leaving the recording site. Some of the newest instruments are totally encapsulated in waterproof plastic and can be placed in direct contact with solid or even within liquid foods. Solid state devices can be very small, effectively tamper proof, and the value of the temperature at set times easily obtained, but the requirement for an associated processing facility substantially increases their cost. In many cases moving chart instruments may still provide the most economic and convenient solution to a monitoring problem. If precise temperature values at certain times are not required, then a quick examination of the chart may be sufficient to show that the temperature of the display cabinet, store room or transport vehicle has kept within the prescribed limits. However, obtaining temperature values from a small chart can be time consuming and inaccurate. In some situations the actual or relative position of the sensing points is important, whilst in others the position of maximum or minimum temperature is required. There are few, if any, commercial sources of multi-point temperature probes and most have to be specially constructed. The sensors are attached to basic probes constructed from composite fibre or wood of 286 Meat refrigeration
Temperature measurement 287 the smallest cross-sectional area that can be used whilst maintaining the required robustness, to minimise heat conduction and achieve a rapid tem- perature response For hygienic reasons, probes are thinly coated with inert epoxy resin. Currently there is no real alternative to the use of an array of individual temperature sensors if data on temperature distribution are required For over 50 years temperature sensors attached to multi-point chart recorders have been used to obtain the temperature history of up to 24 positions and these systems are still common in many processing plants. To differentiate between the sensors on the charts, a range of methods including different colours, line types or numbers have been used As the number of sensors creases it becomes more and more difficult to identify individual sensors and/or ten perature values. If the sole purpose of the recordings is to show that all the temperatures remain within upper and lower limits then chart recording systems are more than adequate. In situations where a more detailed analysis of the data is required then they are being increasingly replaced by microprocessor-controlled data logging systems. Data loggers range from multisensor( typically 2-16 temperatures)ver- sions of the solid state instruments already mentioned, to sophisticated processing systems with thousands of measurement points Two types of portable logging systems are available. The larger type (approximately the size of a large paperback novel) has built in displays and buttons or switches to set start times time intervals between measure ments and to scan through, using the display, the temperatures, that have been measured Instruments can be purchased with between 2 and 16 plug in temperature sensors, and some will display the maximum, minimum and mean temperature recorded by a particular sensor. For more detailed analy- sis of the temperatures, the recorded data are transferred to a personal com puter. The PC is required to program the start time, recording interval and so on, and analyse the data with the smaller loggers. These instruments usually have a maximum capacity of 8 temperature sensors, l of which is often built into the instrument Developments in computer storage chips are continually extending the number of temperature values that can be held in both types of logger Modern instruments would typically be able to take readings of 4 tem- perature sensors at 5min intervals over a 2-3-week period. Further developments in electronics are extending the temperature range over which the instruments will operate. Some instruments will record accu- ately inside blast and spiral freezing systems whilst others can operate ambient temperatures up to 70-80C For extended use at sub-zero tem peratures special batteries are required. Logging systems have been devel- oped which use insulated heat resistant cases to allow operation for several hours at temperatures up to 300C. This allows measurement of product and processing temperatures in batch and continuous baking/cooking operations
the smallest cross-sectional area that can be used, whilst maintaining the required robustness, to minimise heat conduction and achieve a rapid temperature response. For hygienic reasons, probes are thinly coated with inert epoxy resin. Currently there is no real alternative to the use of an array of individual temperature sensors if data on temperature distribution are required. For over 50 years temperature sensors attached to multi-point chart recorders have been used to obtain the temperature history of up to 24 positions and these systems are still common in many processing plants. To differentiate between the sensors on the charts, a range of methods including different colours, line types or numbers have been used. As the number of sensors increases it becomes more and more difficult to identify individual