Foods and beverages: apparatus – Electric – radiant or vibrational treating means
Reexamination Certificate
1999-11-12
2001-01-30
Simone, Timothy F. (Department: 1761)
Foods and beverages: apparatus
Electric, radiant or vibrational treating means
C099S358000, C099S483000, C099SDIG014
Reexamination Certificate
active
06178880
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a treatment chamber in which a homogeneous pulsed electrical field is generated inside a pumpable product. This product is pumped continuously through the chamber in which a treatment is performed. Application of so-called pulsed electrical field treatment is used as a mild preservation method for foodstuffs and pharmaceuticals, In addition to preservation it also can be used to invoke pores in membranes of cellular structures to promote the transport of (macro-) molecular components across the membrane.
BACKGROUND OF THE INVENTION
The application, process description and several embodiments of so-called pulsed electrical field (PEF) treatment chambers for mild preservation have been discussed in literature. Examples can be found in U.S. Pat. Nos. 5,690,978, 5,662,031, 5,447,733, 5,235,905 and DE-3.708.775. The treatment can be performed by pumping the product through a chamber in which an intense pulsed electrical field is generated. In case treatment is applied as a mild preservation method it seems that most vegetative micro organisms (bacteria, yeasts and fungi) are inactivated at a level of typically 30 kV/cm at temperatures that are less than required in a conventional heat pasteurization process. After treatment the organisms have been found not to reproduce. The total time of a treatment depends on the pulse duration and shape, the total number of impulses tat are applied during the residence time in the treatment zone and the volume flow rate of product through the chamber. In order to achieve a sufficient intense field strength temporarily, high voltage pulses of typically microsecond duration are generated in an auxiliary electronic pulse generating system.
The total duration of the treatment is typical in the range of 10-300 microseconds and depends on the specific application. When electrical pulse treatment is performed as a mild preservation method the required reduction of microbiological counts, the type of product and the specific contamination have to be considered, It is preferred to apply several pulses within the treatment zone. High electrical fields should be imposed typically for a total duration in the range of 2 to 200 microseconds. For other field of applications of a treatment, as e.g. the enhancement of induced mass transport through biological membranes by electroporation, the required electrical field strength is in general less than 30 kV/cm.
The electrical fields are imposed to the product using an electrode structure to which high voltage pulses are applied. The electrodes are in physical contact with the product and are contained in a mechanical construction through which the product is pumped. The combination of the electrodes, electrically isolating holders and sealing is referred to as treatment chamber. When produce is pumped through this chamber a treatment is performed in the resident period by applying short high voltage pulses to the electrodes at a sufficiently high rate.
SUMMARY OF THE INVENTION
In this specification a system is considered where a product is pumped through a treatment chamber and where a stationary state in product flow and temperature is reached. In principle, no stationary conditions can be met since pulses are repeatedly applied in the process. However, the energy input by the short electrical pulses can be time averaged. In practice steady state flow and temperature conditions can be reached. In this discussion &phgr; is denoted as the volume flow rate of the product and V the effective volume of the treatment chamber. The average residence time t of a fluid element in the device is given by t=V/&phgr;. In this time the pulse treatment takes place. The required electrical peak power in treatment is given by Pp=&sgr;E
2
V, where V denoted the effective volume of the treatment chamber, &sgr; the mean product conductivity and E the average of the electrical field strength across the treatment device. The mean consumed electrical power is given by the relation Pc=&sgr;E
2
&tgr;&phgr; where &tgr; is the total treatment time of a fluid element. The latter is the total duration tat a high electrical field is imposed on the transversing product. In case that square wave pulses are used with a duration &tgr;
p
the total treatment time is defined as &tgr;=N*&tgr;
p
with N the mean number of pulses applied on the product when resident in the chamber.
The electrical energy is converted into heat inside the product due to Ohmic heating. In general the temperature increase during treatment can be kept below 30 degrees centigrade. However, a small temperature increment results in a strong increase in electrical conductivity of the product. This is known to be the case for many different solutions containing minerals and for foodstuffs in particular. As an illustration: the electrical conductivity of a 0.75% KC
1
solution increases by more than 15% in the temperature range of 18-25 degrees centigrade (CRC, Handbook of Chemistry and Physics, 72
nd
edition, 1991-1992).
The required peak power for a treatment of a column of product with length L and cross-section A is given by P=&sgr;E
2
AL=&sgr;E
2
V in case a homogeneous electrical field distribution is assumed. For a column of product of conductivity &sgr; in which an electrical field is present with the direction along its length, the ohmic resistance is given by R=L/&sgr;A. In general only die real part of the electrical impedance is of importance. The parasitic capacity and self-inductance in a column of product is therefore neglected in this discussion.
The geometry of the treatment chamber determines the largest possible size and shape of a channel through which the product can flow. In principle the size and shape of the channel should be such that the flow resistance is minimised. The magnitude of the electrical field strength and the uniformity of the electrical field distribution are not only determined by the dimensions of the chamber. Also the distribution of the electrical conductivity of He product across the treatment chamber should be considered.
When a local current density of magnitude j is generated in a fluid element of electrical conductivity &sgr;, an electrical field of strength E across the element is imposed. Its magnitude is given by j=&sgr;E (Ohms law). In practice it is important that all fluid elements that are pumped through a treatment chamber should receive a minimal treatment. That is, both the treatment time and the magnitude of the applied electrical field should be sufficiently high. The design of the treatment device and the electrode configuration determine how the electrical field strength is distributed in tie product stream. It is preferred to create a uniform electrical field distribution across the treatment zone.
In U.S. Pat. No. 5,235,905 a coaxial treatment device is considered. A major disadvantage of this design is the limited width of the annulus trough which the product can flow. In addition it has relatively large electrode surfaces. U.S. Pat. No. 5,690,978 describes a co-linear treatment chamber. A disadvantage of the latter is its non-uniform distribution of the electrical field in the treatment zone. For a fixed diameter of this type of chamber a so-called gap distance has to be chosen. In case small gap distances (with respect to the diameter) are considered the electrical field distribution at the entrance and exit of the treatment zone is highly non-uniform. In case the gap distance is enlarged the non-uniformity in the field distribution is of less importance. However, in a treatment chamber with a large gap distance a fairly large temperature gradient will appear across the treatment zone at steady state conditions. This has a negative impact on the electrical field distribution across the fluid column inside the chamber. This is related to the fact that the electrical conductivity of products and solutions containing minerals in general are temperature dependent. The electrical conductivity for these products in
Bartels Paul Vincent
Mastwijk Hendrikus Cornelis
Instituut Voor Agrotechnologisch Onderzoek (ATO-DLO)
Simone Timothy F.
Young & Thompson
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