GB879559A - Improvements in or relating to the preservation of fish - Google Patents

Abstract

In the preservation of fresh fish, the fish are de-gasified and de-aerated by the application of a high vacuum, such that the fish cool down to about freezing temperature due to the evaporation of moisture either originally present on their surface or added during the cooling but are not frozen, and during storage the fish are kept cool and are subjected, by means of an oxygen-free medium having no deleterious effect on the fish, to a pressure higher than that prevailing under the high vacuum. The absolute pressure under the high vacuum may be less than 4 mm. Hg and during storage greater than 80 mm. Hg. Specified oxygen-free media are a gas, a refrigerant vapour and a liquid e.g. degasified and sterilized water. Gas or refrigerant vapour, when employed, may not only serve to increase the pressure, but also to remove heat entering the chamber in which the fish is placed, gas or vapour being sucked out of the chamber, compressed, cooled, allowed to expand or liquefied and sprayed (as the case may be) into the chamber, sucked out again and the cycle repeated continually. Herrings may thus be preserved for about three weeks in an unobjectionable condition at a pressure of 100 mm. Hg using difluorodichloromethane. After being de-gasified, de-aerated and cooled, the fish can be deep-frozen by flooding the fish with refrigerant liquid in a tank and sucking out vapour from above the level of the liquid at a pressure corresponding to a certain decrease in temperature until only vapour remains in the tank the vapour drawn off is compressed, liquefied and collected in another vessel. When the appropriate pressure has been reached in the tank, the compressor is stopped and the liquid collected is returned to the tank so as again to flood the fish. On completion of the deep-freezing, the vapours of the refrigerant are sucked off as far as possible, for instance until a pressure of 1-2 mm. Hg has been reached, which corresponds to a temperature of -20 DEG C. It is stated to be of decisive importance that the temperature drop from 0 DEG C. to -4 DEG C. be completed within 2 to 4 hours to avoid the formation of large ice crystals. According to an example, the pressure in a tank containing herrings which had been de-gasified, de-aerated and cooled to about 1 DEG C. under high vacuum, was raised to a pressure of 300 mm. for storage by introduction of nitrogen. The nitrogen supply was then cut off, the gas in the tank was sucked out, compressed to a pressure of 3 kg/cm.2, cooled in a heat exchanger and expanded back into the tank by means of a nozzle, whereby the gas cooled down to about -60 DEG C.; to avoid uneven flow of gas during re-cycling, the expansion of the gas was carried out intermittently in such a way that pressure fluctuations between 3 and 300 mm. Hg occurred in the tank. Specification 821,369 is referred to.