2011 International Conference on Electrical Engineering and Informatics 17-19 July 2011, Bandung, Indonesia

High Voltage Pulse Generator Design with Voltage Control for Pulse Electric Field (PEF) Pasteurization

Ika Noer Syamsiana#1, Ratna Ika Putri*2 #State Polytechnic of Malang

Jl. Soekarno Hatta No.9 Malang, East Java, Indonesia 1ikanoers@yahoo.com

*State Polytechnic of Malang Jl. Soekarno Hatta No.9 Malang, East Java, Indonesia

2ikaputri_ratna@yahoo.com

Abstract— PEF is the newest non thermal technology for liquid and semi-solid food processing system. Based on previous researches, PEF has the advantage of maintaining better nutritional products, compared with the thermal system. PEF pasteurization requires a high voltage pulse generator that produces electrical shock. Some researches had been conducted to develop a generator circuit which has high efficiency and pulse wave that can be used in the PEF technology. The circuit of a high-voltage pulse generator can be designed either from Resistor-Capacitor (RC) circuits or Resistor-Inductor-Capacitor (RLC) circuits. The high-voltage pulse generator with high voltage transformer used in this paper will be controlled by microcontroller ATMEGA 8535. The paper also explores the design of a high-voltage pulse generator which is equipped with voltage regulation that can be set through the key pad. Constant high-voltage pulse frequency is applied and is controlled by ATMEGA 8535 microcontroller. Chamber is designed as a Tube-shaped chamber with a pasteurized apple juice capacity of 1.5 liters. The high voltage transformer used in this paper is a step-up transformer with a maximum output of 100KV. The designed equipment is expected to produce high voltage in range of 20KV to 100KV with an increase scale of 20KV. The examination result shows there was an error between the expected and the real output voltage from the device/equipment. The average error of this equipment is 5.41% and the smallest error on 60KV output voltage is 0%. This paper also carried out tests on the vitamins A and C content in apple juice to investigate the effectiveness of the designed equipment compared with thermal pasteurization. The content of vitamins A and C in apple juice resulted from PEF pasteurization is remain unchanged. This can be seen from the content of vitamin A. Before pasteurization, the content was about 0.5034µg/gr and after pasteurization treatment using PEF, the content lies between 0.3676 and 0.435µg/gr, while pasteurization thermal treatment cause the content become 0.23g/gr. In general, the PEF treatment will cause the content of vitamin A decrease although it is not significant. A significant decline emerged from apple juice which was pasteurized by heat. The heating process causes a degradation of vitamin A content. Meanwhile, for the content of vitamin C, it was about 0.5034µg/gr before pasteurization treatment and it lies between 11.25 - 18.95 mg/100ml after pasteurization treatment using PEF and thermal pasteurization is 4.00 mg/100ml. Keywords— PEF, Pasteurization, HV

I. INTRODUCTION National Health and Nutrition Examination Survey has

collected data for more than 5 years (1999-2004) and proved that each single adult person who consumes any part of apple such as its fruit, sauce and juice will have less risk from heart attack, stroke, high collesterol and blood pressure up to 30%. Moreover, according to Winneke O (2008), pectine from apple fruit and juice can increase human body potency to protect itself from cancer.

Apple juice has limited storage period. This is caused by microorganism contamination during processing time. These microorganisms, which are a lot in apple fruit, are known as S. Cervisiae. Therefore, it is very important to sterilize apple juice from those microorganisms in order to prolong the storage time and to get better quality [13].

In general, current preservation techniques for apple juice use thermal method called High Temperature Short Time (HTST) pasteurization. This type of pasteurization is done by boiling products at temperature between 76o C and 88o C for 25 up to 30 seconds. This thermal pasteurization is less efficient to kill microbes and can cause some enzymes and vitamins in apple juice will disappear [1]. According to Lee Hee-Kyu (2006), apple juice pasteurization using thermal method will decrease oxydation that will decrease vitamin C content and will loose original taste and flavour of apple.

One of proposed preservation technique choices is non thermal food processing using Pulse Electric Field (PEF). PEF is the newest non thermal technology for liquid and semi-solid food processing system. The process inlcludes giving short pulse for 1 to 100 µs from high voltage electric field between 20 and 80 kV/cm into food material which is placed in between two electrodes at room tempearture [16]. High voltage pulse presence on food material will destroy bacterial membrane that will kill the bacterial.

