Sealed bag for microwave heating

A sealed bag for microwave heating composed of a film having a fusing layer inside, said bag being sealed by fusing both sides of said film longitudinally with each other and both ends thereof laterally, wherein both sides of the film are turned outside to form two rows or turned-up faces, a turned-up line formed by said two rows of turned-up faces is covered with an easily openable fusing tape having an easily openable film inside and reinforced with a base material outside, said tape being fused with the turned-up faces of the film, and at least one end of the fusing tape not fused by a length which can be taken with fingers is left as a pick-up flap in the laterally fused area. When cooked food for a few people is hermetically packaged in this sealed bag and heated with a microwave oven, the bag is not broken until the internal temperature thereof approaches 100.degree. C. and is slightly opened when the inside thereof is sufficiently filled with water vapor, thereby discharging water vapor, and the separation of the tape after the completion of heating permits the contents to be easily taken out.

BACKGROUND OF THE INVENTION 
The present invention relates to a sealed bag for heating food with a 
microwave oven, and more particularly, to a sealed bag for microwave 
heating in which the sealed state is maintained during distribution and 
storage thereof. The breakage thereof does not take place until the 
internal temperature thereof approaches 100.degree. C. by microwave 
heating at that time, a central portion thereof is slightly opened in the 
state in which the inside thereof is sufficiently filled with water vapor, 
thereby discharging water vapor, and the separation of a tape after the 
completion of heating permits the contents to be removed. 
BACKGROUND OF THE INVENTION 
Previously, food has been packed in inner bags having ventilating 
perforations, and the inner bags have been sealed in outer bags to package 
cooked food for a few people. At the time of use, the inner bags are taken 
out of the outer bags, and then, heated in a microwave oven. In this case, 
the inner bags are heated on a tray, because the contents leak from the 
inner bags through the ventilating perforations. 
There have also been sealed bags for microwave heating in which non-fused 
areas narrow in width are opened in longitudinally or laterally fused 
areas and water vapor is released through the non-fused areas in heating 
them. 
Further, sealed bags for microwave heating have been known in which small 
perforations previously formed are covered with separative sheet pieces to 
keep the bags hermetic until heating. In this ease, the sheet pieces are 
separated just before heating in a microwave oven. 
Commercial microwave ovens employ heating systems by use of microwaves, and 
particularly, heating is well performed through water. Food contains much 
water, and is heated from the inside thereof. The surface thereof is 
cooled and not elevated in high temperature because the latent heat of 
vaporization is carried away. 
When the bag is sealed, the inside thereof is filled with water vapor and 
water is prevented from being evaporated from the surface thereof. The 
surface temperature is therefore elevated. In this method, however, the 
internal pressure is increased too high, resulting in abrupt breakage of 
the bag, namely breakage of the bag like an explosion. This method is 
therefore dangerous. 
The present invention provide a technique for keeping a bag hermetic during 
distribution and storage thereof, maintaining the bag hermetic in early 
stages even in heating it with a microwave oven to sufficiently elevate 
the internal temperature, and naturally breaking the bag at a safe time. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a sealed bag for 
microwave heating composed of a film having a fusing layer inside, said 
bag being sealed by fusing both sides of said film longitudinally with 
each other and both ends thereof laterally, in which both sides of the 
film are turned outside to form two rows of turned-up faces, a turned-up 
line formed by said two rows of turned-up faces is covered with an easily 
openable fusing tape having an easily openable film inside and reinforced 
with a base material outside, said tape being fused with the turned-up 
faces of the film, and at least one end of the fusing tape not fused by a 
length which can be taken with fingers is left as a pick-up flap in the 
laterally fused area. Further, the present invention is characterized in 
that the peel strength of the easily openable tape from the film of the 
bag is 100 g/15 mm to 1,500 g/15 mm, and that the easily openable fusing 
tape is fused with the respective turned-up faces over a width of 1 mm to 
15 mm. 
According to the present invention, the sealed state of the bag is 
maintained during distribution and storage thereof and until the sealed 
bag is filled with water vapor to inflate it like a balloon, even when 
heating it with a microwave oven. A part of the bag is gently opened to 
gradually release water vapor when the internal pressure reaches a 
specified value. 
Accordingly, there is no fear of contaminating the contents during 
distribution and storage, and the bag is maintained in the sealed state in 
the early stages of heating, so that the inside of the bag is filled with 
heated water vapor. It becomes therefore possible to heat the contents to 
a fresh state as newly made food.

