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LWT - Food Science and Technology 55 (2014) 232e239

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LWT - Food Science and Technology

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Effects of ohmic heating for pre-cooking of meatballs on some qualityand safety attributes

Ilkin Yucel Sengun a,*, Gulen Yildiz Turp a, Filiz Icier a, Perihan Kendirci a, Gamze Kor b

a Ege University, Engineering Faculty, Food Engineering Department, Bornova, Izmir 35100, Turkeyb Ege University, Institute of Natural and Applied Sciences, Food Engineering Program, Bornova, Izmir 35100, Turkey

a r t i c l e i n f o

Article history:Received 14 January 2013Received in revised form17 July 2013Accepted 10 August 2013

Keywords:OhmicCookingMeatballQualitySafety

* Corresponding author. Tel.: þ90 232 3113028; faxE-mail addresses: ilkinyucel@yahoo.com, ilkin.sen

0023-6438/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.lwt.2013.08.005

a b s t r a c t

Effectiveness of ohmic treatment on some quality attributes of semi-cooked meatballs was studied.Meatball samples were semi-cooked by 15.26 V/cm voltage gradient and 0 s holding time at 75 �C.Although ohmic cooking significantly reduced the numbers of total mesophilic aerobic bacteria, mould-yeast, Staphylococcus aureus and completely eliminated Salmonella spp. from meatball samples(p < 0.05), it was not found efficient to inactivate all Listeria monocytogenes cells. Ohmic semi-cookingprocess was resulted at higher cooking yields, which were supported by high fat and moisture reten-tion values in meatball samples. Metal levels (iron, chromium, nickel and manganese) of ohmically semi-cooked meatball samples were found below the upper level of dietary exposure levels. Ohmic cookingprocedure was found to be safe in terms of PAH formation and mutagenic activity. Sensory evaluationshowed that the overall acceptance of the semi-cooked meatball samples were good. These resultsdemonstrate considerable potential for the application of ohmic process for semi-cooking of meatballs.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Ohmic heating, a well-known electro-heating technique, hasbeen developed during the past two decades and is used in com-mercial scale operations for processing a number of food products(Sastry & Salengke, 1998). The system is based on the passage ofelectrical current through a food product that has electrical resis-tance (Icier & Ilicali, 2005). The electrical energy is converted toheat, while the amount of heat generated through the food productis directly related to the voltage gradient and the electrical con-ductivity (Sastry & Li, 1996). Ohmic treatment is used in a widerange of applications such as preheating, cooking, blanching,pasteurization, sterilization and extraction of food products(Mizrahi, 1996; Lima & Sastry, 1999; Leizerson & Shimoni, 2005a;2005b; Icier, Yildiz, & Baysal, 2005). Shorter processing times,higher yields, maintenance of the colour and nutritional value offoods are some of the advantages of ohmic cookingwhen comparedto conventional heating (Wang & Sastry, 2002; Castro, Teixeira,Salengke, Sastry, & Vicente, 2004; Icier & Ilicali, 2005; Leizerson &Shimoni, 2005a; 2005b; Vikram, Ramesh, & Prapulla, 2005).

Since the uniform heat generation gives uniform temperaturedistribution, especially for liquid foods, USDA (United States

: þ90 232 3427592.gun@ege.edu.tr (I.Y. Sengun).

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Department of Agriculture) and FDA (Food and Drug Administra-tion) suggested the usage of ohmic technologies for pumpablefoods. Although the technique appears both simple and advanta-geous and has proved to be a successful technology to processliquids, the ohmic treatment of solid foods such as meat and meatproducts has not yet been applied industrially due to several dif-ficulties encountered (de Halleux, Piette, Buteau, & Dostie, 2005).

