AU8145482A - Sterilization process and apparatus - Google Patents

Description

-4-

STERILIZATION PROCESS AND APPARATUS

BACKGROUND OF THE INVENTION

This invention pertains to a process and an apparatus for sterilizing by killing bacteria and simi¬ lar organisms within a host.

Macy, U.S. patent No. 2,081,243, discloses apparatus for pasteurizing liquids only, such as milk, by "slow and intensive impulses of alternating electric current". These he produces by an interrupter at 3.3 times per second; the "interruptions literally exploding the bacteria". The temperature of the milk may also be appreciably raised by the process. The apparatus is of the coaxial flow-through type, having numerous parts.

Golden, 1,934, 703, discloses a trough-like " electrical sterilizing apparatus, having an internal central electrode and a pair of external flux- concentrator electrodes.

Smith, 1,975, 805, discloses a dry type steri¬ lizing apparatus, in which a high voltage upon a pair of rotating electrodes, coaxially spaced on opposite sides of a conveyor that carries the material to be steri¬ lized, act upon the material.

OMPI SUMMARY OF THE INVENTION

A host, or a plurality of hosts, containing organisms to be killed are surrounded by a weak electro¬ lyte within the influence of plural electrodes. Successive high-density current pulses of alternating polarity, each having a duration in the microsecond range, are caused to occur approximately 120 times per second. The organisms are electrocuted by the passage of electric current through the host and concomitantly through the organisms. Bacteria are not "exploded". The explosive mode of destruction^* of the prior art likewise destroys the cell structure of the host substance, reducing its quality.-

The current pulses are formed by electronic switches, suited to give a rapid rise of electric current, such as phase-controlled silicon controlled rectifiers (SCRs) .

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic electrical diagram of a single-phase alternating-current powered apparatus for accomplishing processing according to this invention. Fig. 2 is a current vs. time waveform that illustrates the operation of the circuit of Fig. 1. The time scale is not to scale for sake of clarity.

Fig. 3 is a schematic electrical diagram of a dual power-supply powered apparatus.

Fig. 4 is a current vs. time waveform for the circuit of Fig. 3; not to time scale.

Fig. 5 is a schematic diagram of a circuit similar to that of Fig. 1, but including a pair of capa¬ citors. Fig. 6 is a waveform for the circuit of Fig.

5; not to time scale.

Fig. 7 is a diagram of a circuit combining the circuits of Figs. 3 and 5.

Fig. 8 is the waveform for the circuit of Fig. 7; not to time scale. Fig. 9 is a perspective view of a single-phase

apparatus for processing according to this invention.

Fig. 10 is the same view for a three-phase apparatus.

Fig. 11 is a three-phase schematic circuit.

- - DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In Fig. 1 numeral 1 indicated a source of alternating current. This may be the usual electric utility source, typically of 120 volts rms, 230 volts rms, or^" higher. It is the flow of electric current that accomplishes the processing of this invention. The voltage required to obtain a desired current flow depends upon the spacing of the electrodes in the treat¬ ment cell, the conductivity of the electrolyte, and the nature of the material being treated. A typical current density is 1.25 amperes rms per. square centimeter of material treated. This value may be altered, as will hereinafter be evident. Treatment cell 2 typically has parallel spaced electrodes 3 and 4. These are co nected to power source 1 through oppositely poled phase-controlled rectifiers, or thyristors, 5 and 6.

In Fig. 2, dotted sinusoidal wave 8 represents a cycle of voltage of source 1. Solid line spike 9 represents current. The current is limited to a brief interval of time, such as 10% of each half-wave cycle. It is initiated by late triggering of the phase- controlled rectifiers; for instance, by an unijunction circuit 13, of the relaxation oscillator type. Such a circuit is known, being illustrated in the "Transistor

Manual", G.E., Seventh Edition, 1964, pages 312-16.

Fig. 2 is not a true time scale, for sake of clarity. The duration of the current "on" cycle is typically less.

It is desirable that the type of rectifier cho¬ sen be capable of very rapid turn-on of current, pre¬ ferably a few microseconds. A steep turn-on wavefront is most effective in killing unwanted organisms in the host substance.

In Fig. 3, elements 2, 3, 4, 5 & 6 are the same as in Fig. 1. However, power source 1 is supplanted by relatively high voltage power supplies 10 and 11. These supplies provide direct current and charge capacitors 12 and 14 through resistors 15 and 16. The time constants of these resistor-capacitor combinations are equal and are such as to allow a full charge of the capacitors fifty or more times per second. The capacitors are of equal capacitance.

It is desirable that the rise time of the current spike that is obtained by the discharge of the capacitors (one at a time) be extremely rapid. Thus, the inductance of the capacitor, rectifier and treatment cell should be a minimum. This is enhanced by employing low inductance capacitors. A capacitance of 100 micro¬ farads (uf) for each capacitor is suitable.