DETAILED DESCRIPTION OF THE INVENTION 
Base films used in the sealed bags of the present invention are films of 
various plastics such as polypropylene, polyesters, polyethylene and 
nylon, laminated products thereof and products thereof laminated with 
aluminum evaporation layers, paper or polyvinylidene chloride layers. 
These base films have heat fusible layers inside with a low melting point, 
such as low-density polyethylene and ethylene-vinyl acetate copolymers. 
When the base films are films which are heat fusible themselves, such as 
polyethylene films and polypropylene films, they may be monolayer films 
having no additional heat fusible layers. 
The longitudinally fused area used in the present invention is a fused area 
formed in a longitudinal direction of the film forming the bag, and the 
laterally fused area is a fused area formed in a lateral direction of the 
film forming the bag, independently of the ratio of lengths of the 
completed bag. 
In the present invention, easily openable fusing films generally used 
uncovers in tops of containers for jellies and yogurt are employed as the 
fusing tapes. The easily openable films are widely commercially available, 
and include VMX 22RX manufactured by Mitsubishi Chemical Corporation, CMPS 
017C manufactured by Tosero Co. Ltd. and TAF 610C manufactured by Tosero 
Co. Ltd. 
The easily openable fusing films are used by backing them with films of 
polyesters, nylon, polypropylene and the like, and cutting them to the 
form of the fusing tape. The easily openable fusing films may be backed 
after cutting to the tape form. 
The bag is in the so-called pillow form in which one longitudinally fused 
area is formed on one face of the bag. The longitudinally fused area may 
be formed in a central portion on one face of the bag as shown in FIG. 1 
or shifted to an edge on one face of the bag. Further, a gusset type bag 
having gusset folds on both edges can also be used. In this case, the bag 
can be filled with the contents in large. As shown in FIG. 4 and FIG. 5, 
it is also possible to form one longitudinally fused area in one side 
portion of the bag. When the longitudinally fused area is formed in the 
side portion, the internal temperature is higher when heating than when 
the area is formed in one face of the bag. 
Prior to the preparation of the sealed bag of the present invention, both 
longitudinal side edges of the raw material film are brought together in a 
cylindrical form, and folded back over to form turned-up faces. As shown 
in FIG. 1, both edges of the turned-up face are a turned-up line 1 which 
is a fold line, and a side edge line 2 of the turned-up face which is a 
cut line of the film. The width of the turned-up face is preferably 5 mm 
to 20 mm. 
Two turned-up lines 1 are preferably in contact with each other. However, 
they are not necessarily in contact with each other. Some clearance 
between them or some overlapping of them may be allowed, as long as they 
do not come in contact with a longitudinally fused area 4 described later. 
A fusing tape 3 of the present invention is fused on top of the two 
turned-up faces. As the fusing tape 3, an easily openable tape having a 
peel strength of 100 g/15 mm to 1,500 g/15 mm, preferably 200 g/15 mm to 
1,000 g/15 mm, is used. Even when a fusing tape 3 having a low peel 
strength is used, an increase the width of the bag when is fused in it 
being difficult to open the bag by hand. The fused width is preferably 1 
mm to 15 mm, and more preferably 2 mm to 10 mm, per one turned-up face. 
The reference numeral 4 designates the longitudinally fused area, and the 
reference numeral 5 designates the turned-up faces. 
Laterally fused areas 6 are formed in both end portions, crossing at right 
angles to the longitudinally fused area of the bag. The inside of the tape 
is fused with the turned-up faces of the film, leaving a portion 7 which 
is not fused and is of a length easily grasped with fingers, namely 6 mm 
to 15 mm, as a pick-up flap positioned at a least one laterally fused 
area. The portion 7, which is not fused may be formed on the whole surface 
of the laterally fused area. However, when the portion 7, which is not 
fused is under, it results in the use of the film in large amounts, and 
causes the disadvantage that the Actual bag volume is decreased, though 
the apparent bag volume is seemingly increased. 
Then, as shown in FIG. 1, an opening portion 8 having a width larger than 
the tape is formed, and the laterally fused area is curved in the opening 
portion 8. The wide portion 7 is not fused, but an inner laterally fused 
area 9 which is formed by the curve in the opening portion 8 is fused. 
Accordingly, the laterally fused area is continously sealed while forming 
the portion 7, which is not fused. However, it is curved in the form of a 
"U". 
The wide opening portion 8 is outside the inner laterally fused area 9, and 
the fusing tape 3 is not fused in this site and, instead, forms a pick-up 
flap 10. 
In some cases, even when both extended portions of the inner laterally 
fused area 9 are linearly fused and areas between the laterally fused area 
6 and extended fused portions are widely fused, a similar effect is 
obtained. 
Further, both of the laterally fused areas can also be provided with the 
pick-up flaps 10. 