Meat samples commonly have heterogeneous structure, whichaffects the uniform distribution of heat (Shirsat, Lyng, Brunton, &McKenna, 2004). Geometric properties such as the size of thepiece of meat are important factors that limit the use of ohmiccooking technology in meat and meat products (Aymerich, Picouet,& Monfort, 2008). On the other hand, ohmic cooking offers thepotential for safer meat products by effectively inhibiting microbialgrowth through uniform temperature distribution in the productand cooking faster and instantly inside the food (Sastry & Li, 1996;Ozkan, Ho, & Farid, 2004). Several studies have been conductedabout the application of ohmic treatment to meat and meat prod-ucts for cooking purposes (Ozkan et al., 2004; Shirsat et al., 2004;Wills, Dewitt, Sigfusson, & Bellmer, 2006; Vasanthi,Venkataramanujam, & Dushyanthan, 2007; Zell, Lyng, Denis,Cronin, & Morgan, 2009; Bozkurt & Icier, 2010a,2010b). Althoughohmic cooking serves the fast and homogeneous cooking chancefor the meat products, microbiological and toxicological safety ofthe ohmically cooked meat products must also be evaluated.Moreover, there has been no study done regarding the effect of

I.Y. Sengun et al. / LWT - Food Science and Technology 55 (2014) 232e239 233

ohmic cooking on formation of PAHs and mutagenicity in foods,according to the best of the authors’ knowledge. Almost all cookingprocedures cause PAH formation onmeat andmeat products in lowor high amounts according to the cooking conditions. For examplesmoking, grilling and roasting cause PAH formation in high levels,while mild cooking conditions such as steaming can reduce thelevel of PAH formation (Yildiz-Turp, Sengun, Kendirci, & Icier, 2013).Although the temperatures for ohmic cooking procedure is quitemild (<100 �C), this procedure still can cause high levels of PAHformation in meat and meat products because of direct contact ofthe electrodes and the samples (Yildiz-Turp et al., 2013).

In a recent study, meatball samples prepared by same proceduregiven in this paper were semi-cooked ohmically by using 3different voltage gradients (15, 20 and 25 V/cm) and three differentholding times (0, 15 and 30 s). Desirability function in responsesurface methodology was used to determine the optimum ohmiccooking condition as a pretreatment by considering the criteria ofminimizing hardness ratio, maximizing chewiness, resilience, logreduction in microbial load, outside chroma ratio, inside chromaratio and in range of springiness, gumminess and inside L-ratio. Bythis method, optimum ohmic pre-cooking condition in the samecooking system has been determined as cooking up to 75 �C centretemperature by applying 15.26 V/cm voltage gradient and noholding time requirement (Icier et al., 2013). The aim of the presentstudy was to determine the effects of ohmic cooking conducted inthe same system by applying optimum ohmic pre-cooking condi-tion for evaluation of microbiological, toxicological, physical,chemical and sensory attributes of semi-cooked meatball.

2. Materials and methods

2.1. Sample preparation

Lean beef as boneless rounds were supplied from a local pro-cessor (Burdur Güçbirli�gi Meat Facility A.S.) and were transportedto Ege University, Food Engineering Department’s Electrical Oper-ation Laboratory in vacuumed packages with maintaining coldchain (�18 �C). Meat was removed from the vacuum packages andre-packaged in low density polyethylene (LDPE) bags, 0.2 kg each,and stored at �18 �C until used within one month. Thawed meatsamples (at 4 �C for one night) were ground through a 3 mm plategrinder (Arçelik, Turkey), and mixed with the ingredients. Meat-balls were produced according to the following recipe: meat (96%w/w), onion powder (1% w/w), salt (0.5% w/w), sodium carbonate

Fig. 1. Ohmic coo

(0.5% w/w) and distilled water (2% v/w). Mixture was kneaded for15 min by hand, to obtain homogeneous dough. The prepareddough was stored in a refrigerator (at 4 �C) for an hour and thenshaped into cylinder meatballs having 0.025 m diameter and0.05 m length by using a cylinder block. All the production processof meatballs was carried out at room (20 � 1 �C) temperature.

2.2. Ohmic cooking procedure

Ohmic cooking was applied as pre-treatment before finalcooking of meatballs. Experiments were conducted in specificallydesigned custom-made continuous belt type ohmic cooking sys-tem, which consisted of a power supply, an isolating-variabletransformer, a microprocessor board and a data recording system,temperaturemeasurement units, ohmic cooking unit and a rotatingbelt system (Fig. 1). The cooking unit was designed specially, whichincludes rotating (Polyester Monofilament) belt with motor forcontrolling speed, two removable stainless steel electrodes(5 cm� 30 cm), electrically isolated Teflonmountings and electricalconnections to transformer unit. Temperature measurements wereconducted by using Teflon coated electronic temperature sensors(Omega Eng. Inc., Stanford, CT). Teflon coated electronic tempera-ture sensors had special design and isolation to electric current.They could have been immersed into meatball. The occurrence ofthe signal interference in the system was avoided by the usage ofTeflon as a coating material, and other possible signals were takeninto account by the control program of the microprocessor. Thetime constants of temperature sensors were determined by cali-brating them in standard calibration solutions (Omega Eng. Inc.,Stanford, CT). The microprocessor board that was used to monitorthe temperatures, current (A) and voltage applied (V), and ittransmitted this information simultaneously to the microcomputerat constant time intervals (1 s) (Fig. 1).