Additionally, pulse-forming network 33 may be placed in series with treatment cell 2 to increase the rate of change of current rise, dl/dt, thereby obtaining enhanced results. Network 33 is especially effective where there are air bubbles or insulating material in the host. There are capacitative regions. By increasing the dl/dt, Fourier analysis of the wavefront shows that there is generally greater energy at the higher frequen¬ cies. That results in good current conduction through the capacitative regions.

Operating voltages of a thousand volts or more may be produced by power supplies 10 and 11. Note that - these are connected to the capacitors in opposite polarity.

Fig. 4 shows the current waveform as a function of time. It is to be noted that the current spikes are extremely sharp and of brief time duration. The duty cycle is typically a small fraction of one percent.

Alternating current is utilized according to this invention to prevent polarization and plate-out effects in cell 2. Thus, both net positive and negative current flows are equal.

Fig. 5 shows the curcuit for an improved modi¬ fication of Fig. 1. Elements 1 through 6 are the same or similar to the same numbered elements in Fig. 1. Elements 12' and 14' are the same or similar to elements 12 and 14 of Fig. 3.

New element 18 is a diode, typically solid- state, that ceases to conduct at the maximum value of voltage waveshape 8 in Fig. 6. The peak voltage charge is thus retained on capacitor 12' and the current spike 21 at discharge is proportional to the peak voltage of waveshape 8. The current continues at reduced amplitude 9', as in Figs. 1 and 2. Diode 19 performs in the same manner with respect to capacitor 14', but in the opposite polarity, forming current spike 21'.

5 Fig. 7 combines the circuits of Figs. 3 and 5. Analogous to those circuits, high amplitude current spi¬ kes 22 and 22' of opposite polarity are formed, as seen in Fig. 8. The reduced current 9' is also present.

A common return connection 20 is provided in

10 Fig. 7.

As before, the use of power supplies 10' and 11' allows current spikes of relatively great amplitude to be produced.' The reasons for great and small current amplitudes in carrying out the process of this invention

5_. are given later herein. Again, a pulse forming network 33 may be placed in series with the treatment cell to increase -the value of dl/dt. These networks utilize inductors and capacitors and are well ^"known in the art of laser flash tube power supplies.

10

Fig. 9 illustrates in perspective a basic . single-phase treatment cell 2. The shape is that of a hollow rectangular parallelepiped. In the figure the two ends are in phantom, so that the interior can be

5 seen.

Electrodes 3 and 4 are the same as those sche¬ matically shown in the earlier schematic circuit diagrams. The electrodes are normally simply affixed to the adjacent inner surface of the parallelopiped sides.

10 However, these may be moved closer together when the internal electrical resistance of the cell is high. An electrolyte 24 fills cell 2. For solid host material, such as a shellfish 25, the electrolyte may be a salt-bearing liquid, such as sea water. Normally, many items for treatment are present at one time; only one has been shown in Fig. 9 for sake of clarity.

The electrodes are formed of materials that are inert to the electrolytes and the host materials to be processed. One such material is carbon, which may have a "ϋ" channel edge contact of stainless steel, to which connecting wires are attached. Other successful electrode materials include stainless steel, tantalum and titanium.

The host material to be treated may be handled in batch lots in rectangular baskets. Such baskets, for best results, should be non-conductive except for the two sides parallel to the treatment cell electrodes, which sides should be electrically conductive.

Alternately, a slow flow-through hydraulic arrangement should be used, in which the material is incoming at one end of the cell and outgoing at the other.

A preferred embodiment of a treatment cell for three-phase apparatus is shown in Fig. 10. Part of the outer container enclosure 27 has been broken away to show the inner construction, and the forward end is sec¬ tional for the same purpose.

The objective in a three-phase cell is to arrange three electrodes in a symmetrical configuration, such that the electrical flux charges between the electrodes are substantially uniform over the working areas. In Fig. 10, segmented electrodes 28, 29 & 30 occupy much of the volume of outer container 27. This allows parallel electrode surfaces to be presented from one electrode to the other. This is typically a flow- through embodiment. The electrolyte and the material treated are flowed through each of the three channels, as indicated by the arrows in the channel revealed by the break-away portion of the container.

The electrodes are composed of the same or similar materials to those used in the single phase apparatus described above.

Central cylindrical surface 31 is of electri¬ cally insulating material, such as a plastic or glazed refractory material. This prevents a flow of electro- lyte in the central area between the electrodes where the flux charges would otherwise be higher than desired.

