Method for uniform heating of a foodstuff for preservation and apparatus therefor

A foodstuff is located in a container open at its top end in which ambient gas (air) occupies the space inside the container above the product. The container is immersed into a liquid having a low dielectric loss factor, such as water, to such an extent that the liquid level is somewhat below the upper side of the product in the container. Using electrodes located to either side of the container, there is passed through the container from one side thereof to the other a first high-frequency electromagnetic alternating field having a respective first frequency such that the penetration depth of said first field is at least as great as the width of the container. From the open end of the container at which the air filled portion thereof is located, there is passed into the container through the air and into the adjoining portion of the product a second high-frequency electromagnetic alternating field having a respective second frequency such that the penetration depth of the second field is small compared to the penetration depth of the first field.

BACKGROUND OF THE INVENTION 
The invention relates to a process for uniform heating, in particular for 
the purpose of preserving, of organic products (e.g. foodstuffs which are 
filled into a container or a plurality of containers which are permeable 
for alternating electromagnetic fields, the container or containers being 
introduced, up to slightly below the level of the top surface of the 
foodstuff in the container, into a liquid having a suitable dielectric 
constant and having a low loss factor tan .delta. and an adjustable 
temperature, whereupon the containers and the foodstuff present therein 
are exposed to an electromagnetic high-frequency field which acts on the 
foodstuff from opposite sides through the liquid and through the walls of 
the container or containers, the containers and the foodstuff present 
therein being irradiated from above by an electromagnetic 
ultrahigh-frequency field. Such process has been described in U.S. Pat. 
No. 3,974,355, which requires that the containers are closed at the top, 
the space above the top level of the goods being filled with a gas, e.g. 
air. 
During uniform heating, in particular for stabilizing or preserving 
water-containing foodstuffs, problems arise especially if the foodstuff is 
not completely homogeneous with respect to the dielectric loss factor tan 
.delta., such as is approximately the case with yoghurt. Further problems 
arise if the foodstuff has been introduced for the said treatment in 
packages which are not completely closed. 
The above-mentioned prlbems arise in particular in the treatment (whenever 
"treatment" is mentioned this is always to be understood as a uniform 
heating, in particular for stabilizing or preserving water-containing 
foodstuffs) of jams which still contain whole fruit which, during the 
treatment, are to be preserved more or less completely as whole fruit. The 
deviations in the tan .delta. values are to be understood, still in the 
above example, in the sense that in a jam the dielectric loss factor tan 
.delta. of fruit juice with added sugar differs from that of fruit pieces 
or entire fruit also present in the jam. 
In the conventional heat preservation of foodstuffs by using infrared 
radiation or contact heat, a deterioration in the quality of the 
peripheral layers of the goods is almost unavoidable, in the case of 
packaged foodstuff, as the result of heating to excessive temperatures or 
for excessive periods. Unpackaged foodstuffs, however, can be brought more 
or less accurately to the desired average temperature level, and kept 
there, by constant stirring during the heating process. This advantage is, 
however, frequently accompanied by the drawback that the mechanically less 
resistant components of the foodstuff, for example strawberries in a jam, 
are damaged or even pulped during stirring. The more viscous or pasty the 
foodstuff, the more this drawback manifests itself. 
Since however, the quality of a jam is assessed, e.g., by the number of 
largely undamaged fruit contained therein, stirring can easily entail a 
significant reduction in the market value of the jam. 
The fruit preserve industry is thus continually searching for a balance 
between the damage due to overheating and the damage due to stirring. A 
further problem of this industry is the filling of the treated, sterilized 
foodstuff into small containers without the risk of reinfection with 
bacteria or bacterial spores during the filling process if, to save costs, 
the foodstuff had been prepared and stabilized in containers holding large 
quantities. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a primary object of the invention to extend the scope of 
applicability of the process described in U.S. Pat. No. 3,974,355 and, in 
particular, to make it possible to treat foodstuffs by this process even 
in larger batches. It is also an object of the invention to eliminate the 
above-mentioned drawbacks which result from overheating of individual 
portions of the foodstuff or from the necessity of stirring. In 
particular, it is an object of the invention to reliably stabilize jams, 
marmalades or jellies, such as are to be used, for example, for admixture 
to dairy products, such as yoghurt without any damaging or crushing of 
whole fruit or relatively large pieces of fruit contained therein, during 
the treatment. 
