Device for the control and detection of adequate heat levels in microwave ovens

A process for controlling the microwave heating of a product to a temperature which is higher than or equal to a determined temperature for a period which is longer than or equal to a determined period, a marking support and a receptacle for product intended for such a control. Measurement is made of the resistivity of a thermally crosslinkable ink filled with conductive particles, with which the product or the receptacle containing the product has been previously marked and the resistivity of which is arranged to decrease in a predetermined manner in relation to the temperature and the period of microwave heating. The invention is applicable especially to the control of the sterilization of a pharmaceutical or agri-foodstuff product.

FIELD OF THE INVENTION 
The present invention relates to a process and a device for control of the 
microwave heating of a product to a temperature higher than or equal to a 
determined temperature for a period longer than or equal to a determined 
period, a marking support and a receptacle for a product intended for such 
a control. 
It finds its particularly wide, though non-exclusive, application in the 
field of the control of the sterilization of a product, for example of a 
solution capable of being injected into an animal or man. 
BACKGROUND OF THE INVENTION 
Processes for batchwise sterilization of a product in an autoclave are 
known at present. 
Such a sterilization is performed under pressure, for example 3 bar, and 
consists in heating the receptacles containing the product to be 
sterilized to of the order of 120.degree. C. for 10 to 30 minutes. 
Such processes have disadvantages. They do not permit easy continuous 
operation. Once the products are sterilized and then conveyed towards 
their destinations, no subsequent control can be performed in the event of 
doubt concerning the sterilization of said products. 
The present invention is aimed at providing a process, a device, a marking 
support and a receptacle for product to be controlled corresponding better 
than those previously known to the requirements of the practice, 
especially insofar as it permits a true dosimetric control proving that 
the product has been heated to a temperature exceeding a determined 
threshold temperature, for example of the order of 120.degree. C., for a 
period longer than a determined threshold period, for example twenty 
minutes, and does this with extreme ease; the control can be performed at 
any time by an operator who is not specially qualified, with the aid of a 
simple ohmmeter. 
To do this, the invention starts with the idea of employing microwave 
energy, on the one hand for rapidly and homogeneously heating the product 
and, on the other hand, for varying the conductivity of a thermally 
crosslinkable ink with which the product or the receptacle containing the 
product to be heated has been previously marked. 
The inventors have, in fact, had the idea of employing the process of 
crosslinking under microwaves of inks of the polymeric type which are 
filled with conductive particles like metal powders or fibers such as 
silver, copper, iron, manganese, ruthenium, rhenium, rhodium, nickel, 
aluminum or like carbon black, by converting an insulating product into 
conductive product, to make it act as a dosimeter, in order to control the 
sterilization of a product, and especially of an injectable solution. 
SUMMARY OF THE INVENTION 
To this end the invention proposes essentially a process for control of the 
microwave heating of a product to a temperature higher than or equal to a 
determined temperature for a period longer than or equal to a determined 
period, in which the resistivity of a thermally crosslinkable ink filled 
with conductive particles with which the product or the receptacle 
containing said product has been marked beforehand is measured, and the 
resistivity of which is arranged to decrease in relation to the 
temperature and the period of microwave heating in a predetermined manner, 
so that the measurement of said resistivity after heating makes it 
possible to verify the heating of said product to said temperature which 
is higher than or equal to the determined temperature for said period 
which is longer than or equal to the determined period.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In advantageous embodiments, furthermore, either or both of the following 
arrangements is (are) used: 
the process comprises the stage of preliminary marking of the product or of 
its receptacle, said marking consisting in attaching to the product or its 
support a dielectric support tablet on the upper face of which the 
thermally crosslinkable ink has been deposited; 
the product to be controlled being contained in a receptacle, the 
preliminary marking stage consists in marking the receptacle directly with 
a layer of said thermally crosslinkable ink by silk-screen printing. 
The invention also proposes a marking support for controlling the microwave 
heating of a product to a temperature higher than or equal to a determined 
temperature, for a period longer than or equal to a determined period, 
said marking support comprising a support tablet consisting of a 
dielectric material one face of which comprises an absorbent material and 
is capable of being attached to said product or to a receptacle capable of 
containing said product and the other face of which is provided with a 
layer of thermally crosslinkable ink, filled with conductive particles, 
the resistivity of said ink being arranged to decrease in relation to the 
temperature and the period of microwave heating in a predetermined manner, 
with the result that the measurement of said resistivity after heating 
makes it possible to verify the heating of said product to said 
temperature which is higher than or equal to the determined temperature 
for said period which is longer than or equal to the determined period. 