Fig. 11 shows a three-phase schematic electri¬ cal circuit. Therein, entity 35 accepts conventional three- phase, or polyphase, power and transforms it to a higher or lower voltage as required to obtain effective energi¬ zation of treatment cell 27. Both delta and "Y" con¬ nected windings are shown in entity 35 in a generic showing of a transformer. Either mode of connecting the windings may be used. Also, the transformer isolates the apparatus- of this invention from the power mains for safety reasons. The output of entity 35 passes through three conductors into control circuitry 36. This is comprised of three separate single-phase control circuits, such as are shown in Figs. 1, 3, 5 or 7. These separate cir¬ cuits are controlled in concert so that each of the three phases is regulated in the same manner.

The output of control circuitry 36 is indivi- dually connected to electrodes 28, 29 and 30 of the three-phase treatment cell 27, for the electrical energization thereof.

The circuits, apparatus and operating parame- ters of this invention "electrocute" the unwanted orgnaisms. Bacteria are not "exploded", contra to the prior art. Importantly, the cellular structure of the host material is not altered by processing according to this invention. This has been determined by optical microscopy.

It will be understood that a significant alteration in cellular structure is undesirable in the value of the host material as a comestible.

Numerous substances may be treated, among which are shellfish, fish, fruits, vegetables, fowl and meats. The chief advantage of treatment is to reduce the bacterial or other organism count in the host material, thereby to prolong the time before spoilage sets in, and the time prior thereto during which flavor- ful taste is retained. Very significant reductions in bacterial count by this processing have been realized.

The preferred mode of processing was evolved as indicated by the successive schematic diagram figures herein.

First was phase control. When it became apparent that by increasing the conductivity of the electrolyte, thereby shortening the necessary -duration of the conduction periods, the sterilization effect was greatly enhanced a next step evolved.

This was pulse discharge. With sufficient capacitance in the two storage capacitors there was a very significant killing effect on^'the bacterial popula¬ tion.

A third step embraced pulse discharge followed by conduction angle, as 22 and 9^*' in Fig - 8. This gives maximum sterilization effect at minimum heating of the material.

Utilizing the embodiment of Fig. 1, the results for shrimp were:

Control sample 1,600,000 bacteria/gram

Average of treated samples 201,250 bacteria/gram Bacterlogical tests by an independent laboratory. Utilizing the embodiment of Fig. 3, the results for shrimp were:

Control Sample 2,000,000 bact./gram

Average of treated samples 150,000 bact./gram Utilizing the embodiment of Fig. 5 or 7, the results for shrimp were:

Control sample 350,000,000 bact./gram

Average of treated samples 320,000 bact./gram Ditto, the results for scallops were: Control sample 40,000 bact./gram

Average of treated samples zero*

*(At or below the lower limit of laboratory testing resolution) It is possible to establish a treatment proce-

OMPI dure for each substance to be treated. This is based on the substance treated, its initial condition, the degree of sterilization desired, and the parameters of the apparatus used. The latter includes the circuit used and the configuration of the treatment cell. It is unlikely that the treatment procedure needs to be varied during a processing run with a given host material.

Certain examples of the parameters giving the above test results can be given.

Utilizing the embodiment of circuit of Fig. 5 and the cell of Fig. 9, the current was 1.5 amperes per square centimeter (amp/cm 2 ) rms, which was also 60

2 amp/cm ^• peak. The time of treatment was approximately 3 seconds. Salt water was the electrolyte and the electrode spacing was 7 cm.

The parameters were the same for the scallops processing.

It has been- found that a live lobster can be electrocuted and the bacterial count reduced at the same time. This is a rapid, effective and humane mode of processing, rather than the use of boiling water.

The invention is effective in treating materials containing pathogenic multicellure organisms. In certain raw fish, for example, there can exist such organisms, which, when consumed by humans are quite harmful. One is dibothriocephalus latus, a worm found 'in white fish.. ^* Another, is tricini infested port., which causes trichinosis in humans.

OMPI As previously discussed the use of alternating current with rapid rise times allows current flow through voids within the host and through the shells of shellfish. These are capacitors in the electrical sense, through which alternating current easily flows.

Usual tap water may be used as an alternate to sea water as an electrolyte. The conductivity of tap water can be increased if required, by the addition of certain quantities of ionizable chemical salts, or even acid or base chemicals.

For maximum efficiency the electrlyte has slightly less conductivity than does the host material being treated. This causes the current to concentrate to a nominal degree through the host material.

In applying the invention, the greater the peak amplitude of the current the shorter is the processing time. If a reduced degree of sterilization is desired, this is obtained by a shorter processing time or reduced amplitude of the current.

As to the voltage, the greater is the separa¬ tion of the electrodes and/or the lower is the conduc¬ tivity of the electrolyte, the more voltage is required to obtain a given flow of current.

The term "organism" is generic as used herein. It includes bacteria, yeasts, viri, parasitic worms, insect larva and eggs, and spores of bacteria. By and large, any living organism within the host material is subjected to the current pulses and is electrocuted.

