SUMMARY OF THE PRIOR ART The addition of ice-glaze to frozen products is carried out in the first instance to protect the product from dehydration in frozen storage. It also acts as a barrier to oxygen and thus helps to reduce the deterious oxidation and other changes that can take place. It is however a difficult process to control depending on: 1. glazing time 2. product temperature 3. water temperature 4. product size and weight 5. product shape It is accepted that the addition of ice-glaze to frozen products is an essential part of this type of preservation and is required to prevent loss and damage to the product by evaporation of water or ingress of oxygen.

This procedure however can lead to abuses, to the detriment of the consumer and even the retailer. These abuses primarily result from the addition of extra water in the form of ice-glaze to the product to increase its weight. Thus the consumer pays for an increased weight of water rather than the product itself. The uncontrolled addition of ice-glaze can lead to disputes between producers and retailers or retailers and consumers. The interests of the latter are usually represented by local trading standards officers. The disputes are made even more difficult to resolve when it is seen that there is no reliable method for the measurement of this added ice-glaze. In order to'police'this area public analysts and local trading standards officers use a number of published methods, all of which can be extremely inaccurate and subjective. These same methods are all that is available to the industry who are themselves keen to control the

amount of added glaze. Examples of these methods are the CODEX STAN 92-1981, (1981),"Quick frozen shrimps or prawns standard", Section 7.6 Codex Alimentarius Commission, FAO, Rome and the 'Lancashire Method', Lord D. W., Green M. S. and Rhodes J. T., (1986), Environmental Health 94,143-148,"Determination of ice- glaze for IQF cooked and peeled prawns".

A collaborative study by Hodson G. C., Scotter M. J. and Wood R., (1989),"Methods of analysis for the determination of ice-glaze on fish products: Collaborative trial", J Assoc. Publ. Analysts, 27, 85-108, showed that there was considerable variability between the results from different laboratories carrying out basically the same method. Despite this lack of agreement however, a validated method was published by The Ministry of Agriculture Fisheries and Food (MAFF) (1992)"Ice-glaze on quick frozen prawns", MAFF Validated Method for the Analysis of foodstuffs, No. V13, MAFF, London.

The published methods all operate on the same basic principals as the validated method described above. In the validated method a frozen sample is weighed and then placed in a sieve of a known weight. The sieve and sample are then immersed in water and left until the ice-glaze has melted. The sieve is then removed from the water and left to drain for a set time with the sample in it. The weight of the drained sieve and sample is then measured. The ice- glaze content is taken as the difference between the weight of the sieve and sample before the ice glaze is melted and the weight of the sieve and sample following melting and draining.

The major problem with the existing methods lies in the thawing of the samples to be measured. The result is determined from a reduction in weight, which is assumed to be due to a loss of thawed ice. This weight loss may however be due to drip loss of the meat itself, i. e. loss of water from the cells of the sample itself, and it is impossible by existing methods to distinguish between the two. Other factors can also effect the results, the addition of polyphosphate prior to freezing may facilitate the uptake of melted ice-glaze into the tissues of the sample and some water may still not be removed even when the sample has been dried

by using cloths. All these factors contribute to a very large error in the measurements.

SUMMARY OF THE INVENTION The present invention addresses the above problems by measuring the dissolved ice-glaze itself and not weight losses in the food product following removal of the ice-glaze.

The present invention therefore provides a method of measuring the amount of an ice-glaze added to a frozen food product comprising the steps of; dissolving the ice-glaze in a measured quantity of solvent containing no water to form a solution, and measuring the water content of the solution, wherein the amount of ice glaze added to the frozen food product is determined from the amount of water in the solution when all the ice-glaze has been dissolved.

This provides the advantage that the ice-glaze itself is measured and as such the possible inaccuracies and assumptions related to the measurement of weight loss in the food product itself are mitigated.

The method can advantageously comprise the additional step of measuring the electrical conductivity of the solution whilst dissolving the ice-glaze, wherein all the ice-glaze is deemed to be dissolved when the electrical conductivity of the solution reaches a maximum or ceases to change further.

