US 3769028 A
A method for cooking food products packaged in flexible pouches placed between parallel restraining plates to restrict their thickness during heat processing is described. A quantity of food in fluid form and in some instances a predetermined amount of a gas-generating substance (leavening) is placed in each pouch to provide a flat baked or formed product of uniform thickness which substantially fills the package. The product develops a rigid or semi-rigid structure during exposure to heat or after cooling. If gas is generated within the pouch, the product becomes filled with gas cells. An overriding air pressure is controlled by sensing the pressure of a sample of the product placed within a rigid container at processing temperature.
Description (OCR text may contain errors)
United States Patent 1 1191 Katz et al.
1 11 3,769,028 1 Oct. 30, 1973  Inventors: Morris H. Katz; Powell H. Sams,
both of Minneapolis; Duane M. Riley, Shakopee, all of Minn.
 Assignee: The Pillsbury Company,
22 Filed: July 15,1971
21 App1.No.: 162,902
 US. Cl. ..426/345, 99/171 l-l. 99/86  Int. Cl A23i 3/12  Field of Search 99/17] H, 214, 86, 99/1; 21/56, 94
 References Cited I UNITED STATES PATENTS 3,531,300 9/1970 Greenberg et 99/171 H X 3,511,169 5/1970 Fritzberg et al. 99/370 1,485,133 2/1924 White 99/214 3,481,688 12/1969 Craig et al. 99/214 X 3,481,691 12/1969 Trainham 99/214 X 3,215,538 Sada 99 171 H 3,261,140 7/1966 Long.... 99/171 11 ux 3,108,881 10/1963 Shaw et al 99 171 1-1 ux 3,699,875 10 1972 Wilson 99 214 x Primary ExaminerFrank W. Lutter Assistant ExaminerSteven L. Weinstein Attorney-Ronald E. Lund et a1.
ABSTRACT A method for cooking food products packaged in flexible'pouches placed between parallel restraining plates to restrict their thickness during heat processing is described. A quantity of food in fluid form and in some instances a predetermined amount of a gas-generating substance (leavening) is placed in each pouch to provide a flat baked or formed product of uniform thickness which substantially fills the package. The product develops a rigid or semi-rigid structure during exposure to heat or after cooling. lf gas is generated within the pouch, the product becomes filled with gas cells. An overriding air pressure is controlled by sensing the pressure of a sample of the product placed within a rigid container at processing temperature.
9 Claims, 4 Drawing Figures PATENTEU OCT 30 I975 Morris H. Ka tz BY POI/75611 aan I e 477 0fA/E/ METHOD FOR HEAT PROCESSING FOOD PRODUCTS PACKAGED IN FLEXIBLE CONTAINERS FIELD OF THE INVENTION The present invention relates to the sterilizing, cooking, baking, and processing of food products packaged in flexible pouches.
THE PRIOR ART Prior systems for producing sterile pouches containing cooked food products are described for example in U.S.- Pat. No. 3,531,300 and 3,511,169. These systems have not been entirely suitable for some purposes since the food products often do not completely fill the pouches. Sizable voids are left due to compression of the food product by the overriding pressure at the time the structure of the food is set. Other processes previously proposed, for example, the one described in the volume FOOD PROCESSING OPERATIONS, Joslyn 1963, p.456 Volume 2 (which discloses racks of parallel plates between which pouches are placed) have not been entirely suitable since there is no reliable provision for maintaining product density, for assuring that the pouches will be uniformly filled or for preventing compression of the pouches at just the time at which the product structure hardens or becomes set. Thus, assuming the volume of material placed in the pouch is sufficient to just fill the pouch after the leavening reaction is completed, if the crumb structure becomes set when the pouch is in a compressed condition due to overriding gas pressure, gas evolution will merely fill the pouch with a gas pocket and upon removing the pouch from the retort an empty space is apparent within the pouch. In addition to these problems, the flexible pouches are distributed in a large water filled retort so that the initial external pressure on the packages located near the bottom is approximately 2-3 p.s.i. in excess of that at the top due to the water head.
Thus, in the past the food contained in the pouches has not been uniform in thickness and the product density of leavened or aerated foods has varied. The problem is particularly objectionable when the process is used for preparing slices of bread or cake. In these cases, overriding air pressure reliably prevents the pouch from bursting but does not provide a uniform density and often compresses the materials contained in the pouch enough so that it becomes misshapen.
