A METHOD OF HEAT STABILIZATION

The present invention relates to a method for applying microwave energy in a continuous process for heat stabilization of products and providing treatment conditions that guarantee an end quality close to the quality of fresh or unprocessed products, also for products which are not very resistant to processing and which by experience are difficult to process.

The products of interest primarily are food products, for instance meat, fish, fruit and vegetables and also pharmaceutical compounds as dispersions of nutrition.

It is known to apply microwave energy for heat stabilization in continuous processes.

US patent No. 3 809 845 (Stenstrδm) represents the state of the art from which the present invention starts.

The contents of another US patent No. 3 263 052 (Jeppson) represents the idea of applying mutually different power levels along a conveyor tunnel in a continuous process. In that context the idea is to apply a maximum of energy at a first stage and thereafter reduce the energy supply as the water contents of the product decreases.

Because the primary idea of the present invention is to apply processes for heat stabilization that allow sterilization or pasteurization, the first mentioned patent represents to a substantial extent also the problem in the present context, while the latter patent has less relevance because it relates to drying processes having entirely different prerequisites.

The heat stabilization which is meant has for its purpose to inactivate enzymes and microorganisms without Introducing a lower quality, for instance flavour of food stuff. For a long time it has been known that such stabilization should be carried out in a very short time in order to realize the object. Especially for solid or

viscous food, microwaves are the single means for a sufficiently quick heating. However, generally microwaves heat food stuff or similar products not very uniformly - it is not unusual that a portion of the heated product is given three times higher temperature than another portion. However, this can seldomly be accepted because the quality of the product is reduced quickly when the temperature is too high. When packaging products in plastics there, additionally, is a temperature limit above which the packaging material melts, reacts with the product that is packed or is damaged in some other way. A major step towards a uniform processing is disclosed in said US patent No. 3 809 845 where heat stabilization is carried out in a medium surrounding the product and where the medium has a dielectric constant approximatively equal to the constant of the product, and where the temperature of the surrounding medium is controlled such that the medium cools the surface of the product in order not to reach a tempera¬ ture higher than the one desired inside the product.

Some products do require further technique for heat stabilization the product without loss of quality (smell, taste, texture, colour, vitamin contents etc.). One reason for this is that several parameters, individually or together cause a temperature distribution when heating by microwaves.

Such a parameter is inhomogenity of the product. The product may for instance consist of several constituents, for instance potatoes, onion and meat in smaller or larger pieces. Due to the loss factor and the dielectric constant of the microwaves, the several constituents of the product will be given different heating. The distribution and the location inside the product of the several constituents are also important to the heating.

The product may also be a product layered of material types having different loss factors etc. In such a case a layer comprising two materials A and B may give a higher temperature in A when microwave heating at a certain layer orientation, and in B at an other layer orientation.

A piece of side flesh may represent a layered product. Another factor the effect of which is temperature distribution is the existence of standing waves in the microwave field inside the product.

Furtheron, tendencies of a too high heating of the edges of the prouct may be observed if the dielectric constant of the product differs too much from the one of the surrounding medium.

A fourth factor which may give colder or warmer layers inside the product is the fact that the microwaves are attenuated when propagating inside the product, but also counter-measures again the effects of such attenuation in the form of a compensated heating and cooling by means of appropriate media contacting the outside of the product, for instance according to the said US patent No. 3 809 845.

Furhteron, also very well tuned applicators of micro¬ waves do give certain remaining discontinuities in the microwave field resulting in warmer and colder passages inside the product, generally parallel to the transport direction past the applicators.

Additionally, each one of the sources mentioned and causing temperature distribution may coact or interfere with the rest of them resulting in a heating process which may give the product temperature peaks here and there and which would cause severe local quality detoriation or cause formation of vapour which would mean sabotage of the process at the same time, at other locations inside the product there would be regions which would have been heated just noticeably.

The said temperature distribution phenomenon, is of a moderate magnitude in a material having low microwave absorption, for instance plastics, rubber, bread and dry products in general but is considerable in pharmaceutical products and in food products having a high water contents, as meat, fish, vegetables, berries, stews and soups.

Now, in a continuous microwave process where a continuous flow of product units passes microwave applicators, the problem is to increase the temperature

of the regions of such water containing products being most unwilling of heating in the shortest possible time, such that the desired heat stabilization affect may be obtained without a deterioated product quality.

