measurement of wet fish
17.2 Temperature measurement of fish during freezing
17.3 Temperature measurement of frozen fish
17.4 Summary of rules for measuring fish temperature
Temperature measurement is important at all stages of fish handling and processing to ensure that the fish and their environment are at a suitable temperature for maintaining the good quality of the fish. The temperature of the fish is important during the period before freezing since both the eating quality and appearance of the final product depend on the rate of spoilage at this time. Even small differences in the temperature of the fish can result in discernible differences in quality. Checks should therefore be made on the effectiveness of any chilling method used during the prefreezing period by periodically measuring the temperature of selected fish.
Freezing times must be known in order to design freezing plant correctly. Periodic checks on the freezer performance are also useful so that any faults that develop can be quickly corrected. Even after freezing, cheeks are frequently made on the temperature of the frozen product during handling, transportation and cold storage as a means of quality control. All these temperature-measuring requirements need special instruments and special techniques in order to give meaningful results and what follows gives some guidance on the correct methods of temperature measurement to suit each requirement.
In any batch of fish, it is important to know the temperature of the warmest fish. Depending on whether the fish are, at the time of measurement, being cooled or warming up, the warmest fish may be at the centre or on the outside of the batch or container. Even when the location of the warmest fish is known, it is advisable to take a number of random temperature measurements. Fish temperatures should therefore be taken at the outside, the top, the bottom and any other position that may be thought to be significant.
A convenient instrument for measuring the temperature of wet fish is a probe thermometer which has been specially developed for this purpose. The instrument must be robust and have a rapid response so that readings may be taken quickly. The temperature sensitive element in the probe should be small so that the temperature at the point of the probe- only is indicated. The probe should then be inserted in the fish to the point to be measured with sufficient length of the probe in the fish to keep errors due to conduction of heat along the probe to a minimum. It has been found in practice that an instrument used to measure the temperature of wet fish should have an accuracy of within 0.5 Celsius degree.
Since fish is frozen from the outside inwards, it is impossible to judge by the outward appearance or the feel of the fish whether the whole of it is frozen. The surface of the fish, which is close to the freezing medium such as the cold air in a blast freezer or the cold metal of a plate freezer, will very quickly be reduced to a temperature near to that of the freezer. The temperature inside the fish will, however, change more slowly.
The most suitable instrument for measuring freezing times is one which uses a thermocouple wire. The thickness of the thermocouple wire can be chosen to suit the product being frozen, and since it is comparatively cheap and expendable, the wire can be cut off after freezing leaving a short length in the fish which must be recovered when the fish is thawed.
Since the freezing time of a product is the time taken for the warmest point of the fish to reach a desired temperature, it is essential that temperature measurements be taken at the points which are likely to freeze last. In the example shown in Figure 47, the apparent freezing time to -20°C an vary from less than 1 to 21/2 h depending on where in the fish the temperature is measured. The shape of a good temperature time curve is characterised by a plateau at a steady temperature somewhere between 0°C and -3°C followed by a steep plunge to near the temperature of the freezer.
The centre of the fish or package is not necessarily the last part to freeze; this will happen only when freezing is carried out equally from all sides. The thermocouple should therefore be inserted in the fish so that the temperature- sensitive point is likely to be in the part that will freeze last. It is also important that as great a length of wire as possible is in the same layer of fish and hence at the same or nearly the same temperature. This has a twofold purpose: it ensures that there is no error due to conduction of heat along the wire and also that, if the wire is pulled slightly out of position during the loading operation, the temperature-sensitive junction of the thermocouple will remain in a part of the fish that freezes last. Small items such as shrimps are too small to ensure that a sufficient length of the thermocouple is in the fish. In this case a number of shrimp should be threaded on to the thermocouple behind the temperature- sensitive junction which is then located at the centre of the last shrimp.
Thermocouples should be placed in fish which are likely to have the most significant freezing times. Choice of positions in an air blast freezer, for example, would include fish nearest to and furthest from the incoming cold air, fish close to the tunnel walls, at the top and bottom of the load and at any other point where there is a likelihood of fish freezing faster or slower than the average. Once the performance of a freezer with a particular product has been established, subsequent periodic checks need not be so comprehensive.
In the absence of any temperature measuring instruments, some indication can be obtained by examination of the product. The surface of the fish being frozen remains comparatively soft and can be penetrated with a sharp probe down to a temperature of about 4°C. if this penetration can be made, the product is far from being frozen. At the completion of freezing, further examination can be made by breaking open a selected sample of fillet. If the fillet is frozen hard all the way through then the freezing time may have been sufficiently long. If, however, the centre remains soft, a longer period is required in the freezer.
It is sometimes necessary to check the temperature of frozen fish during handling, transport or cold storage and because of the hardness of the product. A spear-type thermometer cannot be inserted in a fish that has a surface temperature lower than -4°C unless it has been specifically designed to be driven into the flesh. This type of spear is often much thicker and robust and may not react quickly enough. If thermocouples have already been used to check the freezing rate of the fish, the ends of the wires remaining in the frozen product can be reconnected to a suitable instrument to measure temperatures at any time during the storage or transport. The fish or packages containing thermocouples should be located at points in the store or vehicle where temperatures are most critical or where they are representative of the bulk of the product. Where there is no thermocouple frozen into the product, it will be necessary to drill a hole so that a thermometer may be inserted but this method is only accurate if the correct procedure is carried out. Errors as great as 20°C are possible when an unsuitable thermometer and an incorrect technique are used (Figure 48). A probe thermometer similar to that described for wet fish temperature measurement should be used and the following measurement procedure should be adopted. Before removing the fish from the cold store drill a neat hole in the fish just large enough to take the thermometer probe, some time before the measurement is required. The hole should preferable be at least 10cm deep to avoid errors due to conduction of heat. This depth of penetration will obviously not be possible with all frozen products. Insert the probe and read the temperature continuously until the lowest reading is reached and the temperature starts to rise again. The lowest temperature observed should then be within 0.5°C of the true temperature. Errors using this technique are mainly due to the fish warming up. The operation should therefore be quick and should not take more than 2 or 3 min. The drilling of the hole has no measurable effect on the temperature of the fish since the heat introduced is quickly dissipated. In the case of fish fillets or small packs which are too thin to drill to a suitable depth the probe can be placed between two fillets or packs. The fillets or packs are then held tightly together, until the lowest temperature is reached.
For routine temperature checks of packaged fish, measurement with a spear sensing unit can be accurate to +1°C but this method is particularly susceptible to error in the hands of an inexperienced operator.
Always measure the most significant temperature, identify and check those fish that are slowest to cool, quickest to warm or are at a high temperature.
The temperature probe should penetrate the fish as deep as possible to avoid errors due to conduction of heat.
Measure the temperatures quickly with little or no handling of the fish.
Use an instrument that responds quickly to temperature changes and that reads to within a 0.5 Celsius degree of the true temperature.
Use an instrument with a small temperature- sensitive element.
Periodically check and recalibrate instruments.