Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: In this paper, an ohmic heater circuit is proposed using low cost, low volume, high power factor, high efficiency, and high frequency power inverters. In order to reduce switching losses while keeping operation frequency high, the well-known techniques of zero-voltage switching ZVS and phase shifting control are described and used. A low frequency control loop is proposed as a way of getting low harmonic input current distortion and thus high power factor.
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Journal of the Saudi Society of Agricultural Sciences www. Ohmic heating of pomegranate juice: Electrical conductivity and pH change Hosain Darvishi a b. Abstract Ohmic heating is an alternative fast heating method for food products.
In this study, the effect of ohmic heating technique on electrical conductivity, heating rate, system performance and pH of pomegranate juice was investigated. As the voltage gradient increased, time, system performance and pH decreased.
The electrical conductivity of the sample increased with temperature rise C. The range of electrical conductivity during ohmic heating was 0. Among the two models tested to t the electrical conductivity of pomegranate juice, the linear model gave the best t for all the data points.
Bubbling was observed above 81 C especially at high voltage gradients. The system performance coefcients for pomegranate juice samples were in the range of 0. Production and hosting by Elsevier B.
All rights reserved. Introduction Currently, Iran is one of the biggest producers and exporters of pomegranate fruit in the world, producing over , ton annually Ghourchi and Barzegar, , the majority of which is converted to juice and juice concentrate. E-mail address: Hosaindarvishi gmail. Acar, The juice of the pomegranate has been found to be effective in reducing heart disease risk factors, including LDL oxidation Sumner et al. Fruit juices in general are characterized by high acidity conditions, which lead to the growth of yeast and mold, in addition to a few types of low-aid-tolerant bacteria.
To avoid microbial spoilage, it is necessary to cause inactivation by applying heat by high temperature heating with very short exposition.
Conventionally heating is the most common method in the heating of foodstuffs. Classic convective methods for heating process uids, using plate heat exchangers, are still the most popular methods in the food industry. The major drawbacks of conventional heating are the low energy efciency and long drying times during heating.
Ohmic heating is a thermal processing method in which an alternating electrical current is passed through food products to generate heat internally Jha et al. Electrical elds, applied during ohmic. Please cite this article in press as: Darvishi, H. Similarly, ohmic heating was found to be more efcient for the required microbial and pectin esterase inactivation due to a shorter residence time while released avor compounds were not degraded as quickly as during conventional pasteurization Leizerson and Shimoni, Ohmic heating yields better products, clearly superior in quality than those processed by conventional heating Allali et al.
Its advantages compared to conventional heating also include the more uniform and faster heating, cleaner and more environmentally friendly; higher yield and higher retention of nutritional value of food Vikram et al. This is mainly due to its ability to heat materials rapidly and uniformly leading to a less aggressive thermal treatment. For example, De Halleux et al. Additionally, it is comparatively less difcult to clean an ohmic heater than traditional heat exchangers because of reduced product fouling on the heaters food-contact surface.
The important parameter in ohmic heating of a liquid food product is its electrical conductivity behavior. It depends on temperature, applied voltage gradient, frequency, and concentration of electrolytes Icier and Ilicali, c; Ye et al. The temperature dependency of the electrical conductivity liquid products follows linear or quadratic relations, depending on product type tested such as strawberry pulps Castro et al.
Icier and Ilicali , a , Darvishi et al. Yildiz et al. Also; they reported that the quality of pomegranate juice such as rheological properties, color, and total phenolic content depends on heating rate. But, they did not report about electrical conductivity and system performance.
The objective of the present work was to evaluate the effect of voltage gradient on electrical conductivity, heating rate, system performance and pH of pomegranate juice during ohmic heating. Materials and methods 2. Sample preparation Pomegranates Punica granatum L.
They were washed in cold tap water, drained, and then manually cut into four or six pieces. The juice was then extracted by pressing the samples with manual press at a pressing pressure of Large particles in the juice were removed using a No. The properties of the pomegranate juice at room temperature before the ohmic heating are listed in Table 1. These values are similar to those observed in the literature for pomegranate juice by Ghourchi and Barzegar , Akbarpour et al.
Ohmic heating unit and procedures Ohmic heating experiments were conducted in a laboratory scale ohmic heating system consisting of a power supply, an isolating variable transformer, power analyzer Lutron DW and a microprocessor board Fig.
The distance between two electrodes was 5 cm resulting in a total sample volume of. Ohmic heating of pomegranate juice: Electrical conductivity and pH change Table 1 Some properties of the pomegranate juice used for ohmic heating.
