Meat Science 96 (2014) 675–681

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Meat Science

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Innovative food processing technology using ohmic heating and asepticpackaging for meat

Ruri Ito a, Mika Fukuoka b, Naoko Hamada-Sato a,⁎a Course of Safety Management in Food Supply Chain, Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, Konan 4-chome, Minato,Tokyo 108–8477, Japanb Department of Food Science and Technology, Tokyo University of Marine Science and Technology, Konan 4-chome, Minato, Tokyo 108–8477, Japan

⁎ Corresponding author. Tel./fax: +81 3 5463 0389.E-mail address: hsnaoko@kaiyodai.ac.jp (N. Hamada-S

0309-1740/$ – see front matter © 2013 Elsevier Ltd. All rihttp://dx.doi.org/10.1016/j.meatsci.2013.10.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 January 2013Received in revised form 31 August 2013Accepted 4 October 2013

Keywords:Ohmic heatingStorageChickenEmergency foodSterilizationEating quality

Since the Tohoku earthquake, there is much interest in processed foods, which can be stored for long periods atroom temperature. Retort heating is one of the main technologies employed for producing it. We developed theinnovative food processing technology, which supersede retort, using ohmic heating and aseptic packaging.Electrical heating involves the application of alternating voltage to food. Compared with retort heating, whichuses a heat transfer medium, ohmic heating allows for high heating efficiency and rapid heating. In this paperwe ohmically heated chicken breast samples and conducted various tests on the heated samples. Themeasurement results of water content, IMP, and glutamic acid suggest that the quality of the ohmically heatedsamples was similar or superior to that of the retort-heated samples. Furthermore, based on the monitoring ofthese samples, it was observed that sample quality did not deteriorate during storage.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Currently, retort heating is one of the main technologies employedfor producing processed foods, which can be stored for long periods atroom temperature. There is a strong demand for retort food fromconsumers as emergency food because of their long shelf life and easeof consumption; furthermore, such foods are popular as everydayfood. However, retort sterilization relies on external heating, using hotwater or steam as a heat transfer medium; this results in poor heattransfer efficiency and, consequently, considerable energy loss.

Ohmic heating is an emerging thermal process technology anddescribes the process when an electrical current is passed directlythrough a food and the resistance imposed by the food leads to thegeneration of heat within the product. The basic principles as well asthe main factors influencing ohmic cooking have been explained bySastry (1992) and Ye, Ruan, Chen, and Doona (2004). Sastry andPalaniappan (1992) reported that ohmic heating can be used in acontinuous flowmode to cook and sterilize liquid food and solid–liquidmixtures. Huixian et al. (2007) reported that the microbial counts andthe calculated decimal reduction time resulting from ohmic heatingwere superior to those resulting from conventional heating, and therewas no difference in the degree of protein denaturation between thetwo methods. Nowadays ohmic heating is viewed as an alternative

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heating system for pumpable foods and there are currently a numberof commercial scale processing plants in various countries (UK, Italy,Mexico) producing fruit and/or vegetables in sauces and alsopasteurized orange juice and liquid egg. Sarkis, Jaeschke, Tessaro, andMarczak (2013), Mercali, Jaeschke, Tessaro, and Marczak (2013), andMercali, Jaeschke, Tessaro, and Marczak (2012), reported on thedenaturation of anthocyanins and vitamin C in acerola and blueberryduring ohmic heating compared to the denaturation of these duringconventional heating. Moreno, Pizzaro, Parada, Pinilla, and Reyes(2012) reported that ohmic heating is the best dehydrating method.And the color and the hardness of osmotically dehydrated strawberrywith ohmic heating and vacuum impregnation was superior to theconventional method. The effect of ohmic heating and vacuumimpregnation changed the shelf-life from 12 days to 25 days. While anumber of the early patents in ohmic heating were in the area of meatprocessing the amount of in depth research conducted to date hasbeen quite limited in spite of the fact that ohmic heating has thepotential to cook meat in a much shorter time than the conventionalcooking procedures. Shirsat, Brunton, Lyng, and Mckenna (2004) andPiette et al. (2004) showed that it is possible to cook comminutedmeat emulsions ohmically to a comparable quality of the conventionalcooked samples. Dai et al. (2013) evaluated the color and sarcoplasmicprotein of pork following water bath and ohmic cooking at 10 °C to80 °C. Ohmic heating of fluids, which may also contain solid foods, hasbeen thoroughly studied and reported in the literature. Bertlini andRomagnoli (2012) showed that the process-target-cost of vegetablesoup was reduced with ohmic treatment and aseptic packaging.