An absorbent material is to be understood to mean any material which is a 
conductor of electricity, such as metals or a carbonaceous product, 
arranged as a thin layer (10 to 100 .mu.m). 
In another advantageous embodiment the marking support tablet-does not 
comprise any absorbent material on its face-which is adhesively coated 
with the product or the receptacle capable of containing it, the product 
itself then acting as the absorbent material, which can be the case 
especially when a liquid is involved. 
Such supports can be manufactured independently of the products whose 
subsequent control they will make possible. 
The marking support advantageously consists of the receptacle itself, which 
comprises an enclosure for retaining said product, said enclosure being 
directly marked externally with the thermally crosslinkable ink. 
The invention also proposes a receptacle for product intended to be 
sterilized or pasteurized by microwave heating, said receptacle comprising 
an enclosure for retaining said product, which is directly marked 
externally, or indirectly on an intermediate support, using a thermally 
crosslinkable ink filled with conductive particles, the resistivity of 
said ink being arranged in order to decrease in relation to the 
temperature and the period of microwave heating in a predetermined manner, 
with the result that the measurement of said resistivity after heating 
makes it possible to verify the sterilization or pasteurization of said 
product, filled with conductive particles, the resistivity of said ink 
being arranged to decrease in relation to the temperature and the period 
of microwave heating in a predetermined manner, with the result that the 
measurement of said resistivity after heating makes it possible to verify 
the sterilization of said product placed in said enclosure. 
The product advantageously cannot be dissociated from its packaging without 
this being detectable. 
In fact, and for example, the receptacle is designed as being tamperproof, 
that is to say that it cannot be opened without such an opening being 
detectable. 
In an advantageous embodiment the product consists of a solution which can 
be injected into man or animal, or else is an agri-foodstuff product. 
An agri-foodstuff product should in particular and for example be 
understood to mean a product of the cooked dish type, but also fresh 
vegetables and the like. 
The receptacle may, for example, be provided made of glass, the microwave 
power density used for sterilizing being 70 w/dm.sub.3. In another 
embodiment the receptacle is made of plastic and comprises a jacket in 
intimate contact with the receptacle containing the product in order to 
ensure good heat conduction. 
The conductive ink is advantageously based on solvent-free plasticized 
phenolic resole to which butyl carbitol has been added to facilitate the 
silk-screen printing. 
The invention also proposes a device for making use of the process for 
controlling the heating of product described above. 
The invention will be better understood on reading the description which 
follows, of a number of examples of embodiment, which are applied to the 
sterilization of solution and are given without any limitation being 
implied. 
The description refers to the drawings which accompany it, in which: 
FIG. 1 gives the stages of the process or the components of a device making 
use of said process, according to one embodiment of the invention. 
FIG. 2 shows a receptacle for solution marked by silk-screen printing for 
control according to the invention. 
FIG. 3 shows a removable support for marking product, for example of the 
agri-foodstuff type, according to the invention. 
FIGS. 4 to 7 are graphs respectively showing the change in the resistance 
of four different inks (A, B, C and D) which can be applied to the 
invention, as a function of the period of heating to a temperature higher 
than of the order of 120.degree. C. 
FIG. 1 shows diagrammatically the stages of a process, or the units of a 
device making use of the process, according to the embodiment of the 
invention which is described more particularly here applied to the 
sterilization of glass flasks of injectable solution. 
The process is also applicable to solutions placed in plastic pouches with 
two superposed walls, one corresponding to the receptacle actually 
containing the solution, and the other to a protective overpackaging. The 
two enclosures are kept in intimate contact with one another, to permit 
good transmission of heat by conduction between solution and outer wall of 
the pouch. 
Such a jacketed pouch is obtained in a manner which is known per se, for 
example by employing a vacuum application technique. 
The first stage 1 is a filling stage which allows an automatic filling of 
the flasks one after another, or in rows of a number of flasks, with the 
solution to be sterilized in a filling unit 1'. 
This may be, for example, a solution of the 5% glucose type. 
Each receptacle is then conveyed, with or without intermediate storage, 
towards the marking unit 2', for a marking stage, which consists in 
marking the receptacles by silk-screen printing in a manner which is known 
per se, with a film of thermally crosslinkable ink. 