According to the invention, this is achieved when, an organic preserve 
(fruit of all types and/or berries and optionally additives, admixed 
thereto, for the purpose of influencing their flavor, odor, appearance or 
maintaining quality and consistency of the foodstuff) is filled into 
containers with an open top end up to a level slightly below their upper 
rim and are then subjected to the treatment with the alternating 
electromagnetic fields. 
According to the invention, it is thus no longer necessary to enclose the 
foodstuffs in completely closed containers for treating them in 
alternating electromagnetic fields. It suffices to introduce the 
foodstuffs to be treated into containers with an open top. The influence 
of the high-frequency field acting from the side is limited upwards by the 
level of the liquid into which the containers are introduced. As already 
explained in detail in my U.S. Pat. No. 3,974,355, this is necessary in 
order to prevent flash-overs (undesirable discharges which may cause 
perforation of the container wall). For this purpose, the liquid level is 
kept below the top surface of the foodstuff so that there are no sudden 
changes of the dielectric constant in the field. In many cases, the top 
surface of the foodstuff to be treated will be uneven because of its 
viscous or pasty form, such as is the case, for example, with jams and 
marmalades. The ultrahigh-frequency field acting from above serves to 
treat this topmost layer of the foodstuff. The treatment of the foodstuff 
can be carried out in containers having an open top as, due to the syrupy 
or pasty consistency of the goods, there is no danger of mixing top and 
intermediate or bottom layers after the treatment. Thus, an infection of 
the parts of the foodstuff which are not open to the surroundings also 
cannot take place. This is especially the case if the foodstuffs to be 
treated are sugar-containing marmalades, jams or gellies which, after 
stabilization or preservation, will keep stable in any case, after the 
treatment, due to their inherent sugar content. In the case that fruit of 
all types and/or berries to be treated are initially present in the frozen 
state, it is advantageous to raise the temperature of the foodstuff first 
to ambient temperature by other types of heating, for example by adding 
heated syrup. The foodstuff is then subjected to the treatment according 
to the invention only after it has been brought to ambient temperatures 
uniformly throughout the foodstuff. 
With regard to the type of liquid, its dielectric constant, its dielectric 
loss factor and its temperature, the description thereof in U.S. Pat. No. 
3,974,355 is incorporated in the instant application by reference. 
Preferably, the frequency of the high-frequency field is provided 
sufficiently low so that, for the foodstuff to be treated, the depth of 
penetration of the field in a direction approximately perpendicular to the 
electrodes is greater than the width of the container in that direction 
(first alternating field). This is important in order to achieve uniform 
heating through the entire thickness of the foodstuff to be treated 
between the two high-frequency electrodes. If necessary, additional 
electrical measures can be taken in order to provide, by an appropriate 
arrangement of the electrodes and the surroundings, that a uniform 
quantity of electrical energy is converted into heat at every point in the 
foodstuff to be treated between the high-frequency electrodes. 
The frequency of the ultrahigh-frequency (second) field is selected to be 
substantially higher than that of the (first) high-frequency field. This 
has the advantage that, on the one hand, the depth of penetration of the 
ultrahigh-frequency field is essentially restricted to that part of the 
foodstuff which is not covered by the high-frequency field and, on the 
other hand, use is still made of the advantage of ultrahigh-frequency 
fields that virtually no flash-overs occur, such as usually take place 
under non-optimum conditions with high-frequency fields of the high field 
strengths required here for the arrangement described. 
