referring to fig1 there is shown a pressure chamber 1 in which a frozen food 2 to be defrosted is placed . the food is here shown as packaged in a fluid - tight bag 3 of a flexible membrane composed of polyvinyl chloride , polyvinylidene chloride , polyester , polyethylene or like plastic and having a thickness generally between 5 and 20 microns . the membrane could alternatively be composed from an aluminum or an aluminum / plastic composite material and may have a thickness between 5 and 10 microns . prior to sealing , the bag 3 is preferably evacuated to expel air therfrom and may incorporate a non - oxidizing gas such as carbon dioxide , nitrogen , helium or argon to protect the food from oxidation . after the sealed food 2 is placed in position in the chamber 1 , a source of fluid pressure 4 , e . g . a compressor , is actuated to supply a pressurized fluid , e . g . air , into the chamber 1 to pressurize the plenum thereof receiving the food 2 , at a superatmospheric pressure , preferably in excess of 2 atm . and practically less than 8 atm . when a predetermined pressure in said range which corresponds to about 1 kg / cm 2 to 7 kg / cm 2 as input gauge pressure is attained , a cock 5 which is provided between the chamber 1 and the source 4 is closed to maintain the plenum at the given pressure . when the sealing bag 3 is not employed , it is desirable to use as fluid medium a suitable non - oxidizing gas of those listed before and to evacuate the chamber 1 before entry of such a gas supplied from the source which is then constituted by a storage tank containing the gas . the chamber 1 is evacuated through a conduit 6 by a pump ( not shown ) whereupon a cook 7 is closed and the cook 5 is opened . as noted previously , a liquid such as water can also be used effectively as the fluid to pressurize the food - sealing bag 3 and to exert an isostatic pressure to the food 2 through the gas in the bag 3 and can be pumped into the chamber 1 from the source . the chamber 1 can be placed in a refrigerator to defrost the frozen food 2 at a natural thawing temperature . a piece of pork of about 150 grams frozen at a temperature of - 18 ° c was placed in a plenum chamber to which carbon dioxide was supplied at a pressure of 3 kg / cm 2 , and the chamber ( hence the meat piece ) was maintained at a temperature of 6 ° c . the defrosting of the meat piece was completed about 2 . 5 hours , and the drip ( the weight of the meat piece after defrosting / the weight of the meat piece before defrosting × 100 %) observed was only about 0 . 35 %. if the same weight of the frozen pork was defrosted naturally in the conventional manner , namely at an atmospheric pressure , the time required for complete defrosting was 12 hours and the drip was about 1 . 85 %. when an increased pressure of carbon dioxide gas of 7 kg / cm 2 was employed according to this invention , the further reduced drip of about 0 . 1 % was observed . fig2 is a graph showing the drip of the meat piece of example i plotted as ordinate with respect to the gas pressure plotted as abscissa , demonstrating the effect of the present inventive defrosting method . carbon dioxide used in example i was replaced by nitrogen gas . with the gas pressure of 3 kg / cm 2 and 6 kg / cm 2 , the observed drip was 0 . 32 and 0 . 1 %, respectively . in general , when a frozen food is defrosted under pressure according to this invenion , it has been found that a reduction of defrosting time : 1 / 3to 1 / 5 and a reduction of drip : 1 / 5 to 1 / 10 are achieved . further , sensuous tests demonstrated that these meats defrosted according to this invention were excellent all in appearance , color and flavor and , if cooked , were tasty as comparable with the cooking of fresh one . to allow accelerated defrosting of a frozen food 2 under fluid pressure , the apparatus shown in fig1 is provided with a heater which is here advantageously constituted by magnetrons or the like high - frequency radiators 8 and 9 energized by a power source 10 having a control circuit 11 , the latter regulating the radiation power . shown also is a rotor 12 adapted to cause a uniform distribution of radiation beams . this heating device produces heat directly through the frozen food 2 by utilizing dielectric loss of the frozen food when it is subjected to an electric field of a high frequency , preferably in excess of 1000 mhz . the &# 34 ; direct &# 34 ; heating is advantageous in that the food is defrosted internally and uniformly . a piece of frozen pork of 150 grams , under pressure of carbon dioxide of 3 kg / cm 2 , was dielectric - heated by applying to it a high - frequency radiation of 2450 mhz and 300 watts intermittently for three times at an interval of 30 seconds with each radiation being 30 seconds , and was defrosted instantaneously . the drip was measured and found to be only 0 . 2 %. when six pulses of radiation of the same frequency and same power with the duration of each pulse of 10 seconds and at an interval of 20 seconds are employed , the drip was 0 . 