AU599608B2

AU599608B2 - Method of obtaining acceptable configuration of a plastic container after thermal food sterilization process

Info

Publication number: AU599608B2
Application number: AU64248/86A
Authority: AU
Inventors: Joseph B. Brito; Lou Kohl; Robert J. Mchenry; WILSON T. PIATT Jr.; Robert J. Reed; Kenneth B. Spencer; Boh C. Tsai; Krishnaraju Vavadarajan; Donald C. Vosti; James A. Wachtel; Mark A. Williams
Original assignee: American Can Co
Current assignee: Rexam Beverage Can Co
Priority date: 1984-04-26
Filing date: 1986-10-21
Publication date: 1990-07-26
Anticipated expiration: 2004-04-26
Also published as: AU6424886A

Description

FIVE DOLLARS P17/2/83 PHILLIPS ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia 599608 a 1

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Applic"'ion Number: cLtf4 t% Lodged: Complete Specification Lodged: Accepted: Published: Priority This docunt com'iin.;tl ametio 49 Atd nim 1. ;iU Section 49aiidi ccX~~i rinting.

Related Art: o ~tO 0 *0 u 00 on 0 00 0 o D 0 010 0~o i i APPLICANT'S REF.: Docket No. 14,084-1 (Div. of 27281/84) Name(s) of Applicant(s): AMERICAN CAN COMPANY Address(es) of Applicant(s): American Lane, Greenwich, Connecticut 06830, United States of America Actual Inventor(s): Address for Service is: PHILLIPS, ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia, 3000 Complete Specification for the invention entitled: METHOD OF OBTAINING ACCEPTABLE CONFIGURATION OF A PLASTIC CONTAINER AFTER THERMAL FOOD STERILIZATION PROCESS following statement is a full description of this invention, including the best method of performing it known to 'ant(s): This application is divided from Australian patent application 27281/84 the entire disclosure of which is incorporated herein by reference.

FIELD OF INVENTION This invention generally relates to containers used for packaging foods and, in one aspect, it relates to a method of improving the configuration of packed plastic containers after thermal processing of the container and its content. In another aspect, the present invention is concerned with attaining acceptable configuration of such containers after thermal processing. In still another aspect, the present invention relates to proper design of plastic container to improve their configuration after thermal processing.

r BACKGROUND OF THE INVENTION j It is common knowledge in the food packaging industry that after the container is filled with food and is S closed, the container and its content must be thermally I4 processed to sterilize the food so that it will be safe for *human consumption.

Thermal processing of such containers is normally St S carried out at temperatures higher than about 190°F. in *141 I various equipment such as rotary continuous cookers, still retorts and the like, and the containers are subjected to various cook-cool cycles before they are discharged, tacked I I and packed for shipment and distribution. Under these thermal processing conditions, plastic containers tend to become distorted or -la-

F

deformed due to sidewall panelling (buckling of the container sidewall) and/or distortion of the container bottom wall, sometimes referred to as "bulging" or "rocker bottom". These deformations and distortions are unsightly, and interfere with proper stacking of the containers during their shipment, and also cause them to rock and to be unstable when placed on counters or table tops. In addition, bottom bulging is, at times, considered to be a possible indication of spoilage of the food thus resulting in the rejection of such containers by the consumers.

One reason for the distortion of the container is that during thermal processing the pressure within the container exceeds the excernal pressure, the pressure in the o equipment in which such process is carried out. One solution S to this problem is to assure that the external pressure always 0 0 exceeds the internal pressure. The conventional means of 09 9 o achieving this condition is to process the filled container in o a water medium with an overpressure of air sufficient to compensate for the internal pressure. This is the means used 0 ,o 0 to process foods packed in the well-known "retort pouch". The chief disadvantage of this solution is that heat transfer in a water medium is not Lts efficient as heat transfer in a steam 0 atmosphere. If one attempts to increase the external pressure 0: in a steam retort by adding air to the steam, the heat transfer S efficiency will also be reduced relative to that in pure stream.

o Several factors contribute to the increase in internal S pressure within the container. After the container is filled with food and hermetically closed, as a practical matter, a small amount of air or other gases will be present in the headspace above the food level in the container. This headspace of air or gas is present even when the container is sealed under partial vacuum, in the presence of steam (flushing the container top with steam prior to closing) or under hot fill conditions (190 When the container is heated during thermal processing, the headspace gases undergo signifJLcant increases in volume and pressure. Additional internal pressures will also develop due to thermal expansion of the product, increased vapor pressures of the products, the dissolved gases present within 39 the container ani the gases generated by chemical reactions in

DH

-2 urr~ the product during its cooking cycle. Thus, the total internal pressure within the container during thermal processing is the sum total of all of the aforementioned pressures. When this pressure exceeds the external pressure, the container will be distorted outwardly tending to expand the gases in the headspace thereby reducing the pressure differential. When the container is being cooled, the pressaire within the container will decrease. Consequently, the sidewall and/or the bottom wall of the container will be disteiided inwardly to compensate for the reduction in pressure.

It has been generally observed that such thermally proccssed plastic containers may remain distorted because of bulging in the bottom wall and/or sidewall panelling. Unless these deformities can be eliminated, or substantially reduced, such containers are unacceptable to consumers.

It must also be noted that it is possible to make a container from a highly rigid resin with sufficient thickness to withstind the pressures developed during thermal processing and thus alleviate the problems associated therewith. However, S 2C practical considerations and economy militate against the use of such containers for food packaging.

Accordingly, it is an object of this invention to improve the configuration of a plastic container after thermal processing.

It is another object of this invention to alleviate tlhe problems associated with bottom bulging and sidewall panelling of a plastic container which result from thermal processing.

It is a further object of this invention to attain an Iacceptable container configuration after such container is packed with food, hermetically closed and thermally processed.

r ic It is still another object of this invention to provide methods, and container configurations which permit plastic containers to have acceptable configurations despite their having been subjected to thermal food processing conditions.

It is yet another object of this invention to facilitate thermal food processing of plastic containers packed with food.

The foregoing and other objects, features and advantag of this invention will be further appreciated from the ensuing 39 detailed description and the accompanying drawings.

3- L I SUMMARY OF THE INVENTION In accordance with this invention, a method is provided for improving the configuration of thermally processed plastic container which is packed with food. Objectionable distortions and deformations rocker bottom and/or sidewall panelling) in the container are eliminated, or substantially reduced, by proper container design, by maintaining proper headspace of gases in the container during thermal processing, by maintaining proper relative pressure during the cooking cycle and cooling cycle of the process, by controlling reforming of the container bottom wall after thermal processing and/or by pre-shrinking the empty container prior to filling and sealing.

Ue BRIEF DESCRIPTION OF THE DRAWINGS I U In the drawings, wherein like numerals are employed to U i designate like parts: Figure 1A is a front elevational view partly in section, S 20 of a cylindrical container of this invention before the container is packed with food and sealed; Figure lB is a front elevational view partly in section, of the container shown in Figure 1A after the container has U been filled with food and sealed under partial vacuum; U* Figure 1C is a front elevational view partly in section, U of the container shown in Figure 1B during thermal processing but before reforming, showing bulging of the container bottom wall; o Figure lD is a front elevational view partly in section, 30 of the container shown in Figure IC illustrating rocker bottom S after thermal processing; Fig £e IE is a front elevational view partly in section, of a container similar to Figure 1D but wherein the container sidewalls are panelled; Figure 1F is a cross sectional view of the container taken along the line IF 1F in Figure IE; Figure 1G is a front elevational view partly in section, of the container shown in Figure 1A illustrating sidewall 39 panelling and bottom bulging; 4

IC

i-i i- headspace of gases in the container during thermal processing, by maintaining proper relative pressure during the cooking cycle and cooling cycle of the process, by controlling reforming of the container bottom wall after thermal processing and/or by pre-shrinking the empty container prior to filling and sealing.

