US3692616A - Method and apparatus for heat treatment of packaged products - Google Patents

Abstract

The packaged product is placed in a container and heated by passing heat-inducing waves through the container walls from an external source while the package is surrounded by a meium which increases in pressure in response to an internal pressure increase in the package, thereby providing an over-pressure around the package to prevent destruction thereof due to said internal pressure increase. A cooling medium under pressure is then directed into the container and caused to flow around the package and maintain said over-pressure until the pressure in the package decreases to a value which is harmless to the package.

Description

P 19, 1972 L. A. STENSTROM 3,

METHOD AND APPARATUS FOR HEAT TREATMENT OF PACKAGED PRODUCTS Filed April 27, 1970 INVENTOR. Lumen Ammo STENSTR 0M QWLAWL, BL

'g i d pStates Patent O1 ice 3,692,616 Patented Sept. 19, 1972 vs. Cl. 161-: 2 Claims I ABSTRACT OF THE DISCLOSURE The packaged product is placed in a container and heated by passing heat-inducing waves through the container walls from an external source while the package is surrounded by a medium which increases in pressure in response to an internal pressure increase in the package, thereby providing an over-pressure around the package to prevent destruction thereof due to said internal pressure increase. A cooling medium under pressure is then directed into the container and caused to flow around the package and maintain said over-pressure until the pressure in the package decreases to a value which is harmless to the package.

THE DISCLOSURE The present invention relates to a method and apparatus for use in heat treating a packaged product in a field of heat-inducing waves, such as an electromagnetic or acoustic field.

Heat treatment of nutritional products in an electromagnetic or acoustic field offers great advantages, as it can take place very quickly. This is especially valuable, for example, in connection with sterilization of nutritional products, where discoloration, deterioration in taste and other impairments in quality can occur if the product is maintained at too high a temperature for too long a time. Heat energy is generated immediately all 7 through the product in a so-called micro-wave heat treatment. This avoids the unfavorable temperature distribution which is the result of heating in a conventional way, that is, a high temperature on the surface of the product and a low temperature in its interior, for such volume heating, micro-waves are usually employed; but for special media with a large acoustic absorption and a small dielectric dissipation factor, acoustic waves can be used instead. 1

However, many problems remain unsolved in connection with micro-wave heating'to high temperatures. When a product is heated, vapors are produced, especially steam; and if the heat treatment takes place at temperatures considerably higher than 100 C., the vapor pressure in the product will be so high that the product may burst. In order to prevent this, the product must be surrounded by over-pressure. Attempts have been made to heat treat nutritional products in open packages and let these pass through a heat treatment tunnel containing compressed air, steam under pressure, or both these media. Such a tunnel, however, must be equipped with lock arrangements both at its entrance and exit and must contain both a heating and a cooling section (possibly also a heat holding section), as the over-pressure around the products must be maintained until the products are cooled down. If not, the internal vapor pressure will burst the products. It is also a problem that vapor from a heated product will be condensed on a product not yet heat treated or on a partly cooled product, or on the walls of the heat treatment equipment. Sterilization in an open package also has the great disadvantage that both the cooling of the product and the sealing of its package must be done aseptically.

Electro-magnetic heat treatment of products can also be performed with the products enclosed in packages. However, this brings about other problems. The package material must then be pressure-strong, cheap and chemically resistant to the enclosed product at the applied treatment temperature. Packages of sheet material cannot be used, since they do not allow the electro-magnetic waves to pass through them, the waves being reflected against the sheet instead (this is so also for acoustic waves). Packages of glass or of a strong heat-resistant plastic will be very expensive, as they must be made very thick in order to resist the internal over-pressure, and they are consequently suitable only for special products, such as pharmaceutical preparations. A thick-walled package also has the disadvantage that it makes a rapid cooling of the enclosed product impossible, whereby the product can be impaired in quality. Moreover, it is difficult to open a thick-walled package.

The present invention provides a simple and inexpensive solution of the above-noted problem encountered in using an electro-magnetic or acoustic field for heat treatment of nutritional products of different kinds.

According to the invention, heat treatment of a product enclosed in a package is effected in an electro-magnetic or acoustic field by placing the product in a container having walls which are transparent to the electro-magnetic or acoustic waves. The method is characterized in that the package placed in the container is surrounded by a medium which, during the heating stage, prevents the package from being destroyed due to a resulting pressure increase within the same, and that a cooling medium under pressure is directed into the container and caused to [flow around the package and maintain the over-pressure created during the heating stage until the pressure within the package decreases to a value which is harmless to the package.

The apparatus for use in carrying out the method of the invention comprises a container with walls as previously described. The container is movable in relation to the electro-magnetic or acoustic field and forms a chamber for the package and for a surrounding medium which will prevent the package from being destroyed due to the previously mentioned internal pressure increase, the chamber having an inlet through which a cooling medium under pressure can be directed and having a normally closed outlet valve operable to open only at a pre-determined over-pressure within the container.

In a heat treatment according to the invention, a product enclosed in a gas-tight and liquid-tight package of very thin material is placed in the container and is surrounded by the mediumwhich prevents the package from being destroyed duirng the subsequent heating stage, which is critical for the package. The container is moved through an electro-magnctic or acoustic field, whereby the product is heat treated in the desired degree. The packaged product is then cooled while it remains in the container. The cooling is eifected by directing a cooling medium into the container and preferably causing it to flow through the container while maintaining an over-pressure around the package in the container, until the product has been cooled sufficiently for the pressure within the package to decrease to a value which is harmless to the same.

Products can be heat treated in order to be sterilized according to the invention. However, the heat treatment can also aim at an inactivation of enzymes in the product so that the taste of the product will remain unchanged or so that a maturing process, for example cheese-making. will be interrupted.