The sample was placed at the inlet of the continuous typecooking unit and sandwiched between the electrodes with thecompression. After the system was sealed, belt was rotated at thespeed of 0.25 cm/s, and the power was given to the system. Theoptimum ohmic condition for precooking purpose for meatball hasbeen determined as 15.26 V/cm voltage gradient and 0 s holdingtime. After ohmic cooking, samples were cooled down to 20 �C inrefrigerated ambient and then immediately transferred to analyses.

Cooking time, which was parameterized that the time requiredreaching 75 �C of the centre temperature of meatball samplesfrom the initial temperature of 20 �C, was 92 s. Temperature

king system.

I.Y. Sengun et al. / LWT - Food Science and Technology 55 (2014) 232e239234

measurement was taken both from centre and lateral surface of themeatball during ohmic treatment. Ohmic cooking was ended whencentre temperature reached to 75 �C. Electrical conductivity ofmeatball was increased with temperature increase during ohmicheating, and it was determined as 1.5 S/m at initial (20 �C) and2.25 S/m at 75 �C.

Before microbiological analysis, all parts of the system weredisinfected by hypochlorite solution (200 ppm/30 min) (Taylor,1993).

2.3. Analysis

2.3.1. Microbiological analysisTo obtain the effect of ohmic cooking on the microbiological

profile of meatball samples and to ensure themicrobiological safetyof the product, total mesophilic aerobic bacteria (TMAB), mouldand yeast, Staphylococcus aureus, Clostridium perfringens, Salmonellaspp., Listeria monocytogenes, Escherichia coli O157:H7 analysis werecarried out.

Each sample (25 g) was transferred into 225 mL 0.1% peptonewater (PW, pH 6.3 � 0.2, Oxoid-L37, Basignstoke, Hampshire, En-gland) and homogenized for 2 min with Stomacher (Lab-blender400, Seward, London, UK). Appropriate ten-fold dilutions of thesamples were prepared in PW and plated on/in growth media induplicate to estimate microbial counts.

TMAB was determined by using pour plate method on PlateCount Agar (PCA, pH 7.1 � 0.2, Oxoid-CM325) and the plates wereincubated at 35 �C for 48 h (BAM, 2001a). The counts of mouldand yeast were determined by surface plating on Dichloran RoseBengal Chloramphenicol Agar (DRBC, pH 5.6 � 0.2, Oxoid-CM0727) containing Chloramphenicol Selective Supplement(Oxoid-SR0078E) and plates were incubated at 25 �C for 3e5

Fat retentionð%Þ ¼ cooked meatball weight� fat in cooked meatballuncooked meatball weight� fat in uncooked meatball

(2)

days (BAM, 2001b). S. aureus was determined by surface platingon BairdeParker Agar (BPA, pH 6.8 � 0.2, Oxoid-CM275) andincubating the plates at 35 �C for 45e48 h (BAM, 2001c). Doubleplated Iron Sulphite Agar (ISA, bioMerieux-42603) plates wereused to count C. perfringens and plates were incubated underanaerobic conditions at 37 �C for 20 � 2 h. Typical colonies wereconfirmed by biochemical tests (motility, nitrate reduction,lactose fermentation, gelatin liquefaction) (ISO, 2004a). Afterpre-enrichment and enrichment steps, Salmonella was detectedby using streak plate method on Xylose Lysine DesoxycholateAgar (XLD, pH 7.4 � 0.2, Oxoid-CM 469) and Bismuth SulphiteAgar (BSA, pH 7.6 � 0.2, Oxoid-CM0201) and incubating the

Reduction in meatball diameterð%Þ ¼ uncooked meatball diameter� cooked meatball diameteruncooked meatball diameter