sensors and/or temperature values. If the sole purpose of the recordings is to show that all the temperatures remain within upper and lower limits then chart recording systems are more than adequate. In situations where a more detailed analysis of the data is required then they are being increasingly replaced by microprocessor-controlled data logging systems. Data loggers range from multisensor (typically 2–16 temperatures) versions of the solid state instruments already mentioned, to sophisticated processing systems with thousands of measurement points. Two types of portable logging systems are available. The larger type (approximately the size of a large paperback novel) has built in displays and buttons or switches to set start times, time intervals between measurements and to scan through, using the display, the temperatures, that have been measured. Instruments can be purchased with between 2 and 16 plugin temperature sensors, and some will display the maximum, minimum and mean temperature recorded by a particular sensor. For more detailed analysis of the temperatures, the recorded data are transferred to a personal computer. The PC is required to program the start time, recording interval and so on, and analyse the data with the smaller loggers. These instruments usually have a maximum capacity of 8 temperature sensors, 1 of which is often built into the instrument. Developments in computer storage chips are continually extending the number of temperature values that can be held in both types of logger. Modern instruments would typically be able to take readings of 4 temperature sensors at 5 min intervals over a 2–3-week period. Further developments in electronics are extending the temperature range over which the instruments will operate. Some instruments will record accurately inside blast and spiral freezing systems whilst others can operate in ambient temperatures up to 70–80 °C. For extended use at sub-zero temperatures special batteries are required. Logging systems have been developed which use insulated heat resistant cases to allow operation for several hours at temperatures up to 300 °C. This allows measurement of product and processing temperatures in batch and continuous baking/cooking operations. Temperature measurement 287
88 Meat refrigeration Most data logging systems that will measure over 20 temperatures are physically too big to be considered as truly portable even though some can be battery powered. Some have a built in display and keyboard but the majority are operated using a video display unit a basic system would consist of a number of input cards to which the temperature sensors are connec a card-based voltmeter to measure the output from sensors when instructed a microcomputer to provide the instructions and convert voltages into temperature measurements, a storage system that could be floppy or hard disc, and a video display lany systems can be expanded to hundreds and in some cases thousands of temperature sensors by the addition of extra input cards, some of which can be up to a mile away from the central system The temperature measurement possibilities of large logging systems are nly limited by the ingenuity of the programmer/operator. Different com binations of temperature sensors can be monitored at varying time inter- vals and the data displayed, analysed, used to control processes or set off alarms or be transmitted to central control rooms hundreds of miles away All three types of temperature sensor thermocouple, thermistor and platinum resistance are commonly used for multi-point temperature measurement Thermocouples are cheap, especially if the wire is purchased in bulk, and very small sensors can be manufactured Thermistors are more expensive, slightly larger but more accurate over limited temperature ranges. Platinum resistance sensors are typically 2-3 times the cost of ther mistors, but are capable of better than 0. 1C accuracy. Thin wire and thin film platinum resistance sensors can be very small Commercial sensors are often enclosed in stainless steel sheaths which makes them more robust but increases their response time. 14.1.3 Time-temperature indicators There are many different types of temperature or time-temperature indi- cators. Almost anything that undergoes a sensibly detectable change with temperature can be used Liquid crystal devices change colour to indicate he temperature at the time they are observed and time-temperature indi cators change irreversibly after a time dependent upon the temperature history or when a temperature threshold is exceeded Temperature indicators are already used as cheap, safe and hygienic ther- mometers in the food chain. Several types have been developed to the point where they have been introduced on some chilled and frozen foods in the USA and on chilled foods in france