The pick-up flap 10 has a length of 6 mm to 15 mm, so that it can be easily 
grasped by the fingers. When the pick-up flap 10 is grasped by the fingers 
and pulled, one side of the bag can be opened all at once and the contents 
can be easily discharged from the bag. 
In making the bags, a continuous bag making and filling method can be 
employed. After longitudinal fusing and filling of the contents, lateral 
fusing and cutting are performed at the same time, thereby continuously 
producing the sealed bags, which can be distributed,and stored without 
fear that the contents will leak or the the bag will break. 
In the present invention, not only the sealing property, but also the easy 
openability of the bag is important. The bag is required to be opened by 
grasping the pick-up flap with fingers and pulling it. Even when an easily 
openable film is used, overall fusing impairs the easy openability. In 
general, the width of a heat bar used for fusing is 1 mm to 15 mm, and 
preferably 2 mm to 10 mm. 
When one longitudinally fused area is formed in one side portion of the 
bag, as shown in FIG. 4, the longitudinally fused area is formed as 
follows. The film used to make the bag is folded along center line and the 
both side edge lines 2 are mated so that they seem to be one line. A 
fusing tape 3 is folded along the center line facing the fusing layer 
outside. The folded fusing tape 3 is inserted between the folded side edge 
lines 2, leaving the both edges of the folded fusing tape 3 slightly. 
Then, longitudinally fused area 4 is formed by fusing between turned-up 
line 1 and side edge line 2. And finally laterally fused areas are fused 
in the same way as described in the bags of FIG. 1. 
The two turned-up faces 5 are overlapped and laterally fused in thus 
produced bag. Generally, an outer surface material of a fusing tape 3 is 
not heat fusible, so that the turned-up faces 5 are not fused with each 
other. A main body of the bag and two turned-up faces 5 form a Y, centered 
on the turned-up lines 1, in a cross sectional view of the bag, and the 
fusing tape 3 is fused on the forked turned-up faces in the V form to form 
a longitudinally fused area 4. In this case, almost all of the fusing tape 
3 is not exposed, so that there is no fear of separation of the fusing 
tape by rubbing during distribution. Further, the appearance of the bag 
itself is also improved. 
Even such a bag is broken at the longitudinally fused area when the 
internal pressure is sufficiently increased by heating it in a microwave 
oven. When a large amount of liquid is not contained in the contents, the 
sealed bag of this type can also be used. 
At the time of use, the bag is placed in a microwave oven, with the face 
having the longitudinal fused area facing upward, and heated by the 
ordinary heating method. In the microwave oven, food is heated from a 
center portion thereof, and when a surface portion thereof is heated, 
water vapor is produced to inflate the bag like a balloon. When the bag is 
sufficiently inflated, a central portion of the fusing tape 3 is separated 
producing a faint explosive sound to break the bag. Then, water vapor is 
appropriately discharged through the broken portion of a lower face of the 
tape, and the inside thereof is sufficiently heated. After the completion 
of heating, the pick-up flap 10 is pulled to separate the fusing tape 3 
all at once, thereby enabling the bag to be widely opened. 
In the sealed bag of the present invention, end portions of the film are 
turned outside in the longitudinally fused area to form the turned-up 
faces, and the tape having a relatively low separation strength is 
narrowly fused on the turned-up faces. According to this sealing method, 
the bag can be maintained sufficiently hermetic during distribution and 
storage. When the bag is heated in a microwave oven as such, the inside 
thereof is filled with water vapor because of its hermetic sealing. 
Accordingly, the latent heat of vaporization is not carried away from the 
surface of the food, resulting in an increase in internal temperature to 
98 to 99.degree. C. or more, and an increase in internal pressure at the 
same time. 
On the other hand, in the sealed bag of the present invention in which one 
longitudinally fused area is formed in one side portion thereof, both end 
portions of the film are turned outside to form the turned-up faces, and 
the tape having a relatively low separation strength is narrowly fused on 
the turned-up faces. 
Accordingly, when the internal pressure is increased, the power of widening 
the inner diameter of the bag is applied to the bag. In the present 
invention, the end portions of the film are turned outside in the 
longitudinally fused area. The bag is therefore widened so as to narrow 
the turned-up faces. Further, the fused face is separated because of its 
low separation strength, finally leading to development of an opening. A 
site most liable to receive the internal pressure is generally the central 
portion of the bag, so that the central portion is broken to discharge 
water vapor. However, the other sites are fused, and the opening is also 
covered with the tape. Accordingly, water vapor is appropriately 
discharged, and the surface temperature is not lowered so much. 