� 100 (4)

plates at 37 �C/24 h for XLD plates and 37 �C/24e48 h for BSAplates. Presumptive Salmonella colonies were confirmed by usingbiochemical tests [Triple Sugar Iron Agar (TSI, pH 7.4 � 0.2,Oxoid-CM0277) and Lysine Iron Agar (LI, pH 6.7 � 0.2, Oxoid-

CM0381) reactions] and serological tests (BAM, 2007). Afterpre-enrichment and enrichment steps, L. monocytogenes isolationstep were applied by using streak plate method on Ottaviani-Agosti Agar (bioMerieux-43461) and Palcam Agar (PA, pH7.2 � 0.2, Oxoid-CM0877) and incubating the plates at 37 �C for24 h. Presumptive colonies were confirmed by using API Listeria(bioMerieux-10300) (ISO, 2004b). Samples firstly screened forE. coli presence by using API 20E (bioMerieux-20100), then E. coliO157:H7 detection were applied for positive results. After pre-enrichment step, isolation step were applied by streak platemethod on TC-SMAC and plates were incubated at 37 �C for 24 h.Typical colonies were confirmed by E. coli O157 latex agglutina-tion test kit (Oxoid-DR0620) (BAM, 2001d).

2.3.2. Proximate analysisMoisture, protein, ash content and pH of each sample were

measured by using the appropriate AOAC (1990) procedures.The conversion factor was 6.25 for protein analyses. Fat contentof samples was determined by using the chloroformemethanolextraction method as suggested by Flynn and Bramblett (1975).

Cooking yield: Percentage of cooking yield was determined bycalculating weight differences of the samples before and aftercooking (Murphy, Criner, & Grey, 1975). The cooked samples werecooled to room temperature for 30 min and reweighed to calculatethe cooking yield

Cooking yieldð%Þ ¼ cooked meatball weightuncooked meatball weight

� 100 (1)

Fat retention: The fat retention value represents the amount offat retained in the product after cooking. Fat retention was deter-mined according to Murphy et al. (1975).

Moisture retention: The moisture retention value representsthe amount of moisture retained in the cooked product per 100 g ofsample and was determined according to El-Magoli, Laroia, &Hansen, 1996.

Moisture retentionð%Þ ¼%yield�%moisture in cookedmeatball100

(3)

Reduction in meatball diameter: Measurements of meatballsamples were made by using a digital micrometer (Mitutoyo,Japan). The reduction in meatball diameter was calculated as fol-lows:

Reduction in volume: The reduction in the volume of meatballwas calculated by determining sample dimensions before and afterthe cooking process.

Table 2Microbiological quality of raw meat, uncooked and ohmically cooked meatball.

Microbiological analysis Raw meat Uncooked Ohmically

Reduction in volume ð%Þ ¼ uncooked meatball volume� cooked meatball volumeuncooked meatball volume

� 100 (5)

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2.3.3. Metal migration levelThe amount of metals such as iron (Fe), chromium (Cr), nickel

(Ni) and manganese (Mn) that were possibly migrated from theelectrode to the sample were determined bymodifying the methodgiven by Jun, Sastry, and Samaranayake (2007). For this purpose,250mg of sample was added to the dissolution pot with 6mL HNO3and 1 mL H2O2. Pot was mixed before closing and was placed in theprogrammed microwave dissolution device (Berghof, SpeedwaveMWS-3). Heating program of microwaves dissolution device isgiven in Table 1.

Prepared samples were measured using ICP-OES (InductivelyCoupled Plasma - Optical Emission Spectrometry, Perkin Elmer,Optima 2000 DV) device. Operating parameters were as follows:using peristaltic pomp flow rate 1.5 mL/min, RF power 1300 W,plasm gas flow rate 15 L/min, auxiliary gas flow rate 0.2 L/min,nebuliser gas flow rate 0.8 L/min, axial measurement type, mea-surement delay time 10 s. Different wavelengths were used for thedetection of metals: Cr: 206.149 nm; Fe: 238.940 nm; Ni: 231.604;Mn: 257.610 nm.

