Variable frequency automated capacitive radio frequency (RF) dielectric heating system

• US 20030205571A1
• Filed: 04/24/2003
• Published: 11/06/2003
• Est. Priority Date: 04/21/1998
• Status: Active Grant

• Current Assignee: Benjamin A. Flugstad, Oregon State University (Oregon University System)
• Original Assignee: Benjamin A. Flugstad, Oregon State University (Oregon University System)
• Inventors: Flugstad, Benjamin A., Kolbe, Edward R., Park, Jae W., Zhao, Yanyun, Wells, John Henry, Ling, Qingyue
• Granted Patent: US 6,784,405 B2
• Time in Patent Office: Days
• US Class Current: 219/497
• CPC Class Codes: A23B 4/01 by irradiation or electric treatment with or without shaping, e.g. in form of powder, granules or flakes
• CPC Class Codes: A23B 9/04 by irradiation or electric treatment
• CPC Class Codes: A23L 3/01 using microwaves or dielectric heating
• CPC Class Codes: H05B 6/50 for monitoring or control

Abstract

A food product is heated by maintaining the food product in an AC electrical field generated by an RF signal. As the heating takes place, maximum energy is delivered to the food product using automatic impedance matching to adjust the rate of the process.

Claims

1. A method for heating a medium that includes a food product, the method comprising:

subjecting the medium to an AC electrical field generated by an RF waveform applied at a selected frequency that heats the medium;

measuring an effective load impedance initially dependent upon the impedance of the medium;

comparing the effective load impedance with an output impedance of a signal generating unit that generates the RF waveform; and

automatically adjusting the effective load impedance to match the signal generating unit output impedance.

2. The method of claim 1 wherein the signal generating unit output impedance is a predetermined constant.

3. The method of claim 2 wherein the signal generating unit output impedance is about 50 ohms.

4. The method of claim 1 wherein measuring the effective load impedance includes measuring a voltage across the medium and the resulting electric field developed in the medium.

5. The method of claim 1 wherein measuring the effective load impedance includes measuring a current of the RF waveform applied to the medium.

6. The method of claim 1 wherein measuring the effective load impedance includes measuring a voltage and a current of the RF waveform applied to the medium, and determining a phase angle based on the measured voltage and measured current.

7. The method of claim 1 wherein measuring the effective load impedance includes measuring a forward power level of the RF waveform applied to generate the voltage across and current through the medium and a reverse power level of the RF waveform reflected from the effective load.

8. The method of claim 7 further comprising calculating a voltage standing wave ratio from the forward power level and the reverse power level.

9. The method of claim 8 further comprising repeating the act of automatically adjusting the effective load impedance until the voltage standing wave ratio is about 2:

1 or less.

10. The method of claim 8 further comprising repeating the act of automatically adjusting the effective load impedance until the voltage standing wave ratio is about 1:

1.

11. The method of claim 1 wherein automatically adjusting the load impedance to the signal generating unit output impedance includes adjusting the selected frequency of the applied RF waveform.

12. The method of claim 1 wherein automatically adjusting the effective load impedance to match the signal generating unit output impedance includes tuning a tunable impedance matching network connected to the load.

13. The method of claim 1 further comprising periodically measuring at least one temperature of the food product during heating, and using the measured temperature in automatically adjusting the effective load impedance to match the signal generating unit output impedance.

14. The method of claim 1 wherein the selected frequency of the RF waveform allows for a wavelength to be at least ten times greater than a longest geometrical dimension of the medium under test.

15. The method of claim 1 wherein the selected frequency of the RF waveform is in the range of 1 MHz-300 MHz.

16. The method of claim 1 wherein the food product in the medium is contained in packaging material.

17. The method of claim 1 wherein the food product is an unsprouted seed.

18. The method of claim 17 wherein the unsprouted seed is coated with an edible coating.

19. The method of claim 17 wherein the coating is HPMC-based or CC-based.

20. The method of claim 1 wherein the food product is a surimi seafood.

21. The method of claim 1 wherein the food product contains at least one meat.

22. The method of claim 1 wherein the food product is a seed, and the selected frequency of the RF waveform is greater than about 30 MHz.

