Design and testing of a prototype ohmic thawing unit

Computers and Electronics in Agriculture

19 (1998) 211–222

Design and testing of a prototype ohmic thawingunit

John S. Roberts, Murat O. Balaban *, Rudy Zimmerman,Diego Luzuriaga

Food Science and Human Nutrition Department, Uni6ersity of Florida, P.O. Box 110370, Gaines6ille,FL 32611, USA

Received 29 May 1997; received in revised form 20 August 1997; accepted 16 September 1997

Abstract

A significant proportion of seafood is imported into the U.S. in the form of frozen blocks.Before these can be further processed, they must be thawed, traditionally by immersion inwarm water. This requires large amounts of fresh water, that become wastewater. It is alsoslow and potentially compromises the quality of the food. Ohmic heating can be used tothaw blocks. The major concern of this method is ‘runaway heating.’ This occurs becauseelectrical conductivity of frozen food is about two orders of magnitude lower than that ofthawed food. The purpose of this study was to design, build and test an automated,computer controlled prototype ohmic thawing unit. Control circuitry was designed andcustom built. Performance was tested with shrimp blocks. © 1998 Elsevier Science B.V.

Keywords: Ohmic thawing; Shrimp

1. Introduction

The total U.S. shrimp catch and imports in 1993 were 133 million and 273million kg, respectively (Current Fishery Statistics, 1994). The shrimp imports

* Corresponding author. Tel.: +1 352 3921991; fax: +1 352 3929467; e-mail: [email protected]

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constituted 37% by weight of the total edible seafood imports in 1993. 70% ofshrimp processed in Florida is imported, mostly in the form of rectangular frozenblocks. These need to be thawed before further processing and handling. Theconventional process is to place them in warm water. Using water for thawing isadvantageous because it has a high heat transfer coefficient (Singh and Heldman,1993), and water reduces moisture loss in shrimp during thawing (James and Bailey,1984). Henderson (1993) reported 1–2% increase in the moisture content of shrimpthawed in water, and this can result in an economic advantage to the processor.There are, however, several problems of using water to thaw shrimp. Warm watercarries heat to the surface of the block by convection, and heat is then transferredby conduction from the surface of the block to its center. This is a slow process.The thermal conductivity of food is greater in the frozen state than in thenon-frozen state, and the heat capacity of food is greater in the non-frozen statethan in the frozen state (Karel and Fennema, 1975). Therefore, the thawed layer onthe surface acts as a thermal insulator, resulting in long thawing times. For 2.3 kgshrimp blocks (30×22×5 cm), thawing time is on the order of 1.5 to 2 h whenimmersed in water at 30°C. The possibility of microbial growth is high for the outerlayers of the block exposed to the warm water for long periods of time. Anotherdisadvantage of using water is the loss of some soluble proteins during thawing,which increases the load in the wastewater generated and reduces the nutrientquality of shrimp (Peplow, 1975). Wastewater generated by the Gulf Coast shrimpindustries is estimated to contain 4.5 million kg/year dissolved and suspended solids(Bough and Perkins, 1977). A most important disadvantage is environmental. GulfCoast shrimp processors use an estimated 3.4 billion liters of water annually (Boughand Perkins, 1977). In locations where the availability of fresh water is limited, thisconstitutes an impediment to the operations and growth of processing plants. Largeplants pay up to $75 000 per month on sewer costs (Personal communication).Therefore, they must find an alternative method to thaw frozen shrimp blocks.

When a current passes through food, its resistance converts electrical energy toheat. This is called ohmic heating, and is used as a method to heat foods (Sastry,1992), mostly in aseptic processing (Skudder and Biss, 1987). This method can alsobe used in thawing frozen foods by placing them between two electrodes andapplying an alternating current. Jason and Sanders (1962a,b) described the dielec-tric thawing of fish. Sanders (1963) described a hybrid method in which frozen fishwere initially warmed by immersion in water, then completely thawed by passing analternating current of ‘mains frequency’ through the blocks by placing thembetween parallel flat electrodes. He mentioned that a number of layers could bethawed by ‘suitably increasing’ the applied voltage and by interleaving the blockswith sheets of conducting material. Thawing times were given for blocks of wholecod as 3 and 5 h for ohmic and warm water thawing, respectively. He estimated thetotal commercial cost of ohmic thawing as one-third of dielectric thawing. Ohtsuki(1990) obtained a U.S. patent for thawing frozen foods by describing a threeelectrode system with voltages between 5 and 20 kV, and current densities between0.02 and 0.3 A/m2. The advantages of ohmic thawing are (1) there is no water usedin the process and no wastewater generated, (2) thawing time can be shorter than

J.S. Roberts et al. / Computers and Electronics in Agriculture 19 (1998) 211–222 213

water thawing, (3) the process can be easy to control, and (4) thawing can berelatively uniform due to volume heating. If the ohmic thawing technology isdemonstrated as technically and economically feasible, its use can save over 12.6billion liters of fresh water in Florida (Henderson, 1993) also freeing seafoodindustries from the cost of wastewater treatment. The cost savings of ohmicthawing over conventional water thawing were calculated as more than $95 000 peryear for a processing plant processing 26 tons of shrimp per day (Henderson, 1993).Ohmic thawing will also eliminate the leaching of water soluble proteins fromshrimp, as well as reduce microbial contamination, thus improving its quality.

