Heat stable microwave energy sterilization method

A continuous method for heating a product having at least one faster microwave heating portion and at least one slower microwave heating portion to a uniform predetermined temperature sufficient to sterilize the product without loss of odor, taste, texture, color or vitamin content quality by transporting the product through a plurality of microwave fields including a first higher energy field and one or more successively lower energy fields, in which the first microwave field is attenuated to an energy level sufficient to heat the fast microwave heating portions of the product to the predetermined temperature, the successively lower energy microwave fields are attenuated to an energy level sufficient to maintain the temperature of the faster heating portions and heat the slower heating portions to the predetermined temperature, and the transport of the product through the successively lower energy microwave fields is continued until the slower microwave heating portions of the product reach the predetermined temperature.

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. 
U.S. Pat. No. 3,809,845 (Stenstrom) represents the state of the art from 
which the present invention starts. 
The contents of another U.S. Pat. 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, microwave 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 U.S. 
Pat. No. 3,809,845 where heat stabilization is carried out in a medium 
surrounding the product and where the medium has a dielectric constant 
approximately 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 temperature 
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 products 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-meansures 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 U.S. Pat. No. 3,809,845. 
Furtheron, also very well tuned applicators of microwaves 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 temperture peaks here and there 
and which would cause severe local quality detoriation or cause formation 
of vapour which wuld 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 phenomenom 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 microwave 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 interest: 
U.S. Pat. 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 U.S. Pat. No. 4,506,127 which also relates to a household oven, the 
output power is reduced "at the end" of a heating process. 
U.S. Pat. No. 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 No. 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 
defrosting" 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 microwave energy is supplied to a product, 
which may be pressurized, when it flows in a pipe. 
The Japanese publication No. 57-189674 deals with retorting by using 
microwave energy and a specific formstable 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 Pat. 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 
constitutents. 
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 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 pharmaceutical products contained in microwave 
transparent packaging material and which are heat treated continuously by 
being transported through a heat treatment channel where microwave 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 
predetermined 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 
pasteurization, 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 ultrasound 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.