Most data logging systems that will measure over 20 temperatures are physically too big to be considered as truly portable even though some can be battery powered. Some have a built in display and keyboard but the majority are operated using a video display unit. A basic system would consist of: • a number of input cards to which the temperature sensors are connected, • a card-based voltmeter to measure the output from sensors when instructed, • a microcomputer to provide the instructions and convert voltages into temperature measurements, • a storage system that could be floppy or hard disc, and a video display unit. Many systems can be expanded to hundreds and in some cases thousands of temperature sensors by the addition of extra input cards, some of which can be up to a mile away from the central system. The temperature measurement possibilities of large logging systems are only limited by the ingenuity of the programmer/operator. Different combinations of temperature sensors can be monitored at varying time intervals and the data displayed, analysed, used to control processes or set off alarms or be transmitted to central control rooms hundreds of miles away. All three types of temperature sensor – thermocouple, thermistor and platinum resistance – are commonly used for multi-point temperature measurement. Thermocouples are cheap, especially if the wire is purchased in bulk, and very small sensors can be manufactured. Thermistors are more expensive, slightly larger but more accurate over limited temperature ranges. Platinum resistance sensors are typically 2–3 times the cost of thermistors, but are capable of better than 0.1 °C accuracy. Thin wire and thin film platinum resistance sensors can be very small. Commercial sensors are often enclosed in stainless steel sheaths, which makes them more robust, but increases their response time. 14.1.3 Time–temperature indicators There are many different types of temperature or time–temperature indicators. Almost anything that undergoes a sensibly detectable change with temperature can be used. Liquid crystal devices change colour to indicate the temperature at the time they are observed and time–temperature indicators change irreversibly after a time dependent upon the temperature history or when a temperature threshold is exceeded. Temperature indicators are already used as cheap, safe and hygienic thermometers in the food chain. Several types have been developed to the point where they have been introduced on some chilled and frozen foods in the USA and on chilled foods in France. 288 Meat refrigeration
Temperature measurement 289 14.2 Calibration Any temperature measuring system should be tested over the operating range at regular intervals to ensure accuracy and should also have a current calibration certificate from its manufacturer or official standards laboratory The system can be checked by means of a calibration instrument, or against a reference thermometer that is known to be accurate Melting ice(which if made from distilled water should read 0C, or-006C if made from tap water with 0. 1% salt)may be used to check sensor accuracy. The ice should be broken up into small pieces and placed in a wide-necked vacuum flask with a depth of more than 50mm. The system should be agitated frequently and the temperature read after a few minutes when stable If differences of more than 0.5C are found, the instrument should either be very carefully adjusted or sent for calibration. Other simple calibration systems are available. These consist of a small stirred tank that can be filled with water or oil. The temperature of the stirred liquid is measured using a standard calibrated platinum resistance thermometer. The temperature sensors to be calibrated are placed in the liquid and compared with the standard measurement. The temperature of the liquid can be raised or lowered to different values by the addition ice, cold liquid or hot liquid 14.3 Measuring temperature data Accurately determining the temperature of chilled meat throughout the cold chain is difficult. Training and experience are required to locate posi tions of maximum and minimum temperature in abattoirs, stores, vehicles and display cabinets. The problem is further exaggerated by changes in posi tion with time caused by loading patterns and the cycling of the refrigera tion plants. Obtaining a relationship between environmental temperatures that can be measured relatively easily) and internal meat temperatures is not a simple process. Relating temperatures obtained in a non-destructive manner with internal meat temperatures again poses problems. Determin- ing the temperature of cuts of meat with regular shapes is quite simpl doing so for irregular cuts of meat is more difficult All the temperature measurement problems associated with chill foods will equally apply to quick-frozen foods. In addition, there are a number of other problems. Many instruments have sensors that will accurately measure temperatures of-20.C and below, but the instruments themselves become inaccurate or fail to operate at low temperatures. If frozen foods tre removed from their low temperature environment to one suitable for the instrument the surface temperature rises very rapidly. However, the main problem is that of actually inserting a temperature sensor into frozen meat