To do this, a layer of line of ink 12 of small thickness, of the order of 
20 .mu.m, is deposited (see FIG. 2) on the outer wall 10 of the receptacle 
11, for example over a length of one centimeter and a width of the order 
of a millimeter, between two studs 13 and 14 which will be used as points 
of contact with the control instrument (ohmmeter) during a measurement. 
The two studs are, for example and quite simply, the ends of the layer of 
ink. 
The composition of the ink is, for example, that described below under the 
heading ink D. 
The solution-filled flasks are next closed again in a known manner and then 
conveyed towards the unit 3' for continuous microwave sterilization 3. 
This unit comprises a cavity for processing the products in a flask for a 
determined period of known type. 
The enclosure is kept at a pressure of the order of 3 bar for 12 minutes. 
The microwave frequency range employed is advantageously between 
approximately 0.1 GHz and approximately 100 GHz, preferably between 
approximately 0.5 GHz and approximately 30 GHz. 
Once the sterilization treatment has been performed, the flasks are 
conducted to the control unit 4' using automatic electrical measurement 4 
of the resistance of the ink layer 12 between the studs 13 and 14, by 
means of an ohmmeter. 
By virtue of the invention such a control can be performed flask by flask 
and not by sampling as in the prior art, with all the risks that this 
involved. 
The nonconforming flasks are automatically rejected towards the unit 5' for 
rejection 5. 
The flasks which have successfully passed the sterilization test are 
automatically directed towards a unit 6' for storage 6 before use. 
FIG. 3 shows a support according to the invention comprising a tablet 20 
made of dielectric material, for example made of epoxy glass of 
thicknesses varying between 8 and 35 microns, the face 21 of which is 
provided with a layer of thermally crosslinkable ink 22 according to the 
invention, deposited by silk-screen printing with two conductive studs 23 
and 24, of the order of 20 .mu.m in thickness. The other face 25 comprises 
a layer of absorbent material 26 which acts as attenuator, for example 
made of carbon, and means for fastening said face 25, for example by 
adhesive bonding, to a receptacle intended for the product to be 
controlled, or even directly to the product. 
The layer 26 makes it possible to avoid the untimely heating of the ink 
layer. 
It is calculated and produced as a function of the product to be treated 
and of the conditions to be adhered to in a known manner, within the scope 
of the person skilled in the art. 
This layer of absorbent material may not even be necessary in the case 
where the product itself acts as attenuator preventing the heating. 
It is recalled here, for memory and relating to the measurements of the 
resistivity of a product formed into a sheet, that the electrical 
conduction characteristic of a conductive or resistive layer deposited by 
silkscreen printing can be expressed in ohms per square unit of area of a 
layer of constant thickness (for example 25 .mu.m in thickness), the 
electrical resistance of such a square unit being proportional to the 
resistivity of the material whatever the area of the layer. 
This characteristic is conventionally employed by a person skilled in the 
art. 
In the field of electronics there are at present in existence inks of 
conductive nature, consisting of a binder (for example a thermoplastic or 
heat-curable polymer) and of a filler (of the silver flake type) which 
make it possible to produce printed circuits or flat keyboards imparting 
the appropriate electrical properties to the required circuit. 
Let us recall that, before crosslinking, the ink filled with electrically 
conductive particles is insulating whereas, after crosslinking, it becomes 
conductive, its electrical conductivity increasing when the cross-linking 
period increases, to reach a limiting value. 
The idea of the invention consists especially in applying the thermal 
crosslinking of the inks to validate the sterilization or the 
pasteurization of pharmaceutical or agri-foodstuff products. 
No currently known ink can be employed as a sterilization dosimeter. 
In fact, no investigation has ever been carried out hitherto into ink whose 
crosslinking kinetics correspond to the periods and temperatures of 
correct sterilization of products such as, for example, injectable 
solutions. 
One of the merits of the invention is precisely to propose such inks and, 
more generally, to detail the manner of obtaining them, inks whose 
crosslinking kinetics are similar to a sterilization cycle of products 
under microwaves and, for example, of injectable solutions in glass 
flasks. 
It is self-evident that, as a function of the required application, 
depending on the following parameters: 
product to be sterilized or heated, 
support employed, 
power of the microwaves employed, 
temperature to be reached, 
period of holding at said temperature, 
load in the microwave applicator, 
a person skilled in the art will be capable of determining the composition 
of the ink to be adopted. 