Accordingly, the current invention provides a method of thermal sterilization of a plastic container packed with food to obtain a thermally sterilized packed container having an acceptable configuration, qhich comprises filling the container with food, sealing the container, either or both of these steps including selecting an initial head space volume and an amount of gas, taking into account an initial vacuum level, if any, at sealing such as to permit bulging and subsequent reformation of the container bottom wall without significant side wall panelling, thermally sterilizing the packed container in a retort operated at a temperature and pressure for a time sufficient to s Arilize the container and its contents and to cause bulging of the container bottom wall, cooling the container and its contents, and, during the cooling step, reforming the bulged container bottom wall to attain an acceptable container configuration by controlling the pressure external of the container and the cooling conditions said controlling step including providing that the plastic of the bulged container bottom wall is at a reformable temperature at which the plastic is soft while providing a pressure differential such that the pressure external of the container exceeds the pressure internal the container, and utilizing the external pressure while said plastic is soft to reform the bulged bottom wall.

-4a- 1 of the container shown in Figure 1A after th ocessing, according to the present inventi Figure 2 arged vertical section schematically Figure 3 is a partial elevational fragmentary sectional view of a multi-layer thermoformed container similar to that shown in Figure 2, showing wall portions having different thicknesses; ao o o 0909 09 0 o )9 9 0o 9' 09

P~

0 9904' 94 0e 4 0 90009 9 0 4 9 Figure 4 is a partial elevational fragmentary sectional view of a multi-layer injection blow molded container similar to that shown in Figure 2, showing wall portions having different thicknesses, Figure 5 is a partial elevational fragmentary sectional view of a container similar to Figure 3 but showing the dimensions of a multi-layer thermoformed container; Figure 6 is a partial elevational fragmentary sectional view of a container similar to Figure 3 but showing the dimensions of a multi-layer injection blow molded container; Figure 7 is a partial elevational fragmentary sectional view of the container shown in Figure 2 illustrating the container bottom wall in neutral, bulged and inwardly distended positions; Figure 7a is an elevational view of the container shown in Figure 6; Figure 7b is a bottom view of the container shown in Figure 7a.

Figure 8 is a schematic representation illustrating the container bottom wall geometry before and after bulging; Figure 9 is a graphical representation illustrating bottom reforming and sidewall panelling as functions of temperature and pressure; Figure 11 is a graphical representation of calculations defining the relationship between the initial headspace of gases in the container and the sealing vacuum in the container; Figure 10 is a graphic representation of experimental data illustrating the relationship between the initial headspace of gases in the container and sealing vacuum in the container.

5 DETAILED DESCRIPTION OF THE INVENTION In a typical operation involving food packaging, the plastic containers are filled with foods and ezch container is then hermetically sealed by a top closure. As it was previously mentioned, the container is typically either sealed under vacuum or in an atmosphere of steam created by hot-filling or by passing steam at the container top while sealing. As it was also mentioned previously, after the container is sealed, there invariably is some headspace of gases in the container. Next, the sealed container is thermally processed at a temperature which is usually about 190 F. or higher depending on the food, in order to sterilize the container and its content, and thereafter cooled to ambient temperature. After thermal processing and cooling, the containers are removed from the thermal go processing equipment, stored and then shipped for distribution.

During the cooking cycle of the thermal sterilization process, the pressure within the con-ainer will rise due to increased pressure of headspace gases, the vapor pressures of the products, the dissolved gases in the container as well as the gases which may sometime be generated from chemical reactions in the container content, and due to thermal expansion of the product. Therefore, during the cook cycle, the pressure within the container will exceed the external pressure and, consequently, the container bottom wall will distend outwardly, it will bulge. As it was also previously mentioned, after thermal processing and cooling, the pressure within the container is decreased and the container bottom wall will flex inward to compensate for this reduction of pressure.

30 Frequently, however,. the container bottom does not fully return to an acceptable position or configuration and remains bulged to varying degrees.

The containers to which the present invention is well suited are plastic containers which are made of rigid or semirigid plastic materials wherein the container walls are preferably made of multilayer laminate structures. A typical laminate structure may consist of several layers of the following materials: 39 -6- -1 yl Lrl outer layer of polypropylene or a blend or polypropylene with high density polyethylene, adhesive layer, barier layer such as ethylene-vinyl alcohol copolymer layer, adhesive layer, and an inner layer of polypropylene or a blend of polypropylene with high density polyethylene.

The adhesive is usually a graft copolymer of maleic anhydride and propylene wherein the maleic anhydride moieties are grafted onto the polypropylene chain.

It must be understood, however, that the nature of the different layers are not per se critical since the advantages Sof this invention can be realized for containers made of other cog plastic materials as well, including those having less or more than five layers, including single layer containers.

o Referring now to the drawings, there is shown in Figure IA o, a plastic container 1 having sidewalls 3 and a bottom wall ~which includes a substantially flat portion 7 and outer and £0 inner convex annualar rings 9 and 9a with an interstitial ring 9b.

After the container is filled, it is sealed with a top closure 11 as shown in Figure lB. As it was previously Smentioned, after the container is filled and sealed, there will S be a headspace of gases at the container top generally designated as 13.

Figure 1C shows the container 1 during thermal processing, or after thermal processing but before bottom reforming. As o shown in this figure, the container bottom is outwardly 310 distended because the pressure within the container exceeds the external pressure. If no proper prior measures are taken, after the container is cooled, the bottom wall may remain deformed as shown in Figure 1D. Such container configuration is unstable or undesirable due to rocker bottom. As will hereinafter be explained, rocker bottoms (Figure 1D) and sidewall panelling as shown in Figures 1E and lF, or both (Figure iG), may be minimized or prevented by pre-shrinking the container prior to filling and closing, by reforming the container 39 bottom wall, by adjusting the headspace of gases in the container -7 7 at each vacuum level, by proper container design, by maintaining proper pressure differential between the inside and outside of the container, or by combinations of these factors. Figure 1H represents the desired container configuration after thermal processing and reforming of the container because it has no rocker bottom or sidewall panelling. This container configuration is the same or nearly the same as the configuration shown in Figure lB.

As it was previously mentioned, during the cooking cycle, the pressure within the container will rise due to the aforementioned factors, and the container bottom wall will be outwardly distended. Unless proper measures are taken, the container may burst due to excessive pressure in the container.

S The container must be designed to deform outwardly at a container internal pressure below the pressure which causes bursting of the container at the particular cooking temperature. For example, io at 250 a temperature commonly used for sterilizing low acid foods vegetables), part of the container will burst if ?o the internal pressure of the container exceeds its external Sao 20 pressure by approximately 13 p.s.i. It will be understood, of ooo course, that this pressure will be different at other cooking temperatures and for other container sizes and designs.

The amount of outward distention of the container bottom wall, and hence the volume increase in the container, during 0 6 S the cooking oycle, must be sufficient as to prevent bursting of Sthe container by reducing the internal pressure. It has been found that this volume increase depends on several factors, such as, the initial vacuum level in the container headspace, Sthe initial headspace, thermal expansion of the product and the 30 container, the container design and its dimensions. Table I below sets forth the volume change for a multi-layer injection blow molded container (303 x 406) at two different thermal processing conditions.

Table I Condition Example A Example B Steam Temperature 9F 230 240 Content Temperature at filling, oF 70 34 Content av.temperature, end of cook,O F 225 235

DG

8 Condition Max. inside metal end wall temp,, F Pressure at closing, psia Internal Pressure before bulge(Pl), psia Internal Pressure after bulge(P 2 psia Internal Pressure External Pressure Unbulged Container PI-14.7, psi Bulged Container r2-1 psi Burst Strength of container, psi Head Space Volume, cu. in.

Initial Volume Volume After Bulge, cu. in.

Volume Increase, cu. in.

Example A 228 6.7 27.4 23.7 12.7 9.0 19 1.48 3.10 2.62 Example B 238 6.7 32.6 28.0 17.9 13.3 16 1.48 3.11 1.63 Example B of Table I illustrates that if the container does not bulge sufficiently to reduce the pressure differential to below 16 p,si. the container would burst. On the other hand, Example A represents conditions under which bottom bulging is Snot required to prevent bursting. It should be recognized that bursting of a container can occur through a failure of the sealing means as well as by a rupture of container wall. It should also be recognized that the decrease in pressure differential as a result o'1 bottom bulging is beneficial even S if the container would not burst at the higher pressure. Such a reduction in pressure differential will reduce the amount of "creep" or "permanent deformation" which the container will 4 4 undergo during the thermal process. As will be discussed later, such creep makes it more difficult to reform the bottom wall S" later in the thermal process.

