Use Of Meter In Quality Control

The Fish Freshness Meter was designed with the needs of the quality controller in mind. The term ‘quality controller’ is meant to include officials responsible for public health inspection or enforcement of mandatory regulations concerning quality, as well as persons concerned with quality control in industry and commerce. In a typical situation, a controller is required to test batches of fish that are being inspected, sold or processed as a unit, and for this a suitable sample must be taken. The meter simplifies this process of assessing a batch by sampling.

If the freshness of individual fish within a batch is measured with the meter, it will be found to vary. This fish-to-fish variation in meter readings is caused by two factors:-

(a) Variation in intrinsic freshness among individual fish.
(b) Variation of meter readings among fish of the same freshness.

Both are expressions of the variability of biological organisms. A batch of fish caught at the same time and handled and stored identically will spoil at slightly different rates because of variations in chemical constitution and bacterial activity. Hence the individual fish in the batch will have a range of freshness; a range that will tend to increase as spoilage increases.

The meter readings themselves are not direct measures of freshness, as defined in sensory terms, but are strongly associated with it. The relationship between meter readings and freshness as measured by a sensory panel will once again be different for each individual fish. For this reason it is not recommended that the meter be used to assess the freshness of single fish, except within rather wide limits.

These deviations from average behaviour tend to cancel out when the means of samples from batches are taken, and the bigger the sample the better the correlation between the mean meter reading and freshness score. It follows from sampling theory that the number of fish which ought to be sampled from a batch depends mainly on the desired precision of the mean score. It also follows that, provided the batch is large enough, the required sample size does not depend on the size of the batch. From observations made, it has been decided to use a sample size of 8 Fish as the basis for the averaging in the Torry Fish Freshness Meter.

In most marketing and distribution systems the fish are held in containers with a capacity of 50 kg. Several containers constitute a batch. In this situation fish from as many containers as possible should be sampled i.e. one from each container if the batch has eight or more, four from each container if that is all there is in the batch. If the controller or inspector wants greater confidence in the batch average, more than one set of eight fish can be sampled, and a grand average of the results calculated.

Note that the precision of an averaged result increases only as the square root of the sample size. An average of 64 readings is only twice as precise as an average of 8 readings.

Relationship Between Meter Readings and Other Measures of Freshness

There are many ways of expressing the freshness of fish. Perhaps the simplest for fish held in ice is to quote the storage time. However, as is well known, the initial quality of many species of fish varies with the seasons of the year, due to such factors as the spawning cycle and the availability of food. These factors also affect the rate of spoilage, so that the time in ice is not necessarily a good measure of freshness unless related to the time of the year. It has been found that a better way of expressing freshness is to construct a system of numerical scales based upon methods of objective sensory assessment by a trained taste panel. Such a system is better because it is directly related to eating quality rather than depending on knowledge of the storage conditions.

Often the fish is allocated to one of a few quality grades based on definitions in sensory terms. An example of this is the European Economic Community’s system of grading fish for marketing purposes. An extreme form of this type is the public health inspector’s grading into fit and unfit. A quality controller may not even consciously put a score or a grade to a batch, but must accept or reject it for the purpose in mind.

In practice the quality controller in industry adopts the most convenient system for his purpose, but an official inspector will have to use whatever is laid down in the regulations. It is quite possible to define grades, boundaries or acceptance/rejection ranges of freshness for various outlets, etc, in terms of meter readings and indeed this is a good way of using the instrument. However, until the user is familiar with the instrument, it is best to employ it in parallel with the accustomed method of assessing freshness. After some experience, the relationship between the quality levels the user is familiar with and the meter readings can be established. Nevertheless, the meter can be used by an unskilled operator after very little instruction.

As a guide to the values that can be expected from some species of interest to European Fish Processors, average readings for different levels of freshness are shown in the set of Charts within APPENDIX 1 starting on p45. These results were obtained by Torry Research Station at their laboratories in Aberdeen and Hull, and at various fishing ports in the UK. The sensory scores are based on the scales used at Torry Research Station, and have been described in their publications.

These scales were interpreted as follows:-

1. Perfectly fresh fish is given a score of 10.
2. Good quality fish has a score of 6 or more.
3. Below a score of 4, the fish are considered unfit to eat.

These freshness scores are only a rough guide. Users are strongly advised to obtain equivalent levels from scores given by their own trained panel, as the scores may be based upon different criteria, or obtained under different conditions, from those obtained by Torry Research Station and Distell.

For comparison, the approximate number of days of storage in ice which would cause the fish to reach a given freshness score is also included in the tables (appendix section). An explanation of the different columns of meter readings is given in the next section.

The examples given are typical demersal fish. The situation with fish of high and variable fat content is rather more complex, and the relationships between the meter readings and sensory scores have not yet been elucidated. However, extensive work carried out by the staff of Torry Research Station with herring has established that the manner in which meter readings vary with time of storage in ice is very dependent on:-

(a) The fat content of the fish.
(b) Whether or not it is iced immediately after catching.
(c) The extent to which it is subjected to handling.