These living organisms include the range of from micro-

_,OMPI Inherent in this invention is the concept of phase control; that is, passing a current pulse through the host medium for only a brief period of each half cycle of incoming alternating current. This is as shown in Fig. 2.

Adding pulse discharges from capacitors, as 12 and 14 of Fig. 3, enhanced the killing effect of the process upon the unwanted organisms while reducing the power dissipation. Note Fig. 4. It was then found that a combination of the two current waveforms gave a superior killing effect than either of them alone. This is accomplished by the apparatus of Fig. 5 and is illustrated by the waveforms of Fig. 6, having portions 21 and 9'. Where greater killing effect is required the apparatus of Fig. 7 and the waveform of Fig. 8 is used. The current spike 22 is of greater amplitude than in Fig. 6.

Claims (14)

1. A process of kiling organisms resident within a host, which includes the process steps of;

(a) immersing the host in an electrically conduc¬ tive liquid, (b) flowing an electric current through both the host and the liquid, in the form of successive strong pulses of opposite electrical polarity, each of said pulses having a time duration in the microsecond range, with a- repetition rate of greater than approximately fifty pulses per second, and

(c) continuing the flow of current for a period of up to several seconds.

2. Apparatus for killing organisms resident within a host, comprising;

(a) an electrically non-conductive container (2 or 27) for receiving the host,

. (b) an electrically conductive liquid (24) within said container and surrounding the host, ^' (c) electrodes (3, 4 or 28, 29, 30) symmetrically disposed within said container and contacting said liquid, and

(d) a pulse^'d electric .source (1, 5, 6, 13, etc.) connected to said electrodes to pass electric current through the host and concomitantly through the orga¬ nisms, in the form of short pulses of successive oppo¬ site polarity.

3. The apparatus of claim 2, in which; (a) the electrically conductive liquid is tap water.

4. The apparatus of claim 2, in which;

(a) the electrically conductive liquid is sea water.

5. The apparatus of claim 2, in which;

(a) said electrodes are equidistantly spaced, one from the other.

6. The apparatus of claim 2, in which said pulsed electric source comprises;

(a) a source of alternating current power (1) ,

(b) a pair of thyristors (5, 6) oppositely poled and connected to said source of alternating current power, (c) an electrical connection from both of said thyristors to one said electrode (3) , and

(d) an electrical connection from the. other said electrode (4) to said source of alternating current power.

7. The apparatus of claim 6, which additionally includes;

(a) an oscillatory trigger means (13) connected to each of said thyristors (5, 6), to trigger each thyristor into conduction near the end of each half- cycle of current from said source of alternating current power (1) .

8. The apparatus of claim 7, in which said oscillatory trigger means comprises;

(a) a unijunction relaxation oscillator.

_OMPI

9. The apparatus of claim 2, in which said pulsed electric current source comprises;

(a) a source of alternating current power (1) ,

(b) a pair of thyristors (5, 6), oppositely poled, connected to said source of alternating current power through like poled diodes (18, 19), and

(c) a pair of capacitors (12', 14'), connected to said thyristors to accumulate oppositely poled electric charges, and also connected to said electrodes (3, 4) through said thyristors to pass said current pulses through said host and said organisms.

10. The apparatus of claim 2, in which said pulsed electric source comprises;

(a) a pair of oppositely poled direct current power supplies (10, 11),

(b) a capacitor (12, 14) separately connected across each said direct current power supply,

(c) oppositely poled thyristors (5, 6) connected to said capacitors and to one of said electrodes, and (d) a return connection (20) from the other said electrode (4) to said direct current power supplies.

11. The apparatus of claim 10, which additionally includes; (a) a diode (18, 19) connected to each of said oppositely poled thyristors, in the same polarity as that of said thyristors, and

(b) a source of alternating current power (1) con¬ nected to both of said diodes and to said return connec- tion.

12. The apparatus of claim 2, in which said pulsed electric source includes; (a) a pulse-forming network (33) interposed betwen the source (5, 6, etc.) and said electrodes (3, 4) to enhance the rate of change of current with respect to time (dl/dt) fo the pulses of said source.

OMPI

13. The apparatus of claim 2, in which said container comprises;

(a) an outer cylindrical enclosure (27) ,

(b) plural equally extensive and equally spaced apart electrodes (28, 29, 30) , coextensive within said enclosure, and

(c) a connection from each of said plural electro¬ des to a phase of a pulsed electric source (35, 36) having plural phases.

14. The apparatus of claim 2, in which said pulsed electric source comprises;

(a) a plural phase source of alternating current electrical enegery (35) ,

(b) plural control circuits (36) individually con¬ nected to a phase of said plural phase source, and,

(c) a connection from each of said plural control circuits to a separate electrode of a container (27) having plural electrodes (28, 29, 30).