The cell constant of the ohmic heater was 1. Temperature uniformity was checked during previous heating experiments by measuring the temperatures at different locations in the test cell.
Since the temperature variation at different points inside the test cell was 1. Therefore, only the temperature in the center of the test cell was measured. Similar results are found to correspond well with those existing in the literature Assiry et al. Temperature was continuously measured with a K-Type, Teon coated thermocouple to prevent interference from the electrical eld.
A hole with diameter of 1 cm was created on the surface of the cell to observe the bubbles formation, insertion of thermocouple, and exit of vapor in the cell. The samples were placed in the test cell; the thermocouples were inserted and tted into the geometric center of the sample.
Temperature, current and voltage applied were monitored and this information was passed to the microcomputer with an RS port at 1second intervals. The accuracy of ohmic system was compared and calibrated with the standard conductivity. The calibration results for the accuracy of electrical conductivity of 0. The electrodes were thoroughly rinsed using a brush and dematerialized with twice-distilled water after each run.
System performance coefcient SPC The ohmic heating system performance coefcients SPCs were dened by using the energies given to the system and taken up by the juice samples. To simplify the calculation of SPC, the following assumptions were made: i specic heat capacity of the pomegranate juice is constant within the range of temperatures considered; ii SPC is constant, iii Prior to commencing ohmic heating it is assumed that the entire sample is at a uniform temperature of 20 C.
The energy given to the system and the heat required to heat the sample to a prescribed temperature were calculated by using the current, voltage and temperature values recorded during the heating experiments. The energy given to the system. Properties measurement. The energy loss is the sum of the heat required to heat up the test cell, the heat loss to the surroundings by natural convection, the heat loss for physical, chemical and electrochemical changes of juice, and the electrical energy which has not been converted into heat.
The energy loss calculations for the experimental data were performed by using the method in Icier and Ilicali b. Pomegranate juice density was determined by applying the pycnometric method. Specic heat was measured using the method described by Magerramov , based on an adiabatic calorimeter. Total titratable acidity TA was determined potentiometrically using 0. The results were reported as an average of three replicates.
Results and discussion 3. Heating rate The voltage gradient had a signicant effect on the heating rate of pomegranate juice samples during ohmic treatment. The calculated natural convection heat transfer coefcients were small, roughly 3. The increase in the surface temperature of the test cell at the end of the ohmic heating experiments was between 15 and 42 C.
The heat transfer area was also small. Due to these reasons, the heat loss to the surroundings was very small and could be neglected without any loss in accuracy. Results of one-way analysis of variance of the parameters. The heating rates may be affected by varying either the electric-eld strength or product electrical conductivity. The ohmic heating rate of pomegranate juice is shown in Fig. At higher voltage gradients, the current passing through the sample was higher and this induced the heat generation faster.
When higher voltage gradients were applied, samples showed an ideal range where there was an exponential or linear trend of temperature rise from 20 to 85 C. The ohmic heating rates were 4. Formation of bubbles was observed during the heating process, especially when the temperature of heated samples reached around 81 C, and heating was stopped when bubbling started. The reason for this phenomenon could be the release of gas in the liquid due to some electro-chemical reactions.
Palaniappan and Sastry reported that fruit juices are acidic resulting in the potential electrolytic hydrogen bubble formation. Zhao et al. The bubbles occurred much more quickly in high voltage gradient operations.
Therefore releasing the bubbles needs serious consideration in designing the static ohmic heaters. Also, the paired comparison t-test for the voltage gradient dependence showed that of the six comparisons made for electrical conductivities, voltage gradient was signicant between each of the voltage gradients. The changes in electrical conductivity of pomegranate juice with temperature during ohmic heating at four different voltage gradients are given in Fig. Electrical conductivity increased with temperature, as is expected and consistent with literature data Kumar et al.
Icier and Ilicali a reported that the increase in the electrical conductivity values with temperature has been explained by reduced drag for the movement of ions. It was observed that electrical conductivities decreased with temperature rise after bubbling started. The decrease in electrical conductivity may be caused by increased concentration of solids due to evaporation of water causing a drag in the ionic movement.
The highest value of the electrical conductivity of pomegranate juice was 1. Palaniappan and Sastry found that the drag for ionic movement increased when the solid content increased, which might be a reason for the decreasing trend in electrical conductivity with increasing solid content. The values of electrical conductivity are comparable with the reported values of 0. Akbarpour et al.
Calentamiento Ohmico Articulo
Adekunte, A. Tiwari, A. Scannell, P. Cullen and C. Modelling of yeast inactivation in sonicated tomato juice. International Journal of Food Microbiology —
Ohmic heating of food by means of high frequency power inverters