Previous research (Henderson, 1993) to thaw shrimp blocks using manuallycontrolled ohmic heating showed that as the frozen product thaws, current passesthrough the thawed portion of the block more readily, following a path of leastresistance. There is a direct and strong relationship between temperature (Biss et al.,1989), phase (Luzuriaga et al., 1996) and electrical conductivity. Conductivityincreases with temperature for a given phase (Jason, 1965; Palaniappan and Sastry,1991), and frozen shrimp has a two order of magnitude lower electrical conductivitythan thawed shrimp (Luzuriaga et al., 1996). With the current flowing through thethawed portion of the block, enough heat is generated to cook the shrimp in thatportion, while the rest of the block is still frozen. This is called runaway heating, orthe formation of hot spots. This hindered the adoption of ohmic thawing by thefood industry (Burgess et al., 1967). If this problem were solved, a new method tothaw foods would be available to processors. Although ohmic thawing may beapplied to different kinds of frozen foods, the primary food studied in this researchwas shrimp.

The overall objective of this research project was to design, build and test aprototype automated ohmic thawing unit to serve as a basis for modification andscale-up for industrial use. The specific objectives were:1. To design a prototype portable ohmic thawing unit, with surface temperature

sensing and computer automated capabilities, that can thaw two shrimp blocksat the same time,

2. To conduct extensive testing of the operation of the unit to assure the elimina-tion of runaway heating, and to develop guidelines and suggestions for scale-up.

2. Apparatus design and construction

2.1. Design considerations

The ohmic unit was designed to have the capability to simultaneously thaw twoshrimp blocks. Two is the minimum number to demonstrate that dissimilar blockscan be thawed simultaneously using this control method. The goal was to eliminatehot spot formation by automated control. Current flow through a block is affectedby the size, ratio of meat to water, temperature, salt content, and phase (frozen orthawed) among other things. Since many of these factors varied with shrimpspecies, packing and freezing practices, seasons, etc. it was not feasible to design a


thawing strategy to account for all this variation. Previous experience with ohmicthawing (Henderson, 1993) showed that hot spots occur simultaneously on bothsurfaces of the block. Therefore, localized heating could be monitored at onesurface only, and the supply of electrical current controlled. It was also necessaryto detect the current and voltage at any given time and to accumulate the totalenergy used to decide on the end of operation. It was also desirable to distribute thecontrol of the operation to different circuit boards, rather than concentrating it atthe computer. Although this is not significant in a prototype, it is crucial in ascaled-up industrial design to improve reliability and speed.

2.2. Electrode design

Four electrodes were manufactured, each pair to thaw one shrimp block. Theywere made of 1 cm thick 304 stainless steel (24×30 cm), which was slightly largerthan a typical shrimp block. The top and bottom electrodes were divided into fourequal quadrants (Fig. 1) electrically isolated from each other by a Silicone coating(Dow, Midland, MI). Each quadrant had its own power on/off relay, and wasseparately connected to the power supply. A quadrant had 32 thermistors (Fenwal,Milford, MA) in eight rows and four columns covering the surface uniformly, sothat the maximum distance from any point to a thermistor was less than 2 cm.

Fig. 1. Design of the electrodes.


Electrically isolated thermistors resided in 0.3 cm holes drilled into the plates andhad enough slack to drop down to touch the surface of the frozen block duringoperation. If excess temperature was detected at the surface of one quadrant, itspower was shut off while the other quadrants continued heating the block. Thisresulted in quicker and more controlled thawing. A Lexan plastic plate (Faulkner,Gainesville, FL) with dimensions 31×31×1.3 cm provided a sturdy backbone toeach top electrode, and an area to mount the circuit boards, relays, volt and currentsensors, and microprocessor board. The bottom electrode plates, also subdividedinto four quadrants, were placed onto support stands made of Lexan plastic to keepthe shrimp block from contacting the collected thaw water. They had raised edgesto prevent the block from sliding off, and channels at each corner to providedrainage for the melted ice. Each bottom quadrant had its power relay working intandem with that of the top electrode.