Fig. 1. Five locations of thermocouples for evaluation of temperature distribution duringheating. A: Center of the cell. B: Bottom center of the cell. C: Upper center of the cell.D: Upper part of the cell near the electrode. E: Lower part of the cell near the electrode.

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However, these products represent a relatively small proportion of totalcooked meats and no results have yet been presented on the quality ofohmically cooked noncomminuted meats. The direct application ofohmic heating to solid food is limited (De Alwis & Fryer, 1992). Thereare no studies on the production technology for solid food withcommercial-level sterility attained by heating at 100 °C or higher or byohmic heating without the use of conductive liquids.

The ohmic method requires uniform conductivity values within themeat which means that a perfectly even distribution of injected salt orbrine solutions must be achieved in the case of non-comminutedmeats. A lot of research was done on electrical conductivity of foods(Palaniappan & Sastry, 1991) and on the changes in electricalconductivity of foods during ohmic heating (Halden, De Alwis, &Fryer, 1990). Sanjay, Sudhir, and Lynn (2008) published a paperabout the change of electrical conductivity values over a specialtemperature range in a very small unit. This research team lookedmainly into the electrical conductivity changes of fruits and also afew details about the behavior of different meat pieces werepublished but there is still a lack of research in the ohmic heatingof full meat products.

A novel cooking method such as ohmic heating may offer a numberof advantages, such as quicker cooking and less power consumption andsafer product, however, the important considerations for a food productare its taste, quality, and customer satisfaction.

There have been no studies on ohmic heating combined withaseptic packaging for meat nor on meat measured for one yearas shelf-stable food of meat. In this study, we developed novelfood processing technology, which supersedes retort processing,using ohmic heating and aseptic packaging. Chicken cooked bycombined ohmic heating and aseptic packaging was tested andcompared with chicken heated by retort heating. We examinedthe temperature history, electrical conductivity data and lethalrate during current application. Additionally, we assess quality andsensory tests on the sterilized packaged food heated by those twomethods.

2. Materials and methods

2.1. Material

Chicken meat was purchased from Miyagawa Shokucho Keiran andstored in a freezer at−80°C until experimentation.

2.2. Equipment

We used a high-frequency power unit (HJU3000-HF-30, HanoManufacturing). The output voltage was 10–100 V, output frequencywas 20kHz, andmaximumpower outputwas 3kW. A polyphenylsulfone(PPSU) container with an internal diameter of 3cm and a length of 10cm(Sunny) was used as the heating cell. Titanium foils (30-μm thick) wereused as electrodes.

2.3. Preparation of samples

The stored meat was removed from the freezer and thawed in arefrigerator at 5 °C. Then, the meat was shaped into a cylindrical form(approximately 30mm×100mm, 70g±0.5 g) so that it could fit intothe heating cell, and was wrapped with polyvinylidene chloride film.Wrapping the sample with the film made it easier to clean the cell andprepare it for the next test.

Themeatwas inserted into the heating cell. Thereafter, silicone ringsand stainless caps, in that order, were attached to the cell, and the cellwasfixed using a stainless steel retainer. A type T thermocouple coveredwith an insulator was inserted into the cell through a hole in one of thestainless steel caps and fixed in place. Given that the pressure inside thecell increases during heating, the cell was retained securely to prevent

steam leakage during heating. The electrodes of the high-frequencypower unit were connected to the stainless steel caps and an electriccurrent was then applied to the heating cell. The voltage was set atthe maximum, 100V. The current was set to 5 A because the maximumeffective current is around 1.5 A for chicken breast when the measuredtemperature is between 10°C and 140°C.