A particular difficulty in treating foodstuffs results if the upper surface 
thereof is very uneven due to relatively large undamaged fruits which 
partly protrude from the surface or due to the very viscous consistency of 
the foodstuff. In any case, it is necessary to prevent the presence of air 
gaps or gas gaps in the path of the high-frequency field, the upward 
extent of which is determined by the height of the liquid level, since 
these gaps give rise to undesired discharges. If the surface of the 
foodstuff to be treated is very uneven, the liquid level must be kept 
sufficiently low so that it is still below the lowest upper surface 
regions of the foodstuff. Thus, considerable differences in height between 
the highest and lowest upper surface regions of the foodstuff can occur. 
If the differences between the highest and lowest surface regions of the 
foodstuff to be treated are relatively large, the frequency of the 
ultrahigh-frequency field must be reduced in order to ensure the greater 
depth of penetration, which is then necessary, down to the region treated 
by the high-frequency field (first field). 
Preferably, the containers for the foodstuff to be treated have at least 
approximately rectangular or cubic cross-sections. As a result of this, 
the layer thickness of the foodstuff to be treated is approximately equal 
everywhere between the high-frequency electrodes so that a uniform heating 
of the foodstuff during the treatment can be achieved. To achieve an 
automation of the high-frequency or ultrahigh-frequency energy supplied, 
it has proved advantageous to run the containers through the treatment 
zone not as discrete units but as a virtually uninterrupted chain of 
containers, virtually no free space being present between the containers 
as viewed in the conveying direction. The ideal in this respect is a 
virtually endless container which, for example, can be formed by an 
endless conveyor belt, the lateral regions of which are bent upwards in 
the region of the treatment zone so that the cross-section, transversely 
to the conveying direction, of the endless belt has approximately the 
shape of a U in the region in which it is loaded with foodstuff to be 
treated. 
To prevent a reinfection of the foodstuff being treated, as far as 
possible, the treatment is carried out in an atmosphere which is as low in 
bacteria as can practically be ensured. 
As already explained further above, sugar-containing marmalades, jams and 
jellies which have been stabilized by the process according to the 
invention are virtually free from any danger of perishing in regions which 
are not exposed to the ambient air. It is essentially the sugar content of 
these foodstuffs which maintains their food qualities stable. Marmalades, 
jams or jellies of this type are liable to be reinfected exclusively at 
the surface. If, after the treatment according to the invention, such 
marmalades, jams or jellies are filled into small packages, such as, for 
example, marmalades jars or plastic beakers, it is sufficient, after 
closing containers of this type with a lid or closure which is permeable 
for electromagnetic waves, to subject these small containers to an 
after-treatment with an ultrahigh-frequency field on their upper surface 
which is accessible to the air in the air-filled head space inside the 
container. In this way, the treatment of the foodstuff filled into small 
packages can be carried out very rapidly and economically. 
Small packages and their lids which are to be used in conjunction with the 
above-mentioned process and for which a synthetic plastics material is 
customarily chosen as the container material and lid material, are usually 
joined by heat-sealing or, less frequently, by welding. In the former 
case, a layer of sealing lacquer which joins the two parts to one another 
under a corresponding pressure and temperature is applied either to the 
rim of the container or to the lid. When using appropriately selected 
types of plastics material, welding can be accomplished in a similar 
manner--but of course without heat-sealing lacquer. 
The materials customarily selected for the lid, for the container and for 
the heat-sealing lacquer have only low dielectric losses and, for this 
reason, they are also only slightly warmed in the ultrahigh-frequency 
field radiated in from above. Difficulties result, however, from the fact 
that, due to the high filling speed conventional in industry, it is 
unavoidable that droplets or splashes of the product stick onto the rim of 
the container. The solid constituents thereof remain as residues which 
interfere with sealing and are aligned horizontally flat, at right angles 
to the ultrahigh-frequency irradiation, between the plastic material 
surfaces which are to be sealed to one another and disturb the contact of 
the heat-sealing lacquer with the two plastic material surfaces. 
