FIELD OF THE INVENTION
The present invention relates to a method of producing preserved
foods and, more particularly, to a method for performing cooking and sterilization
of food materials or foodstuffs, following packaging the same into vessels.
Up to now, there have been used various methods of producing preserved
foods. The most widely used methods are boiling and retorting methods. In the boiling
method, previously cooked foods are packed in vessels such as bags, trays or bottles,
and then sterilized by eating with hot water of 85 to 95 °C for 45 to 90 minutes.
The produces produced by the boiling process includes bottled foods, boiled wild
plant packs, seasoned and fried bean curds, and the like. In the retorting process,
food materials are heatead in a steam boiler under pressure at a temperature of
115 to 132 °C for 15 to 60 minutes. The products by retorting includes curry bags,
packed rice trays, canned goods, and the like.
However, these conventional methods involve heating of previously
cooked foods or food materials at a high temperature for a long period of time
to perform sterilization following or simultaneously with cooking. Thus, the previously
cocked foods or food materials lose taste thereof. In particular, if the food
materials contain protein substances and heated at a temperature of about 100 °C
and above for a long time of period, they are considerably denatured by heat. Further,
there is a fear of causing bacteriogenic secondary pollution during packaging since
the cooked foods are exposed to air after cooking. The products occasionally become
hard by vacuum packaging so that the preserved foods taken out of the bag have
to be broken into flakes.
In addition, the conventional methods are followed by discharge of
a large quantity of heat and/or steam into the working space, so that workers are
obliged to work under harsh conditions of a high temperature and a high humidity.
To solve these problems, the inventor has developed a method of producing
preserved foods, comprising the steps of vacuum packaging foods to be processed
with a packaging medium of a gas impermeable thermoplastic material which allows
infrared rays to pass therethrough, and irradiating infrared rays of wavelengths
ranging from 1.5 to 30 µm and a lower limit of wavelengths of 1.5 to 2.0 µm onto
the packaged food under the reduced pressure while maintaining an ambient temperature
of the package to 65 to 100 °C to cook and sterilize the food.
The above method makes it possible to improve the working environment
and tastes of the preserved foods since the foods are cooked and sterilized at
a lower temperature within a short time. However, the packaging medium or vessel
change the shape with increase of the degree of vacuum, causing depreciation of
the commercial value of the products. In addition, the products may change the
shape because of increase in the internal pressure of the packaging vessel at the
time of heating of the products.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method
of producing preserved foods, which makes it possible to prevent packaging vessels
from being deformed by residual air in the vessel during packaging as well as to
pasteurize and cock foods at a relatively low temperature while preventing the
foods from losing its taste.
The above and other objects are solved by decreasing the degree of
vacuum and replacing the residual air with inert gas to prevent packaging vessels
from deformation, and by applying infrared rays to the packaged food materials
or foodstuffs at relatively iow temperatures to perform cooking and sterilization
without causing loss of taste of the food materials or foodstuffs.
According to the present invention, there is provided a method of
producing preserved foods, which comprises the steps of:
- placing one or more food materials or foodstuffs in an open-topped packaging
vessel of synthetic resin;
- replacing air in the vessel with an inert gas;
- sealing an open-top of said packaging vessel with a sealing member; and
- applying infrared rays to the sealed packaging vessel to cook and sterilize
the food materials or foodstuff packaged in the vessel.
In a preferred embodiment of the present invention, the infrared
rays have wavelengths ranging from 1.5 to 30 µm and have a lower limit wavelength
of 1.5 to 2.5 µm. Preferably, the packaged food materials are treated by infrared
rays at an ambient temperature of not lower than 65 °C but not higher than 100
The method of the present invention may be applied to food materials
or foodstuffs such as, for example, fishes, shellfisches, meats, food plants and
The packaging vessel may be made of any known packaging materials
of plastics conventionally used for food packaging. In particular, it is preferred
to use synthetic resins such as, for example, polyolefines (e.g., polyethylene
(PE), polypropylene (PP)), polyamide, polyaminde 6, polyaminde 66, polyaminde 12,
polyaminde 11, polyaminde MDX 6, poly(ethylene terephthalate) (PET), poly(vinylidene
chloride) plastics (PVDC), polycarbonate (PC), polystylene (PS), cellulose acetate
(CA), cellulose acetate propionate(CAP), and cellulose acetate butyrate (CAB),
poly(vinyl alcohol) (PVA), poly(vinyl chloride) plastics(PVC), ethylene-vinyl
alcohol copolymer (EVCH), KOP, CPP, EVOP, and the like. Also, the material for
the vessel may be plastics containing a filler such as, for example, talc, calcium
carbonate, silica or the like.