OBJECTS OF THE INVENTION The primary objects are to prevent pouch bursting during the cooking or baking of foods contained in flexible pouches while at the same time providing food pieces in the form of slabs or slices of uniform thickness and uniform density substantially filling each pouch.
THE FIGURES FIG. 1 is a semidiagra'mmatic vertical sectional view of a retort and control system embodying the invention.
FIG. 2 is an exploded perspective view of a sensing cell employed in accordance with the invention.
FIG. 3 is a perspective viewpartly in section of the pouches after it has been filled with the product and baked within the apparatus of FIG. 1.
FIG. 4 is a perspective view of several pieces of finished product as they appear after being removed from the pouches.
BRIEF DESCRIPTION OF THE INVENTION Briefly, in accordance with the present invention there is provided a vessel containing a heat transfer medium, e.g., water, steam or hot air for cooking food products contained in pouches. Restrainers comprising parallel restraining elements or plates are provided to engage both sides of each pouch for preventing expansion thereof beyond predetermined limits. A sensing cell within the vessel is formed from a rigid container having temperature and pressure sensing elements therein. A quantity of the material to be baked or cooked is placed in the cell. The temperature and pressure sensing elements are operatively connected to a source of fiuid pressure to maintain the pressure in the vessel slightly less than that in the pouches. A controller having a predetermined i.e., programmed temperature rise, dwell and fall controls the heat supplied by the heat transfer medium. The pressure maintained in the vessel is on the order of from about 1 to 5 pounds less than the pressure within the pouches. The food product may or may not contain a leavening system including components adapted to generating leavening gas. If gas is generated, the gas provides a finished product of a predetermined density with a total volume which is the sum of the volume of solid and liquid food placed in the pouch and that of the leavening gas generated. The total volume thus achieved is made to equal the expanded volume of the pouch when placed between the restraining plates.
A pouch 1 containing the food 4 to be cooked as best seen in FIGS. 1 and 3 is formed from a'flexible or semiflexible sheet material 2 which is imperforate and relatively gas impervious. The seals of the pouch or package 1 are weak enough so that they could burst when subjected to a substantial increase in internal pressure. A number of packaging materials which can be used to form the pouch 1 will be apparent to those skilled in the art. In one preferred form of the invention, the package 1 consists of sheet material formed from several layers having different properties. Thus, it has been found that a metal foil laminated to a resinous organic film provides the advantages of both the foil and the resin. One suitable composite sheet material of this type consists of aluminum foil of 0.5 mils thickness laminated to a polyester film of 0.5 mils in thickness and a third film layer consisting of a polyethylene film of 3.0 mils in thickness, e.g., the product sold under the registered trademark Mylar by the E.I. DuPont Company, Inc., Wilmington, Del. The package is heat sealed at 3 along its edges. The seals should retain a substantial degree of strength when heated to processing temperature, which may vary from about 212 F to 350 F. For this reason, a high melting point polyethylene layer is preferred.
For the purpose of evaluating the invention, pouches measuring 4 %x7 inches were produced. These were heat sealed conventionally along each edge 3, the final seal being made after the batter dough or other food product 4 was inserted. Other film materials can be employed if desired, such as a flurohalocarbon film sold under the trade name Aclar by GeneralChemical Division, Allied Chemical Corporation of New York. Still another suitable film material consists of a 0.75 mil Capran Polyamide Nylon film laminated to Vic'an dispersuch foods are farinaceous materials, gelatin, pudding and meat loaf, etc. Among the farinaceous materials with which the invention is concerned, are bread, cake, biscuits, cookies, crackers and the like.
A' variety of oven's or cooking vessles can be employed with the invention. The vessel should include a I source of heat and control means for regulating the heat. Preferably, a cooling means is also provided. A means is also provided for fluid pressure within the cooker and a pressure control means is used for maintaining the pressure within the chamber at selected levels during different portions of the cooking cycle. The fluid can be either a gas or a liquid.