The Charts on pages 58 to 63 within APPENDIX 1 show the approximate age in ice corresponding to given meter readings for boxed herring of various fat contents. The age-in-ice is valid only for fish which has been boxed and iced immediately after catching. Delayed icing always gives rise to lower meter readings during subsequent storage in ice compared with those shown in the chart.

The extent of the effect varies with fat content and temperature during storage before icing. For example, lean fish held for 8 hours at a temperature of about 5°C before icing will generally show meter readings about one unit lower than those on page 58, for any given number of days in ice, while for fatty fish the effect can be up to twice as great for the same time and temperature before icing. There is evidence to show that the influence of still higher initial temperatures can easily double this effect.

Mackerel generally behave in much the same way as do herring, although detailed information is not at present available. The charts on pages 64 and 65 give some idea of readings to be expected from carefully handled fish, boxed and iced.

Although less is known of the relationship between objective sensory scores and meter readings than for white fish, it is believed that meter readings give a fair indication of the intrinsic quality of unhandled, boxed fatty fish, irrespective of fat content.

The Tables on pages 69 to 74 show relationships between the Fish Freshness Meter readings and time of storage in ice for a few species of African fish. The data has been supplied by the Tropical Products Institute, London.

Effects of Handling and Processing

The Torrymeter measures certain properties of fish muscle and skin that change in a systematic way during storage in the wet state. Any other process that affects the structure of the muscle at the cellular level will also affect the measurements, almost invariably to lower them. Handling is an important factor.

A common method of handling fish in Europe is to stow it, mixed with ice in bulk aboard the vessel, then unload it onto the quayside and sort the fish into containers. The pressure it is subjected to in the hold, the handling during discharge, and the sorting process tend to lower the meter readings when compared with fish that have been stowed carefully in boxes with ice and kept undisturbed until measured. The difference is indicated in the Charts within APPENDIX 1 on pages 46 to 74

The meter can also be used to grade fillets. For skin-on fillets make the measurements on the skin side in the usual way.

The meter measures properties of both skin and muscle, but it can still be used with skin-off fillets. In this case the meter should be applied to the bone side of the fillet. The readings though, are much lower than for whole fish or skin-on fillets of equivalent freshness. Also discrimination in fish material with a freshness score below 6 is not possible. Typical values for cod are shown in the chart on page 49.

The quality of herring and mackerel is most sensitive to handling, and in each case this is reflected by the readings obtained with the meter. Even careful handling of the fish can lower the instrument readings by 1 or 2 units. Normal commercial handling reduces the readings still further.

Effects of Freezing and Brining

Freezing has a drastic effect on the cellular constituents of muscle. It is not possible to determine the original freshness of thawed fish since meter readings are invariably in the range of 0-3, whatever the quality before freezing. In fact, this property can be used in many cases to determine whether fish has been frozen at some time in its history. If the sample, judging by odour, appearance and flavour appears to be fresh i.e. should have a high meter score, but in fact gives a low reading then there are grounds for suspecting it has been frozen at some time. Once again judgment should not be based on one fish only; several fish should be examined.

Brining also lowers the reading though not quite as much as freezing. It is therefore not possible to check the freshness of brined fish.

Cleaning and General Care

The Torrymeter is a sealed unit and is waterproofed prior to leaving the factory. However, we do not recommend that the unit be immersed in water or any other fluid. Anyone tampering with the unit, opening the seals, etc, will immediately invalidate the warranty.

The meter is encased in impact resistant abs plastic, with the sensing probes made from stainless steel and carbon materials. It may be cleaned with a damp cloth, using a mild solution of soap or detergent in water. Be aware that some household cleaners may contain chemicals which could seriously damage the plastic housing. Avoid the use of any petroleum based solvent cleaners. Do not let slime dry out on the electrode surfaces. Do not attempt to clean the electrode surfaces with abrasives.

The unit should be charged regularly and the meter retained within its case when not in use, for safekeeping.

Your Torrymeter is a precision instrument! Ensure that the unit is stored securely in transit. Do not drop or otherwise misuse the equipment, as this may invalidate the warranty.

Before Calling for Service

If you have reason to believe that a fault has occurred, please first of all check the following:

• Is the unit switched on?
• Has the battery pack been charged?

If no obvious fault is apparent please telephone our Help Desk for advice and to report the fault.

Warranty and Service

Your Torrymeter is fully warranted against manufacturing defect for a period of 12 months. See Appendix 3 on page 81 for fuller details.

If you need to return the unit for repair, please contact your supplier for advice on how to do so. The meter should be carefully packed, together with an explanatory note as to the nature of the fault.

Should you have difficulty in returning your Torrymeter to your supplier for servicing, you can always contact Distell.Com direct by telephoning our Help Desk, reporting the fault and asking for a Returns Number. Please quote date of purchase and give a full description of the fault.