23. The method of claim 1 wherein the food product is an ungerminated seed, and the heated ungerminated seed has a germination rate higher than a germination rate of an untreated seed.

24. The method of claim 1 wherein the food product is a seed, and wherein the heated seed has a measured microbial activity less than a measured microbial activity of an untreated seed.

25. The method of claim 1 wherein the food product is at least one seed, and the food product is heated for a duration of about 30 seconds or less.

26. The method of claim 1, wherein the food product is surimi seafood, and the food product is heated for a duration of about 3 minutes or less.

27. The method of claim 1 further comprising immersing the food product in a bath of deionized water.

28. The method of claim 27 wherein the immersed food product is heated while immersed in the bath.

29. The method of claim 27 wherein the immersed food product is heated while immersed in the bath having a temperature of about 75 to 85 degrees C.

30. The method of claim 1 further comprising cooling the food product under predetermined conditions after the food product reaches a desired final temperature.

31. The method of claim 30, wherein the food product is at least one seed, and wherein the desired final temperature is about 75 degrees C.

32. The method of claim 30, wherein the food product is surimi seafood, and wherein the desired final temperature is about 85 degrees C.

33. The method of claim 30, wherein a hold time at which the food product is maintained at the desired final temperature is about three minutes or less.

34. The method of claim 30, wherein the food product is at least one seed, and wherein a hold time at which the food product is maintained at the desired final temperature is about 30 seconds or less.

35. The method of claim 30, wherein the food product is at least one seed, and wherein after the food product reaches the desired final temperature, the food product is cooled to room temperature within about 4 minutes.

36. The method of claim 30, wherein the food product is contained in a package, and wherein the method further comprises immersing the package containing the food product in a water bath after the desired final temperature is reached to cool the food product and package.

37. The method of claim 36, wherein the water bath is maintained at a temperature below room temperature.

38. The method of claim 1, wherein the food product is at least partially frozen and heating the food product causes it to thaw.

39. A food product made by the method of claim 1.

40. A method for heating a food product, the method comprising:

subjecting the food product to an AC field produced by applying an AC waveform at a known frequency with a signal generating unit, the signal generating unit having a known, generally constant output impedance;

measuring an actual impedance of the food product;

determining an effective load impedance initially dependent upon the actual impedance of the food product, the effective load impedance being determined by at least one of measuring the voltage and current of the applied RF waveform and computing a phase angle difference, and measuring a forward power level of the RF waveform applied to the food product and a reverse power level of the RF waveform reflected from the food product with the signal generating circuitry;

comparing the effective load impedance with the signal generating unit output impedance; and

automatically matching the effective load impedance to the signal generating unit output impedance by at least one of adjusting the frequency at which the RF waveform is applied and tuning the tunable impedance matching network such that the effective adjusted load impedance is approximately equal to the signal generating unit output impedance.

41. A method for heating a food product, the method comprising:

maintaining a food product that is contained in packaging material in an AC electrical field generated by an RF waveform at a frequency not greater than 300 MHz provided by signal generating circuitry; and

controllably heating the food product by automatically maintaining an impedance match between the food product automatically matching an impedance of the food product and the signal generating circuitry.

42. The method of claim 41, wherein the food product is at least partially frozen, and wherein the act of controllably heating the food product includes thawing the food product.

43. A method for heating a food product comprising:

maintaining a food product that is contained in packaging material in an AC electrical field provided by an RF waveform;

periodically sensing the impedance of the food product and the packaging material to produce a sensor output signal;

determining impedance mismatch based on a difference between a most recently sensed impedance and a known impedance, and generating a corresponding control signal output that corresponds to the difference with a computer; and

as the food product and the packaging material increase in temperature, adjusting the frequency of the RF waveform by the control signal output of the computer such that the sensed impedance matches the most recently sensed impedance.