However, the concept of completion of crosslinking of the inks based on 
thermoplastic or heatcurable resin epoxy, alkyd melamine or acrylic 
melamine resins is a relative datum. In fact, whatever the method of 
treatment (microwave or thermal), there always remain reactive sites which 
have not reacted and the crosslinking can therefore develop as a function 
of the period-temperature pair. Consequently, the inventors have been led 
to find the best criterion of good crosslinking of an ink. 
By a thermal route, experiment has shown that after 5 hours at 150.degree. 
C. the mean resistivity of the circuit stabilizes around a value R, the 
latter could subsequently decrease by 0.1 R in the case of an additional 
treatment of 5 hours at 150.degree. C. 
This 10% variation also corresponds to the accuracy which can be expected 
from a printing method using silk-screen printing. 
In practice, therefore, the inks to be tested are thus standardized in an 
oven at 150.degree. C. for 5 hours. 
The investigation of thermally crosslinkable inks and the determination of 
an optimized ink, as applied to the sterilization of injectable solution 
in glass flasks will be now described by way of example, no limitation of 
any kind being implied. 
APPLICATION OF THE INKS 
The inks were silk-screen printed manually by means of a 140-mesh polyester 
screen. The test vehicle was designed specially to be silk-screen printed 
on adhesive tape known under the name of Kapton, 19 mm in width. This test 
vehicle is represented by a 100 .quadrature. circuit 1 mm in width for 
measuring resistivity. 
This test vehicle is then simply adhesively bonded onto 1/2-liter glass 
flasks filled with injectable solution. 
MICROWAVE APATUS 
The microwave equipment used comprises 4 pressure-resistant glass tubes, 
for example made of glass known under the name "Pyrex", in which a 
pressure of 2 to 3 bars can be established with the aid of a compression 
and decompression air-lock. Each tube comprises 5 applicators known under 
the name Stereomode and manufactured by the French company M.E.S., each 
powered by an 800-watt generator. 
Each applicator can receive 21/2-liter flasks. 10 flasks per tube can thus 
be treated at each sterilization cycle. 
To speed up the cooling of the flasks the apparatus is equipped with forced 
air circulation with a heat exchanger. 
EXPERIMENTAL RESULTS FOR VARIOUS INKS 
Ink A 
This is a thermoplastic conductive ink based on polyester of high molecular 
weight, manufactured by the German company Huls. 
As in the case of all thermoplastic resins, the ink resistivity decreases 
in line with the evaporation of the solvents to reach a limiting value 
which approaches asymptotically the resistivity of the silver-filled 
resin. 
It is therefore a matter of finding the solvent equilibrium which makes it 
possible to combine, on the one hand, a good stability on the screen and, 
on the other hand, evaporation kinetics matching the sterilization cycle. 
Among the inks that were investigated, a more particular study was made of 
the silver ink AG C31 from the French company Comptoir Lyon Alemand Louyot 
(CLAL) (ink A). 
The composition of ink A is the following: 
______________________________________ 
70% Ag JV4 flakes (mean diam. of the silver particles of the 
order of 3 .mu.m) 
30% polyester binder at a concentration of 
35% in carbitol acetate 
100% 
______________________________________ 
Expression of the results for ink A (see FIG. 4): 
______________________________________ 
Time, min Resistivity of the circuit in .OMEGA. 
______________________________________ 
2 188 
4 66 
5 48 
6 39 
7 32 
8 30 
9 24 
10 23 
11 18 
12 17 
+30 min in the 150.degree. oven 
13 
+60 min in the 150.degree. C. oven* 
11 
______________________________________ 
Note 
*In practice, an additional treatment of 1 h in an oven at 150.degree. C. 
corresponds to the heat treatment of the same ink in the oven for 5 hours 
at a temperature of 150.degree. C. 
Ink B 
To obtain a better reactivity under microwaves, binders based on 
water-soluble polyester resins were employed next. 
These resins, directly soluble in water without any neutralizing amine, 
then appeared to offer a major advantage for the reactivity under 
microwaves. 
For example, the product known under the name Hydrorob 101100 from the 
Robbe company was employed to advantage. 
Its characteristics are the following: 
______________________________________ 
Solids content 80% .+-. 1% 
Solvent Butyl glycol 
Hydroxyl value 280-320 
______________________________________ 
Different binders were studied by starting with this resin, one comprising 
a certain proportion of water, the other without the presence of water, 
containing only solvents of high polarity. Butyl glycol with an 
evaporation value of 163 was finally chosen as base solvent. Butyl 
diglycol with a higher evaporation value was adopted as tension solvent. 