In order to elucidate the huge interest of heating, in certain cases heat stabilizing, products by applying micro¬ wave energy, it may be proper to refer to the following publications which, however, completely avoid dealing with continuous process technique but still might be of interst:

US patent No. 4 370 535 relates to a household oven where a magnetron is arranged for thawing food stuff such that the output power varies depending on the temperature of the product. Such temperature is measured in discrete measuring points.

In US patent No. 4 506 127 which also relates to a household oven, the output power is reduced "at the end" of a heating process.

US 4 508 948 relates to a microwave oven having a variable output power. A micro computer is used and controls the output power based on the weight of the product and empirically established optimum parameters. In UK 963 473 there is a water buffer system which compensates for a decreased water contents in the product as the product is supplied by microwave energy which is maintained at a constant level.

European patent application having publication No. 64 082 relates to a microcomputer controlled microwave oven system where a "natural de rosting" is obtained during a short time period.

Microwave applications in connection with food stuff may also be found in a number of abstracts of Japanese patent applications. For instance Japanese patent application having publication No. 53-77360 relates to a control system for a microwave oven where, in order to reveal the effects of different original temperatures of products which are placed in the oven for heating, a continuous measurement of the temperature is carried out for terminating the heating at a desired temperature level

without any need for additional time.

In the Japanese publication No. 57-150371 there is described a sterilizing system for food stuff where micro¬ wave energy is supplied to a product, which may be pressur¬ ized, when it flows in a pipe.

The Japanese publication No. 57-189674 deals with retorting by using microwave energy and a specific form- stable closure of a packaging container.

The Japanese publication No. 57-202275 relates to a further system for microwave heating of a pressurized food stuff product.

In the Japanese publication No. 58-13372 there is described a packaging system where water having an acceptable dielectric constant is used as a medium for absorption of microwave energy for preventing vapour formation in a sealed package.

The Japanese publication No. 58-23774 also relates to a method of preventing "explosion" of a sealed package due to vapour formation. A powder having a high dielectric constant is added along a sealing area.

In the Swiss patent No. 647 131 there Is described a method of processing a product having constituents of different characteristics. The method is based on the fact that solid constituents are separated from floating constituents.

In a further European patent application having publication No. 70 728 there is described a multi-step procedure for thawing where a microcomputer is used for controlling the supply of energy relative the weight.

As previously mentioned the basic object of the present invention is to provide a method useful for a continuous process and which means a total heating time which is so short that the quality of the product will not be deteriorated as far as taste and other characteristics are concerned.

The heating time should be minimized relative the time allowed by the continuous process, and of course with due consideration of the characteristics of the product.

The comprehensive litterature list given does not solve the present problem.

Therefore, the invention uses a method of heating where the heating should be carried on to a desired temperature for stabilizing water containing food products or pharma¬ ceutical products contained in microwave transparent packaging material and which are heat treated continuously by being transported through a heat treatment channel where microwaves of different strength are supplied for heating the product and where the product when being passed through the microwave channel first will be affected by a microwave field having a certain strong heating effect, whereafter the product is moved out of this strong field and moved into a field of a lower heating effect.

The method is characterized in that the microwave energy which is transferred to the product in the field having a strong heating effect is selected such that a quickly heated portion of the product reaches a pre¬ determined temperature which is higher than the desired temperature, and such that the power of the field having the lower heating effect is selected such that said portion of the product maintains said predetermined temperature or a temperature just below, and that the heating in the field of the lower heating effect is given at least such a duration that a portion of the product which is heated slowly reaches said desired temperature.

In a practical embodiment the said predetermined temperature Is selected generally equal to the maximum allowed product temperature.

In most cases the desired temperature is selected generally equal to the temperature required for heat sterilization, for instance sterilization or pasteuri¬ zation, of the product.

In one embodiment of the invention the product enclosed in a microwave transparent material is encompassed by a liquid medium, for instance water, having a dielectric constant of the same magnitude as the product. The temperature of the liquid medium is controlled such that

the medium cools the surface of the product and the temperature of the medium is allowed to reach the temperature desired for the product at the end of the heating.

Knowledge of the temperature distribution in the product which is processed in the continuous process is obtained, according to one embodiment of the invention, by means of a number of temperature sensors which are stuck into the product at empirically found suitable positions in samples of the product, whereby the continuous heating process may be adjusted successively and manually according to the basic principal of the Invention.