2.3.4. PAH formationPAHs (Acenaphthene-Ace, Anthracene-Ant, Benz[a]anthracene-

B[a]A, Benzo[b]fluoranthene-B[b]F, Benzo[k]fluoranthene-B[k]F,Benzo[ghi]perylene-B[ghi]P, Benzo[a]pyrene-B[a]P, Chrysene-Chr,Dibenz[a,h]anthracene-DB[ah]A, Fluoranthene-Flt, Fluorene-Fln,Indeno[1,2,3-cd]pyrene-I[cd]P, Naphthalene-Naft, Phenanthrene-Phe and Pyrene-Pyr) were extracted from the samples by using asimilar method suggested by Chung et al. (2011). 30 g of choppedsamples were lyophilized and stored at �20 �C before the analysis.In the extraction step, firstly lyophilized samples were placed into500 mL round flask, then 100 mL KOH (2 M, prepared by usingmethanol:water [9:1]) solution and 100 mL hexane were addedrespectively, and flask was held in a 80 � 2 �C water bath underreflux for 2 h. The mixture was cooled to approximately 40 �C byadding 100 mL cold water, and allowed to stand overnight in thedark. Hexane phase was transferred into 250 mL round flask andconcentrated to w2 mL at 50 � 2 �C by using a rotary evaporator.The concentrate was purified by passing through a Sep-Pak florisilcartridge, which had been previously activated with 10 mLdichloromethane and 20 mL hexane. Elution solvents consisting10 mL hexane and 8 mL hexane:dicholoromethane (3:1) werepassed through the cartridge respectively. All the eluates werecollected, dried under a nitrogen stream, dissolved by using 1 mL ofacetonitrile, filtered through 0.45 mmmembrane filter and collectedin to 2 mL amber vials. 20 mL of extract was injected to High Pres-sure Liquid Chromatography (HPLC) device equipped with a fluo-rescence detector (Agilent Tech. 1200 Series G1321). A reverse-phase C18 column (Vydac-201TP5415, 150 mm � 4.6 mm � mmparticle size) was used. The gradient solvent system started with

Table 1Heating program of microwaves dissolution device.

Heating program of the device *T (�C);applied temperature step *ta (min);outlet rate step *t (min);heating time step

Steps 1 2 3 4T (�C) 125 150 180 100ta (min) 5 5 5 5t (min) 10 15 5 5

50% acetonitrile in water (v/v) and linearly increased to 100%acetonitrile within 30 min at a flow rate of 1.25 mL/min. Theexcitation and emission wavelengths were 270/340 nm for Naft,Ace and Fln, 254/375 nm for Phe, 254/390 nm for Pyr, B[a]A and Chr,260/420 for Ant, Flt, B[b]F, B[k]F, B[a]P, DB[ah]A and B[ghi]P and293/498 nm for I[cd]P (Chen,Wang, & Chiu,1996; Chiu, Lin, & Chen,1997; Lorenzo, Purrinos, Garcia Fontan, & Franco, 2010; Chung et al.,2011; Lorenzo et al., 2011). The PAHs were quantified by usingexternal calibration curves obtained for each PAH by using workingsolutions of standards (Supelco PAH Kit 610-N, Bellafonte, PA, USA)with the concentrations from 0.05 to 500 mg/kg in acetonitrile.

2.3.5. Mutagenicity assayThe mutagenicity of meatball ohmically cooked at 15.26 V/cm

voltage gradient and 0 s holding time, was evaluated by measuringits ability to induce reverse mutations in Salmonella Typhimuriumstrains TA98 and TA100. Additionally, assay has been conductedwith metabolic activation with S9 rat liver enzyme extract. Theywere performed by using test kit (TheMuta-ChromoPlate�), whichis based on themost generally used and validated bacterial reverse-mutation test, known as the ‘Ames Test’ (Ames, McCann, &Yamasaki, 1975). The test employs a mutant strains, of SalmonellaTyphimurium, carrying mutation(s) in the operon coding for his-tidine biosynthesis. When these bacteria are exposed to mutagenicagents, under certain conditions reverse mutation from amino acid(histidine) auxotrophy to prototrophy occurs (Ames et al., 1975).The method for the extraction of meatball was based on liquideliquid extraction as explained in the test kit manual. For thebioassay reaction, the mixture contained extracted samples, bac-terial strains with or without S9 mix, DaviseMingioli salts, D-glucose, Bromocresol purple, D-Biotin and L-histidine, were pre-pared and filled to each well in the plate at three different dilutionsof the sample extracts. After the incubation period at 37 �C for 3e6days, purple wells were scored as negative, while yellow wells aspositives. Obtained results of each treatment plate were evaluatedagainst the background mutation (negative control).