In order to prevent reliably an inadmissibly strong heating in the zone of 
the seal seam between the lid and the container, caused by the irradiation 
with the ultrahigh-frequency field, but without affecting or even 
preventing the effectiveness of the process in heating the upper layers of 
the product or the air-filled headspace present in the container above its 
contents, another feature of the invention provides that the containers 
which preferably have the largest cross-section at their top side, in the 
zone of the opening of the container, always have a rim which is angled 
outwards and on each of which a lid is fixed, after the main treatment 
according to the invention, by means of hot-sealing or welding and that, 
at least for the duration of the subsequent after-treatment by 
ultrahigh-frequency irradiation, the zone in which the container and the 
lid are joined together is screened against the ultrahigh-frequency field. 
This selective screening, on the one hand, does not impede the desired 
heating of the product close to its surface, and of the headspace in the 
container as well as of the particles of product and the bacteria in this 
headspace, by the ultrahigh-frequency field but, on the other hand, 
undesired heating up of the region of the joint seam between the lid and 
the container, as a consequence of the ultrahigh-frequency irradiation, is 
effectively prevented by the selective screening of that region, whereby 
the stress resistance of the joint seam between the lid and the container 
remains unimpaired by the ultrahigh-frequency irradiation. 
Of course, the screening can be carried out in various ways but, according 
to the invention, it must always meet the two demands already mentioned, 
namely that, on the one hand, an ultrahigh-frequency irradiation of the 
joint seam present between the lid and the container is prevented and, on 
the other hand, the ultrahigh-frequency irradiation, coming from above, of 
the top surface of the product and of the headspace also enclosed in the 
container is not impeded. 
According to a preferred embodiment, screening is effected by applying to 
the lid a mask of relatively thin metal or a self-adhesive layer having a 
satisfactory radiation-absorbing metal content. This can be accomplished 
either by applying a metal foil or the like, in the form of a 
corresponding template which only covers the joint seam itself, to the lid 
or between the layers of a lid composed of several films, or it is also 
possible to provide the lid in the zone of the joint seam on its top 
surface or between the lid layers with a metal-containing coating. in a 
preferred mode, the metal-containing layer can be printed on, in which 
case several successive printing steps may be necessary in order to 
achieve the metal content which is required for the aforesaid purpose. For 
example, printing can be carried out as colour printing using a metal 
powder--preferably aluminum--as the pigment, the metal content of the 
imprint and hence the degree of screening being adjusted by multiple 
printing. 
The process according to the invention can be automated by measuring the 
temperature of the foodstuff to be treated immediately before and after 
the treatment with the electromagnetic fields and automatically 
determining the field energy to be supplied as a function of the 
difference between the two values and the difference between the actual 
value and the set value of the temperature after the treatment with the 
alternating electromagnetic fields, and automatically supplying the 
corresponding amount of field energy.

DETAILED DESCRIPTION OF THE EMBODIMENT SHOWN IN THE DRAWING 
A tank or trough 1 is filled, up to a level 2a, with a liquid 2, for 
example water, having an adjusted dielectric constant .epsilon. and having 
a low loss factor tan .delta.. Water having these properties can be 
obtained, for example, by careful purification. In the tank or trough 1, 
two guide elements 3 constituting channels are provided, each of which 
forms a guide for a successive row of containers 4, which are open at the 
top. Each row of containers 4, only one of which per row is shown in the 
drawing in cross-sectional view, is moved through its respective channel 3 
in a direction perpendicular to the plane of the drawing. Each container 
is charged up to a level 5a with foodstuff 5 to be treated therein. The 
walls of the channels 3 and likewise the walls of the containers 4 are 
manufactured from a material which is permeable to alternating 
electromagnetic fields and has a low dielectric loss factor tan .delta.; 
these walls are manufactured preferably from a suitable synthetic plastics 
material. Polytetrafluoroethylene, e.g. the product marketed as Teflon or 
Hostaflon is a suitable material for this purpose. 