In order to attain the desired increase in volume of the container, it has been found that the container bottom wall must be so designed as to provide a significant deformation of the bottom wall of the container, Such bottom wall design is a significant consideration during the cook cycle and reforming as will hereafter be explained.

It has been discovered thac in order to accommodate the requirements of volume increase of the container without bursting during the cook cycle, and inward distention of the 39refo to attain an acceptable bottom bottom wall on reform to attain an acceptable bottom 9 configuration, the container must be appropriately designed.

Thus, the container bottom wall must be so designed and configured as to include portions which have lower stress resistance relative to other portions of the bottom wall, as well as relative to the container sidewall. Such container configuration is shown in Figure 2 wherein the bottom wall includes portions such as shown at 15, 17, 19 and 21 which are configured to have lower stress resistance than the portion of the bottom wall designated by 7, and the sidewalls as shown at 23 and Although the bottom wall of the container may be made to include portions of less stress resistance by varying the bottom configuration, such lower stress resistant areas can be formed by varying the material distributions of the container so that its bottom wall include weaker or thinner portions.

Thus, as shown in Figures 3 and 4, the thicknesses of the S bottom wall at T 5 and T 6 are less than T 7 the thickness S of the remaining segment of the bottom wall. Similarly, T and Tg are less than T 2

T

3 and T 4 the thicknesses at different portions of the sidewall, Similar differences in material distribution are shown in Figure 3.

Another example of a bottom configuration which includes portions of less stress resistance is one having segmented indented portions preferably equal, such as a cross configuration wherein the indented portions have less stress resistance than the remainder of the bottom wall e.g. remaining segments thereof, and than the container sidewall. Preferably the inde ited segments of the cross meet at the axial center of the bottom. Deeper indentations assist reformation, and while 310 shallower ones help to prevent excess of bulging.

A large outward deformation of the container bottom wall is usually best achieved by unfolding of "excess" material in the container bottom rather than by simple stretching of the plastic wall. The preferred container bottom wall should therefore be designed so as to have approximately the sam surface area as would a spherical cap whose volume is Iho of the undeformed volume of the bottom of the coitaine r the desired volume increase. The volume of the hemisphoie 3 cap shown in Figure 8 can be determined from the equation (1)

DG

10 I as follows V 1/67rh(3a 2 h 2 (1) where is the volume, is the height of the dome of the spherical cap and is the radius of the container at the intersection of the sidewall and bottom wall of the container.

The surface of the spherical cap may be calculated from equation 2 as follows:

S

2 i(a 2 h 2 4 (2) 7 where "S is the surface area of the spherical cap, and "a" and are as discussed above.

The design volume and the surface area of the spherical cup required for satisfactory bulge and reform over a wide range of food processing conditions for a container of any given size (within a wide range of sizes) may be calculated by the following procedure: The ratio of the dimension to the dimension is expressed as

I

20 o a 84400 030 a 39 k h/a or h ka where and are as described above. It has been discovered that is about .47 for satisfactory containers.

Therefore the required volume and surface are of the spherical cap required for a satisfactory container of a given size may be calculated as follows: V 1/6 (.47)a (3a 2 (.47a) 2 S2=7 (a 2 (.47a) 2 wh, re "S 2 and are as discussed above for the given size container.

The bottom is designed to have a surface "S in the folded portion so that "S" 1 1 is approximately equal to S 2 As it was previously explained, at the conclusion of the thermal sterilization cycle, the container bottom wall is distended outwardly and must therefore be refor-,i:. to attain an acceptable bottom configuration. The bulged bottom will not return to its original configuration merely by eliminating the pressure differential across the container wall. This failure to return to its original configuration is a result of "creep" or "permanent deformation" of the plastic material.

Creep is a well-known property of many polymeric materials.

11 I _I The bottom wall can be reformed by imposing added external pressure, or reducing the internal pressure in the container, so that the pressure outside the container exceeds the pressure within the container. This reformation can best be effected while the bottom wall is at "reformabl temperature". This temperature will of course vary depending on the nature of the plastic used to form the bottom wall but, for polyethylene -polypropylene blend, this temperature is about 112 F.

Reformation by imposing an "overpressure" can be readily attained by introducing air, nitrogen, or some other inert gas at the conclusion of thermal processing but before cooling.

Where the contents can be degraded by oxidation, it is preferable to use nitrogen or another inert gas rather than 3 n A oxygen since at the prevailing reform temperatures, the oxygen oi and motsture barrier properties of the plastic are reduced.

The advantages of adequate overpressure during S reforming of the container bottom wall is illustrated in the o following series of tests.

a 00 Several thermoformed plastic containers (401 x 411 i.e.

9 0 4-1/16 inches in diameter and 4-11/16 inches high) were filled with water to a gross headspace of 10/32 inch, closed at atmospheric conditions and thermally processed in a still retort under an atmosphere of steam at 240 F. for 15 minutes.

At the conclusion of the thermal sterilization process, air was introduced into the retort to increase the pressure from S 10 to 15 p.s.i.g. Thereafter, the container content was cooled to 160 F. by introducing water into the retort. The resulting containers were observed to have severely bulged S bottom and sir"ewall panelling.

8 0 The foregoing procedure was repeated for another set of S identical thermoformed plastic containers under the same conditions except that the pressure during rjform was increased to 25 p.s.i.g. prior to introducing the cooling water. The resulting containers had no rocker bottoms or sidewall panelling and the containers had an acceptable configuration.

The results are shown in Table II below.

39 DG 12 r7l a **i o 0 0 0 *0 CI oQ 00 C0 0 or 0 0 zoo 4 0 0 0 a 3 0 3 0Z03 4o OCO 0 0 0 0 C 2 0 04 0 CODO C J TABLE II COOKING CYCLE REFORM CYCLE CONTAINER CONFIGURATION COMMENTS Fill Temp. Pressure Pressure at 160 F Sidewall Bottom Panelling Bulge (4) 160 F.

160 F.

175 F.

Severe Severe Severe Severe Severe Severe Severe Severe Severe Severe Severe Severe All Containers Bad Objectionable Configuration 160 F. 10 25 OOR-1 OK-125 All 160 F. 10 25 OOR-2 OK-120 Containers 160 F. 10 25 OOR-1 OK-145 Bad 175 F. 10 25 OOR-1 OK-245 Acceptable 175 F. 10 25 OOR-1 OK-168 Configuration 175 F 10 25 OOR-1 OK-140 (1) (2) (3) (4)

DG

Steam cook at 240 F. maximum temperature.

Air pressure during cooling maintained until container content was cooled at 16? F.

"OOR" designates out of roundness with OOR of 1 indicating almost perfect roundness and OOR OF 5 indicating almost panelled.

Numbers following OK measure center panel depth in mils. Thus OK-125 indicates inward bottom distention of 118 inch.

F

L.

must be maintained during reform in order to obtain acceptable container c'onfiguration. From the above, it can be seen that "overpressure" herein means the retort cooling pressure, is usually greater than the retort cooling pressure. Overpressure does not refer to the pressure outside the container relative to the pressure inside the container.

In another series of tests, plastic containers (303 x 406) were filled 8.3 ounces of green beans cut to 1-1/4 to 1-1/2 inches in size, A small quantity of concentrated salt solutions was added to each container and the container was filled to overflow with water at 200 0 F. to 205 0 F. Each container was topped to approximately 6/32 inch headspace and then steam flow closed with a metal end. The containers were then stacked in 04 S a still retort, metal ends down, with each stack separated from S the next by a perforated divider plate. Two batches of containers (100 containers per batch) were cooked in steam at 250 0

F.

for 13 minutes. At the conclusion of the cooking cycle air was 0 introduced into the retort to increase the pressure from o,0 p.s.i.g. to 25 p.s.i.g. and the container was then cooled by water for 5-1/2 minutes. The retort was then vented to atmospheric pressure and cooling continued for an additional 5-1/2 minutes. Examinations of the containers showed no rocker bottom or sidewall panelling and all the containers had acceptable configurations.

In another series of tests plastic containers (303 x 406) were filled with 10.2 ounce of blanched fancy peas.