Recommended control mutagens [sodium azide (NaNO3),2-nitrofluorene (2-NF), 2-amino-anthracene (2AA)] have been usedto verify the Ames test. Sodium azide (NaNO3) was used withTA100, whereas 2-nitrofluorene (2-NF) was used with TA98.Among positive controls, 2-Amino-Anthracene (2AA), whichrequire S9metabolic activation, was used in the presence of S9 mix,contained MgCl2, KCl, Glucose-6-Phosphate, Nicotine Amide Di-nucleotide Phosphate, Phosphate Buffer, distilled water, rat liverextract as positive control for TA100 and TA98.

meatball cooked meatball

TMAB (log10 CFU/g) 5.71 � 0.15 5.77 � 0.13 3.30 � 0.20Mould and yeast ((log10 CFU/g) 3.00 � 0.35 3.41 � 0.56 <1S. aureus (log10 CFU/g) 2.54 � 0.55 2.61 � 1.27 <1C. perfringens (log10 CFU/g) <1 <1 <1Salmonella spp./25 g Positive Positive NegativeL. monocytogenes/25 g Positive Positive PositiveE. coli O157:H7/25 g Negative Negative Negative

Table 3Chemical composition and pH values of raw meat, uncooked and ohmically cooked meatball.

Sample Moisture (%) Fat (%) Ash (%) Protein (%) Cooking loss (%) pH

Uncooked meatball 76.63 � 0.18 1.44 � 0.19 1.64 � 0.03 21.24 � 0.23 e 6.44 � 0.03Ohmically cooked meatball 73.93 � 0.38 1.59 � 0.12 1.77 � 0.17 23.59 � 1.17 15.57 � 1.61 6.51 � 0.02

*There are no significant differences between the results in the same column (p > 0.05).

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2.3.6. Sensory analysisOhmically cooked samples were evaluated by attendance of

seven experienced assessors from Food Engineering Department ofEge University. Samples were given to the panel in porcelain dishesjust after cooking process and round table discussion was con-ducted for defining the surface color, inner color, surface textureand inner texture, visually. Visual acceptance of the samples wasalso evaluated individually by using five-point scoring test (5-excellent, 4-good, 3-satisfactory, 2-poor and 1-unacceptable)(Altug & Elmaci, 2005).

2.3.7. Statistical analysisExperiments were conducted in three replicates. The differences

in cooking properties, chemical, toxicological and microbiologicalattributes were analyzed by ANOVA using the SPSS softwareversion 15 (SPSS, 2004). Differences among the means werecompared by using Duncan’s Multiple Range test. A significancelevel of p � 0.05 was used for all evaluations.

3. Results and discussion

Cylindrical shape of the meatballs has been selected on the basisof shape of traditional meatballs consumed in Turkey. Since cylin-drical meatball was contacted to electrodes by sandwiched be-tween its circular end surfaces, it had similar configuration forelectrical treatment to round meatball (infinite slab having circularflat area in its end surfaces) studied in literature. In other words,shape of meatball used in this study can be said as having higherheight than round meatball. Height of the meatball and the voltageapplied were taken account on application of voltage gradient (V/cm) during ohmic heating. The homogeneity of the heating wasincreased by supplying of turning around of its own axis androtating on belt of continuous ohmic heater. Therefore, the homo-geneity of heating was comparable to roundmeatballs. The heatinghomogeneity, which was defined as ratio of highest temperature tolowest temperature in the meatball heated, was obtained as in therange of 1.0e1.3 during ohmic heating. Since meatball heated wasopen to surrounding ambient in the continuous ohmic heater, itslateral surface was exposed to slight cooling due to heat lossoccurred. On the other hand, it was thought that the heating ho-mogeneity was sufficient since ohmic cooking was used as pre-treatment before following final cooking process. Since this studywas only one part of the comprehensive project, the followingcooking process (not mentioned in this study) had been alsoapplied to obtain acceptable browning in the surface of the meat-ball and ensure the microbiological safety of the final product.

Table 4Cooking properties of ohmically cooked meatball.