Each of the containers 4 has the shape of an oblong, and in each row of 
containers 4 passing through the respective channel 3, the containers are 
in contact with one another, the rear end face of a preceding container 
being in contact with the forward end face of the next following 
container, so that virtually no free space remains between the frontal 
faces of successive containers. In practice, each container may be, for 
instance, 30 cm wide, 25 cm high and 45 cm long. 
In the walls of the channels 3, perforations (not shown) are provided which 
permit an exchange of the liquid 2 in the trough 1. This design of the 
channels 3 virtually completely eliminates the formation of undesired 
waves in the trough 1. 
Adjacent to the channels 3, high-frequency electrodes 6 and 7 are located 
in the liquid 2 in the trough 1, the inner electrodes 6 which are 
connected to the high-frequency generator via a line 8 being the live 
electrodes and the electrodes 7 being the grounded electrodes. The 
electrodes reach downwards to approximately the underside of the 
containers 4 and preferably end slightly above the underside of the 
containers 4 and, on the other hand, protrude upwards beyond the level of 
the liquid. Paraboloid ultrahigh-frequency radiators 9 are located above 
the container 4. 
The entire treatment installation is accommodated in a Faraday cage 10 in 
order to prevent electromagnetic waves from reaching the surroundings. The 
containers 4 are conveyed through the channels 3 with the aid of plastic 
belts 13 which are guided in guide tracks 13a below the underside of the 
channels 3. The liquid is fed from the trough 1 through an overflow pipe 
11 to a purification unit and is recycled from the latter via a feedline 
12 into trough 1. At the same time, the height of pipe 11 is adjustable 
and serves to set the liquid level in the trough 1. 
The containers 4 in a row, the front faces of which are in contact with one 
another or interlock, form together one elongated container which takes up 
the total length of the treatment space or can even exceed the length of 
the latter. Before they enter into the treatment stations equipped with 
the high-frequency electrodes 6 and 7 and the paraboloid 
ultrahigh-frequency radiators 9, the containers 4 are continuously and 
uniformly charged with the foodstuff 5 up to the specific set filling 
level 5a, in such a way that the foodstuff 5 forms a layer of 
predetermined thickness which reaches from one side wall of the container 
4 to the other and always covers the entire bottom of the container, it 
being necessary to take care that the layer contains no air and gas 
inclusions and, if necessary, such inclusions are eliminated by shaking or 
the like. If a relatively even surface of the foodstuff does not form by 
itself on its topside after filling, the evenness of the surface of the 
foodstuff can be further improved by smoothing it mechanically. The 
containers 4 are then passed through the treatment space at a constant 
speed which can be regulated from a gearbox (not shown). 
The level 2a of the liquid 2 present in the tank 1 is always set to be a 
little lower than the surface level 5a of the foodstuff 5 in the 
containers 4. Preferably, the liquid level should be about 6 to 10 mm 
below the surface of the foodstuff 5 in the containers 4. If the surface 
of the foodstuff is uneven, the liquid level should be lowered yet 
further. If necessary, the difference between the surface of the foodstuff 
and the liquid level below the former can, for example, be up to 18 mm in 
the case of certain jams, when an ultrahigh-frequency field of 2450 
megacycles is used. This difference can be up to 35 mm, if an 
ultrahigh-frequency field of 915 megacycles is used. 
The alternating electromagnetic field existing between the high-frequency 
electrodes 6 and 7 extends through the channels 3, the containers 4 and 
the foodstuff 5 and is delimited upwards by the liquid level 2a. The upper 
layers of the foodstuff to be treated, which are not reached by the 
high-frequency field, and also the air-filled space present above the 
foodstuff, are treated by means of the ultrahigh-frequency field from 
above through the open top of the containers 4. 
The throughput speed depends on the tan .delta. value of the foodstuff and 
on the available generator power. To control the temperature, it has 
proved simpler in practice to vary the power of the electric generators 
than to vary the throughput speed of the container 4. 