14.2 Calibration Any temperature measuring system should be tested over the operating range at regular intervals to ensure accuracy and should also have a current calibration certificate from its manufacturer or official standards laboratory. The system can be checked by means of a calibration instrument, or against a reference thermometer that is known to be accurate. Melting ice (which if made from distilled water should read 0°C, or -0.06 °C if made from tap water with 0.1% salt) may be used to check sensor accuracy. The ice should be broken up into small pieces and placed in a wide-necked vacuum flask with a depth of more than 50 mm. The system should be agitated frequently and the temperature read after a few minutes when stable. If differences of more than 0.5 °C are found, the instrument should either be very carefully adjusted or sent for calibration. Other simple calibration systems are available. These consist of a small stirred tank that can be filled with water or oil. The temperature of the stirred liquid is measured using a standard calibrated platinum resistance thermometer. The temperature sensors to be calibrated are placed in the liquid and compared with the standard measurement. The temperature of the liquid can be raised or lowered to different values by the addition of ice, cold liquid or hot liquid. 14.3 Measuring temperature data Accurately determining the temperature of chilled meat throughout the cold chain is difficult. Training and experience are required to locate positions of maximum and minimum temperature in abattoirs, stores, vehicles and display cabinets.The problem is further exaggerated by changes in position with time caused by loading patterns and the cycling of the refrigeration plants. Obtaining a relationship between environmental temperatures (that can be measured relatively easily) and internal meat temperatures is not a simple process. Relating temperatures obtained in a non-destructive manner with internal meat temperatures again poses problems. Determining the temperature of cuts of meat with regular shapes is quite simple but doing so for irregular cuts of meat is more difficult. All the temperature measurement problems associated with chill foods will equally apply to quick-frozen foods. In addition, there are a number of other problems. Many instruments have sensors that will accurately measure temperatures of -20 °C and below, but the instruments themselves become inaccurate or fail to operate at low temperatures. If frozen foods are removed from their low temperature environment to one suitable for the instrument the surface temperature rises very rapidly. However, the main problem is that of actually inserting a temperature sensor into frozen meat. Temperature measurement 289
90 Meat refrigeration 14.3.1 Contact non-destructive methods The surface temperature of a food or pack can be measured by placing a temperature sensor(such as those discussed above) in contact with the surface. In practice there are very large temperature gradients on both sides of the surface and the presence of the sensor can influence the temperature being measured. Extending the surface of the sensor to measure the average temperature over a larger surface area is one method used to minimise these problems. This method is recommended for such applications as between-pack measurement Since it is impossible to measure the temperature of an exposed surface accurately, the next best thing is to take a measurement of the temperature between two food items. As long as good thermal contact is achieved between the temperature sensor and the packs, a between-pack method should provide an accurate measurement of the pack temperature. If the thermal conductivity of the packaging material is high and the food makes a good thermal contact with the pack then the temperature measured will be close to that of the product With a product such as skin-wrapped chilled sausages the above require ments are satisfied. A temperature sensor, especially a flat-headed probe, can be sandwiched between two packs. An accurate measurement is obtained owing to the combination of a flexible food and a thin wrappin With chilled food in cartons or bubble packs the accuracy is much lower The contact problems are much greater with a frozen product. Since the surface of a frozen product is not flexible, only point contact can be achieved between the surface of the product and that of the pack or probe. Using a flat probe with extended contact surfaces does not necessarily improve the accuracy of temperature measurement. In extreme cases, for example with frozen sausages, the contact surfaces may extend out into the ir stream and measure air, not product temperature. With packs of smal items such as diced meat the accuracy will be much better. Care must also be taken to precool the probe before temperatures are measured. This especially important with low heat capacity packaging materials. Alternatively'temperature sensitive' paints can be painted directly onto the surface of interest and will accurately determine its temperature However, painting foods is not a practical solution 14.3.2 Non-contact non-destructive method Non-contact temperature measurement devices measure the amount of energy in an area of the infrared spectrum that is radiated from the surface being measured. Basic instruments measure the average temperature of the area in a small field of view. More complicated systems of thermal imaging provide a temperature picture of all the objects over a much wider area. There are two types of detector currently used in low temperature infrared thermometers, thermopile detectors and pyroelectric detectors