Sample Cooking yield (%) Fat retention (%) Moisture r

Ohmically cooked meatball 84.43 � 1.61 96.23 � 11.28 62.18 � 1.

3.1. Microbiological properties of meatball

In the study, total mesophilic aerobic bacteria (TMAB) counts ofraw meat and prepared uncooked meatball were found as 5.71 logCFU/g and 5.77 log CFU/g, respectively (Table 2). As it can be seenfrom the table, TMAB count of meatball sample was significantlyreduced 2.47 log unit by ohmic cooking applied at 15.46 V/cm ofvoltage gradient (p < 0.05). According to the microbiologicalcriteria recommended for cooked meatballs, the maximum limitsfor mould and yeast counts are given as 3 log CFU/g (Anonymous,2009). In this study, mould and yeast counts of meatball sampleswere reduced to undetectable levels by ohmic cooking (Table 2).S. aureus counts of raw meat and uncooked meatball samples werefound as 2.54 log CFU/g and 2.61 log CFU/g, respectively (Table 2).Ohmic cooking reduced S. aureus counts of samples to an unde-tectable level. In the other study, meat and meatball samplesinoculated with S. aureus at 104e105 CFU/g levels were completelyeliminated after ohmic treatment at 81 �C for 5 min total processtime (Mitelut et al., 2011).

E. coli O157: H7, L. monocytogenes, Salmonella spp. andC. perfringens are pathogenic microorganisms commonly found inmeat and meat products and cause food borne diseases if theproducts containing these pathogens are not well cooked (ICMSF,1998). In this study, C. perfringens and E. coli O157:H7 were notisolated from raw meat, uncooked and ohmically cooked meatballsamples (Table 2). S. aureus and C. perfringens can be tolerable forcooked meat products at levels between 2 and 3 log CFU/g, whileSalmonella spp., L. monocytogenes and E. coli O157:H7 are notallowed to be present in cooked or uncooked meat and meatproducts (Anonymous, 2009). In this study, rawmeat and uncookedmeatball samples were found positive for Salmonella andL. monocytogenes. Salmonella spp. were eliminated from meatballsamples by ohmic cooking process, however L. monocytogenescould not be eliminated completely from the product (Table 2).These results showed that ohmic cooking condition used in thisstudy had lethal effect on Salmonella cells, while this applicationwas not sufficient for killing all L. monocytogenes cells on meatball.It is reported that Salmonellae are sensitive to heat, while somespecies of Salmonella are more resistant than others such as Sal-monella Senftenberg (ICMSF, 1996). Therefore, it should be takeninto account for effective Salmonella inactivation in foods whenusing thermal possessing technology. Although it is known thatthere is no pathogenic microorganism which show resistance toohmic heating (Knirsch, Santos, Vicente, & Penna, 2010), the effectsof ohmic heating on pathogenic microorganisms depends on theohmic conditions used. Similar results were reported by Zell, Lyng,

etention (%) Reduction in meatball diameter (%) Reduction in volume (%)

56 2.55 � 3.95 19.74 � 6.76

Table 5Levels of metal ions in meatball samples before and after ohmic cooking procedure.

Sample Fe Cr Ni Mn

Uncookedmeatball(mg/kg)

3.481 � 0.001 0.129 � 0.003 0.085 � 0.003 0.221 � 0.001

Ohmicallycookedmeatball(mg/kg)

4.780 � 0.275 0.266 � 0.037 0.108 � 0.015 0.396 � 0.053

I.Y. Sengun et al. / LWT - Food Science and Technology 55 (2014) 232e239 237

Cronin, and Morgan (2010), who inoculated meat samples withListeria innocua at 7.05 log CFU/g level. Although they couldcompletely eliminate L. innocua cells on meat by ohmic cooking athigh temperature and short time application (95 �C, 7 min totalprocess time and no holding time), same reduction level (7.05 log)in L. innocua number was achieved after additional 15 min of 72 �Cof ohmic treatment.

3.2. Chemical composition and cooking properties of meatball

The chemical composition of meatball samples is given inTable 3. As an expected result from a cooking process, ohmiccooking increased protein, fat and ash contents of meatball sampleswhile it decreased the moisture contents. It was observed that thepH value of meatball was increased after the ohmic cooking pro-cess. Similar results were obtained in studies which differentcooking systems of meat products were applied. The pH of cookedmeat batter was determined higher than uncooked meat batter(Choi et al., 2009, 2010). Also cooking of meat burgers with elec-trical grill increased pH value of the samples (Gök, Akkaya, Obuz, &Bulut, 2011).