The curve portion denoted by reference numeral 10 in FIG. 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. The level 13 
represents the temperature which is to be reached by the entire product, 
in this case the temperature necessary for sterilization. 
The tangent of the curve 10 at origo shown by broken lines 14 represents 
the microwave heating of the fast heated portion, while the tangent 
denoted by the reference numeral 15 represents the corresponding parameter 
of the portion which is slow to heat. 
The curve represented by the reference numeral 16 represents the average 
temperature of the product. The microwave power supplied as a function of 
time is represented by a pulse 17. 
At the initial state it is assumed that the product which is to be 
microwave heated has a certain uniform temperature (60.degree. C. in the 
example). 
When the supply of microwave power is started, the fast heated portion of 
the product raises its temperature according to curve 14, several times 
faster than the portion 15 which is slow to heat. As soon as the 
temperatures of said two portions start to deviate from each other, heat 
is transferred, by conduction and possibly convection, inside the product 
from the hot portion(s) to the cold portion(s). The curve 10 is therefore 
obtained instead of 14 for the portion which is fast heated and the curve 
11 instead of 15 for the colder portion. Between said curves the average 
temperature curve 16 is to be found. The inclination of 16 basically is 
directly related to the magnitude of the microwave power 17. To get the 
curve 11' of the colder portion to reach the predetermined temperature 13 
the average temperature 16 has to pass said level. This means that there 
is necessary a certain surface contents of the microwave puls 17, i.e. 
there is required a certain supplied power. 
When the average temperature 16 has passed the level 13, in the 
conventional process, it is generally unavoidble that the temperature 10, 
10' of the fast heated portions exceeds the allowed level 12 during a 
certain time period. This means quality detoriation of said portions and 
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 charcteristics 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 t.sub.1 the microwave pulse 17 is terminated and the 
temperature curves 10 and 11 start to converge. At the time t.sub.2 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.sub.3). 
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 considerable 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 FIG. 2 the improved process according to the present 
invention will be describe. The reference numerals 10, 10', 11, 11' etc. 
to 16, 16' and 17 have the corresponding meaning as in FIG. 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'but stays at the level 12 
according to 10". The reason that 10' fall is that the fast heated portion 
of the product are cooled buy 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 
t.sub.1 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 teminated 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 to 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 F.sub.O is reached also in the portions which have the lowest 
temperature. This has happened at the time t.sub.3, when the product is 
ready to be cold. 
Due to the fact that the lowest temperature of the product according to 
11"' in FIG. 2 is somewhat higher than the corresponding temperature 11' 
in FIG. 1, there is required a substantially shorter stand-by time in FIG. 
2 for reaching one and the same sterile value F.sub.O. Additionally, the 
temperature level 12 has not been exceeded, which was the case in FIG. 1. 
Both said phenomena coact to maintain the product quality of the process 
in a uniquely way compared to the known processes. At the same time the 
absolute respect for the temperature level 12 means that the new process 
gives safety, as far as damages of the packaging material is concerned, 
against vapour formation which would destroy the bacteria safety or 
require a proces equipment designed for all to high pressures and 
therefore an expensive equipment. 
It has been listed a number of parameters which each or together contribute 
to the temperature distribution represented by the vertical distance 
between the curves 10 and 11 in FIGS. 1 and 2. 
FIG. 3 shows a side view in which three pairs of arrows, 20 and 21, 22 and 
23 and 24 and 25 show the main direction for the application of three 
successively lower microwave fields by respective field applying means 26 
and 27, 28 and 29 and 30 and 31, against product units 32, enclosed in 
microwave transparent material 33 and surrounded by liquid medium 34. The 
construction of the field applying means 26, 27, 28, 29, 30 and 31 is not 
further disclosed as it may be conventional. The continuous transport of 
the product units 32 through the successively lower fields is obtained by 
means 35. 
If one of said factors has a dominating importance this may give an extra 
good reward when carrying out the method after having identified said 
parameter and put in countermeasures just for this parameter. The reward 
is a still more increased product quality depending on the shorter heating 
time. This depends on the fact that the heating 18 as a result of the 
counter-measure is given a better initial condition, to such an extent 
that the curve portion 11" at t.sub.1 will be placed closer to the curve 
portion 10" (at level 12) than without said counter-measure. The level as 
well as the duration of the necessary heating 18 according to the 
invention will be considerably reduced. As a compensation the microwave 
heating 17 will be carried out by using a somewhat increased power. The 
process will not be any longer due to this without any counter-measures. 
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 FIG. 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 producted by magnetrones, which are 
supplied by a rectified 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 consequtive 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 juxtapositioned contributions should vary with the 
time according to the criteria of the main claim. The reason for not 
splitting up 17 or 18 in FIG. 2 in pulses, which as a rule will be the 
case in practice, is an ambition not to further contribute to the 
complexity of the figures. The vention according to the claims should 
therefore be valid in that case where the microwaves are delivered to the 
product in the shape of pulses or as continuous power. 
The method according to the accompanying claims has been exemplified in an 
application where it is started out from a product at 60.degree. C. and 
which aims at obtaining a nondestructive sterilization at a temperature 
somewhat above 123.degree. C. As mentioned, the method is suitable also 
for other type of heat treatment or processing where speed is desired, for 
instance pasteuriztion. In such a case the desired temperatures could be 
for instance 70.degree. to 90.degree. C. 
A third application is preheting before a sterilization which thereafter 
basically is carried out according to the example in FIG. 2. Also in this 
case the problem is to heat quickly and such that end temperature of the 
product is given an unnoticeable distribution. In such a case the initial 
temperature could for instance be 10.degree. to 20.degree. C. and the 
desired temperature about 60.degree. C. It may also be valuable to reach 
this preheating temperature quickly. Not at least as far as food products 
having fat as well as protein are concerned. In such products it is of 
advantage if the denaturation of proteins, which nevertheless occurs at a 
certain temperature, will take place so quickly that fat that melts at 
said temperature does not have time to leave the food but will be blocked 
inside in a dispersed form. 
Of course, it is essential that the method according to the invention may 
be put into real practice. In this case information is neccessary on 
temperatures which may arise in the product during the processing thereof. 
A method which has been tested to a large extent and found to work is to 
measure the temperature at certain suitable portions at some selected 
times. A suitable time is close to t.sub.1, whereby the measurement is 
carried out at that point which according to experience will be the 
hottest. Thereafter it is suitable to measure some further 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 possiblity 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 ditribution 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 servocircuits 
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.