06 %. these pork pieces was broiled and their taste were excellent . a piece of mackerel ( saury ) pike of 100 grams frozen at - 18 ° c was defrosted under air pressure of 5 kg / cm 2 and by applying to it a high - frequency ( 2450 mhz ) radiation ( 600 watts ) twice in 2 seconds . the continued radiation for 2 minutes resulted in cooking of the fish without collapse of the shape which could be encountered by the conventional rapid defrosting . a piece of pork of 150 grams frozen at - 18 ° c was packaged in a polyvinyl - chloride bag filled with carbon dioxide . the sealed pork piece was placed in a pressure chamber to which an air was pumped at a pressure of 3 . 5 kg / cm 2 by a compressor . to this pork piece a high - frequency radiation of the same frequency and power as the preceding examples was applied twice at an interval of 30 seconds with each radiation being 30 seconds . the pork piece , which then completed defrosting instantaneously , showed a drip of only 0 . 19 % and natural flavor and appearance and , practically no development of oxidation . an apparatus shown in fig3 is designed to heat a frozen food 2 resistively by utilizing the electric conductivity thereof . in this arrangement which can be used in combination with or without a radiation system described previously , a pair of electrodes 13 and 14 slidably mounted through the chamber 1 sandwish the food to be defrosted therebetween under pressure applied by springs 15 and 16 and are connected to a source of electric current 17 . the current source 17 can be a direct - current , alternating - current or combined direct - current and alternating - current source or a source of unidirectional or alternating pulses and is designed to pass through the food 2 a resistive current in the order of milliamperes . metal screens or textiles 18 and 19 can be used between the electrode 13 and the food 2 and between the latter and the electrode 14 to increase electric contact at these interfaces . fig4 is a sectional view diagrammatically showing a freezing apparatus which may be used to practice the present freezing or freezing / storing method . the apparatus includes a freezing container 20 which receives a pressure chamber or box 21 in which a food to be frozen is placed . in fig5 the box 21 is shown as opened to receive the food and after entry of the latter , the box 21 is tightly closed . the box 21 has a first duct 22 provided to evacuate the space therein and a second duct 23 through which a pressure fluid is supplied to exert a superatmospheric pressure isostatically to the food in the closed box 21 . as noted previously , the pressure preferably is in excess of about 2 atm . and we may set its practical upper limit as 8 atm . in the container of fig4 a freezer system 24 feeds a freezing current through a distributing duct 25 and circulates the current throughout the freezing space within the container 20 to bring and , if desired , maintain the food in the box 21 to its preselected freezing temperature . pieces of pork of various weights around 400 grams were frozen at a temperature of - 24 ° c , using carbon dioxide as pressure medium pumped at various pressures between 0 and 7 kg / cm 2 into the box 21 . each of the pork pieces was retained under each respective pressure at the temperature above for 24 hours and thereafter its sublimation was measured and its proportion to its weight before freezing was calculated . results are shown in the table below . ______________________________________ weight beforepressure freezing sublimation proportion ( kg / cm . sup . 2 ) ( grams ) ( grams ) (%) ______________________________________0 345 1 . 49 0 . 430 396 1 . 63 0 . 412 330 0 . 53 0 . 162 426 0 . 77 0 . 184 495 0 . 54 0 . 114 465 0 . 37 0 . 087 375 0 . 30 0 . 087 399 0 . 36 0 . 09______________________________________ from the above table , it is clear that pressure process according to the invention markedly reduces sublimation during freezing and storing . this demonstrates the fact that the present process is highly effective to protect the food against drying and volatilization of flavor which could be encountered in the conventional course of freezing and storing . several leaves of lettuce were placed in a pressure box ( as shown in fig5 ) which was then tightly closed and filled with a mixed air and carbon dioxide gas ( containing 55 % co 2 ) at a gauge pressure of 2 . 5 kg / cm 2 . the box was placed in a freezer container ( as shown in fig4 ) and cooled to a temperature of - 10 ° c . the lettuce was stored under this condition for 1 month , and then naturally defrosted by bringing the freezer space to a temperature of 6 ° c which was maintained for about 30 minutes . at the end of this time , the pressure in the box was released gradually . the appearance , color and tast of the defrosted lettuce was similar to that of the unfrozen fresh lettuce . freezing and defrosting of example vii was followed except that the gas pressure was replaced by 50 kg / cm 2 and the lettuce was frozen and maintained at a temperature of - 18 ° c . the defrosted lettuce showed no substantial difference in color and freshness from the unfrozen lettuce but was somewhat sour . this is attributable to the fact that carbon dioxide , due to elevated pressure , was dissolved in cells of the lettuce . freezing and defrosting of example vii was followed except that the lettuce was packaged in a polyethylene bag in which air is present . the defrosted lettuce was excellent all in color , freshness and tast . freezing of a red - ripe tomato results in collapse of the skin . when such a tomato was frozen under fluid pressure , no collapse occurred . the tomato was frozen under air pressure of 3 kg / cm 2 at a temperature of - 10 ° c . after freezing , the pressure was decreased to 1 to 1 . 