Among the many types of ovens or cooking vessels which can be used in connection with the invention are gas fired ovens, electric ovens, microwave ovens such as the Mark- V Radarange, sold by Raytheon, Inc;-of Waltham, Massachusetts, as well as steam cookers, autoclaves or hot water cookers. A preferred form of hot water cooker will now be described in connection with FIGS. 1, 2 and 3. .1
As can be seen in FIG. I, there is provided a retort 10 consisting of a vertically disposed cylindrical steel tank having a side wall 12, a bottom wall 14 and a removable cover 16, which is held in place during operation by means of screws 18 that are threaded at 19 within brackets 20 --which are mounted circumferentially on the upper end of the retort 10. By placing the fasteners 18in position and turning the handles at their upper ends, cover 16 canbe securely locked and positioned. The retort 10 is provided with a pressure release valve (not shown). A pipe 130 is provided for supplying a pressurizing medium such as air to the upper end of the tank. The retort 10 includes an opening 38 having a pipe 40 connected thereto to the end of which is fastened a glass window 42 that serves as an inspection port. I
Within the'retort 10 is a support means comprising horizontally disposed angle irons 44. Supported upon the horizontal angel irons 44 are three shelves or support racks 50. The racks each include horizontally disposed frame members and a plurality of vertically disposed parallel plates 52 that engage and serve as restrainers for the pouches 1. The plates 52 are welded or otherwise secured to the bars 50. The plates 52 are properly spaced to provide a compartment within which the packages I of a predetermined thickness D (FIG. 3). In the bottom of the tank 10- below the support 44 is a heat supply means such as a heat exchanger 57 formed from a helicallywound tube'to which a suitable heating fluid such as steam is supplied through a'line 58. Control of the steam entering line 58 is provided by means of an inlet control valve 60 and a coolant return valve 66. The opposite end of the exchanger 57 is connected to a line 64 coupled to an outlet control valve 62 and a cooling water inlet valve 68. At the bottom of the tank 10 is provided a circulating means for carrying a heat transfer medium such as water with which the tank is filled to just cover the supporting racks. The circulating means indicated generally at comprises a motor 72 to which current is supplied through conductors 74. The shaft 75 of the motor 70 extends upwardly through a rotary seal and has secured to its upward end, a fluid circulating propeller 76. It will thus be readily seen that during operation, the motor will impart circulating movement to the heat transfer medium within the tank 10.
In FIG. 2 is shown a preferred form of test cell designated generally 80. The test cell comprises a fluid tight chamber 82 composed of a rigid vertical heat coni ducting wall defining a cavity 84 with top and bottom plates 86 and 88 rigidly secured thereto. Plate 88 seals the bottom and the top plate 86 is provided with a removable cover 90 which is sealed by a gasket 92 and held in place with six hold down studs 94 that extend through corresponding openings in the plate 90. Plate 90 is tightened in place by wing nuts 96 only one of which is shown. A thermocouple 98 or any other temperature sensor. provided within the cell is coupled to a control 120 byconductors 100 and 102. A pressure sensing unit such as a stainless steel capillary tube 106 extends through the cover 90. The capillary tube 106 is open at its lower end and is capable of sensing the pressure withinthe cell 80 by transferring the gas under pressure to-a pressure transducer l10-All joints and connections to the cell are tightly sealed so that the cell is capable of retaining and withstanding internal pressure without leakage. Just before the retort 10 is to be used, the cell 80 is filled with a measured quantity of the batter or other food product 4 which is placed in the pouches. The cell 80 is then sealed and placed within the retort as shown in FIG. 1. The cell 80 is then evacuated to maintain the amount of residual air approximately equal to that present in the pouches. This can be achieved in many cases by reducing the pressure in cell 80 to a pressure of about 200 mm of mercury in the case of bakery goods about 800 mm of Hg for nonbakerygoods.