2.3. Integrated circuit boards

The thermistor leads for each electrode quadrant were connected to an electroniccircuit board custom designed by Microtherm (Gainesville, FL). The signal fromthe thermistors were continuously compared by the board to a dial-adjustable setpoint temperature above the melting point of shrimp and well below cookingtemperatures. There was one set point temperature per electrode. If any tempera-ture was greater than the set point, an LED light for that thermistor was lit, theboard sent an alarm signal to a microprocessor board (MC68HC05B5 microproces-sor, Motorola, Proto Circuits of Florida, Ft. Lauderdale, FL) with erasableprogrammable read-only memory (EPROM). This board controlled the relays, onefor each quadrant (Fig. 2). If the current to a quadrant was shut off, a time delaywas allowed for heat to dissipate and temperature to fall below the set point, afterwhich the microprocessor board turned power on to that quadrant. This is apositive method to control overheating regardless of size, moisture content andsalinity of different shrimp blocks. The microprocessor board also monitored thecurrent and voltage of each quadrant and passed this information to the microcom-puter via an RS232 port at regular time intervals. This allowed real-time calculationof the total power input to a block at any given time.

2.4. Control of 6oltage

From previous experience, the operation needed relatively high voltage levels atthe beginning, with relatively low current passing through the block, since electricalresistance of the block was high (Henderson, 1993). As heating continued, thetemperature of the block increased and its resistance decreased. Therefore, reduc-tion of voltage was necessary to keep the maximum current to any quadrant at aset level, adjustable by the operator. The computer monitored the maximumcurrent passing through any quadrant, and adjusted the voltage output of thevariable transformer (6020CT-25, Staco Energy, Dayton, OH) using a steppermotor connected to the parallel port of the computer. The circuit board to control


Fig. 2. Conceptual schematic of the ohmic thawing system.

the stepper motor was designed by the Industrial and Systems Engineering Dept.,University of Florida. For detailed descriptions of the various electronic circuitboard diagrams, refer to Roberts (1994). Since the ‘intelligence’ of the system is notconcentrated at the microcomputer but distributed to several boards, the flexibility,reliability, and speed of the operation is increased.


2.5. Control program

The nomenclature for the equations in this section is given below: Cp, heatcapacity, kJ/kg°C; E, energy, W h; I, current, A; m, mass, kg; P power, W; Q heat,kJ; T, temperature, °C; t time, h; V, voltage, volt; l, latent heat, kJ/kg; subscript a:above freezing; subscript b: below freezing; subscript fp: freezing point.

The control program that supervised the operation was written in Turbo Basic(Borland, Santa Clara, CA). The control consists of reading voltage and currentvalues, and calculating the heat generated in each quadrant, and keeping a setcurrent going through each quadrant. The energy used in each quadrant wascalculated as follows:

E=Pt=VIt (1)

The energy calculated throughout the thaw experiments is termed the actual energyinput, Qactual. The theoretical energy, Qtheor, needed to thaw the block was calcu-lated as:1. Sensible heat below the freezing point:

Qb=mCpb(Tfp−Tinitial) (2)

2. Latent heat:

Qlatent=mlfreezing (3)

3. Sensible heat above the freezing point:

Qa=mCpa(Tfinal−Tfp) (4)

The total theoretical energy input was calculated by:

Qtheor= (Qb+Qlatent+Qa)shrimp+ (Qb+Qlatent+Qa)water (5)

The energy equations given above were applied to the shrimp and the ice in theblock, corrected by their mass fraction. The theoretical energy calculated providesa reference to compare the actual energy used to thaw the blocks to the energytheoretically required to thaw the blocks. A coefficient of performance, COP, wasdefined as:

COP=Qactual

Qtheor

(100) (6)

The COP is the percentage of electrical energy used to thaw the shrimp blocks.The sequence of operation of the program is as follows:

1. Turn on the heating relays to all four quadrants of an electrode.2. Set up a fast loop where the voltage, currents, temperature alarm status, and

relay on/off status to each quadrant are read, the W h calculated and accumu-lated for each quadrant, and for the total shrimp block.

3. At predetermined intervals, these values are shown on the computer screen, andwritten to an output file.


4. The time interval in which a quadrant is turned off due to a temperature alarmis monitored. When the temperature alarm for that quadrant is off, additionaltime is allowed for the temperature to equilibrate before the relay to thequadrant is turned on again.

5. The maximum current flowing through the system is kept constant. This is setby the operator. The stepper motor turns the voltage up or down to keep thecurrent at the set level.

6. The end point of the process is reached when the total accumulated W h reachesa predetermined value corresponding to the heat necessary to thaw that partic-ular block.

The flow diagram of this program is shown in Fig. 3.