The current application was stopped when the temperature of thecold point exceeded 121 °C for four continuous minutes. When thetemperature decreased to below 100 °C, the sample was placedaseptically in a sterilized retort pouch andwas sealed using a heat sealer(Ishizaki Electric MFG). Following cooling for 20 min under flowingwater, the sterilization-test sample was stored in an incubator at 35 °Cfor 14 days; the quality-test sample, at 25 °C for 2, 14, 28, 56, 84, 112,168, 224, 280, or 365 days; and the sensory-test sample, at 25 °C for14days.

In this study, the ohmically heated sample was compared with aretort-heated sample in the quality and sensory tests. The retort-heating sample was shaped into a cylindrical form with the same sizeand weight as the ohmic-heating sample, frozen at −80 °C, and sent toNihon Senshoku, Fukuoka Prefecture (shipping temperature −20 °C),where it was retort heated. The retort-heated sample was stored for 2,14, 28, 56, 84, 112, 168, 224, 280, or 365days. Furthermore, prior to retortheating,we ensured that the samplewas stored at the same temperatureas the ohmically heated sample.

2.4. Temperature measurement at different locations

Every point was measured three times. The heating cell and thelocations of thermocouples placed on the cell for temperaturemeasurement are shown in Fig. 1. The thermocouples were placed atfive locations (A–E) (see Fig. 1) and temperature changes until 121 °Cwere recorded.

2.5. Electrical conductivity calculation

Measurement was made five times. The distance between theelectrodes d [m], electrode contact area A [m2], applied voltage V [V],and measured current I [A] were substituted in Eq. (1) below. Thisequation was derived from Ohm's law, the relational expressionbetween electrical resistance and electrical resistivity, and the relationalexpression between electrical resistivity and electrical conductivity, to

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calculate the electrical conductivityσ [S·m] of the sample under currentapplication.

σ ¼ I·dð Þ= V·Að Þ ð1Þ

2.6. Sterilization test

The sterilization test was conducted in accordance with theconstant-temperature test and the microbacterial test for retortfood stipulated in the Food Sanitation Act. The test was done fivetimes. The ohmically heated samples, held at 35 °C for 14 days,were cooled to 20 °C before their morphology was observed. Thesamples whose packaging containers were inflated or from whichthere was content leakage were considered microorganism-positive.The samples considered as microorganism-negative were subjected tothe microorganism test. A portion of each sample, 25 ± 0.1 g, wascollected aseptically and placed in a sterilized bag (PYXON-30, Elmex)with 225 ml of sterilized water. This mixture was then ground in ahomogenizer (Stomacher 400-T, Organo) for 60 s to obtain a sampleextract; 9ml of sterilized water was added to 1ml of the sample extractto make a 1:100 diluent. One milliliter of the 1:100 diluent wasdispensed in five tubes, each containing thioglycollate medium (EikenChemical) heat-dissolved in distilled water and sterilized (hereinafterreferred to as the TGC liquid medium). Furthermore, one milliliter ofsterilized water was added to the TGC liquid medium as negativecontrol. Then, the sample and control solutions were incubated at35 °C for 48h to check for any microbial growth.

2.7. Temperature changes during retort heating

Tomonitor sample temperature during retort heating, a temperaturedata logger (button-shaped, diameter 8.675mm, height 6.4mm) (HyperThermochron, KN Laboratories) was inserted into the center of eachsample. The monitoring was done five times.

2.8. F0 measurement

F0 value was calculated as the accumulated thermal death effect onthe spores of Clostridium botulinum, one of the most common foodpoisoning-causing bacteria. Lethal rate, L, was calculated using thefollowing equation:

L ¼ 10 � T–121ð Þ=10½ � ð2Þ

where T is temperature (C˚).F0 is calculated using Eq. (2) as follows:

F0 ¼Z

Ldt ¼Z

10 � T–121ð Þ=10½ �dt: ð3Þ

Based on temperature history, F0 was calculated using (3) for ohmicheating and retort heating.