S A small quantity of a concentrated salt solution was added to each container and the container was filled to overflow with 0 0 a water at 200 F to 205 F, Each container was topped to approxi- 30 mately 6/32 inch headspace and then steam flow closed with a S on metal end. The containers were stacked in a still retort, metal ends down, in 4 layers, with 25 containers in each layer separated by a perforated divider plate. The containers were then cooked with steam at 250 F for 19 minutes. One batch of the containers was cooled with water at the retort pressure of 16 p.s.i.g. The ru-ulting containers did not reform properly due to bottom rocker and sidewall panelling. Another batch was reformed at 25 p.s.i.g. by passing air into the retort 39 and then cooled with cold water for approximately 6 minutes after 14 r- js"9 which the retort was vented to ambient pressure and cooled for another 6 minutes. No rocker bot:tom or sidewall panelling was observed and all the containers in this batch had acceptable configuration.

As has been discussed a container which is subjected to a normal thermal processing cycle will bulge outwardly at the end of the heating cycle. If at that time the container should be punctured so that the inside to outside pressure differential across the container wall would be eliminated and the container then cooled, the bulged condition would persist and the bottom would not reform. In order to reform the container, the pressure outside the container must exceed the pressure inside the container.

so Figure 9 shows the pressure differential required to reform the bulged bottom wall of a particular multi-layer on oo injection blow amolded container (curve A) and also the pressure 0o differential above which the sidewall panels (curve B) This relationship is shown over the range of 330F to 2500F.

6 The data for Figure 9 were developed by heating the 0# f 20 container in an atmospheric hot air oven to 250 F and subjecting it to an internal pressure of about 6 psig for few minutes. The container temperature was then adjusted to the 0% various temperature values shown on the graph and the internal pressure was then decreased until reform and panelling occurred and the corresponding pressure differentials were recorded.

From Figure 9 it is noted that if the container material is 150 F or above and a pressure differential (P outside P inside) is applied across the container walls, the container 30 will reform satisfactorily whereas if the container wall is at o° 75 F or lower, and a pressure differential is applied it will panel at a lower pressure than is necessary to produce bottom refom., In addition it is noted that for this design, and in the 150 to 250 F temperature range, there is a difference between the pressure differential required for proper reform and that which causes sidewall panelling.

It is further noted that curves and cross at about 112 F, indicating a temperature below which satisfactorz 39 reform cannot be accomplished. In observing the containers 15

I

during testing it was noted that at 150 F or above, reforming appeared to occur gradually and proportionally with the pressure change. At 75 F and below reform and panelling occurred abruptly.

The increase in external pressure while the plastic is warm can be readily accomplished in most still retorts by introducing air or nitrogen at the end of the steam heating cycle but before the cooling water is introduced. Although air and nitrogen are equally effective in reforming the container, the use of air could result in some undesired permeation of oxygen into the container since the oxygen barrier properties of some containers are reduced by the high temperatures and moisture conditions during retort. We have found that the introduction of such an air or nitrogen overpressure is also effective in many 9:p continuous rotary cookers.

In other cases, it is impractical to impose such an °0 added gas overpressure, either because there is no provision for maintaining such a pressure during cooling or because the 0 pressure limitations of the equipment are such that the pressure required for reforming exceeds the allowable pressure limits. It has been found that under certain conditions, the desired reformation can be achieved even without such an 6, externally applied pressure or with an external pressure 0 000 o o insufficient for reformation at the internal pressures existent S at the end of the heating cycle. The key to proper reformation Sunder these restrictions is to cool gradually the container in t 0 such a manner that the plastic will still be relatively soft at the time when the container contents have cooled sufficiently to reduce the internal pressure below the external pressure.

o This can be accomplished with the use of relatively warm cooling O water, at least during the initial stages of cooling.

39 DG -16 JI -r In connection with the above, it has been found that under certain conditions less than the previously mentioned large overpressure of about 10 to 15 psig is sufficient to obtain successful reformation. It has been found that the retort or external pressure during cooling can be moderately higher, about the same as, or even below the retort cook pressure. This would apply whether the retort is still or continuous.

The following series of tests will further illustrate this aspect of the invention.

Several injection blow molded multi-layer plastic containers (211 X 215, i.e. 2-11/16 inches in diameter and 2-15/16 inches high) were filled with 135°F water to leave a series of different headspaces, closed by a double seam with a steel end at 20 inches of vacuum and thermally processed in a still retort at 250F (15.3 psig equilibrium steam pressure) for 90 minutes. At the conclusion of the thermal sterilization process, air was introduced to attain an air pressure of about 15 psig. Thereafter, the container content was cooled for 12 minutes to below 165 0 F with water sprayed onto the plastic end of the container while the container was resting on its metal end. Table IIA below shows that plastic containers having a head-pace in the six through ten cc range when still retorted as above were successfully reformed with a pressure during cooling about the same as pressure during cooling.

0 0 0 us 0oi *000~ ro O00 0 0P0 S S TABLE IIA CONTAINER CONFIGURATION Head space Volume (cc) 2 2 2 2 2 4 4 4 4 4 4 6 6 6 6 6 6 8 8 8 8 8 8 10 10 10 After Retorting Rocker Rocker Success Success Success Success Success Success Rocker Rocker Rocker Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Panel 0 0 0 0 8 008 8a 4 oo 8o 8 'a 6 00 6 00 8 9 8 8 4 Success Kink* Kink* Success Success Success Success Success Panel Panel Panel Panel *Kink: A distortion of the bottom of the container caused by a local thin spot around one of the rings of the bottom. It is related to panelling in that it is aggravated by too much headspace and vacuum.

_I under the conditions of Table IIA during thermal processing are shown below in Table IIB.

TABLE IIB Condition in Retorts Time, minutes Container psig Retort psig Mid Cook 50 End Cook 93 Cooling before Reform 95 Container Reform 98 End of Cooking 109 Pressure Released 110 21.5 21.0 18.5 13.0 13.0 -0.3 15.0 15.0 14.5 14.0 14.0 0 is shown in 0 0 0 0 0 i o e o 0 o S 2a 0 S0 0 0 The successfully reformed container whose history Table IIB had a headspace of 8cc.

In another test, a container packed as in the previous case was thermally sterilized and cooled under "overpressure" cooling. The results are shown in Table IIC below.

000.

0 60 00 0 4 0 3.'0 0) 000 SlbO 0 a TABLE IIC Condition in Retorts Time, minutes Mid Cook 55 End of Cook 109 Start overpressure 109.5 Start water spray 113.5 Container Reformat 118.5 End overpressure cool 130 Pressure released 131 Container psig 15.2 15.2 21.0 20.0 18.0 18.0 -0.2 Retort psig 10.5 10.5 17.0 19.5 19.0 19.2 0 The successfully reformed container whose history is shown in Table IIC had a headspace of 8 cc.

As shown in Table IIB, the retort pressure during the cooling cycle may be less than the retort pressure during cooking cycle. This is evident by comparing the pressure of 15.0 psigat the end of the cooking cycle with the pressure of 14.0 psig 39 during cooling cycle (container reform).

'1 -I In case of "overpressure" cooling, as it is seen from Figure IIC, the retort pressure in the cooling cycle (container reform) is 19.5 psig compared to a retort pressure of 10.6 psig at the end of the cooking cycle. This indicates that the retort pressures during cooling and reform need not be as much as 15 psig higher than the retort pressure during cooling.

In both cases, the resulting containers had acceptable container configuration.

Results similar to Table IIC were attained by packing the container with Chili and Beans instead of water. These results are shown in Table IID below.

TABLE IID Condition in Container Retort Retorts Time, minutes psig psig Mid Cook 60 17.0 10.6 End ofCook 115 17.2 10.6 Start overpressure 115.5 19.8 19.5 Start water spray 119 21.0 20.8 Container Reformed 123.5 18.5 19.5 End overpressure cool 130,5 18.0 19.5 Pressure released 131 1.2 0 *The multi-layer plastic containers successfully reformed under the conditions shown in Table IID were 211 X 215 inches and closed with a steel end.

As shown in Table IID, the retort pressure at the end of the cook is 10.6 psig, and during cooling (container reforir', the retort pressure is 19.5 psig. Once again, it is noted that this difference is less than 15 psig but the container configuration was still acceptable and had no rocker bottom or sidewall panelling.

While the above test results indicate that acceptable container configurations are readily obtainable with a still retort, acceptable container configurations are also readily attainable with a Steritort with continuous retorts The following test results show successful reformation of containers in a Steritort cooker/cooler.

