Measuring Fish with Torrymeter

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.
    1. 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.