44. A method for killing an undesired organism associated with a food product, the method comprising:

maintaining a food product that is contained in packaging material in an AC electrical field provided by an RF waveform;

periodically sensing the impedance of the food product and the packaging material to produce a sensor output signal;

determining impedance mismatch based on a difference between a most recently sensed impedance and a known impedance, and generating a corresponding control signal output that corresponds to the difference with a computer;

as the food product and the packaging material increase in temperature, adjusting the frequency of the RF waveform by the control signal output of the computer such that the sensed impedance matches the most recently sensed impedance.

45. A method for heating a food product comprising:

testing a first sample of a food product to determine impedance of the food product at several different temperatures;

storing the resulting impedance vs. temperature information for the food product in a memory of a computer;

flowing a signal through a second sample of the food product, the signal being at an RF frequency not greater than 300 MHz for the food product;

sensing the impedance of at least one portion of the second sample;

determining, by operation of the computer, a relationship between the most recently sensed impedance of the food product and a heating rate of the food product; and

adjusting the heating rate of the food products based on the relationship.

46. A method for treating seeds, the method comprising:

maintaining seeds in an AC electrical field provided by an RF signal at a frequency not greater than 300 MHz; and

controllably heating the seeds while matching an impedance of the seeds to a predetermined constant.

47. A capacitive RF dielectric heating apparatus for treating food products, the apparatus comprising:

a source of an AC RF signal at an RF frequency not greater than 300 MHz, the source being connected to a pair of electrodes on opposite sides of a product treatment zone to cause an RF signal to generate an AC electric field in the product treatment zone;

a frequency controller to adjust the frequency of the RF signal between different RF frequencies;

a mathematical model that predicts impedance of the food products as a function of temperature;

an impedance sensor to sense an impedance of the food products;

a computer programmed to receive impedance data from the impedance sensor, to process the impedance data using the model for the product, and to apply a control signal to the frequency controller to adjust the frequency of the RF signal to match the sensed impedance to a predetermined impedance.

48. The apparatus of claim 47 wherein the source of the AC signal includes a frequency generator connected to a power amplifier.

49. The apparatus of claim 48 further comprising an impedance matching network tunable to match the output impedance of the power amplifier to the impedance of a load comprising the pair of electrodes and any product in the product treatment zone between the two electrodes.

50. The apparatus of claim 48 further comprising a directional coupler coupled to a transmission line leading from the power amplifier to receive signals proportional to levels of power supplied from the amplifier.

51. The apparatus of claim 50 wherein the directional coupler includes a forward power portion that receives signals proportional to the power supplied by the amplifier and a reverse power portion that receives signals proportional to power reflected back to the amplifier.

52. The apparatus of claim 51 comprising a measurement instrument connected to receive the respective signals from the forward and reverse power portions.

53. The apparatus of claim 52 wherein the measurement instrument computes a voltage standing wave ratio.

54. The apparatus of claim 53 wherein the measurement device computes a load reflection coefficient.

55. The apparatus of claim 52, wherein the computer is connected to and receives input signals from the measurement instrument, and the received signals are processed with the temperature data in generating the control signals.

56. A capacitive RF dielectric heating apparatus for treating food and seed products, the apparatus comprising:

a source of an AC RF signal at a frequency not greater than 300 MHz;

a first electrode that is connected to the source;

a second electrode that is connected to the source and that is spaced from the first electrode so that a product treatment zone is defined between the electrodes and an RF signal flows through the product treatment zone;

impedance matching means for matching an impedance of the heated food or seed products to a predetermined constant by adjusting the frequency of the RF signal.

57. The apparatus of claim 56 wherein each of the first and second electrodes have multiple electrode elements which are electrically isolated from one another, individual elements of the first electrode being located opposite corresponding individual elements of the second electrode to provide multiple pairs of opposed electrode elements.

58. The apparatus of claim 56 wherein a computer-controlled switch is connected in the RF signal supply circuit for each pair of electrodes so that individual electrode pairs can be turned off and on by the computer.