Several injection blow molded multi-layer plastic containers (211 X 215) were filled with 135°F water to leave a series of different headspaces, closed by a double seam with a steel end at inches of vacuum and thermally processed in a Steritort at 250°F (15.3 psig equilibrium steam pressure) for 30 minutes. At the conclusion of the thermal sterilization process, air was introduced to obtain an air pressure of 13.3 psig. Thereafter, the container content was cooled for 5 minutes at that air pressure by continually or intermittently submerging the containers in water during rotation of the Steritort reel on which the containers are mounted and during the rotation of the container in the water in the lower portion of the Steritort shell housing. The container content was cooled to below 165°F, were then additionally cooled to below 110F in the same manner but at atmospheric pressure.

Table IIE below shows that plastic containers having a headspace in the four through ten cc range when Steritort processed in the manner described above were successfully reformed with a cool pressure about 2 psig below the cook pressure.

0 00 000 4 4 4 0 0 oa S0000 4 400 4 0 6000 9 a t a a 4 4 6 44t '444 TABLE II E Headspace Volume (cc) 2 2 2 2 2 2 4 4 4 4 4 4 6 6 6 6 6 6 8 8 8 8 8 8 10 10 12 12 12 12 12 12 Container Configuration After Retorting Success Rocker Success Rocker Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Success Panel Success Panel Success Panel Success Success Panel Panel Panel Panel Succeos Success D 0r While the above test results show plastic containers can be successfully reformed using a Steritort process, they also indicate plastic containers can be successfully reformed in continuous retorts, since it is well known that steritorts are used in laboratories to simulate, and predict performance of containers thermally processed in, commercial continuous, e.g. rotory, retorts.

Although the test results demonstrate successful container reformation with containers filled to within certain headspace ranges, it is to be noted that the headspace range may be different and may be wider than reported above, since, as discussed herein, bottom bulging, panelling and succesful reformation will depend on various factors such as container size, wall thicknesses, design, and material properties, initial vacuum level in the container headspace, initial headspace, thermal expansion of the product and the container, whether the container has been pre-shrunk, and, as will be discussed in detail, the cuoling process including the type employed, and especially the rate and uniformity of cooling.

In addition to achieving a condition, however obtained, during the cooling cycle wherein the pressure outside of the container (Po) is greater than pressure inside the container (Pi) to obtain successful reformation, it has been found that the type, rate and uniformity of cooling of container body also are very important factors to be considered for successful reformation, particularly in relation to how and when the aforementioned pressure differential will occur. These cooling factors affect the headspace range in which successful reformation can be attained, given other factors such as the container's characteristics and its contents.

4 0 03 0 E l f o d a 0 (3 £443 a 6 n, 644 e 4 44 4 4 I I ~VL -T I sr As previously stated, reformation is best effected at a temperature at which the plastic is reformable. In reformation during cooling it is desirable that Pi be reduced below Po when the plastic is reformable, preferably soft. Since cooling the plastic affects its softness and reformability, the cooling factors are important. During cooling, Pi, which in the cook cycle exceeded Po, will initially be about the same as or slightly above Po. When the container is gradually cooled, Pi drops below Po primarily because the vapor pressure in the container decreases as the contents are cooled. This pressure differential provides the driving force for container reformation. Thus, under the cooling conditions, the reformation process begins and the bottom bulge begins to reform or invert.

In certain applications the more gradual the cooling rate the wider the headspace range will be. It has been found that with a still retort, the cooling rate of the plastic body may be faster, cooling is less unlfcrm and the headspace range for reformation to o acceptable configurations may be narrower, than with Steritort and continuous retorts.

a pt2nL 0 8 0 0 o 0 VV -r Ii z I- PP~~.P r~lF~ l~ icontainer bottom adjacent to which is any headspace, since the container is inverted and rests on the metal end which usually is its top end. Not being in direct contact with the heated contents, the plastic bottom wall cools and stiffens relatively more quickly then it does in a Steritort where the water contact is different. Cooling of the container body is less uniform than in a Steritort in the sense that the container's bottom which is in first contact with the water and is not in contact with the heated contents, cools more rapidly than the sidewall which is in direct contact with the heated contents. The above will occur in any still retort in which containers are so inverted during the thermal processing.

In a Steritort, vid increasingly so for a continuous retort, cooling of the plastic is more gradual. In a Steritort, S the containers are in a horizontal position on the Steritort o reel and the containers are rotated about the axis of the reel 6 S and about their axes as they are repeatedly submerged in the water at the bottom portion of the shell housing. The heated contents are more uniformly mixed or agitated and more uniformly So"0 in contact with the container sidewalls and bottom wall, and the container is more uniformly cooled than in a still retort. Thus, *the plastic of the container particularly, its bottom stays warmer longer, is in reformation condition longer and stiffens

S

0 later. This is particularly desirable because it has been °o found that in any cooling cycle, it is particularly important that cooling be effected in a manner that when the internal pressure of the container drops below the pressure exterior of the cortainer, e.g. in the cooler, the temperature of the S plastic bottom not be so much cooler than that of the sidewall such that the bottom would be sLiff and more stable than the side walls and the side walls would panel before the bottom reforms, sucks in or inverts. Thus, in a Steritort or continuous coolinq process this condition is avoided since conditions can be such that a significant temperature differential between the bottom and sidewall temperature is avoided, and their temperatures are more uniform duriihL cooling.

39 As it was previously described, the bottom bulge will not properly reform unless the relative rigidity of the bulged bottom wall is less than that of the sidewalls. This relative rigidity depends on the temperature of the plastic walls at a time when the external pressure exceeds the internal pressure.

Even if this rigidity relationship is such that the bottom does reform inwardly from its bulged position, it will not always reform far enough to form a acceptable container at the end of the cooling phase of the process. In particular, it has been found that if the initial vacuum level in the 0 00 0 0 0 0 00 0000 0 0000 0 0 0 0 00 oo o aO ao 0 It tll

I

container is not sufficient, the bottom wall will not always be uniformly reformed. Thus, the bottom wall will in many cases be distended inwardly in one area of the bottom while still remaining distended outwardly in another portion, thereby producing a "rocker" bottom. Even when tie more extended -ortion does not extend beyond the base of the sidewall so as to form a "rocker" bottom, the appearance of such an unevenly formed bottom is undesirable. This non-uniform reformation is believed to result primarily from non-uniformities in the plastic thickness as formed in the container manufacturing process.

We have discovered, however, that we can produce satisfactorily uniform reformation of the bottom even with So such imperfect containers by filling the containers under B conditions which will result in all areas cf the bottom being I largely inverted. In particular, we have found that for a Si given fill height and hence a given initial headspace volume, o there is a given minimum vacuum level required for full o inversion. For a smaller initial headspace volume, the minimum S2Q vacuum level required would be less. We have found that the proper relationship of these two variables can be defined by how much inward deflection of the bottom would be required to i. increase the pressure in the final headspace to nearly S" atmospheric. If the deflection required to compress the headspace is too low, the bottom will not fully invert and rocker bottoms can result. For the preferred container shown in figure 6, the headspace and initial vacuum levels should be sufficient to invert the bottom of the container by at least 14 cubic centimeters before the headspace gasses would be S"'O compressed, at room temperature, to approximately atmospheric pressure.

It will be obvious to one skilled in the art that any gasses dissolved in the product will alter this relationship in the same was as if those dissolved gas&es had been present initially in the headspace. Curve A on figure 11 represents the relationship between headspace and initial vacuum level in the container in cases where there are no signific- amount of dissolved qasses(i.e. water) in the container cont, 39 It will further be recognized that the initial vacuum can be generated either with a vacuum closing machine or by displacing some of the air in the headspace with steam by impinging steam into the headspace volume while placing the closure onto the container by the well known "steam flow closure" method.

If the vacuum level in the container is very high, the bottom wall will distend inwardly as long as it continues to be less resistant to deflection than is the sidewall. Once it has distended inwardly to the point where it has formed a concave dome, it will start to become more resistant to further deflection than is the sidewall. If there is still sufficient vacuum remaining at that point, the sidewall will panel giving an undesirable appearance. As in the minimum allowable vacuum 'J level described previously, the maximum allowable vacuum level 0 depends on the fill height. Again it has been found that the proper relationship of these two variables can be defined by how much deflection of the bottom would be required to increase S the pressure in the final headspace to atmospheric. For the °o preferred container shown in figure 11, the headspace and .o20 initial vacuum levels should be sufficient to invert the bottom "a 0 of the container by no more than 26 cubic centimeters. Curve B on figure 11 represents the relationship between these two i variables 1or the case in which there is not significant amount of dissolved gasses; i.e. water.