The most important parameter in processing system using PEF method is the process parameter including electric shock power, pulse width, pulse number and chamber design. According to Van Heesch Bert et al (2004), bacterial

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membrane cells will be damaged that will cause bacterial died if they receive electric shock more than 25 kV/cm with pulse width 100-200 ns. Electric shock power depends on high voltage pulse given to the chamber, whilst the number of pulses depends on processing time period [14]. To get appropriate electric shock to activate microorganisms, high voltage pulses that will be given to the chamber must be controlled.

II. HIGH VOLTAGE PULSE GENERATOR Several researches have been developed to produce a high

efficiency and pulse wave generator circuit which can be used in the PEF technology.

Fig 1. High Voltage Pulse Generator Circuit (a) Square Pulse Generator (b)

Square Pulse Generator with LC (c) Overdamped Oscillator Pulse Generator High-voltage pulse generators circuit can be designed

from some types of circuit such as Resistor-Capacitor circuit (RC) and resistor-inductor-Capacitor (RLC) circuit. According to De Haan SWH and Willock PR (2002) RLC circuit has higher efficiency than RC circuit. RLC circuit reaches maximum efficiency 40-50%, while the RC circuit reaches only 38%. Maximum efficiency of RLC circuit depends on the ratio of threshold voltage (Vth) and the initial capacitor voltage (VCO).

VCO optimal value for the RLC circuit must be 1.6 to 3 times larger than the Vth. While the highest efficiency for some forms of pulse signal is square pulse that has a maximum efficiency reached 100%. The maximum efficiency of square pulse depends on short rise time pulse.

For the over damped oscillator pulse, the maximum efficiency only reach 52%. Based on energy performance comparison, the square pulse has maximum energy efficiency by adjusting the pulse rise time as short as possible.

Rise time setting depends on the selection of the value of resistor and capacitor components which are used. Figure 1 shows a series of pulse generator circuits and their generated pulses.

Meanwhile, according to Lee Hee-Kyu (2006) the use of RLC circuit as a source of PEF as an impulse wave generator will effectively kill S. cerivisiae in apple juice if the circuit can generate 20KV/cm high voltage pulses.

The use of PEF effectively kills microorganisms without loss of vitamin C, flavour and original taste of apple juice.

This impulse pulse generator consists of a high voltage DC power supply that supplies 0.1 μF capacitor.

The capacitor will remove the charge into the electrode by setting a controller. Time constant of the pulses generated depends on the value of resistors and capacitors. They have been determined by fixed beforehand, but the inductor value can be set to generate produce a good oscillation on the voltage pulse output.

A high voltage pulse generator with RC and RLC circuit generates oscillations and rise time pulse which depends on the value of used resistors and capacitors component. The circuit is then developed by using high voltage (HV) transformers.

According to Liang Ziwei et al (2006) high-voltage pulse generator with a HV pulse transformer has a shorter rise time than that with RLC circuit. Rise time pulse circuit with HV transformers are in nanoseconds range.

This research applies a HV transformer in 30kV high voltage DC pulse generator for apple juice processing.

The power supply comes from 110V alternating voltage (AC) which is passed through a step-up transformer to increase voltage. The step-up transformer output voltage is passed through rectifier circuit to generate low voltage DC. This voltage through a capacitor bank will activate tyristor then will raise the voltage through the HV transformer and fill the HV capacitor. HV capacitor will discharge through the thyratron in processing container. Thyratron is driven by a controller circuit which generates square pulse signals, so the frequency to activate the thyratron can be arranged. Thus, generated pulse frequency can be set manually or automatically at a frequency of 1-200 Hz. So with the HV transformer pulse frequency setting range is wider and easier than that with the RLC circuit.

Birbir Yasar (2006) applied pulse width modulation (PWM) technique to control the quasi-resonant flyback converter as a high voltage pulse generator. With this circuit, the frequency range becomes very wide and has a high efficiency. Because it is operated at high frequency, a fast switching device, called MOSFET, is used.

PWM control technique is activated by microprocessor unit to generate high voltage pulse. If it is compared with other converter circuits such as full-bridge, half-bridge and push pull converter, fly-back converter circuit has the simplest circuit configuration and high efficiency. The fly-back converter merely needs one switching component, while full-bridge and half bride need more than one switching component, and Push pull converter needs three sets of transformer. The efficiency of fly-back converter is very high at full load and decrease used power consumption. Meanwhile, Ratna (2007) designed a high-voltage pulse generator circuit using a flyback transformer. The output voltage is a 20kV square pulse signal which is used to pasteurize milk with PEF method. Microcontroller is used to drive a MOSFET that serves to regulate the frequency of high-voltage pulse generated. The number of pulses can be adjusted by controlling the pulse output from the microcontroller. However, by using a flyback transformer, the maximum

output voltage is 20kV, whereas in order to kill microorganisms located on a particular media will require a specific high-voltage pulse.