59. The apparatus of claim 56 further comprising temperature sensors, and wherein at least some of the temperature sensors are supported on the first electrode.

60. A capacitive dielectric (RF) heating apparatus from treating food products, the apparatus comprising:

at least one pair of spaced-apart electrodes defining therebetween a food product treatment zone that can accommodate a food product to be treated;

signal generating circuitry connected to the electrodes, the signal generating circuitry being capable of providing an AC RF signal to charge the electrodes and generate an AC electric field in the food product treatment zone;

impedance measuring circuitry connected to the electrodes and to the signal generating circuitry, the impedance measuring circuitry measuring an impedance of the electrodes and the food product within the food product treatment zone; and

a controller linked to the impedance measuring circuitry and the signal generating circuitry, the controller controlling the signal generating circuitry and the AC electric field generated thereby based on the impedance measured by the impedance measuring circuitry.

61. The apparatus claim 60, wherein the signal generating circuitry includes a variable frequency RF signal generator.

62. The apparatus of claim 60, wherein the signal generating circuitry includes an amplifier connected to the variable frequency RF signal generator.

63. A capacitive RF dielectric heating apparatus for treating food and seed products, the apparatus comprising:

a source of an AC RF signal at an RF frequency not greater than 300 MHz, the source being connected to a pair of electrodes on opposite sides of a product treatment zone to cause an RF signal to flow through the product treatment zone;

a frequency controller to adjust the frequency of the RF signal between different RF frequencies;

a mathematical model that predicts Debye resonance frequency as a function of temperature for a product to be heated by the apparatus;

a temperature sensor to measure the temperature of a product located in the zone;

a computer programmed to receive temperature data from the temperature sensor, to process the temperature data using the model for the product, and to apply a control signal to the frequency controller to adjust the frequency of the RF signal to a Debye resonance frequency of the product at the sensed temperature in the zone.

64. The apparatus of claim 63 wherein:

the model provides Debye resonance frequency information for multiple types of products; and

the apparatus further comprises an input device to tell the computer what type of product is located in the zone.

65. The apparatus of claim 63 further comprising:

a model that provides Debye resonance frequency information for multiple types of packaging materials; and

an input device to tell the computer whether a product located in the zone is contained in packaging material and, if so, what type of packaging material,

the computer being programmed to signal the frequency controller to adjust the frequency of the RF signal to an RF frequency that is not a Debye resonance frequency of the packaging material.

66. The apparatus of claim 65 wherein the input device input device also tells the computer what type of product is located in the zone.

67. The apparatus of claim 65 wherein:

the model is a data table that contains Debye resonance frequencies for at least one product at various temperatures.

68. The apparatus of claim 65 wherein the model is a mathematical model that predicts the Debye resonance frequencies for various products based on the dielectric properties of the products.

69. The apparatus of claim 65 further comprising a field strength controller that responds to signals from the computer to adjust the power level of the RF signal in the zone.

70. A capacitive RF dielectric heating apparatus for treating food and seed products, the apparatus comprising:

a source of an AC RF signal at a frequency not greater than 300 MHz;

a first electrode that is connected to the source;

a second electrode that is connected to the source and that is spaced from the first electrode so that a product treatment zone is defined between the electrodes and an RF signal flows through the product treatment zone;

multiple temperature sensors positioned to measure the temperature at multiple regions of a product located in the zone; and

a computer which receives temperature data from the temperature sensors, processes the temperature data using a model for the product, and adjusts at least one characteristic of the RF signal in response to changes in the sensed temperatures in the zone.

71. The apparatus of claim 70 wherein each of the first and second electrodes have multiple electrode elements which are electrically isolated from one another, individual elements of the first electrode being located opposite corresponding individual elements of the second electrode to provide multiple pairs of opposed electrode elements.

72. The apparatus of claim 71 wherein a computer-controlled switch is connected in the RF signal supply circuit for each pair of electrodes so that individual electrode pairs can be turned off and on by the computer.

73. The apparatus of claim 70 wherein at least some of the temperature sensors are supported on the first electrode.