This step gives a measurable point at which the ice glaze is considered to be dissolved which allows for a consistent approach.

The'transition point', i. e. the point at which the conductivity of the solution reaches a maximum, results from the fact that the ice-glaze is usually added in a process which involves the addition of salt, the salt has a higher conductivity than the water and as such when the conductivity of the solution reaches a maximum the transition point is reached which indicates that all the glaze is dissolved. The dip in conductivity which follows is

due to the subsequent dilution of the salts present in the glaze water by the beginning of the osmotic diffusion regime.

Alternatively a set time period can be used which would allow for all the ice-glaze to dissolve or the user can judge by eye or touch when the ice-glaze is all melted.

The method for measuring the water content of the solution preferably comprises the steps of; measuring the dielectric properties of the solution and the temperature of the solution whilst the glaze is being dissolved; comparing the dielectric properties of the solution to the dielectric properties of known solutions of water and solvent in varying ratios, to give a water to solvent ratio for the solution, and correcting the water to solvent ratio of the solution in relation to any measured changes in temperature.

Such calibration data for known solutions of water and solvent in varying ratios and temperature corrections can be achieved in one experiment and standard tables produced for specific solvents, for ease of reference.

Alternatively the water content of the solution can be measured by determining any other property of the solution which is known to be affected by the solution's water content, such as density or thermal conductivity, and which can provide a provide a suitable calibration.

Whilst the ice-glaze is being dissolved the solution can be stirred. This is particularly helpful to prevent the formation of air bubbles on the surface of the measuring apparatus which can effect measurements but it is also helpful to ensure good heat and mass transfer within the solution.

Isopropanol has been found to preferable as the solvent as it provides good sensitivity for the measurements and has a suitable density so that samples do not float in the solvent which could effect the measurements.

According to the present invention there is also provided a device for determining the amount of an ice glaze added to a frozen food, the ice glaze having been melted in a solvent to form a solution, comprising means for measuring the dielectric properties of the solution, and processing means for determining the amount of ice glaze from the dielectric properties of the solution.

Alternatively, such a device could comprise means for measuring any other property of the solution which is known to be affected by the solution's water content, such as density or thermal conductivity, and which can provide a suitable calibration.

Such a device may provide for the automatic generation of results without the need for operator intervention.

Advantageously the means for measuring the dielectric properties of the solution comprises a microwave circuit configured so as to measure the reflection coefficient of an open ended sensor.

A device for use in the measurement of the dielectric properties of materials can comprise; a microwave circuit which comprises a microwave source connected via an isolator to a direction coupler, a first output of which is connected to a reference signal detector and an amplifier, a second output of which is connected to a reflected signal detector and an amplifier and a third direct output of which is connected to a microwave transmission line terminated in an open-ended coaxial sensor. The outputs of the detector amplifiers can be fed via a digitising circuit to a microprocessor or a personal computer.

In addition there can also be provided a means for detecting changes in the conductivity of the solution, the output of which can be fed to a processing means such as microprocessor or personal computer.

There can also be provided a means for measuring the temperature of the solution, the output of which can be fed to a processing means such as a microprocessor or personal computer.

The microprocessor or personal computer can perform a number of operations including the storage of calibration equations, the

temperature correction of the calibrations, the determination of. the point when the ice-glaze has all melted and the calculation of added ice-glaze from the dielectric measurements at a specific instant.

