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.