2.9. Water content measurement

Five different samples were used for themeasurements. Differencesbetween groupswere evaluated using a two-sample t test. Five grams ofeach sample was placed in an aluminum cup (Toyo Aluminum EkcoProducts) and was leveled using a medicine spoon. Following dryingby heating at 135 °C for 120 min, the water content of each samplewas measured using infrared moisture testers (FD-600, FD-620, KettElectric Laboratory).

2.10. Glutamic acid measurement

Five different samples were used for the measurements.Differences between groups were evaluated using a two-sample t test.A YAMASA L-Glutamate Assay Kit II (Yamasa Corporation) was usedfor this purpose.

2.11. Inosine monophosphate (IMP) measurement

Five different samples were used for themeasurements. Differencesbetween groups were evaluated using a two-sample t test. High-speedliquid chromatography was used for measuring IMP.We used a ShodexAsahipak GS-320 HG column (φ7.6mm×300mm, Showa Denko) at atemperature of 30 °C. An 875-UV (JASCO) detector was used, and thedetection wavelength was 260 nm. As the mobile phase, we used200 mM sodium dihydrogen phosphate (pH2.9) filtered through amembrane filter (pore diameter 0.20μm); its flow rate was 0.6ml/min.

A portion of each sample, 2.0± 0.3 g, was homogenized in 5ml ofchilled 10% perchloric acid. Then, the solution was centrifuged at16,000 g using a centrifuge (CF15R, Hitachi Koki). The supernatantwas decanted; 5ml of 10% perchloric acidwas then added to the residueand mixed well. This mixture was centrifuged for collecting thesupernatant. The supernatant and the 10% perchloric acid were mixedto obtain a 25-ml supernatant diluent. One milliliter of the diluent wasneutralized using 10M KOH and 1M KOH, and the neutralized diluentwas centrifuged at 14,000 g. The supernatant was decanted, and 1 mlof Milli-Q was added to the residue and mixed well; the mixture wasthen centrifuged at 14,000 g, and the supernatant was collected. Thesupernatant was centrifuged again and decanted, and then, Milli-Qwas added to the decanted supernatant to prepare 10ml of test liquid.The test liquid was filtered through a syringe filter unit (pore diameter0.20 μm), and 20 μl of the test liquid was injected into the sample formeasurement. An autosampler (Model 09, System Instruments) wasused for injection.

2.12. Sensory test

The sensory test was conducted on about thirtymen andwomen. Theohmically heated and retort-heated samples, each 2.0±0.1g,were placedon separate trays and the subjects evaluated them in terms of appearance,flavor, taste, tenderness, flavor in mouth, and comprehensive evaluation(six items) on a five-point scale (−2 to 2). As for comprehensiveevaluation, a two-sample preference test was also conducted.

2.13. Statistical analysis

The results were expressed as average± standard deviation, anda t-test was used for determining statistical significance. MicrosoftExcel was used for this analysis.

3. Results and discussion

3.1. Cold point determination

The changes in temperature at locations A–E inside the cell areshown in Fig. 2. The average time it took each of the five samples toreach at 121 °C is shown in Fig. 3. The time taken to reach 121 °C wasthe shortest at A, the center of the cell; the time taken was the longestat D, the upper part of the cell near the electrode. The following reasonswere suggested for these results: temperature increase at A was thefastest because itwas farthest from the cell surface andwas less affectedby heat transfer. In comparison, B, C, D, and E were closer to the cellsurface; D and E were closer to the stainless steel cap than B and C,making D and E more susceptible to the effect of heat transfer. Thus,the temperatures at these locations were lower. Furthermore, D waslocated higher on the cell than E, thus releasing more heat than E. This

Fig. 2. The averages of times which the 5samples took to reach at 121 °C. Fig. 4. Correlation of temperature and electrical conductivity. It show temperatureand calculated electrical conductivity using σ = (I·d)/(V·A). Solid line means y =4E-05x2 + 0.001x + 0.369, R2 = 0.983.