At values of initial vacuum and headspace volume i D falling below curve A, the containers will form rocker bottoms tesbeo Sand at values above curve B, the containers will panel. Values falling between curves A and B are therefore desired.

I The above calculated relationships correspond approximately to the experimental results for a group of containers which have been specially treated by a process of this invention known as annealing. The data on these containers are represented by the cur res marked A' and B' in figure For containers which have not been so treated, rocker bottoms are observed under conditions which would be calculated to invert acceptably. Data on these containers are represented by the curves and in the figure We have found that this increased tendency to form 39 rocker bottoms after thermal processing is the result of a DG i r C rlshrinkage which occurs in these cc tainers at the temperatures experienced in the food sterilization process. As a result of this shrinkage, the volume of the container after processing will be less than would otherwise be expected.

Correspondingly, the amount of bottom deflection which would be required to compress the headspace to approximately atmospheric pressure is reduced and the bottom will no longer fully invert under conditions which would have achieved full inversion without such shrinkage. As will be apparent from the above discussion and from the experiment results presented below, improved container configuration after processing can be achieved by annealing or pre-shrinking the containers before filling or sealing.

The pre-shrinking of the container may be achieved by annealing the empty container at a temperature which is approximately the same, or preferably higher, than the thermal processing temperature. The temperature and time required for thermal sterilization of food will vary depending on the type of food but, generally, for most packaged foods, thermal processing is carried at a temperature of from about 190 F.

(for hot-filling) to about 270 0 for a few minutes to about several hours. It is understood, of course, that this time need only to be long enough to sterilize the food to meet the commercial demands.

For each :ontainer, at any given annealing temperature, there is a ccrresponding annealing time beyond which no significant shrinkage in the container volume can be detected.

Thus, at a given temperature, the container is annealed until no significant shrinkage in the container volume is realized upon further annealing.

In addition to pre-shrinking the container by a separate heat treatment step conducted in an oven or similar device, it is possible to achieve the same results by preshrinking the container as a part of the container making operation. By adjusting mold cooling times and/or mold temperatures, so that the container is hotter when removed from the mold, a container which shrinks less during thermal processing can be obtained. This is shown below for a series of 303 x 406 containers made by multi-layer injection blow I iroulding in which the residence time in the blow mold was deliberately varied to show the effect of removing the container at different temperatures on the container's performance during thermal processing.

Mold Closed Temp. on Time-Sec. Leaving Mold Container Designation Capacity-cc 1 510 2 505 3 498 Shrinkage 250°F, 15 Minutes cc. 10.2 8.5 1.7 4.4 0.9 2.4 1.2 0.1 Lowest Intermediate Highest t t ttr CIl (P I 0 9 Note that the container 3 had partially shrunk on cooling to room temperature and had less shrinkage at 250 F than containers 1 and 2. All these containers were filled with water at a range of headspace, and a 20" closing vacuum, and retorted at 2500F for 15 minu:es to determine the range of headspace that would be used to achieve good container configuration.

Container 1 1 High Temperature Annealing No Yes No Yes No Yes Allowable Headspace cc 39-40 20-40 25-40 18-40 22-40 17-40 Note that container #1 when unannealed had only a 1 cc range in headspace, Container #2 and #3 without annealing had a much larger range. Of particular importance is the fact that container without a separate heating step, had virtually as broad a range as container #1 which had a separate high temperature annealing step.

The amount of residual shrinkage in the container when 39 it is filled and closed has a major effect on the range of DG $5

I-L

i C L i~allowable headspace and vacuum levels. When shrinkage exceeds about 1-1/2% (at 250 F for 15 minutes) it becomes extremely difficult to use the containers commercially unless they are deliberately pre-shrunk. The containers discussed above were made by either injection blow molding or thermoforming and had shrinkage of 1.4 and 4% respectively. There are other plastic containers being developed for thermal processed foods which have about 9% residual shrinkage and will also benefit from this pre-shrinking invention.

These containers are the Lamicon Cup made by Toyo Seikan in Japan using a process called Solid Phase Process Forming, and containers made using the Scrapless Forming Process by Cincinnati Midacron who is developing this process.

The advantages of using an annealed container in the process of the present invention can be further appreciated by reference to figure 10. As shown in this figure, the use of annealed containers increases the headspace range which may be fit$ maintained in the container at closing. Thus, for example, for a typical multi-layer injection blow molded container of 303 x 406, filled with 70 F deionized water, of the container is S closed at an initial sealing vacuum of 20 inches, ugsable Sheadspace which can be tolerated at reform for an unannealed S container is 26-40 cc. This corresponds to a headspace range for 14cc. If, however, the container is annealed, the usable headspace is 21-40 cc, then measuring the headspace range to 19 cc.

Sr The increased usable headspace range allows for less 9 41 accuracy during the filling step. Since commercial filling and closing equipment are generally designed within an accuracy of 8 cc, the annealed container will not require much modification of such equipment.

It has also been discovered that further improvements in container reformation may be realized by using a container which has been pre-shrunk prior to thermal processing. The use of pre-shrunk container permits greater range of filling conditions as will hereinafter be explained.

For each container, at any given annealing temperature, there is a corresponding time beyond which no significant 39 shrinkage is attained in the container volume. Thus, at any V- given temperature, the container is annealed until no furhter significant shrinkage in the container volume is detected upon further annealing. Obviously, this will vary with the different resins used to make the container and the relative thickness of the container wall.

Instead of pre-shrinking the container by annealing as aforesaid, it is possible to use a pre-shrunk container wherein the container volume has been reduced during the container making operation. Thus, whether container is made by injection blow molding or by thermoforming, the container made may be essentially non-shrinkable since its volume has been reduced during container making operation.

The following examples will serve to further illustrate the present advantages of the use of annealed (pre-shrunk) containers.

EXAMPLE 1 Two sets of thermoformed multilayered plastic containers (303 x 406, i.e. 3-3/16 inches in diameter and 4-6/16 inches high) were used in this example. The first set was not 2C annealed but the second set was annealed at 250 F. for minutes in an air oven, resulting in 20 cc volume shrinkage of the container measured as follows: A Plexiglass plate having a central hole is placed on the open end of the container and the container is filled with water until the surface of the Plexiglass plate is wetted with water. The filled container and Plexiglass plate are weighed and the weight of the empty container plus the Plexiglass plate is subtracted therefrom to obtain the weight of water.

H The volume of the water is then determined from the temperature and density at that temperature.

The above procedure was carried out before and after annealing of the container. The overflow volume shrinkage due to annealing was 20 cc, or 3.9 volume percent, based on container volume of 502 cc.

Both sets of containers were filled with 75 0 F. deionized water and the containers were sealed by a vacuum closing machine at 20 inches of vacuum. All containers were then retorted in a Steritort at 250e. for 20 minutes and then 39 cooled at 25 p.s.i. The results are shown in Table I below, wherein "Rocker" signifies that the containku is unsatisfactory i i due to bulging in the container bottom, "Panel" designates sidewall panelling and, again, unsatisfactory container, and "OK" indicates that the container is satisfactory because it has no significant bottom bulging or sidewall panelling.

TABLE III Condition after Condition After Closing Machine Retorting Headspace Volume, cc Annealed Not Annealed Annealed Not Annealed 16 OK OK Rocker Rocker 18 OK OK OK Rocker OK OK OK Rocker 22 OK OK OK Rocker 24 OK OK OK Rocker 26 OK OK OK Rocker 28 OK OK OK Rocker OK OK OK Rocker 32 OK OK OK Rocker 34 Panel Panel OK Rocker 36 Panel Panel Panel Panel As shown in Table III, the annealed, and hence, preshrunk containers are free from bottom bulging or sidewall panelling, whereas the non-annealed containers largely fail due to rocker or panel effects. In addition, the use of annealed containers permits greater range of headspace volume as compared to the containers which were not annealed prior Sto thermal processing.