14.3.1 Contact non-destructive methods The surface temperature of a food or pack can be measured by placing a temperature sensor (such as those discussed above) in contact with the surface. In practice there are very large temperature gradients on both sides of the surface and the presence of the sensor can influence the temperature being measured. Extending the surface of the sensor to measure the average temperature over a larger surface area is one method used to minimise these problems. This method is recommended for such applications as between-pack measurement. Since it is impossible to measure the temperature of an exposed surface accurately, the next best thing is to take a measurement of the temperature between two food items. As long as good thermal contact is achieved between the temperature sensor and the packs, a between-pack method should provide an accurate measurement of the pack temperature. If the thermal conductivity of the packaging material is high and the food makes a good thermal contact with the pack then the temperature measured will be close to that of the product. With a product such as skin-wrapped chilled sausages the above requirements are satisfied. A temperature sensor, especially a flat-headed probe, can be sandwiched between two packs. An accurate measurement is obtained owing to the combination of a flexible food and a thin wrapping. With chilled food in cartons or bubble packs the accuracy is much lower. The contact problems are much greater with a frozen product. Since the surface of a frozen product is not flexible, only point contact can be achieved between the surface of the product and that of the pack or probe. Using a flat probe with extended contact surfaces does not necessarily improve the accuracy of temperature measurement. In extreme cases, for example with frozen sausages, the contact surfaces may extend out into the air stream and measure air, not product temperature. With packs of small items such as diced meat the accuracy will be much better. Care must also be taken to precool the probe before temperatures are measured. This is especially important with low heat capacity packaging materials. Alternatively ‘temperature sensitive’ paints can be painted directly onto the surface of interest and will accurately determine its temperature. However, painting foods is not a practical solution. 14.3.2 Non-contact non-destructive methods Non-contact temperature measurement devices measure the amount of energy in an area of the infrared spectrum that is radiated from the surface being measured. Basic instruments measure the average temperature of the area in a small field of view. More complicated systems of thermal imaging provide a temperature picture of all the objects over a much wider area. There are two types of detector currently used in low temperature infrared thermometers, thermopile detectors and pyroelectric detectors. 290 Meat refrigeration
Temperature measurement 291 Thermopile detectors consist of a collection of rods that act as thermocou ples to sense emitted thermal radiation. Pyroelectric detectors contain a crystal which exhibits temperature-dependent polarisation and requires the ncident radiation to be cut by a 'chopping device to prevent currents building up within the crystal that nullify this charge A certain amount of knowledge is needed in order to interpret the values that such instruments give(Evans et al., 1994; James and Evans, 1994). The first point to bear in mind when using infrared thermometry is that the tem- perature measured is the surface temperature. If the meat has been in surroundings that have not changed in temperature for a long period of time, then it is likely that the surface temperature will be very close to that of the meat beneath the surface. However, if the temperature of the sur roundings is changing or has changed over the previous 24 h, then it is likely that the surface temperature will not be the same as the temperature deep within the meat whe or example, transfer a carton of frozen meat from a refrigerated lorry where the temperature has been maintained at -18C to a refrigerated oading bay at 4C, and the surface of the carton will warm very rapidly Therefore, if an infrared thermometer is to be used to check the meat tem- perature the value must be taken before it is removed from the lorry if any ccuracy is to be obtained. Even temperature fluctuations of +2C caused by normal control fluctuations will mean that the surface tem- perature will differ significantly from the deep meat temperature. If the neat is still cooling down, its surface temperature will be warmer than the air temperature and, in turn, the interior of the meat will be warmer still These problems can be overcome if the operator is aware of them. For example, the temperature of the