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result was consistent with previous studies. Marra, Zell, Lyng, Morgan,and Cronin (2009) reported that in ohmic heating mashed potatoeach of the same positions showed similar results.

In retort heating, temperature is measured at the cold point in food(region with the slowest temperature increases rate). In this study, itwas assumed that the ohmically heated samples would be storedunder the same conditions as the retort-heated samples; therefore,temperature measurement at the cold point was essential. In thisstudy, we selected D as the temperature measurement point forohmic heating because it is the farthest from the cell center and releasesthe most heat. In the following sections, temperature measurementswere made at D, unless specified otherwise.

3.2. Electrical conductivity

In Fig. 4, data are plottedwith the coldpoint temperature on the x-axisand the electrical conductivity, estimated fromcurrent andvoltage values,

Fig. 3. Temperature history distribution during ohmic heating at the five locations ofthermocouples in the cell.

on the y-axis. The temperature–electrical conductivity relationship isgiven as y = 4E-05x2 + 0.001x + 0.369 with a contribution rate ofR2 = 0.983, showing high correlation. Based on these results, it wasshown that sample temperature could be estimated by calculatingelectrical conductivity based on the applied current and voltage values.

3.3. Sterilization test

It was confirmed that the aseptic condition of the ohmically heatedsamples could be maintained at 35 °C for 14days. This implies that ourmethod conforms to the sterilization method stipulated in the FoodSanitation Act. Thus, it was demonstrated that the reliability of foodscan be enhanced using our sterilization method.

3.4. Comparison of temperature history with retort heating

Temperature changes during processing were compared betweenohmic and retort heating (Fig. 5). Temperature increase from 14 °C to121 °C was faster by approximately 15min under ohmic heating thanretort heating. Under retort heating, initially, the temperature increasedquickly, but the time taken to reach 70 °C was almost the same as thatunder ohmic heating. After reaching 70 °C, however, temperatureincreased more rapidly under ohmic heating. Temperature historyduring cooling was nearly the same for the two methods. Furthermore,under retort heating, the temperature gradient between the hot waterand food, the two heat transfer media, serves as a driver of heating—temperature increase is slower at higher food temperatures. Underohmic heating, however, electrical conductivity generally increaseswith food temperature; therefore, temperature increase is quicker asat higher food temperatures.

It has been reported that when food is sterilized by heating, qualitydeterioration can be reduced by decreasing the heating time. Therefore,it is expected that the shorter heating time under ohmic heating willresult in the production of higher-quality food.

3.5. F0 value

The changes in the lethal rate during ohmic heating and retortheating, estimated based on temperature history at the cold point, are

Fig. 7. Transition of water content rate of chicken processed by various heating methodsfor 365 days.

Fig. 5. Evaluation of temperature during ohmic heating and retort. Temperature reached121 °C faster by ohmic heating than by retort.

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shown in Fig. 6. The F0 values for ohmic heating and retort heatingwere5.74 and 10.28, respectively (both exceeding 4). Therefore, it wassuggested that in addition to satisfying the requirements of 121 °C and4min, ohmic heating reduces the quality deterioration of food causedby heating.

3.6. Water content

The changes in water content are shown in Fig. 7.For all storage periods, the ohmically heated samples had higher

water content than the retort-heated samples; however, the differenceswere not statistically significant.

Fig. 6. Evaluation of Lethal rate during ohmic heating and retort. Temperature reached121 °C.