The control cell 80 pressure sensing capillary 106 is connected by-its upward end 104 to a transducer 110 such as a differential pressure transmitter e. g., Honeywell Company Model No. Y292l2 to convert the pressure of the gas within the capillary 106 to a controlled pneumatic signal. The interior of the vessel 10 is also connected to an auxilliary pressure recorder 114 by means of a pipe 116 and to the transducer 110 by pipe 117. In this way the pressure recorder 114 is able to continually record the pressure within the vessel 10. The transducer 110 is connected by means of pneumatic tubing 118 and 119 to a control mechanism 120 which utilizes the pressure information for regulating the pressure within vessel 10. The amount of steam introduced through valve 60 is controlled by a preestablished program of temperaturerise followed by constant temperature followed by cooling within controller 61. The controller 120 is connected via pneumatic tubing 122 and 124 to a pneumatically operated valve 126 which when opened supplies pressurized air from tank 128 to the vessel 10 through a pipe 130 to prevent the pouches I from bursting. Control 120 is also coupled by means of pneumatic tubing 130 and 132 to a pneumatically operated valve 134 to vent compressed air from the vessel 10 when the pressure therein is to be reduced. t
The internal pressure in cell 80 is not identical to that within thepouches 1 because of. the rigid walls 82 but the temperature is almost identical to that in the 'pouches. The pressure within the retort 10 is controlled by the pressure differential between cell 80' and that in the vessel 10. This is made possible by measuring the pressure within the product in cell 80 without sensing and feeding back the pressure within the vessel by means of cell 80. The pressure within retort 10 is separately sensed and fed to transmitter 110 by duct 117. The differential pressure transmitter 110 is preset to the desired pressure difference between the retort and the cell 80 which is typically from 1 to 6 p.s.i. and preferably from about 3 to 5 p.s.i. in excess of that within the retort. In any case, the pressure differential must be sufficient to press the walls of each pouch against the restraining plates throughout the process and not enough in excess of this pressure to burst the pouches. When the retort pressure is the same as that within the controlcell 80, for example at the beginning of the heating cycle during the early stages of temperature, the differential pressure transmitter 110 will send a weak signal to the pressure controller 120. The air pressure control valve 126 will remain closed and the air exhaust control valve 134 will remain open. As the control cell 80 pressure rises and approaches the desired preestablished (e.g., 3 p.s.i.) pressure differential due to heating of the material 4 within the cell, the differential pressure transmitter 110-will send a relatively strong signal to the controller 120 to open valve 126 and close valve 134 thereby sending compressed air into vessel 10. As the pressure differential between the cell 80 and the vessel 10 exceeds preestablished value, an even stronger signal is transmitted to the pressure controller 120 and the air pressure control valve 126 will open thereby maintaining the differential always at about l-6 p.s.i.
The valve 126 is preferably set to operate over the full range at signal pressures between 9. andlS p.s.i. As the pressure differential falls below the set value, a weaker signal is transmitted to the controller 120. The air pressure control valve 126 then closes and the exhaust valve opens. In one-apparatus embodying the invention, it was found that outstanding results could be obtained by setting the exhaust valve 134 to open and close with the pressure differentials in the range of between 3 and 9 p.s.i. control line pressure with closing occuring as the pressure is reduced and the opening occuring as the pressure increases and inlet valve 126 set to operate in the range between 9 and p.s.i. control line pressure pressure differential with opening occuring at the lower end of the range.
The present invention duplicates very closely atmospheric baking conditions by controlling the following variables: (a) the maximum expansion widths of the packages 1 through the use of the spaced restraining plate, (b) the quantity of food in the package, and (c) the volume of leavening gas generated (ifany), (d) a programmed pressure differential ofsucha value that the internal pressure within the pouchis slightly higher e.g., 1-6 p.s.i. higher than that in the-retort 10 so that the pressure throughout theprocess cycle creates an expansion of the package to the volume determined by the restraining plates whereby the sum of the volumes of fluid product V and generated gas V is just equal to the volume of the pouchwhen placed between restricting plates of a defined spacing W.
The pouch volume is calculated as follows as seen in.
I the length of the pouch not including the seals. w the width of the pouch not including the seals.
V lwd 11' (PM (H'w) F 11' (1 /6 Assuming that the theoretical volume thus determined is 10 cubic inches, and further assuming that the finished product is to be one-fourth gaseous and the remaining fraction non-gaseous the amount of leavening used should be approximate to generate a volume of gas lO-V and the remaining volume should be water and solids in the amount l0V The sum of V and V equals 10, the theoretical volume calculated as above. In this way the materials that are employed in each a pouch can be accurately determined and the pouch will be. completely filled with a product having the desired density composed of solid and liquid components with gas cells uniformly distributed therethrough. Moreover, the pieces as seen in FIG. 4 after being removed from the pouch will have a uniform thickness throughout as well as being uniform in density.
The invention will be better understood by reference to the following examples.
EXAMPLEI I Pound cake was prepared using the following formulation:
In preparing pound cake batter, a Readco jacketed Sigma blade mixer is used. Cold water is circulated through the jacket while blending to retard bench action of leavening system. Blend all dry ingredients thoroughly using slow speed on mixer (assure no lumps in either leavening) 5 to 10 minutes. Blend inshortening gradually. Mix at medium speed untilwell blended for about 5 minutes. In a separate blender (A200 Hobart with a wire whip), blend the water and fresh eggs. Add the water and eggs to the mix and blend-thoroughly at a medium speed for about 5 minutes. The batter may be transferred to filler by pump or by hand.