EXAMPLE 2 Example 1 was repeated under similar conditions S except that the plastic containers used had been obtained by injection blow molding. Shrinkage due to annealing was 7.9 cc or 1.6 volume percent. The results are shown in Table IV.

-r n l--uapp e TABLE IV Headspace Condition After Closing Machine Condition After Retorting Volume, cc Annealed

OK

Panel Panel Not Annealed

OK

Panel Panel Annealed Not Annealed Rocker

OK

Panel Rocker Rocker Rocker Rocker Rocker Rocker

OK

Panel SIf The results in this example also illustrate the ,iI, advantages which result from annealing of the containers prior to retorting.

m 2 EXAMPLE 3 This example was similar to Example 1 except that retorting was carried out at 212 F. for 20 minutes. As i I shown in Table III, similar results were obtained as in the 24 previous examples.

llt <3t 7 rl;~ lll*.ii CIL-I-----CI CI~ i -_1I~U 3L3--- 1LI~-~ 9 TABLE V Condition After Closing Machine Condition After Retortina Headspace Volume, cc Annealed Not Annealed Annealed Not Annealed

OK

20 OK OK 21 OK OK 22 OK OK 23 OK OK 24 OK OK 25 OK OK o 26 OK OK 27 OK OK 28 OK OK 29 OK OK 2 30 OK OK 31 OK OK 32 OK

OK

I 33 OK OK 34 Panel Panel Panel Panel EXAMPLE 4 Rocker Rocker

OK

Panel Rocker Rocker Rocker Rocker Rocker Rocker 2ocker Rocker Rocker Rocker Rocker Rocker Rocker Rocker Rocker Rocke Rocker Rocker Rocker

OK

Panel The procedure of Example 3 was repeated except that the containers had been obtained by injection blow molding.

Table V shows the same type of advantageous results as in the 33 previous examples.

TABLE VI Condition After Closing Machine Condition After Rpf- n rl-. ino Headspace Volume, cc 17 19 21 23 27 29 31 33 Annealed

OK

OR

OK

OIK

OK

Panel Panel Not Annealed Annealed Not Annealed

OK

OP.

OK

Panel Panel Rocker Rocker Rocker

OK

Panel Rocker Rocker Rocker Rocker Rocker Rocker

OK

Panel.

The increased usab1'- headspace range allows for less accuracy in the filling steps. since commercial filling and closing equipment are generally designed within an aco'uracy of 8 cc, the annealed container will not require much modification of ouch equipment.

In the foregoing examples the advantages of 2pre-shrinking of the container by annealing are illustrated utilizing containers filled with water because of experimental, simplicity. These advantages can also b,* realized, however, in other cases where the contaiiner is '4m A illed with fruits, vegetable or other edible products. For example, injection blow molded multilayer plastic containers (303 x 406) were filled with fresh pears and syrup (130 0

F.,

20% sugar solution) and retorted at 212 0 F. for 20 minutes, Prior to filling a set of the containers was annealed at 32 250 0 F. for 15 minutes, while the other set was not MK 1, ann~ealed. When 7500 containers were annealed prior to retorting, the success rate was as high as 95 percent, with only about 5 percent reform failure. In the case of non-annealed containers, the success rate was Qonsiderably les.s since reform failures were observed in most retorted containers.

4 o a 8t Mx 981

Claims

1. A method of thermal sterilization of a plastic container packed with food to obtain a thermally sterilized packed container having an acceptable configuration, which comprises filling the container with food, sealing the container, either or both of these steps including selecting an initial head space volume and an amount of gas, taking into account an initial vacuum level, if any, at sealing such as to permit bulging and subsequent reformation of the container bottom wall without significant side wall panelling, thermally sterilizing the packed containe- in a oo", retort operated at a temperature and pressure for a time ono sufficient to sterilize the container and its contents and to cause bulging of the container bottom wall, cooling the S" container and its contents, and, during the cooling step, 00 Q .s.o reforming the bulged container bottom wall to attain an 0 acceptable container configuration by controlling the pressure external of the container and the cooling conditions said controlling step including providing that the plastic of the bulged container bottom wall is at a 0000 o reformable temperature at which the plastic is soft while .o providing a pressure differential such that the pressure external of the container exceeds the pressure internal the o o container, and utilizing the external pressure while said plastic is sofx to reform the bulged bottom wall.

2. A method as claimed in either one of claims 1 or 2 00,0 wherein the cooling conditions are controlled by effecting coolinq gradually such that as the pressure internal the S container decreases, reforming occurs when the plastic of the bottom wall is at a reformable temperature at which the plastic is soft.

3. A method as claimed in either one of the preceding claims wherein the pressure external of the container is controlled during reforming 6o that it is tht same as or about the same as that employed during the sterilizing step.

4. A method as claimed in any one of claims 1 or 2 wherein the pressure external of the container is controlled during reforming so th, it is moderately or slightly higher T than that employed during the sterilizing step. A method as claimed in claim 4 wherein, during reforming the pressure exceeds the pressure during sterilization by from about 8 psig to 10 psig.

6. A method as claimed in any one of claims 1 or 2 wherein the pressure external of the container is controlled during reforming so that it is less than that employed during the sterilizing step.

7. A method as claimed in claim 6 wherein, during reforming the pressure in the retort is from about 1 to about 2 psig less than the pressure during sterilizing.

8. A method as claimed in any one of the preceding claims wherein reforming of the bulged container bottom wall is effected in the retort.

9. A method as claimed in any one of the preceding claims wherein the method is effected in a still retort. A method as claimed in any one of claims 1 to 8 where the method is effected in a continuous retort.

11. A method as claimed in any one of claims 1 to wherein reforming of the bulged container bottom wall is effected by esta lishing a preselected ambient gas pressure Sin the retort at the conclusion of thermally sterilizing the ,container and its contents.

12. A method as claimed in claim 1 wherein cooling is effected in a continuous cooler.

13. A method as claimed in any one of the preceding claims wherein during reforming, the temperature of the container side wall and the temperature of the container bottom wall are such that the bottom wall reforms before the side wall panels.

14. A method as claimed in any one of the preceding claims wherein during cooling and reforming, a significant temperature differential betwer the container side wall and container bottom wall is avoided. lF. A method as claimed in claim 12 wherein, during reforming, the container side wall and container bottom wall are at about the same temperature.

16. A method as claimed in claim 12 wherein the "\control;lng step includes establishing a preselected air 3 f. y. .A pressure prior to, at or during the initial stages of cooling, providing a rate of cooling such that as the container contents cool and the pressure and volume internal the container decrease, reforming occurs prior to side wall panelling while the plastic of the bottom wall is at a reformable temperature at which the plastic is soft.

17. A method as claimed in any one of claims 12 to 16 wherein the pressure external of the container is the ambient pressure in the continuous cooler.

18. A method as claimed in any one of the preceding claims wherein the cooling step is effected gradually.

19. A method as claimed in any one of the preceding claims wherein reforming is initiated by subjecting the container to a gas pressure and it then is further affected by contacting the container with water.

20. A method as claimed in any one of the preceding claims wherein the controlling of cooling conditions includes controlling the rate of cooling.

21. A method as claimed in any one of the preceding claims wherein the cooling conditions include the cooling temperature.

22. A method as claimed in any one of the preceding claims wherein the controlling of cooling conditions takes into 0 0 account the temperature of the Elastic of the container. i i 1 o 23. A method as claimed in any one of the preceding claims r wherein the retort has an environment which includes steam.

24. A method as claimed in any one of the preceding claims wherein the plastic container packed with food is thermally sterilized in a retort operated at a temperature and I pressure for a time sufficient to sterilize the container and its contents and to cause bulging and creep of the plastic of the container bottom wall. A method as claimed in any one of the preceding claims wherein the method includes selecting as the container to be thermally sterilized, one which has wall portions of less stress resistance relative to other portions of the wall and relative to the side wall to allow controlled bulging of the wall during thermal sterilization. PIA4/ 4 X26. A method as claimed An any one of claims 1, 2, 4, 5, 8 _Ili--Wl~. -26' or 9 wherein at the conclusion of thermally sterilizing, there is included the step of introducing air into the retort to increase the pressure to an amount moderately or slightly greats: than what it was during the thermally sterilizing step.

27. A method as claimed in any one of claims 1 to 3, or 6 to 9 wherein at the conclusion of thermally sterilizing, there is included the step of introducing air into the retort to maintain the pressure at an amount about the same or less than what it was during the thermally sterilizing step.