3.7. IMP content

The changes in IMP concentration are shown in Fig. 8. ATP-relatedcompounds the samples stored for 14 days are shown in Fig. 9. Duringthe storage period between two days and one year, the ohmicallyheated samples had significantly higher IMP concentration than theretort-heated samples (p b 0.01). The x-axis represents the measuredsample, and the y-axis shows the content of ADP, AMP, ATP, and HxR.ATP changes into ADP, ADP changes into AMP, AMP changes into IMP,IMP changes into HxR, and HxR changes into Hx. The decreases inATP, ADP, and AMP lead to an increase in IMP content. Conversely,increases in HxR and Hx lead to a decrease in IMP content. IMPconcentration changed to HxR, Hx and so on. IMP concentration inboth the sample types remained nearly constant across storage periods.This suggests that IMP concentration did not change during storage. IMPis themain flavor-enhancing component of chicken and is an important

Fig. 8. Transition of IMP concentration of chicken processed by various heating methodsfor 365 days.

Fig. 9. ATP. ADP, AMP and HxR. Hx content of the samples stored for 14 days.Fig. 11. The result of sensory test. The appearance of ohmically heated samples wassignificantly better than that of retort-processed samples, and retort-processed sampleswere significantly more tender than ohmically heated samples.

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indicator of the quality of chicken products. The above results show thatthe ohmically heated samples contained more IMP than retort-heatedsamples, allowing for the preparation of higher-quality samples. Thisis ascribed to the higher temperature increase rate under ohmic heating,which restricts IMP reduction caused by heating.

3.8. Glutamic acid content

The changes in glutamic acid concentration are shown in Fig. 10. Forall storage periods, the ohmically heated samples had higher glutamicacid concentration than the retort-heated samples. However, significantdifferences were found only in the samples stored for 14, 84, 168, and280 days (p b 0.05). Glutamic acid did not fluctuate greatly in bothsample types. Based on these findings, it was suggested that theglutamic acid concentration did not change during storage.

Fig. 10. Transition of Glutamic acid concentration of chicken processed by various heatingmethods for 365 days.

3.9. Sensory test

The results of the sensory test are shown in Fig. 11. On average,between the two sample types, the ohmically heated samples receivedhigher points in terms of appearance, taste, tenderness, flavor inmouth,and comprehensive evaluation; the retort-heated samples receivedhigher points in terms of flavor. However, significant differences werenot found in all of the items. Therefore, it was suggested that therewere nomarked differences in sensory quality between the two sampletypes.

4. Conclusion

In this study, we ohmically heated chicken breast samples andconducted various tests on the heated samples. Our findings are asfollows:

1) Temperature changes during current application were monitored atseveral locations inside the heating cell. Temperature increase wasthe slowest in theupper part of the cell near the electrode. Therefore,this locationwas selected as the temperaturemeasurement location.

2) There was a high correlation between electrical conductivityduring current application and measured temperature (y = 4E-05x2 + 0.0010x + 0.3690 with a contribution rate of R2 = 0.98).Thus,we demonstrated that sample temperature could be estimatedwith a certain level of confidence based on current and voltagevalues.

3) A sterilization test was conducted on the prepared ohmically heatedsamples. The samples conformed to the constant-temperature testand the microbiological test for retort food stipulated under theFood Sanitation Act. Therefore, it was suggested that the shelf lifeof the ohmically heated samples was almost the same as that ofthe retort-heated samples.

4) Temperature changes during heating were compared for the twoheating methods. Temperature increase was quicker under retortheating immediately after the start of heating. However, bothsample types reached 70 °C at nearly the same time; thereafter,temperature increase was quicker under ohmic heating. Thus, it

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was demonstrated that heating time could be reduced using ohmicheating.

5) The measurement of water content, IMP, and glutamic acid suggeststhat the quality of the ohmically heated samples was similar to thatof the retort-heated samples. Furthermore, based on themonitoringof these samples, it was observed that sample quality did notdeteriorate during storage.

6) The sensory test did not yield significant differences in the testedsensory qualities. It was suggested that the ohmically heatedsamples had similar sensory qualities to the retort-heated samples.

Acknowledgments

This study was partially funded by Japanese Society of TasteTechnology. We are grateful to Mr. Toshio Nakai for providing technicalsupport for retort treatment and to Mr. Daiki Yokoyama for providingtechnical support for ohmic heating.

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