A material for sealing materials and for bag-shaped vessels may be
one of single films, laminated films, coated films, metal-deposited films or other
composite films made of a synthetic resin mentioned above. Metals used for laminate
films or metal deposited films include aluminum, silver or the like. If the packaging
vessel is made of a laminated film or metal-deposited film, it is necessary to
provide one or more windows through which infrared rays can pass into the interior
of the packaging vessel.
As the inert gas, there may be employed any gas, which is stable
and is never activated by heating with infrared rays, but the most preferred inert
gas is nitrogen gas in view of food hygienics.
The method of the present invention may be carried out by use or
an infrared cooking machine disclosed in a specification of unexamined Japanese
utility model publication (Utility model application serial number 62-2230). However,
the method of the present invention may be carried out by use of any other cooking
devices provided that it can radiate infrared rays ranging from 1.5 to 33 µm.
The range of wavelength and lower limit of wavelength of the infrared rays depends
on the temperature of an infrared heater or an infrared heater. For this reason,
the infrared heater is generally set to a temperature within the range of 250 to
500 °C so that it generates infrared rays having a lower limit of wavelength falling
within the range of 1.5 to 2.5 µm.
The present invention will become more fully understood from the
detailed description given hereinbelow and the accompanying drawings which are
given by way of illustration only, and thus are not limitative of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
- Fig. 1 is a schematic diagram of an infrared cooking device used in a method
of producing preserved foods according to the present invention;
- Fig. 2 is a sectional view illustrating steps of packaging food materials in
a vessel; and
- Fig. 3 is a graph illustrating changes of temperatures of a sealed vessel containing
food materials when infrared rays are applied thereto.
As illustrated in Fig. 1, an infrared cooking device generally comprises
a body 1, plural infrared heaters 2 arranged in two rows parallel to upper and
lower walls of the body 1 at certain intervals, reflecting plates 3 each being
arranged on a backside of the infrared heater 2, and supporting means 4 for supporting
half-stuff 5 of food materials or foodstuff.
The heaters 2 are slidably mounted on frames of the body 1 to adjust
a distance between upper and lower rows of infrared heaters 2. The distance between
the upper and lower rows of infrared heaters 2 is generally set to 15 to 50 cm.
If the distance between the upper and lower heaters is less than 15 cm or exceeds
50 cm, the heaters have much effect on the packaging vessels and decrease in radiation
efficiency of infrared rays.
The infrared heaters 2 are so controlled as to keep them to a temperature
of 250 to 500 °C. If the temperature of the infrared heaters 2 is lower than 250
°C, it is difficult to generate infrared rays with desired wavelengths, in particular,
far infrared rays which enter into an interior of each food material or foodstuff
and directly heat the food material or foodstuff to be treated. If the temperature
exceeds 500 °C, the radiation energy of the heater increases, the lower wavelength
becomes considerably short, resulting in deformation of the package, or increase
of electric power consumption, which in turn causes failure in energy-saving due
to the use of infrared rays.
The heaters 2 maintained within the above temperature range radiate
infrared rays which have wavelengths of 1.5 to 30µm having the lower limit of wavelengths
of not less than about 1.5 µm but not more than 2.5 µm, thus making it possible
to cook and sterilize food materials effectively. The reflecting plates 3 are made
of a metal having a nigh reflectivity reflecting, such as aluminum, nickel or
The supporting means 4 comprises a pair of motor-driven chain belts
4a provided with a plurality of flat panels or trays and so arranged that the trays
moved along a middle plane of the body 1. Any other known supporting means may
be used for this purpose. For example, the supporting means 4 may be composed of
a wire netting, woven wires or the like and may be removably or fixedly arranged
in the body of the cooking device. Preferably, the supporting means such as wire
netting or trays have a non-bright surface or a dark surface coated with a black
Table 1 shows characteristics of the infrared heaters 2. The radiation
characteristics were measured at 296 °C, using, as a specimen, powdered radiating
of the infrared heaters 2, An infrared detecting means is separated by 20 cm from
Infrared quantities (W/cm2)
0.7 - 3.5
1.18 x 10-3
1.5 - 14
8.58 x 10-3
7 - 14
4.32 x 10-3
7 - 30
4.38 x 10-3
As can be seen from the result shown in Table 1, the radiation energy
of infrared rays with wavelengths within the range of 0.7 to 3.5 µm is 1.18 x 10-3
W/cm2, which is about 1/4 of the radiation energy (8.58 x 10-3
W/cm2) of infrared rays with wavelengths within the range of 1.5 to
14 µm. Thus, the radiation energy of infrared rays having wave lengths of less
than 1.5 µm is negligible and ineffective to cooking of the food materials or foodstuffs.