III. METHOD The design of high pulse generator consists of some steps,

as shown in figure 2, with specifications: 1). Generated high voltage pulse can be controlled through keypad with maximum value of 80kV. 2). The high voltage pulse frequency is constant. 3). Production time period can be controlled through keypad in seconds up to 90 seconds with increment of 10 seconds. 4). Capacity of the tube-shape chamber is 1.5 litre of pasteurized apple juice.

Fig 2. High Voltage Pulse Generator Block Diagram

This research used a high voltage step up transformer with

maximum output of 100kV. The output voltage depends on generated PWM pulse. According to previous research, generated PWM pulses from microcontroller have 50% duty cycle.

The output voltage is controlled by microcontrollers through HV transformer input controller. Higher the HV transformer input, higher the output voltage will be. The output voltage and process time are entered using keypad.

Microcontroller acts as duty cycle controller of PMW pulses, LCD viewer and pasteurization processing time controller. So, microcontroller has three output signals. The first output used to view keypad setting on LCD, the second one to control the voltage and the output, the third one used to control processing time period.

The output of microcontrollers connected to the voltage controller which will activate a relay on a voltage controller circuit. The voltage controller circuit utilizes relay circuit in which each relay is connected to a rectifier circuit. Based on the design specifications, controllable high voltage is in the range of 20 to 100 kV with increment of 20 kV scale.

Therefore, five relay circuits of which each is connected to the rectifier circuit are needed. This circuit produces appropriate voltage for HV transformer to get appropriate output level. Whilst the output of three microcontrollers is connected to a driver circuit and switching which separates HV transformer and microcontroller used to control processing time period and to produce 0.5 duty cycle.

Microcontroller circuit is the centre of data processing and equipment driver. There are four ports to connect with LCD, keypad, switching circuit and voltage controller. Microcontroller will receive input data from keypad, process

and view it on LCD, and activate the HV transformer. This research used ATMEGA8535 type microcontroller. This microcontroller has complete facilities and easily found in the market.

The software used in this research is BASCOM programming language. In order to activate HV transformer, microcontroller has to generate PWM signal with duty cycle of 0.5 and frequency of 20 kHz. This frequency is adjusted to HV transformer frequency to produce maximum high voltage.

The circuit of voltage controller is function to generate voltage needed by HV transformer, to generate the desired high voltage. Higher the voltage, higher the output voltage generated by HV transformer will be.

Voltage controller circuit consists of rectifier and relay circuit. Based on the design specification of equipment, controllable high voltage is in the range of 20 to 100 kV with increment of 20 kV scale.

To generate 5 high voltage types, it needs 5 output voltage from voltage controller. Therefore, five rectifier circuits are needed of which each generates different output adjusted to HV transformer requirement. Voltage control circuit equipped with transistor BC108 with current amplifier (βdc) of 163 refering to datasheet.

HV transformer driver circuit is designed as HV transformer driver and desired process time controller circuit. Microcontroller will generate pulse voltage during chosen time and set through keypad. Microcontroller output is used as driver circuit input through transistor C2211. Transformer OT is used to isolate microcontroller circuit from HV transformer.

The design of Pasteurization Chamber Prototype for chamber treatment uses stainless steel plate with 4 mm thickness. The shape of treatment chamber is four-footed cylinder equipped with output tap and cover at the top. The dimesnsion of the chamber is 45 cm in height and 7 cm in diameter. Output tap is made from stainless steel with 1.25 cm in diameter. This chamber is able to accommodate apple juice up to 1.5 liter.

IV. RESULT

A. Examination of High Voltage Pulse Generator The examination result shows that the voltage controller

circuit generates output voltage which is appropriate with the need and generates pulse signal with frequency of 20 kHz to activate driver circuit and HV transformer switching. Therefore, microcontroller can generate square-shaped signals with appropriate processing time. If the processing time is set at 10 seconds, the microcontroller will generate square pulse for 10 seconds and after 10 seconds, it will “off”.