______________________________________ 
Binder 1 Ink B No. 1 
______________________________________ 
80% water-soluble polyester 
25 75% Silver JV7 
Methylated urea formaldehyde 
60 20% Binder 1 
resin 
Butyldiglycol 5 5% Butylglycol 
Water 5 
Butyl glycol 4.5 100% 
Blocked PTSA catalyst 
0.5 
100% 
______________________________________ 
Binder 2 Ink B No. 2 
______________________________________ 
80% water-soluble polyester 
25 75% Silver JV7 
Methylated urea formaldehyde 
60 21% Binder 2 
resin 
Butyldiglycol 5 2% Butylglycol 
Butyl glycol 9.5 2% Butyldiglycol 
Blocked PTSA catalyst 
0.5 
100% 
100% 
______________________________________ 
The difficulty of storing inks based on resins (water-soluble 
polyester/amino resins/catalyzed with blocked PTSA) makes it necessary to 
keep these inks at temperatures of the order of maximum 10.degree. C. To 
circumvent this difficulty the inventors have been led to employ much more 
reactive amino resins, which resulted in ink B No. 3. 
______________________________________ 
Binder 3 Ink B No. 3 
______________________________________ 
15 Hydrorob 101100 
75% Silver flake JV7 
40 HMM 066 (BASF) 21% Binder 3 
18 Butyldiglycol 2% Butyl glycol 
18 Butyl glycol 1.9% Ethyl glycol acetate 
8 Ethyl glycol acetate 
0.1% Byk 052 (antibubble) 
100% 100% 
______________________________________ 
Expression of the results for ink B (see FIG. 5): 
______________________________________ 
Time, min Resistivity of the circuit in .OMEGA. 
______________________________________ 
1 250 
2 111 
3 43 
4 25 
5 18 
6 16 
7 16 
8 15 
9 13 
10 13 
11 12 
12 10 
+30 min in the 150.degree.oven 
9 
+60 min in the 150.degree.oven 
9 
______________________________________ 
Although satisfactory overall, ink B nevertheless has the disadvantage of a 
very high hydroxyl value of the resin, of the order of 300; it is 
therefore necessary to employ a high proportion of amino plastic needed 
for the crosslinking, resulting in a risk of loss of the film flexibility. 
Ink C 
Ink C is an ink based on heat-curable acrylic resin (ref. Plexisol DV 606 
from the German company Rohm). 
______________________________________ 
Binder 4 Ink C 
______________________________________ 
Plexisol DV 606 68 70% Silver XRP 5 
Methylated melamine resin of high 
10 30% Binder 4 
reactivity 
Butyl glycol 9 
Butyl carbitol acetate 
9 100% 
Butyldiglycol 4 
100% 
______________________________________ 
Expression of the results for ink C (see FIG. 6): 
______________________________________ 
Time, min Resistivity of the circuit in .OMEGA. 
______________________________________ 
3 169 
4 73 
5 56 
6 49 
7 40 
8 35 
9 30 
10 23 
11 18 
12 15 
+30 min in the 150.degree. oven 
10 
+60 min in the 150.degree. oven 
9 
______________________________________ 
Ink D 
Ink D is a conductive ink (ref. VF 53 from CLAL) based on imprenal 31A from 
the Rashig company. 
Imprenal 31A is a solvent-free plasticized phenolic resole. 
After curing the films exhibit a high resistance to chemical products, to 
solvents, to acids, to bases and to oxidizing agents. 
Ink D is compatible with epoxies and vinyl, acrylic amino plastic polyester 
resins. 
______________________________________ 
Composition of the ink D 
______________________________________ 
63% Silver JV5 (as Flake) 
17.3% Imprenal 31A 
6.5% Butyl carbitol 
13.2% Methyl cyclohexanol 
100% 
______________________________________ 
However, the wettability of the pigment filler is mediocre and results in 
an imperfect ink rheology, the deposits being nonuniform in appearance. 
An addition of butyl carbitol makes it possible to make the ink fit for 
silk-screen printing with good reproducibility. 
Expression of the results for ink D (see FIG. 7) 
______________________________________ 
Time, min Resistivity of the circuit in .OMEGA. 