BRIEF DESCRIPTION OF THE DRAWINGS The device according to the present invention will now be described by way of example only, with reference to the accompanying drawings in which: Fig 1 is a block circuit diagram of a device according to the present invention Fig 2 shows the device according to the present invention being used to determine the ice glaze of a frozen product Fig 3 shows the coaxial sensor being used Fig 4 shows the circuit diagram of the conductance measurement Fig 5 shows the circuit diagram of the temperature measurement DETAILED DESCRIPTION As shown in Fig 1, a device for use in the measurement of the dielectric properties of a solution has a microwave circuit which measures the magnitude of the reflection co-efficient of an open ended sensor immersed in the solution of interest comprising a microwave source 1, connected via an isolator 2 to a directional coupler 3. One port 4 of the directional coupler is connected to a reference signal detector 5, another 6 is connected to a reflected signal detector 7 and the other 8 is connected to a transmission line 9 itself connected to an open ended coaxial sensor 10. Outputs of the reference signal detector 5 and the reflected signal detector 7 are each connected to low gain, voltage-follower type amplifies 11 and 12. Each of these are connected to inputs of a 16-bit multi-channel analogue-to-digital converter 13 from which the digitised signals are fed to a microprocessor or personal computer 14.

The coaxial sensor 10 is shown in Fig 3 and consists of a piece of coaxial transmission line with a 3mm diameter dielectric core 15 of some solvent resistant material and inner 16 and outer 17

conductors of copper. This is connected to the microwave circuit via sma type connectors 18.

The measurement of the conductivity of the solution is achieved by the use of the circuit shown in Fig 4. A pair of simple electrodes 19 are immersed in the solution 20 and is connected in series with a known resistance 21 across a 5V dc voltage 22. The voltage dropped across the electrodes is fed to an input of a 16 bit analogue-to-digital converter 13.

The temperature of the solution is measured as shown in Fig 5 using a thermistor 23 connected in series with a known resistance 24 across a 5V dc voltage 22. The voltage dropped across the thermistor is fed to an input of a 16 bit analogue-to-digital converter 13.

Fig 2 shows the device in operation. In use the sensor 10, the conductance electrodes 19 and the temperature measuring thermistor 23 are immersed in pure alcohol in this description being isopropanol in a beaker 25. The liquid is stirred by a stirrer 26 and its temperature is maintained by some heating or cooling device or both 27.

In the reflectometer unit 28 (Fig 1) the microwave source 1 in this description operates at 2 GHz with an output power of 2-3 milliwatts. Power from the microwave source 1 is fed via the isolator 2 to the directional coupler 3. The purpose of this isolator is to prevent the reflected power from the sensor 10 from interfering with the operation of the microwave source. The directional coupler 3 feeds a fraction of the power, in this case 20dB, to the reference signal detector 5. The major part of the power is fed through the transmission line 9 to the sensor 10 at the open end of which some of it is reflected, the rest being transmitted into the liquid and absorbed. The reflected power is fed back through the transmission line to the directional coupler 3 which feeds a fraction of the power, in this case 20dB, to the reflected signal detector 7. Both detected signals are digitised 13 and fed to a microprocessor or personal computer 14 at the same time as the digitised temperature and conductance measurements.

The ratio of the reflected microwave signal to the reference signal is a measure of the magnitude of the complex reflection co- efficient at the sensor and liquid interface. The reflection co- efficient changes as the dielectric properties of the liquid change. This can be as a result of change in temperature or change in the amount of water mixed with the alcohol. The logarithm of this ratio (ratio expressed as dB) is found to be proportional to the percentage water content of the alcohol when this percentage is less than 30% of the total weight of solution.

Calibration equations involving these two parameters are found by adding known quantities of water to the solution at various measured temperatures. These equations are stored by the microprocessor or the personal computer.

One method for the use of the device is as follows. An amount of 250g of isopropanol is placed in a 500ml beaker which is placed either on a temperature controlled heater or in a temperature controlled bath at 20°C. The sensor, thermistor and conductance electrodes are also immersed the liquid being agitated vigorously by a stirrer. The data collection begins and at a suitable moment a known weight of frozen glazed prawns of about 50g is dropped into the liquid. The conductance changes rapidly as ice is dissolved in the alcohol and the output from the reflectometer changes more slowly. At the instant that it is detected that the conductance has reached a maximum the water content of the solution is calculated from the difference between the reflectometer output at the instant that the conductance reaches a maximum and the output when only isopropanol was present using the calibration equations corrected for the measured temperature at that instant.

The device described above, is not restricted for use with prawns and may be used for a variety of frozen food products.