carton of frozen meat could be measured in the lorry, or if removed the carton could be opened and the temperature of an inner surface of one of the packs inside immediately read with the infrared thermometer. However, in doing this two of the principle advan- tages of using infrared have been removed: namely, being quick and totally non-destructive It is also necessary for the operator to know how much of the surface is 'seenby the infrared instrument, as it will measure the average'tempera ture over the whole of this area. The target area can vary significantly from instrument to instrument and with the distance between the instrument and the surface A further complication in the use of infrared thermometers is reflected radiation. The instruments will 'see the radiation emitted from a surface and also an amount of radiation from the surroundings that is reflected by that surface. The reflected radiation will therefore constitute an error. For warm objects at a temperature greater than their surroundings, the amount of reflected radiation will be small in relation to that from the surface and consequentially the error will be small. With frozen meat, the temperature of the meat is no warmer than, and often colder than, the temperature of
Thermopile detectors consist of a collection of rods that act as thermocouples to sense emitted thermal radiation. Pyroelectric detectors contain a crystal which exhibits temperature-dependent polarisation and requires the incident radiation to be ‘cut’ by a ‘chopping device’ to prevent currents building up within the crystal that nullify this charge. A certain amount of knowledge is needed in order to interpret the values that such instruments give (Evans et al., 1994; James and Evans, 1994). The first point to bear in mind when using infrared thermometry is that the temperature measured is the surface temperature. If the meat has been in surroundings that have not changed in temperature for a long period of time, then it is likely that the surface temperature will be very close to that of the meat beneath the surface. However, if the temperature of the surroundings is changing or has changed over the previous 24 h, then it is likely that the surface temperature will not be the same as the temperature deep within the meat. For example, transfer a carton of frozen meat from a refrigerated lorry where the temperature has been maintained at -18 °C to a refrigerated loading bay at 4 °C, and the surface of the carton will warm very rapidly. Therefore, if an infrared thermometer is to be used to check the meat temperature the value must be taken before it is removed from the lorry if any degree of accuracy is to be obtained. Even temperature fluctuations of ±2 °C caused by normal control fluctuations will mean that the surface temperature will differ significantly from the deep meat temperature. If the meat is still cooling down, its surface temperature will be warmer than the air temperature and, in turn, the interior of the meat will be warmer still. These problems can be overcome if the operator is aware of them. For example, the temperature of the carton of frozen meat could be measured in the lorry, or if removed the carton could be opened and the temperature of an inner surface of one of the packs inside immediately read with the infrared thermometer. However, in doing this two of the principle advantages of using infrared have been removed: namely, being quick and totally non-destructive. It is also necessary for the operator to know how much of the surface is ‘seen’ by the infrared instrument, as it will measure the ‘average’ temperature over the whole of this area. The target area can vary significantly from instrument to instrument and with the distance between the instrument and the surface. A further complication in the use of infrared thermometers is reflected radiation. The instruments will ‘see’ the radiation emitted from a surface and also an amount of radiation from the surroundings that is reflected by that surface. The reflected radiation will therefore constitute an error. For warm objects at a temperature greater than their surroundings, the amount of reflected radiation will be small in relation to that from the surface and consequentially the error will be small. With frozen meat, the temperature of the meat is no warmer than, and often colder than, the temperature of Temperature measurement 291