28. A method as claimed in either one of claims 26 and 27 wherein the method includes continuing the air introducing step for a period of time during cooling to maintain the Ott pressuring during cooling by an amount and for a time sufficient to prevent the container bottom wall from bulging S excessively such that it would no longer be reformable to an acceptable configuration. fits

29. A method as claimed in any one of the preceding claims wherein the cooling step is effected by introducing water into the retort.

30. A method as claimed in any one of claims 27 to wherein the air introducing step is effected prior to cooling,

31. A method as claimed in any one of the preceding claims wherein the selecting step is effected to provide a full inversion of the container bottom wall upon reformation.

32. A method as claimed in any one of claims 12 or 15 to 4414 S17 wherein the cooling step is effected gradually by contacting the container with relatively warm cooling water in the continuous cooler at least during the initial stages of cooling.

33. A method as claimed ir. any one of the preceding claims wherein there is included the step of preshrinking the plastic container and utilizing the preshrunk plastic container throughout the rest of the steps of the method.

34. A method of thermal sterilization of a plastic container packed with food to obtain a thermally sterilized S packed container having an acceptable configuration, which comprises filling the container with food, sealing the ills. C A container, either or both of these steps including selecting an initial head space volume and an amount of gas, taking into account an initial vacuum level, if any, at sealing such as to permit reformation of the container bottom wall without significant side wall panelling, thermally sterilizing the packed container in a retort operated at a temperature and pressure for a time sufficient to sterilize the container and its contents and to cause bulging of the container bottom wall, cooling the container and its contents and, during the cooling step, reforming the bulged container bottom wall to attain an acceptable container configuration by establishing a preselected ambient air pressure and controlling the ambient pressure and the cooling conditions, said controlling step including providing a gradual rate of cooling such that as the container contents cool and the pressure and volume internal the container decrease, _jforming occurs when the plastic of the bottom wall is at a reformable temperature at which the plastic is soft. A method as claimed in claim 34 wherein the reforming step is effected at an ambient pressure which is higher than that employed during the sterilizing step.

36. A method as claimed in claim 34 wherein the reforming step is effected at an ambient pressure which is less than that employed during the sterilizing step.

37. A method as claimed in claim 34 wherein reforming step is effected at an ambient pressure which is about the same as that employed during the sterilizing step.

38. A method as claimed in any one of claims 34 to 37 wherein reforming of the bulged container bottom wall is effected by establishing the preselected ambient air pressure in a retort at the conclusion of thermally sterilizing the container and its contents.

39. A method as claimed in claim 38 where the retort is a continuous retort. A method as claimed in claim 38 where the retort is a still retort.

41. A method as claimed in ony one of claims 34 to 37 TM4-/ wherein the cooling step is carried out in a continuous I~ Pll~~_r~ll cooler and the preselected ambient air pressure is established prior to the initial stages of cooling.

42. A method as claimed in any one of claims 34 to 41 wherein the plastic container packed with food is thermally sterilized in a retort operated at a temperature and pressure for a time sufficient to sterilize the container and its contents and to cause bulging and creep of the plastic of the container bottom wall.

43. A method as claimed in any one of claims 34 to 42 wherein the method includes selecting as the container to be thermally sterilized, one which has wall portions of less stress resistance relative to other portions of the wall and relative to the side wall to allow controlled bulging of the wall during thermal sterilization.

44. A method as claimed in claim 43 wherein the selecting step includes selecting a container whose bulged bottom wall would have approximately the same surface area as would a spherical cap whose volume is the same as that of the unbulged volume of the bottom of the container plus the desired volume increase wherein the volume is determined 2 2 by V=l/6 rh (3a h wherein is the dome of the spherical cap, and is the radius of the container at the s intersection of the side wall and bottom wall of thA container, the surface of the spherical cap can be calculated as follows: S 2 (a2+h 2 )4/3 where S2 is the surface area of the spherical cap, and "a" and are as defined above, and wherein the ratio of the dimension to the is expressed as: k=h/a or h=ka where and are as defined above, and k is about 0.47. A method as claimed in claim 44 wherein the selecting step includes selecting a container whosj bottom wall in its unbulged state b a folded portion whose surface area is wherein "S "S2

46. A method as claimed in any one of claims 34 to ,./\wherein at the conclusion of thermally sterilizing, there is ii: included the step of introducing air into the retort to increase the pressure to an amount moderately or slightly greater than what it was during the thermally sterilizing step.

47. A method as claimed in any one of claims 34 to 46 wherein at the conclusion of thermally sterilizing, there is included the step of introducing air into the retort to maintain the pressure at an amount about the same or less than what it was during the thermally sterilizing step.

48. A method as claimed in either one of claims 46 or 47 wherein the method includes continuing the air introducing step for a period of time during cooling to maintain the pressure during cooling by an amount and for a time sufficient to prevent the container bottom wall from bulging excessively such that it would no longer be reformable to an acceptable configuration.

49. A method as claimed in any one of claims 34 to 48 wherein the cooling step is effected by introducing water into the retort. A method as claimed in any one of claims 47 to 49 wherein the air introducing step is effected prior to cooling.

51. A method as claimed in any one of claims 34 to wherein the selecting step is effected to provide a full S° inversion of the container bottom wall upon reformation.

52. A method as claimed in any one of claims 34 to 37 or 41 to 48 wherein the cooling step is effected gradually by contacting the container with relatively warm cooling water in the continuous cooler at least during the initial stages of cooling.

53. A method as claimed in any one of claims 34 to 52 wherein there is included the step of preshrinking the plastic container and utilizing the preshrunk plastic container throughout the rest of the steps of the method.

54. A method as claimed in any one of claims 34 to 53 wherein the reLorming step includes utilizing the ambient pressure external the container for reforming the container bottom wall. A method of thermal sterilization of a container which I _I I_ T6 Y~ has a plstic end wall and is packed with food to obtain a thermally sterilized packed container having an acceptable configuration, which comprises filling the container with food, sealing the container, either or both of these steps including selecting an initial head space volume and an amount of gas, taking into account an initial vacuum level, if any, at sealing such as to permit bulging and subsequent reformation of the container end wall without significant side wall panelling, thermally sterilizing the packed container in a retort operated at a temperature and pressure for a time sufficient to sterilize the container and its contents, and to cause bulging of the container end wall, cooling the container and its contents, and, during the cooling step, reforming the bulged container end wall to attain an acceptable container configuration by controlling the ambient pressure external of the container and the cooling conditions, and utilizing the ambient pressure external of the container at a level which is about the same as or less than that employed during thermal sterilization to reform the container end wall while providing that the plastic of the bulge is at a reformable temperature at which Sthe plastic is soft.

56. The method of claim 55 wherein the container is comprised of plastic.

57. The method of claim 56 wherein the end wall is the container bottom wall.

58. A method as claimed in claim 55 where the retort is a still retort.

59. A method as claimed in any one of claims 55 to 58 wherein the plastic container packed with food is thermally sterilized in a retort operated at a temperature and pressure for a time sufficient to sterilize the container and its contents and to cause bulging and creep of the plastic of the container bottom wall. A method as claimed in any one of claims 55 to 59 wherein at the conclurion of thermally sterilizing, there is included the step of introducing air into the retort to increase the pressure to an amount moderately or slightly X' greater than what it was during the thermally sterilizing i LU cc L -r 1 I~ I I- I II step.

61. A method as claimed in any one of claims 55 to wherein the selecting step is effected to provide a full inversion of the container bottle wall upon reformation.

62. A method as claimed in any one of claims 55 to 61 wherein the reforming step includes utilizing the ambient pressure external the container for reforming the container bottom wall.

63. A method as claimed in any one of claims 55 to 62 wherein there is included the step of preshrinking the plastic container and utilizing the preshrunk plastic container throughout the rest of the steps of the method.

64. A method substantially as hereinbefore particularly described with reference to any one of the tests or examples. DATED: 23 April 1990 PHILLIPS ORMONDE FITZPATRICK Patent Attorneys For: AMERICAN CAN COMPANY 04'wk- 0 4 44 4 i S64 ft I 4 a a t aol 4 4< 00 4 4 4 o 0 a 44 9 (7053h) 6 066 a o a 4oo o 0 4