______________________________________ 
4 231.5 
5 131.8 
6 90.5 
7 67.3 
8 51.7 
9 39.1 
10 28.2 
11 16.0 
12 11.2 
+30 min in the 150.degree. oven 
9.6 
+60 min in the 150.degree. oven 
8.7 
______________________________________ 
INTERPRETATION OF THE RESULTS 
The inventors subsequently reduced the possibility of final exploitation of 
these results to two parameters: 
on the one hand, the change in slope of the conductivity curve as a 
function of the period of microwave treatment. The sensitivity of the 
method will, in fact, be proportionally higher the greater the change in 
slope in the vicinity of the end of the microwave treatment cycle (12 
minutes in the case of a power of 800 W and 1/2-liter flasks in the 
example more particularly described here); 
on the other hand, the stability of the conductivity curve after the end of 
the cycle, to permit long-term sterilization checks. 
One of the merits of the inventors has been to decide the choice of the 
inks from their crosslinking curve while being guided by the optimization 
of these 2 parameters. 
It is obvious that this method, described more particularly with reference 
to the sterilization of flasks of solution, is, of course, similarly 
applicable to the investigation and the optimization of the thermally 
crosslinkable ink which would be intended for heating another product 
and/or another receptacle. 
The following table has been established so as to make it possible to 
compare the results between the inks: 
______________________________________ 
Ink A Ink B Ink C Ink D 
______________________________________ 
R (.OMEGA.) 
18.5 12.1 18.26 16.02 
R' (.OMEGA.) 
17.5 10.4 15.5 11.2 
R" (.OMEGA.) 
11 9.0 9.3 8.7 
.increment..sub.1 
-5.4 -14.0 -14.8 -30.0 
.increment..sub.2 
-37.1 -13.5 -40.0 -22.3 
______________________________________ 
with: 
R: resistance of the ink in Q after 11 minutes under microwaves; 
R': resistance of the ink in Q after one 12-minute sterilization cycle; 
R": resistance of the ink in Q after a second 12-minute sterilization 
cycle. 
##EQU1## 
Bearing in mind what has been stated above, the optimum dosimeter ink must 
give a maximum change in its resistivity between 11 and 12 minutes 
(corresponding to the sterilization period), and must give as little 
change as possible in its resistivity after the end of the cycle, for 
example less than 10 or 20%. 
From the above table it is therefore concluded that ink D fulfills best the 
conditions determined by the inventors, since it exhibits a change of the 
order of 30% between 11 and 12 minutes and since the change in resistivity 
with time is of the order of 20%. 
The crosslinking kinetics of ink D therefore constitute a good compromise 
between a sufficiently rapid change at the end of the sterilization cycle 
and a correct stability after the end of the cycle, as follows, 
furthermore, from the inspection of FIG. 7, when compared with FIGS. 4 to 
6. 
Let us note, however, that inks A, B and C, while less optimized for the 
present application, also exhibit acceptable results, making it possible 
to meet the criteria of the invention. 
This choice of ink therefore permits: 
immediate verifications of a proper sterilization of the flask by 
measurement of the electrical resistance of the test circuit as soon as 
the sterilization ends; 
subsequent, and especially very long term verifications of the 
sterilization of the flask, whatever its geographical location, insofar as 
a value of resistance which is lower than 20 Q is impossible to obtain 
without the flask having been treated with microwaves in appropriate 
conditions. 
However, the chosen ink formulation necessarily demands that the following 
parameters be taken into account: 
1) The sterilization must be done using microwaves and not using steam or 
another thermal means. 
2) The flask must be full of solution (whatever its kind) because the 
kinetics would be different with an empty flask. 
3) The nature of the container is important. The formulation made up for a 
glass flask must therefore be more or less adjusted in the case of, for 
example, a plastic pouch package. 
4) The rate of sterilization is a function of the microwave power density 
used. The use of 2 kW instead of 0.8 kW generators would therefore result 
in a reformulation of the ink in order to readjust the crosslinking and 
sterilization kinetics. 
Such adjustments are within the competence of a person skilled in the art, 
especially when employing the optimization method developed by the 
inventors. 
Once again, one of the chief advantages of a process according to the 
invention is to permit the individual and continuous control of the state 
of sterilization of the products by measurement of an electrical 
resistance, with the possibility of automatic rejection of dubious flasks, 
for example at the end of the treatment. 
It also permits a second on-site check using an ohmmeter, especially just 
before the flask is used, this providing the user with a high degree of 
safety.