92 Meat refrigeration the surroundings. Therefore, the amount of reflected radiation coming from the surface constitutes a significant error However, the proportion of radiation emitted by a surface relative to that of a perfect black body is the same proportion of incident radiation that would be absorbed by the surface. If the absorbtivity or emissivity of the surface is known and the surface is not transparent, which is true for all packaging materials except some plastics, the reflectance of the surface will also be known by subtracting the absorbtivity from 1. Hence it is possible to calculate the extent of the error Unfortunately this requires a lot of information about the emissivity and eflectance of the various packaging materials and takes a long time. There- fore, the advantage of taking quick accurate readings will be lost The Meat Research Corporation in Australia(1995a)recommend that an infrared thermometer should be permanently located in beef chill rooms. It should be positioned to measure the surface temperature of one of the last sides to be loaded. It can then be used to provide a permanently logged output of surface temperature and control the refrigeration system. When the surface temperature has reached ca. 2C the fan speed can be auto- matically reduced and the suction pressure raised. This will reduce both weight loss and operating cost Infrared systems have also been used to monitor the surface tempera- ture of pig carcasses in chill rooms(Metternick-Jones and Skevington 1992). The thermometer required a minimum stabilisation time of 120min 1C of other methods and were repeatable to the same accurat e within in the chill room. After this time the temperatures measured wer 14.3.3 Contact destructive methods Determining the temperature of small cuts of meat with regular shapes is quite simple. Determining the temperature of irregular cuts of meat, par ticularly large pieces, is more difficult. Possibly the most difficult problem is ascertaining deep leg temperature in beef carcasses The Meat Research Corporation in Australian(1995b)recommend that the temperature sensor should touch the trochanter major (aitch bone), which is the knob'of bone on the opposite side of the femur to the hi nt. To locate the in this position it should be inserted through the pope's eye' at an angle about 15-20 below the horizontal(Fig 14.1).It should be aimed at an imaginary vertical line approximately one-third of the distance from the achilles tendon to the last tailbone Because conduction occurs along the steel shaft of a probe it is impo tant that the probe is inserted as far as possible into the meat. For example, to take the temperature of a cut of meat it is better practice to insert the probe to its full depth along the long axis of the cut rather than to insert the probe to half its length through the short axis(Csiro, 1991) For manufacturers to produce accurate data on the freezing of their meat
the surroundings. Therefore, the amount of reflected radiation coming from the surface constitutes a significant error. However, the proportion of radiation emitted by a surface relative to that of a perfect black body is the same proportion of incident radiation that would be absorbed by the surface. If the absorbtivity or emissivity of the surface is known and the surface is not transparent, which is true for all packaging materials except some plastics, the reflectance of the surface will also be known by subtracting the absorbtivity from 1. Hence it is possible to calculate the extent of the error. Unfortunately this requires a lot of information about the emissivity and reflectance of the various packaging materials and takes a long time. Therefore, the advantage of taking quick accurate readings will be lost. The Meat Research Corporation in Australia (1995a) recommend that an infrared thermometer should be permanently located in beef chill rooms. It should be positioned to measure the surface temperature of one of the last sides to be loaded. It can then be used to provide a permanently logged output of surface temperature and control the refrigeration system. When the surface temperature has reached ca. 2 °C the fan speed can be automatically reduced and the suction pressure raised. This will reduce both weight loss and operating costs. Infrared systems have also been used to monitor the surface temperature of pig carcasses in chill rooms (Metternick-Jones and Skevington, 1992). The thermometer required a minimum stabilisation time of 120min in the chill room. After this time the temperatures measured were within 1 °C of other methods and were repeatable to the same accuracy. 14.3.3 Contact destructive methods Determining the temperature of small cuts of meat with regular shapes is quite simple. Determining the temperature of irregular cuts of meat, particularly large pieces, is more difficult. Possibly the most difficult problem is ascertaining deep leg temperature in beef carcasses. The Meat Research Corporation in Australian (1995b) recommend that the temperature sensor should touch the trochanter major (aitch bone), which is the ‘knob’ of bone on the opposite side of the femur to the hip joint. To locate the sensor in this position it should be inserted through the ‘pope’s eye’ at an angle about 15–20° below the horizontal (Fig 14.1). It should be aimed at an imaginary vertical line approximately one-third of the distance from the Achilles tendon to the last tailbone. Because conduction occurs along the steel shaft of a probe it is important that the probe is inserted as far as possible into the meat. For example, to take the temperature of a cut of meat it is better practice to insert the probe to its full depth along the long axis of the cut rather than to insert the probe to half its length through the short axis (CSIRO, 1991). For manufacturers to produce accurate data on the freezing of their meat 292 Meat refrigeration