In industrial operation, the foodstuffs are delivered for treatment at 
different temperatures; the deviations from the envisaged standard can be 
up to 15 centigrades or more. In the case of certain types of foodstuff, 
the end temperature of the foodstuff to be treated, which must be adhered 
to after treatment in order to achieve the purpose of the treatment, lies 
within a relatively narrow temperature range with a tolerance of about 
.+-.6 centigrades. In order to reach an end temperature after treatment 
within the particular permissible tolerance range, the particular quantity 
of energy which is to be supplied is controlled as a function of the 
delivery temperature and the desired end temperature. Infrared sensors can 
be provided to continuously transmit the value of the initial temperature 
of the foodstuff to an appropriately programmed electronic computer, to 
which the end temperatures after treatment are also fed by a second 
infrared sensor from the other end of the installation. In accordance with 
these two data, the computer meters the quantity of energy supplied by 
means of the two fields so that the end temperature of the treated 
foodstuff always remains within the permissible relatively narrow 
tolerance range of the end temperature. If the inlet temperature is so low 
that the energy available from the high-frequency or ultrahigh-frequency 
generators is insufficient to reach the desired end temperature at a 
predetermined speed, the computer emits an alarm signal and switches off 
the treatment installation. The generator power is selected so that the 
computer still has sufficient scope for compensating the fluctuations, 
which are customary in practice, of the delivery temperature of the 
foodstuff to be treated. 
The passage through the installation described completes the stabilization 
of mass products which are intended for further processing in industry, 
for example of jams for the dairy industry which are used for the 
manufacture of yoghurt preparations and cream cheese preparations having a 
fruit taste, since the material is filled into sterile tanks under 
precautions which exclude a reinfection, and is transported to the 
customers in these tanks. 
The foodstuff which has been stabilized in the manner described can also be 
used for packaging in portions into small units by introducing it into 
appropriate small containers using filling installations suitable for this 
purpose. The filling installation need not work "aseptically"; it suffices 
if the storage tank of such a station, its filling pumps and its 
feed-lines are kept low in bacteria content since most of the foodstuffs 
for which the process can be used are very rich in sugar and hence are 
poor breeding grounds for bacteria. However, they are the more susceptible 
to mould-producing fungi. 
During the actual filling process, the foodstuff comes into contact with 
the ambient air between the end of the filling spout and the container, so 
that reinfection becomes possible. Since, however, the bacteria which may 
be present are aerobic, they can develop only on the surface of the 
foodstuff. If the small container--which, in a special case, can also be 
manufactured from a metal--is now covered with a film which is permeable 
to electromagnetic waves or with a lid of this type and is then subjected 
again to an after-treatment in an ultrahigh-frequency field which is 
radiated in from above and has a sufficiently high energy to destroy the 
bacteria which may also have been enclosed on the surface of the foodstuff 
and in the headspace of the package which may be present, a well preserved 
small package can thus be produced. 
This last-mentioned ultrahigh-frequency field can have a frequency which is 
identical or similar to that which was used in the main stabilization 
process but--depending on the desired depth of penetration and on other 
features--it can also have one of the other frequencies approved for 
industrial use. 
This preferred arrangement of an apparatus working in accordance with the 
invention has a treatment space with an open top and a horizontal passage. 
Many other arrangements are possible amongst which only those may be 
mentioned here which have treatment spaces with a downward gradient in the 
direction of movement of the goods--up to a tube in a vertical position, 
with a circular, oval or rectangular cross-section--and in all of which 
conveying of the foodstuff through the treatment space could be 
accomplished more easily, in some cases even automatically utilising 
gravity. Nevertheless, the horizontal arrangement is to be preferred 
because the elimination of occluded air enclosed in viscous or pasty 
material is still most readily accomplished in this arrangement. If 
necessary for this purpose, the installation can be preceded by a short 
shaking feeder. 
Amongst the manifold possibilities of eliminating occluded air in treatment 
spaces with a gradient, a conically tapering feed path of about 35.degree. 
inclination toward the horizontal plane affords the best results. 
This can also be used in the preferred arrangement for increasing and 
further stabilizing the layer thickness of the foodstuff being conveyed. 
