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 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 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