In order to generate maximum high volatge, HV transformer mus be controlled by signal wth frequency of 20 kHz. The microcontroller will activate driver circuit and HV transformer switching must generate 20 kHz signal. Equipment used for the examination includes PE1540 DC Power Supply 40V 3A, Oscilloscope GwInstek GOS-622G dan Digital Multimeter Sanwa. Figure 3 shows measurement results of microcontrollr output voltage connected to

switching and driver circuit.

Fig 3. Measurement Results of Microcontroller Signals The number of pulses generated by this pulse generator

circuit in one second is 20000 pulses. The processing time can also be controlled. For 10 seconds processing time set, the number of pulses given to the chamber is 200000 pulses. Longer processing time, higher the number of pulses giben to the chamber will be.

The examination of high voltage pulse generator is an entire examination of electtronic circuit of high voltage pulse generator. The examination is done by connected high voltage pulse generator equipment to the chamber. Then, the average of voltage and current flowing through the chamber are measured. The measurement is done by using HV probes with 1000 times weakening, oscilloscope and multimeter SANWA. Table 1 shows the measurement results of circuit voltage and current with voltage variation.

Table 1. Measurement Results of Equipment’s Voltage and Current

No Voltage Seting

Voltage Measured

Current Measured

Voltage Error

1. 40KV 35KV 4.25mA 12.5% 2. 60KV 60KV 12.76mA 0% 3. 80KV 77KV 15.32mA 3.75%

Based on the measurement results there is an error between

desired voltage and output voltage generated by the equipment. The average error of this equipment is 5.41% and the smallest error of 0% occured at the output voltage of 60kV.

B. Characteristic of Apple Juice Nutrition Content Resulted from Pasteurization Apple juice pateurization using this equipment is aimed to

examine the effectivity of this equipment. The examination is done through 9 treatments with voltage variation of 40kV, 60kV, and 80kV. Each voltage level applied for 10, 20, and 40 seconds respectively. Then, the contenct of vitamin C and vitamin A will be examined.

Nutrition content that will be analized including the content of vitamin A and C in apple juice resulted from PEF pasteurization which is compared with those from thermal pasteurization. Initial vitamin C content is 14.77 mg/100ml while the content in PEF pasteurization treatment is about 11.35-18.95 mg/100ml. The vitamin content of juice apple pasturized by thermal is 4.00 mg/100ml. The graph showing

the change of vitamin C content to the processing time with voltage variation is shown in figure 4.

Figure 4. The Change of Vitamin C Content to the Processing Time with Voltage Variation

Pasteurization treatment using PEF did not cause

significant change of vitamin C content. There is a trend of increasing vitamin C content which caused by the amount of water decrease from the sample during analyse process. In other words, entire vitamin C content is relatively constant compared with the condition before pasteurization treatment.

In boiling treatment, vitamin C content decrease occured. According to Gahler et al (2003) vitamin C is damaged because of boiling process as oxidation velocity is higher at higher temperature. However, electric field shock treatment will decrease the vitamin C content as well. This is assumed that the presence of electric field can cause oxidation process on vitamin C due to the availability of energy for reaction is higher.

Initial content of vitamin A is 0.5034 µg/gr and become 0.3767 – 0.435 µg/gr after PEF pasteurization treatment. While vitamin A content in apple juice pasteurized by thermal is 0.23 µg/gr. The graph shows the change of vitamin A content to the voltage and processing time is shown in figure 5.

Figure 5. Change of Vitamin A Content to the Time with Voltage

Variation

Generally, PEF treatment cause vitamin A content decrease but not significant. Significant decrease occured in apple juice pasteurized by thermal. The presence of boiling process cause degradation of vitamin A content. PEF process based on short pulsation at high voltage (20-80 kV/cm) applied on the food placed between two electrodes. PEF is a non thermal process because the food is processed at room temperature even less for some seconds. This can minimize nutrition lost caused by boiling process, including vitamin A content of apple juice ([11],[18]).

V. CONCLUSIONS The high voltage pulse generator circuit which is designed

can be used for pasteurization treatment using Pulsed Electric Field (PEF) method with average error 5.41%. Processing time can be controlled up to 99 seconds with increases each 1 second while high voltage pulses be controlled at 20kV, 40kV, 60kV, 80kV, and 100kV. Content of vitamin C and A in apple juice is not significantly changed compared with those in fresh apple juice. PEF pasteurization can protect the nutrition content of juice apple rather than thermal pasteurization.

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