EXAMPLE 
A jam prepared from frozen strawberries and sugar as well as the usual 
ingredients is to be sterilised. It is known that it is infected--via the 
fruit--with yeast cells, mould spores and Escherichia coli. The fruit is 
thoroughly rinsed with water before preparing the jam. 
The jam is delivered for treatment at a mean temperature of about 
38.degree. C. Since it is to be heated to 75.degree. to 80.degree. C, the 
.DELTA.t value is about 40.degree. on average. 
The end temperature which is to be adhered to is set to 76.degree. to 
79.degree. on the controller of the electronic computer, the liquid 2 is 
heated to about 50.degree. and its level is brought to the normal height, 
that is to say to about 10 to 12 mm below the set level of the surface of 
the jam 5 to be treated, by appropriately setting the overflow pipe 11. 
When the availability signal has been received from the generators, the 
liquid circulation pumps, the conveying device and finally the feed device 
for the goods are switched on. The mutually joined, product-filled 
containers 4 of each row are set in motion by the chain belt 13. The jam 5 
filled into the containers 4 from the dispenser initially does not lie 
flat in the containers; for this reason, it is pressed into the correct 
position by means of a surface-shaping device conventional in the 
industry. 
Since the computer at the start receives only the inlet value, the 
treatment of the first filling in the installation is unreliable. It is 
branched off at a point provided for this purpose and returned to the 
dispenser where its temperature equilibrates with that of the much larger 
quantity stored there. 
Even during the course of the treatment, the surface of the jam is not 
completely level: individual fruits can protrude or leave holes. For this 
reason a frequency of relatively deep penetration, namely 915 megacycles, 
was selected also for the ultrahigh-frequency energy radiated in via the 
paraboloid radiators 9, whilst the high-frequency field between the 
electrodes 6 and 7 has a sufficient depth of penetration at a frequency of 
27.12 megacycles. 
After a passage time of about 5 minutes, nearly 1.5 minutes of which are 
accounted for by the treatment itself, a product is obtained at an average 
temperature of 78.degree. C with a maximum deviation of a little less than 
1 centigrade. 
A bacteriological test indicated sterility. 
The installation described here is suitable for small and medium firms. 
However, installations for a larger output can also be constructed, even 
though the emitter electrode cannot be lengthened indefinitely. It is 
designed as a twin electrode installation with two adjacent treatment 
spaces. In this model, the grounded electrodes are on the outside of the 
treatment spaces. 
A part of the jam treated in this way was introduced, immediately after the 
treatment described above had ended, into a sterile tank which could serve 
as the storage tank of a filling installation. The jam did not come into 
contact with the atmosphere during this transfer. 
The parts of the filling installation which come into contact with the jam 
had beforehand been reduced to a very low content of bacteria, for example 
by flushing with steam. The jam was filled in the conventional manner, 
i.e. while in contact with the ambient air, into small round aluminum 
dishes of about 10 mm depth and about 40 mm diameter. The small dishes 
were then sealed with lids consisting of synthetic plastics material and, 
since the temperature had fallen to about 55.degree. C in the meantime, 
the dishes were again heated to a surface temperature of the contents of 
almost 80.degree. C by means of ultrahigh-frequency irradiation. An 
ultrahigh-frequency field having a frequency of 2450 megacycles was used 
therefor. 
The jam portions treated in this way proved to be free from mould and yeast 
and withstood storage at room temperature for more than 6 months. 
Compared with known types of treatment, the treatment according to the 
invention has the further advantage that lower maximum temperatures (about 
80.degree. instead of about 100.degree.) and shorter times, during which 
the material is kept warm are sufficient for stabilizing. 
To achieve an even finer dosification of the high-frequency and 
ultrahigh-frequency energy supplied by the computers, it is possible 
separately to determine the initial temperatures and end temperatures in 
the parts of the foodstuff to be treated, which are to be influenced or 
have been influenced by the high-frequency field and the 
ultrahigh-frequency field.