In a further embodiment of the present invention, information of temperature is obtained by measuring ultra¬ sound velocity which facilitates automatization of the heat processing method.

Still one embodiment of the present invention is based on collecting the necessary temperature information for carrying out the heat processing method also automatically by sensing and computer processing from the surface of the product and the inside thereof outgoing electromagnetic radiation in a manner known within the art of meteorology.

In order to Illustrate the inventive idea a description will now be made by reference to the accompanying drawings where as a specific issue sterilization of food having a neutral pH and a high water contents, for instance a concentrated meat stew containing vegetables, has been selected. Figure 1 shows a temperature time graph for a conventional process for heating by microwaves, and Figure 2 shows a temperature time graph for the process according to the present Invention.

The curve portion denoted by reference numeral 10 in Figure 1 represents the fast heated portion of the product, for instance the surface thereof. The reference numeral 11 refers to the temperature of the portion of the product which is "slow" to heat.

The level 12 represents the maximum allowed temperature

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certain risks. The packaging material may be damaged if it is affected by peak temperatures. Vapour may be formed in the product if the process is not carried out in an environment having an over-dimensioned over-pressure. The vapour immediately destroys the process in that the heat transfer changes the characteristics thereof in the product and the microwaves get wild due to geometric changes in the load thereof, i.e. the product. That which has been said Is an explanation to the fact that so many failures exist in the microwave sterilization field In spite of the fact that very expensive development work has been put in.

After the time i the microwave pulse 17 is terminated and the temperature curves 10 and 11 start to converge. At the time t2 the temperature curve 11 of the colder portion intersects the temperature level 13 necessary for the sterilization. After a certain stand-by time at or just above said level, the product has obtained the desired sterile state. Then the product Is cooled (at the time t~ ) .

A minor increase in the level 13 would mean that the desired sterile state would be reached considerably faster implying a better maintenance of the original quality of the product. But the level 13 cannot be increased, instead it is much too high in the example, indicating that the temperature of the fast heated portion raises a considerabl bit over the allowed temperature with the results pointed out above.

Actually the level 13 has to be drastically decreased or the microwave energy 17 distributed over a substantially longer time for being approved by the fast heated portion of the product. However, in order to reach the sterile state, the time at a heated condition has to be Increased drastically which leads to a prolonged heat treatment of the entire product and as a result of this a lower quality.

Now, with reference to Figure 2 the improved process according to the present Invention will be described. The reference numerals 10, 10 ¹ , 11, 11 etc. to 16, 16 and 17 have the corresponding meaning as in Figure 1.

Two Important steps are clearly indicated in the new

process. One is the fact that the energy of the microwave pulse 17 is so selected that the temperature 10 of the fast heated portion just reaches the allowed temperature 12. In this case the average temperature 16 , after the energy supply, may be allowed to stay below the sterilization temperature 13_ which was not allowed in the conventional process.

The second step means a microwave heating 18 such that the fast heated portion of the product does not fall below 10 ^l but stays at the level 12 according to 10". The reason that 10' fall is that the fast heated portions of the product are cooled by the portions which are heated slowly. Thus, it is a question of applying microwave energy to such an extent that the heat loss of the fast heated portions to the colder portions is compensated.

However, that which is of interest is to quickly raise the temperature to sterilization temperature of the portions of the product which are heated slowly. However, this is exactly what takes place when applying the method of the present invention. The portions which are heated slowly are namely heated in two ways and both are at an optimum. The slow heated portions are heated partly by microwaves proportional to the power 18 of the microwaves. And such power is maintained as high as possible, having in mind the fact that the fast heated portion of the product may not reach a temperature above the allowed temperature. Additionally, the slow heated portions are heated by heat from the fast heated portions. This transfer of heat is more efficient the higher the temperature difference is. And the temperature difference is maintained at a maximum if the fast heated portion is maintained at the highest temperature according to the present invention.

The heating 11" of the slow portions apparently will be as quick as possible because it is the sum of two heating phenomena, both of which are at a maximum at the same time, each as a result of one and the same measure. It is to be . noted that the power at the microwave heating 18 is far below the power at the microwave heating 17. The difference

is represented by the difference between the inclinations of the curve portions 16" and 16. From the comparison it is realized that at the time ti there really is a drastic reduction of power, a fundamental discontinuity.