Further, the above infrared heaters 2 radiates infrared rays substantially within
the range of 1.5 to 30 µm under the conditions of about 300 °C.
As illustrated in Fig. 2, the packaged food 5 to be treated is prepared
by placing foodstuffs 7 and flavoring materials in a commercially available packaging
vessel or tray 6 made of PBP (polypropylene + EROH) as shown in Fig. 2 (a) in a
packaging machine, pre-sealing an opening of the charged vessel with a sealing
material such as a laminated film (PP + NY) as shown in Fig. (b), evacuating the
pre-sealed vessel of air in the packaging machine while introducing an inert gas
(e.g., nitrogen gas) into the packaging machine to replace the residual air with
an inert gas, and sealing the vessel hermetically with a sealing film 8 as shown
in Fig. 2(C).
The thus packaged foods 5 are put into the infrared cooking device
shown in Fig. 1 and exposed to infrared rays to perform cooking and sterilization
of the foodstuffs. By applying the infrared rays to the vessel, the foodstuff 7
in the vessel is directly heated and sterilized by infrared rays passing through
the packaging vessel 6 and sealing film 8.
Since the space in the vessel is charged with the inert gas to keep
its internal pressure equal to or slightly lower than the atmospheric pressure,
the vessel is prevented from deformation due to evacuation, which may occur in
the method of the prior art. Further, the air in the vessel has been replaced with
the inert gas so that the packaging vessel has an internal pressure equal to or
slightly lower than the atmospheric pressure, which makes it possible to avoid
deformation of the vessel during radiation of infrared rays as the inert gas prevent
the vessel from increase in internal pressure.
There were prepared packaging vessels (18cm in lenght, 12 cm in width,
2.5 cm in depth of PBP and a part coated film of a synthetic resin (i.e., biaxially
oriented polypropylene film partially coated with polyamide 6). A piece of raw
mackerel (100g) was put into a packaging vessel together with 85 g of flavoring
materials, and then hermetically sealed with heat after replacing air in the vessel
with nitrogen gas.
The sealed vessels were put into an infrared cooking device of Fig.
1 and placed on trays arranged in the middle of upper and lower rows of the heaters
spaced by 20 cm. The sealed vessels were then heated by infrared heaters maintained
at 320°C for 12 minutes. During irradiation of infrared rays, measurements were
made on temperatures of the center and flange of the vessel together with ambient
temperature. Results are shown in Fig. 3.
As can be seen from the results shown in Fig. 3, the center temperature
is risen to 80 °C after seven minutes and 20 seconds, while the flange temeprature
is 97 °C. After 12 minutes from the start of irradiation, the center temperature
is risen to 115 °C while the flange temeprature was rised to 98,5 °C. A bacteriological
examination after heating showed than coliform bacteria is 0 and than general
viable cell number is not more than 100 per gram. The vessel showed no deformation
during heating and even after heating.
It will be seen that according to the present invention, it is possible
to produce fully cooked, preserved foods under the above conditions by applying
infrared rays to the vessel or tray of which an inert gas is replaced for the remaining
air in the head space.
For comparison, some of the sealed vesseis prepared in the above
Example were heated with an electromagnetic oven (Power ouptut: 600W), but cooking
was never completed as the vessel was burst after 3 minutes.
As can be seen from the above, the present invention makes it possible
to produce preserved foods within a short time without causing secondary pollution
since food materials or foodstuffs are charged into packaging vessels together
with an inert gas and then immediately cooked and sterilized under the packaged
conditions. Further, the packaging vessels are prevented from deformation during
cooking since the cooking and sterilization of the food materials are carried out
by infrared rays at an ambient temperature of 65 to 100 °C and since the internal
spaces of the packaging vessels are charged with an inert gas. Thus, the products
can be sold at a store under warmed conditions by keeping them in an infrared
cocking device with a temperature of the above range.
According to the present invention, it is possible to prevent preserved
foods from hardening during production even if the products are in the form of
pouches. Further, since the foodstuffs are cooked and sterilized by infrared rays
directly, it is possible to use commercially available plastic vessels and films,
which in turn makes it possible to reduce the production cost of preserved foods.
Further, the cooking and sterilization are carried out by infrared
rays directly, it is possible to minimize the energy loss, as compared with convective
heating or conductive heating employing hot water or steam. In addition, the use
of infrared rays scarcely discharges heat or steam into the working atmosphere,
thus making it possible to prevent the working space from elevation of temperature
and humidity, which in turn makes it possible to improve working conditions.