Products which can be heat treated to advantage according to the invention are provisions or other nutritionals for man and animal; but other products such as nutrient substrata for microcultures for production or research can also be treated in this way.

When heat treating according to the invention, it is a great advantage that the treatment container does not need to have the same form as the product to be heat treated or as the package in which the product is enclosed. This is especially important when heat treating products which are irregularly shaped and/or irregularly shaped among themselves or of diiferent sizes.

The invention also has the advantage that it enables an exceedingly even heating of the product. It is not only the mobility of the container (for example, its quality of being rotatable in the actual magnetic or acoustic field) that contributes to this but also the fact that the medium situated outside the package in the container, and which is preferably a liquid or a vaporized liquid, can cool the warmest parts and warm the coolest parts of the package.

Another advantage of the invention is that a very rapid cooling of the heated product is possible. The cooling medium directed to the container will come into direct contact with the entire outside of the package. A suitable packing material is a thin plastic with a sufficient thermal and chemical resistance.

The packaged products placed in containers according to the invention may be transported through a heating tunnel which extends through an electro-magnetic field. By the relative movement between the container and the electro-magnetic field, the effects of possible differences in the field strength are eliminated. The products can be cooled thereafter in the manner described above.

Due to the relatively high pressure created in the container during the heating, the container must be resistant to pressure. Therefore, the container is shaped as a cylindrical casing which is strong and permanent in shape. A suitable material for the container is plastic, for example, plastic reinforced with glass fibers.

The medium which surrounds the package during the heating stage is not necessarily a liquid but can consist of a gas or of a gas/liquid mixture. This medium preferably has the same electro-magnetic or acoustic absorption qualities as the product to be heat treated, as in this way the surface temperature of the product can be made to correspond to the temperature in the interior of the product. Further, the advantage is achieved that even if the product has a varying thickness, the electro-magnetic or acoustic waves must penetrate a mass of equal thickness, which is homogeneous in respect of absorption qualities. This mass consists of the product and the surrounding medium in the container. There is then no risk that different parts of the product will be unequally heated. The micro-wave absorption of a sodium chloride solution varies with the concentration of sodium chloride. Such a solution can easily be given absorption qualities which correspond to the qualities in the heat treated product.

In the following, an example of an apparatus for use in performing the method according to the invention will be described with reference to the attached drawing, in which the single illustration is a longitudinal sectional view of the example.

In the drawing, there is shown a container 1 with a detachable end wall 2 closing one end of the container. The end wall 2 is secured to the container 1 by a socalled bayonet mounting (not shown), but other kinds of locking arrangements such as a spring lock can be used instead. When the end wall 2 is secured to the container 1, there is formed an hermetically closed chamber 3 which constitutes the treating chamber for one or more packaged products. Two such packages 4 hermetically packed in thin plastic film are shown in the drawing, placed in the treatment chamber 3.

The detachable end wall 2 and the opposite undetachable wall 5 of the container have openings 6 and 7, re-

spectively. These openings 6 and 7 are closed by valves 8 and 9, respectively, which are shown schematically in the drawing. The valve 8 in the opening 6 is constructed so that it is normally closed but will unblock the opening 6 in response to a pre-determined over-pressure within the container 1. The valve 9 in the opening 7 is constructed in a similar way except that it is opened when a medium from outside the container 1 is forced into the opening 7 with a pre-determined pressure.

In the use of the illustrated apparatus, one or more packaged products 4 to be heat treated are placed in the container 1, which is thereafter closed by means of the end wall 2. The products are surrounded by liquid, wholly or partially, which is introduced into the container 1 either before the end wall 2 is applied or thereafter through the opening 7. The container is then moved in or through an electro-magnetic or acoustic field so that the packaged products 4 are rapidly heated to a desired temperature. This creates a greatly increased vapor pressure in the products, which results in an over-pressure within the packages. This over-pressure, however, is counteracted by the pressure acting on the package from the outside and which is caused by the liquid present in the container 1. This liquid is also heated (partly vaporized) in the electro-magnetic or acoustic field.

When the products have been heated to a desired degree, they are cooled by directing a cooling medium under over-pressure to the container 1 through the opening 7. The cooling medium has such an over-pressure that it flows around the packaged products and overcomes the closing force on the valve 8 so that it discharges from the container through the opening 6. The force tending to close the valve 8 insures that a pre-determined overpressure is maintained in the container 1 while the cooling medium flows through the same. This over-pressure must be maintained until the products are so cool that their internal vapor pressure no longer can' endanger the durability of the packages. Only then can the packaged products be removed from'the container 1. When the invention is utilized in a larger scale, many containers of the kind described are connected together to an endless band and moved along a track, such as a horizontal track, each of the containers passing different stations where the different working operations are performed automatically.

I claim:

1. In the heat treatment of a product enclosed in a package, wherein the packaged product is placed in a container and subjected to a heating stage by passing heat-inducing waves through the container walls from an external source, the improvement'which comprises surrounding the package in the container, during said heating stage, by a first medium which increases in pressure in response to an internal pressure increase in the package, thereby providing an over-pressure around the package to prevent destruction thereof due to said internal pressure increase, and then, while maintaining said overpressure, directing a cooling medium under pressure into the container and causing said cooling medium to fiow around the package during discontinuance of said heating stage, until the pressure in the package decreases to a value which is harmless to the package.

2. The improvement of claim 1, in which said cooling medium is directed into the container while expelling said first medium from the container.

References Cited UNITED STATES PATENTS 3,536,129 10/1970 White "-30 CHARLES SUPALO, Primary Examiner US. Cl. X.R.' 165-17, 30

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