Where the microwave heating 18 is terminated, the curve portions 10", 11", 16" merge into new curve portions 10"' , 11"' , 16"' , of which the last mentioned is horizontal. The portion 11"' is still Increasing indicating that the heating 18 could be terminated somewhat before 11" has reached the sterilization level 13. In this case the curve portion 11"' , after a short time, reaches the sterilization temperature. The important feature, apparently, is the fact that the heating is not terminated earlier than allowing the curve portions 11" or 11"' reach the sterilization temperature without unnecessary time delay. Especially, it would be a disaster to interrupt the heating 18 already at the time when the average temperature of the product 16" reaches the sterilization level 13. In such a case it would be necessary to wait a long time before the slow product portion would be sterilized. It is feasible to continue the heating 18 somewhat after the curve portion 11" has reached the level 13. In the coldest region of the product there will be obtained very quickly a temperature somewhat above the predetermined level 13. This means a possibility of an extra fast sterilization (short stand-by time) which is desirable for most products suited for microwave sterilization.

In the figure the case has been shown where the heating 18 is terminated just when the curve portion 11" reaches the sterilization level 13. This does not _. exclude a termination of the heating 18 somewhat before this happens, in a case where the curve portion 11"' is so raising that the sterilization level 13 nevertheless will be reached without any noticeable delay.

After the microwave heating 18, the product is left without any thermal influence during a certain stand-by time, such that the desired sterile value Fo is reached also in the portions which have the lowest temperature.

of the product for giving the interior thereof the necessary heating, a suitable counter-measure resides in cooling the surface of the product by a surrounding medium, for instance water, in such a good time that the effect will be sufficient also a distance inside the product. The temperature of the surrounding medium Is not allowed to exceed the temperature desired in the product, but is controlled such that it reaches this temperature at the end of the microwave treatment or process, meaning that also the surface layer reaches such a treatment degree.

In the description just made and in Figure 2, the heating 17 and 18 have been represented as a continuous heating. This is not the case in practice. The microwaves as such do not, accurately seen, give a continuous heating. Analogue to an ordinary AC current in a resistor, the microwaves give the maximum heat at the wave peaks. If for instance microwaves of a frequency of 2450 MHz are used the heating will be periodic and have a frequency of 4900 MHz. If the microwaves are produced by magnetrones, which are supplied by a re.ctified one phase 50 cycle current, the microwave heating discloses pronounced maxima, each one occuring at an interval of one hunderedth of a second.

If the products are heated by a number of microwave applicators during the transport thereof and the applicators are arranged at a certain distance from each other, there is also obtained a certain modulation or pulse effect. This means that each one of the simple power graphs 17, 18 actually should be replaced by a number of conse- qutive pulses having a surface contents and a distribution making them generally equivalent to 17 and.18. The characteristic details of the microwave heating 17, however, are not important, but so is the total energy and the duration.

For the microwave heating 18 the same is valid with the addition that the smeared-out power of juxtaposltioned contributions should vary with the time according to the criteria of the main claim. The reason for not splitting up 17 or 18 in Figure 2 in pulses, which as a rule will be the

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times during an estimated time for the heating 18. The hottest point is measured and at the end of the treatment also the coldest. For a certain product the correct times for tracing the measurement points will be quickly established and the regions which will assume extreme temperatures will be Identified. If the measurement is carried out by thermoelements which are placed Inside the product, the measurement may be facilitated by stopping the transport of the product and switching off the microwaves during such a very short time of measurement. If the measurement is carried out by a not conductive measuring element, for instance an optical fibre having a suitable temperature sensor at the end thereof, the microwaves do not have to be switched off during the measurement.

Another method is based on the known fact that the temperature in a medium is affected by the speed of sound in the medium. By measuring the travel time of sound pulses in suitable directions through the product there will be obtained, under certain conditions, a possibility to translate the measured values to temperature information.

A third method means use of electromagnetic waves from the surface of the product and from layers further down in the product for identifying the temperature distribution inside the product. There Is only needed a passive sensing. Several frequences are observed. Computer processing which translates the character of the radiation field to a temperature distribution inside the product is complex, but basically known from the meteorology.

In that case where sound waves or electromagnetic waves are used for temperature measurement such measurement may be a continuous one and without any need for puncturing the packaging material of the product. This means that the control of the microwave heating according to the present invention may be obtained automatically by simple servo- circuits and switches.

It is to be noted that the point which during the processing or treatment will appear as the hottest, necessarily does not have to be stationary inside the

product but may have different positions from time to time during the process. This is also true for the coldest point.