5 kg / cm 2 while the temperature was maintained to keep the tomato frozen . after 1 month , the pressure was increased to 3 kg / cm 2 and the temperature to 6 ° c . the defrosted tomato had appearance , color and flavor substantially the same as those of the unfrozen one . a fresh tomato similar to that used in the preceding example was frozen under air pressure of 5 kg / cm 2 at a temperature of - 20 ° c . after freezing , the pressure was decreased to 2 to 2 . 5 kg / cm 2 while the temperature was maintained . after 1 month , the pressure was increased again to 5 kg / cm 2 and the tomato was defrosted at 6 ° c . the appearance , color and tast of the defrosted tomato was excellent . peaches and cucumbers were also frozen and defrosted in the manner of examples x and xi . with these , results were again excellent . in the preceding examples , it should also be noted that no substantial reduction in the content of vitamin c was observed in all of the frozen and defrosted vegetable or fruit . in the case of vegetables and fruits , especially those which have less strong tissues and cell membranes and abundant cellular liquid , it has been found essential that they , even after freezing , be kept still under pressure of a superatmospheric level to achieve best defrosting results . this pressure to be maintained during storage period is preferably the pressure used in freezing but , as is apparent from the above examples , may be reduced level which should , however , be substantially above the superatmospheric pressure , preferably in excess of about 2 atm . or 1 kg / cm 2 as measured in usual guage pressure . we have also found that fruits and vegetables can be rapidly or forcedly defrosted without cooking . tomatoes and peaches frozen and maintained under air pressure of 3 kg / cm 2 and - 10 ° c were defrosted . when several pulses of high - frequency ( 2450 mhz ) dielectric - heating radiation ( 600 watts ) with each pulse of 10 seconds were applied to them , this took only about 1 / 2to 1 / 3of the time required without radiation . the defrosted foods are quite good all in appearance , color and flavor . as noted already , it has also been found advantageous to apply a sterilization light beam to oxidizable foods during freezing , storing or defrosting . freezing and defrosting of example vii was followed except that ultraviolet light of 2746 a was applied to the lettuce being defrosted . this treated and defrosted lettuce developped an appreciable oxidation in 10 hours while an unfrozen lettuce which was of similar freshness to that of the defrosted one before freezing developed the same degree of oxidation only in about 6 hours . the merely defrosted lettuce developed such oxidation in about 3 hours . as fluid medium in the pressure freezing , storing and defrosting steps according to the invention , we may also use a vapor such as steam pressurized preferably in excess of about 2 atm . leaves of lettuce were placed in steam environment plenum chamber and a number of tests were conducted with various steam pressures ranging between 0 and 6 atm . in each test , the chamber and hence the steam and the lettuce was cooled and maintained at - 10 ° c for 30 minutes . pressurized steam under such low temperature formed an ice shell of substantial thickness ,* the shell compressing it isostatically . yet , it has been found that with the increase of steam pressure the ice - crystalization of cellular liquid is reduced and the reduction occurs sharply from the vapor pressure of about 2 atm . this evidently means that a very favorable condition is created in which the lettuce is rendered less frozable in its internal structure and yet is maintained at a low environmental temperature at which its storage is very suitable . the lettuce leaves were stored at their respective conditions in which they had been frozen and , after storage , were defrosted at 6 ° c . it has been found that the amount of drip is reduced as a function of the steam pressure under which the lettuce is defrosted . here again , it has been discovered that pressure in excess of about 2 atm . is preferred .
frozen foodstuffs are defrosted under an atmospheric pressure between about 2 atm . and 5 atm . using a liquid or gaseous environment at a defrosting temperature , thereby retaining the cellular structure , texture and appearance of the foodstuffs during defrosting . when the liquid is water , the foodstuffs may be maintained under pressure by immersing them to a depth in excess of 10m , while the medium is preferably a non - oxidizing gas or a gas capable of excluding atmospheric oxygen , preferably carbon dioxide , nitrogen , helium or argon . the defrosting temperature is preferably provided by internal heating of the foodstuffs , e . g . by high frequency dielectric heating , inductive heating or resistive heating .