The pound cake batter is filled inthe foil pouches measuring 4 54 X 7 inches using abottom fill technique.
at grams, 2 grams. Vacuum scaling is not required for this product, but residualair should be removed by a mechanical means prior to sealing. In lieu of such mechanical means, a very slight vacuum is acceptable. The filled pouches are to be top sealed. All packaging should be accomplished within 2 hours after blending.
Thermoprocessing of pound cake begins no later than 3 hours after batter is blended unless batter or packages are held at refrigerated conditions. When approximately half the pouches from a batch have been filled, the pressure control cell 80 is filled with product in the vessel 10 throughout the process cycle. With the filled pouches at room temperature, the processing water temperature at the beginning of a retort run should be at 80 F 1 5 F and the rate of temperature rise should be set at 8 F/minute consistent within 1-. 2
F throughout the retort. The processing unit should be maintained at 250 F for 20 minutes with temperatures throughout the unit consistent to i 1 F. The unit cool down rate should be at 8F/rninute with temperatures throughout being consistent to 1 2 F. Products may be removed whenwater temperature falls below 100 F. The above processing profile yielded F values of greater than 6 during product development the term F0 is defined on Page 160 of Modern Food Microbiology 1970, by James M. Jay, Van Nostrand Reinhold Company.
After a lengthy storage period, the pouches were opened and the pieces of pound cake therein were found'to be of uniform thickness and substantially filled each of the pouches as shown in FIG. 4. Moreover, the density of each of these pieces was uniform throughout and was about 0.70 which is highly desirable for a cake product.
TOTAL PREPARATION OF BATTER In preparing chocolate nut cake batter, a Readco jacketed Sigma blade mixer is used, cold water is circulated through the jacket while blending to retard bench action of leavening system. Blend all dry ingredients except chocolate drops and nuts thoroughly using slow speed on mixer. (Assure no lumps in either leavening) to 10 minutes. Blend in shortening gradually. Mix at medium speed until well blended. 5 minutes. In a separate blender (A200 Hobart w/wire whip), blend the water and fresh eggs. Add the water and eggs to the'mix and blend batter thoroughly at medium speed. 5 minutes. Add chocolate pieces and nuts tomix and blend. Slow speed about seconds. Batter may be transferred to filler by pump or by hand.
The chocolate nut cake batter is filled in the foil pouches using a bottom till technique at 113 grams, i
cal means prior to scaling. In lieu of such mechanical means, a very slight vacuum is acceptable. The filled pouches are to be top sealed. All packaging is accomplished within two hours after blending. Minimum attrition of the batter in handling and filling is important to prevent discoloration and loss of discrete particles due to breaking of chocolate or nutmeats.
Thermoprocessing of chocolate nut cake should begin no later than 3 hours after batter is blended unless batter or packages are held at refrigerated conditions. When approximately half the pouches from a batch have been filled, the pressure control can should be filled with product (l 13 grams) and the residual air evacuated through the pressure tap and shut off valve. The differential pressure transmitter should be set at 2 p'.s.i. allowing the internal package pressure to exceed the overriding (retort) air pressure throughout the process cycle. With the filled pouches at room temperature, the processing water temperature at the beginning of a retort run should be a F i 5 F and the rate of temperature rise should be set at 8 F/minute consistent within i 2 F throughout the retort. The processing unit should be maintained at 250 F for 20 minutes with temperatures throughout the unit consistent to i 1 F. The unit cool down rate should be at 8 F/minute with temperatures throughout being consistent to i 2 F. Products may be removed when water temperature falls below F. The above processing profile yielded F0 values of greater than 6 during product development.