That is, the bag of the present invention is maintained sufficiently 
hermetic during distribution and storage and in early stages of heating 
with a microwave oven, and a part of the bag is only gently broken when 
the internal pressure reaches a specified value. 
EXAMPLE 1 
A sealed bag for a microwave oven of the present invention having the shape 
shown in FIG. 1 was produced. Both sides of a 30 cm-wide film were each 
turned outside by 1 cm to form turned-up faces 5. Respective turned-up 
lines 1 were allowed to approach to each other, and a fusing tape was 
fused on both the turned-up faces 5, thereby turning the film to the 
cylindrical form. In longitudinal fusing, a roughly central portion of 
each turned-up face 5 was heated with a heat bar having a width of 3 mm. 
One end of this cylindrical film is linearly laterally fused leaving a 
slight portion not fused outside, and the other end was fused by a 
laterally fused area 6 curved in the U form so as to leave an opening 
portion 8. The opening portion 8 was formed in a position in which it was 
not in contact with a fusing tape 3. Accordingly, a pick-up flap 10 was in 
a separated state. Concurrently with lateral fusing, the bag was cut to a 
length of 22 cm, and filled with two small-sized Chinese buns 11. When the 
bag was heated in a microwave oven, either one of the sites indicated by 
the arrows in FIG. 1 was opened. 
In this example, the longitudinally fused area was formed in a central 
portion on one face of the bag, and a material in which an inner surface 
of a 15 .mu.m-thick nylon film dry laminated with a 50 .mu.m-thick linear 
low-density polyethylene film was used as a material for the bag. A 18 
mm-wide fusing tape in which an inner surface of a 16 .mu.m-thick 
polyester film was laminated with a 50 .mu.m-thick easily openable film 
was used as the fusing tape 3. As the easily openable film, CMPS 017C 
manufactured by Tosero Co. Ltd. was used. 
When heated in a microwave oven, the bag was kept for a while in the state 
shown in FIG. 2(a). However, the bag began to inflate gradually to narrow 
the width of the turned-up faces as shown in FIG. 2(b). When inflated like 
a fully inflated balloon, the bag was broken at one of the fused areas 6 
as shown in FIG. 2(c), and the discharge of water vapor started. However, 
the discharge of water vapor was not so violent that the bag was kept 
inflated as shown in FIG. 3., because the tape was present on the opening 
portion as before. The reference numeral 12 designates the water vaper 
filled in the bag. 
After the completion of heating, the bag could be easily opened by taking 
the pick-up flap 10 with fingers and pulling it. Further, the peel 
strength of the fusing tape was decreased by heating, so that the whole 
bag could be opened only by pulling the broken site. Chinese buns do not 
contain much water and crumble when heated in a microwave oven. However, 
when the Chinese buns 11 sealed in the bag of the present invention were 
heated, water was sufficiently supplied to the whole to provide wet ones 
similarly to the case that they were heated by use of a steamer. The 
reference numeral 12 designates water vapor. 
Frozen food and chilled food which can be eaten as such by microwave oven 
heating can be packaged in the bag of the present invention. Examples of 
such food include hamburgers, prawn chili sauce, sweet-and-sour pork, meat 
dumplings, various cooked foods, shao-mais, omelets, spaghetti, pilaf, 
various lunches and dried fishes, as well as Chinese buns. 
When the contents are taken out of the bag without use of a microwave oven, 
the bag is easily opened by pulling the pick-up flap 10 as such. 
EXAMPLE 2 
Using the same film and fusing tape 3 as with Example 1, two kinds of 
sealed bags for a microwave oven having a width of 167 mm, a length of 237 
mm and a maximum volume of 1100 ml were produced in the same manner as 
with Example 1. One had the shape shown in FIG. 1 in which a 
longitudinally fused area was formed in a central portion of the bag, and 
the other had the shape shown in FIG. 4 in which a longitudinally fused 
area was formed in a side portion of the bag. Each bag was filled with 300 
ml of water and sealed. The sealed bags were heated in a microwave oven, 
and the time from the start of heating to the breakage of the bags and the 
temperature on the breakage of the bags were measured. The measurements 
were repeated 5 times and the mean values thereof were determined for each 
bag. 
For the bag having the shape shown in FIG. 1 in which the longitudinally 
formed area was formed in the central portion, the time until the breakage 
of the bag was 3.59 seconds, and the internal temperature was 98.9.degree. 
C. On the other hand, for the bag having the shape shown in FIG. 4 in 
which the longitudinally fused area was formed in the side portion, time 
until the breakage of the bag was 4.06 seconds, and the internal 
temperature was 99.3.degree. C.