2.6. Experiments

Frozen shrimp blocks (2.3 kg) were obtained from Singleton, Tampa, FL.Extensive testing of the operation of the unit was necessary to assure that therunaway heating problem was eliminated. Experiments with two blocks thawingsimultaneously were conducted in the lab. The blocks were placed in a −20°Cfreezer overnight. Before placing between the electrodes, each block was measuredand weighed and the information entered into the computer program. Afterthawing was complete, the shrimp and thaw drip were collected separately andweighed. Thawing times at different conditions were compared, and the properfunctioning of the unit was tested. Shrimp were visually inspected for color changeindicating cooking, and the final temperature of the shrimp was measured atdifferent locations.

3. Results and discussion

The computer program stored all the voltage, current, and experimental cumula-tive W h versus time. From these data, plots of the above properties vs time weregenerated to determine a particular shrimp block’s thawing characteristic. Typicalvoltage, current, and cumulative W h of the quadrants in one unit vs. time areshown in Fig. 4. In this block, quadrant 1 governed the voltage since it conductedcurrent immediately. It also accumulated the greatest amount of energy at a fasterrate than any other quadrant. Other quadrants also accumulated a significantamounts of energy. As shown by this example, a shrimp block can be quitenonhomogeneous, and the thawing profile from one quadrant to the next can becompletely different.

With two blocks thawing simultaneously, eight quadrants were analyzed duringa single experiment. One of the eight quadrants, with the greatest electricalconductivity, determined the voltage level applied to the entire ohmic system.

During the initial period of thawing, the frozen block has a low temperature.Therefore, resistance is high, and a high voltage level is necessary to pass the setcurrent. During the later periods of thawing the temperature of the block is close


to the freezing point, and has partially thawed regions. Therefore, the resistance islower, and the voltage level necessary to pass the set current is low. In this period,the operation is controlled by the surface temperature, indicated by frequent LEDand relay off/on sequences (Fig. 4).

Fig. 3. Flow diagram of the control program.


Fig. 4. Voltage vs time, current vs time, W h vs time profiles for each quadrant, and cumulative W h vstime for all quadrants.


Table 1Results of ohmic thawing tests

Shrimp/waterTest Shrimp used COP (%)Thickness (cm) Thaw timeNet. Wt. (g)(min)(g/g)

1021 a 5.4 2027 2.48 19182.07b 1026.03 1918

1.79 872 36d 3.81 1482

263 2.12a 894.45 1814

3.84 494 c 344.76 2223

120 145 2.16a 5.72 18782.54 119 13c 5.08 1606

a: White shrimp, butterfly, peeled, deveined, tail-on, \90–110 per kg.b: Pink shrimp, Marbella brand, peeled, round, 200–264 per kg.c: White shrimp, Yolita brand, butterfly, peeled, deveined, tail-on, 90–110 per kg.d: Pink shrimp, Moon Star brand, peeled, round, 200–264 per kg.

It is important to note that in all experiments, the temperature of the shrimp wasbelow the set point temperature (12.7°C) at all times. The surface temperature ofwater immersion thawed shrimp reached water temperature (30°C). In all experi-ments, ohmically thawed shrimp was colder than water immersion thawed shrimp.

It was observed that if the temperature of the room where the ohmic unit wasoperating was high (between 27 and 30°C), then the surface of the block heats andstarts to trigger temperature alarms prematurely. This increased thawing times. Theproblem was not observed at room temperatures below 17°C. Therefore, operatingin chilled rooms may be more efficient for thawing times. Optimization of theohmic process should further reduce thawing times.

Table 1 summarizes the results of experiments. It can be seen that there was acorrelation between the thawing time and COP: the longer the thawing time, thelower the COP. The longer the thawing time, the more heat was absorbed by theblock from the environment, and the smaller was the fraction of electrical heat inthe total heat necessary to thaw the block. It can also be seen from Table 1 that twoblocks of shrimp that are different (sets 1 and 5) can be thawed at the same timein the prototype unit. The control method adjusts for the differences and thaws theblocks in approximately the same time. Single blocks could also be thawed, andthawing time would depend on the block properties (sets 2, 3 and 4).

4. Conclusion

The ohmic system worked as designed, with the capability of thawing two shrimpblocks simultaneously. Ohmic thawing did not use any water, or generate muchwastewater, and was more energy efficient. The test results proved that the time forohmic thawing is comparable to water immersion time, without the incidence of hot


spots. Thus, the major obstacle of ohmic thawing has been solved and this methodcould be used to thaw shrimp blocks.

Acknowledgements

Financial support for this project was provided by the National Coastal Re-sources Research and Development Institute, Grant ST93-197-5630-03. Raw mate-rials and expertise was provided by Singleton Seafood, Inc., Tampa, FL. The circuitboards were designed by MicroTherm, Inc., Gainesville, FL.

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Design and testing of a prototype ohmic thawing unit