.After a substantial storage period, the cake was removed from the pouches and was found to have a very appealing taste and texture. The pieces, moreover, were uniform in thickness and in density and the baked products filled substantially the entire pouch.
l. A method of preparing a food product packaged in a flexible pouch comprising placing the food product in the pouch, completely sealing the pouch, placing the pouch in a pressure vessel and applying mechanical restraint to the sides of the pouch to prevent expansion thereof beyond a predetermined thickness, heating'the pouch while thus restrained mechanically, controlling the pressure within the vessel by applying a gas pressure externally of the pouch which is somewhat less than the pressure developed within the pouch sufficient such that the pressure within each pouch presses the .wall of each pouch against the mechanical restraint throughout the process and said gas pressure external of the pouch is not sufficiently low to cause the pouches to burst whereby the pressure maintained withinthe pouch is at all times somewhat in excess of that surrounding the pouch, and the volume of the food product after heat is applied being substantially equal to the volume of the pouch when expanded between the mechanical restraints whereby the food product within wthe pouch substantially fills the pouch after being heated in the vessel, cooling the food product after it is cooked, the food product being composed of a substance which develops a set structure during the heating and cooling cycle to form a fabricated food piece having a substantially predetermined thickness.
2. The process of claim 1 wherein the food product comprises a fluid having a volume V, and contains a leavening adapted to generate a volume of gas V, during the heating cycle within the vessel and wherein the total volume V V equals a volume V which is substantially the same as the expanded volume of the pouch when placed between the restraints.
3. The process of claim 1 wherein the applied gas pressure is continuously controlled and maintained so as to be increased continuously during the initial heating phase as the temperature of the product within the pouch is increased, is maintained at a constant level during the time the pouch and its contents are no longer being heated and is decreased as the temperature and pressure within the pouch decreases at the end of the heating cycle, the pressure within the vessel being thereby maintained at all times slightly less than the pressure within the pouch, whereby the contents of the pouch will expand the pouch against the restraints during the entire heating and cooling cycle.
4. The process of claim 1 wherein the volume of the pouch is as follows:
the length of the pouch not including the seals w the width of the pouch not including the seals d the final thickness of the pouch and the volume of fluid product including liquid and solid components V plus the gas generated V is substantially equal to V.
5. The process of claim 1 wherein a quantity of the product is placed within a closed rigid container within the vessel and in the heat conductive relationship with the interior of the vessel which is maintained at the same temperature as the pouch at all times, the pressure within the container is sensed continuously throughout operation and the pressure thus sensed is used to control the gas pressure applied externally of the pouch to maintain the gas pressure such that the pressure in the container and the pouch is in excess of the gas pressure externally of the pouch by an amount which is sufficient to press the walls of the pouch against the mechanical restraint throughout the process but not enough in excess of this pressure to burst the pouch.
6. The process of claim 5 wherein the differential pressure is on the order of about 1-6 psi.
7. A method for cooking a farinaceous product within a pouch, said product comprising a mixture of flour, sugar, shortening, a chemical leavening agent and water; said method comprising mixing the aforesaidingredients to prepare a batter, placing a predetermined volume V of the batter in the pouch, the chemical leavening being present in a predetermined quantity adapted to generate a volume of leavening gas V and the sum of volumes V V wherein:
l the length of the pouch not including the seals w the width of the pouch not including the seals d the final thickness of the pouch completely sealing the pouch, placing the pouch in a pressure vessel and applying mechanical restraints to the sides of the pouch to prevent expansion thereof beyond a predetermined width, the width of the pouch being maintained substantially uniform by the restraining means, heating the pouch while thus restrained between the plates, applying a gas pressure externally of the pouch which is somewhat less than the pressure developed within the pouch at all times sufficient such that the pressure within each pouch presses the walls of each pouch against the mechanical restraint throughout the process and said gas pressure externally of the pouch is not sufficiently low to cause the pouches to burst whereby the pressure maintained within the pouch is in excess of that in the surrounding area throughout the process while the food is at elevated temperature, cooling the food product after it is cooked whereby the food product substantially fills the pouch after the heating and cooling cycle is completed and the product is composed of a food substance which develops a set structure during heating and cooling to form a cooked food piece having a substantially uniform thickness.
8. The process of claim 7 wherein a quantity of the product is placed within a closed rigid container within the vessel which is maintained at the same temperature as the pouch at all times, the pressure within the container is sensed continuously throughout operation and the pressure thus sensed is used to control the gas pressure applied to the pouch to maintain the gas pressure such that the pressure in the container and the pouch is in excess of the gas pressure externally of the pouch by an amount which is sufficient to press the walls of the pouch against the mechanical restraint throughout the process but not enough in excess of this pressure to burst the pouch.
9. The process of claim 7 wherein the pressure differential between the pouch and the surroundings is on the order of about l-6 p.s.i.