It is well-established that meat cooked quickly to a given in-ternal temperature has a lower cook loss value and is juicier thanmeat cooked slowly to the same temperature (Lawrie, 1998). Thereduction of cook loss and improved juiciness is one of the majoradvantages of ohmic heating compared to other heating technol-ogies (Zell et al., 2009). In this study, cooking properties of ohmi-cally cooked meatball samples are given in Table 4. Reducedcooking times with ohmic cooking process resulted with highercooking yield value in meatball samples. Also high fat andmoistureretention values supported higher cooking yield values of thesamples. It was observed that ohmic cooking of low fat meatballswas resulted at higher cooking yield, moisture retention and fatretention values compared to the literature values on cooking of

Table 6Levels of PAHs in meatball samples after ohmic cooking procedure.

PAH PAHs levels (mg/kg) in meatball

Naph 0.03 � 0.04Ace 0.97 � 0.85Fln 1.44 � 0.15Phe 0.05 � 0.09Pyr 0.29 � 0.12B[a]A <0.012a

Chr <0.009a

Ant 0.33 � 0.18Flt 0.33 � 0.18B[b]F 0.19 � 0.04B[k]F 0.09 � 0.08B[a]P 0.09 � 0.08DB[ah]A 0.71 � 0.61B[ghi]P 0.18 � 0.16I[cd]P <0.015a

Total 4.44 � 1.90

a Under detection limit.

low fat meatballs with electrical grill (Ulu, 2006; Gök et al., 2011),and teflon coated pan (Yildiz-Turp & Serdaroglu, 2010).

The reduction in volumewas relatedwith the structural changeswithin the sample during cooking. In ohmic treatment, the samplewas contacted with electrode surface perfectly. Hence the heatgeneration occurred in the system due to higher current valuesbeing passed would result sudden expansion in the sample tissue,especially at higher voltage gradients. This simultaneous expansionand contraction occurred during ohmic cooking would have beenresulted to less overall volume changes (Bozkurt & Icier, 2010b).

In this study, it was determined that the volume reduction ofohmically cooked meatball samples was higher than the commi-nuted meat samples which were ohmically cooked by Bozkurt andIcier (2010b), while it was lower than the same samples cookedtraditionally. Since Bozkurt and Icier (2010b) used a rigid samplecell surrounding the sample during ohmic cooking and the usage ofthis cell could explain the prevention of the changes in samplevolume.

3.3. Metal migration levels

Table 5 shows the Fe, Cr, Ni and Mn levels of meatball samplesbefore and after application of ohmic cooking. It can be observedfrom the table that ohmic cooking procedure caused significantincrease in Fe, Cr and Mn levels of samples, while this increase wasnot significant in Ni level (p < 0.05). Similar results were observedby Jun et al. (2007) during ohmic cooking of tomato soups by usingstainless steel electrodes, where the process caused significant in-creases in Fe, Cr and Mn levels while Ni level was constant.

There is no information related to the acceptable limits of Fe, Cr,Ni and Mn levels in foods in Turkish Food Regulations as well as inEuropean Directives. Besides the Scientific Committee on Food (SCF,2006) offers an upper tolerable limit only for Cr, which should beless then 1 mg/day. The Food and Nutrition Board, Institute ofMedicine, National Academies (USA) has established TolerableUpper Intake Levels for iron, nickel and manganese as 45 mg/day,1 mg/day and 11 mg/day, respectively (Anonymous, 2013). By usingthe data given in Table 5, metal levels of 1 portion (120 g) ofohmically cooked meatball sample were calculated as 0.57 mg Fe,0.03 mg Cr, 0.01 mg Ni and 0.05 mg Mn, respectively. Based onthese calculations, it can be seen that the metal levels of 1 portionof ohmically cooked meatball is far below from the tolerable upperintake levels.

3.4. PAH formation

PAH analysis was applied to the meatballs before and afterohmic cooking process and none of the PAHs were found in theuncooked samples. Table 6 shows the PAH levels of meatballsamples after application of ohmic cooking. According to EC di-rectives, the max. B[a]P limit that is permitted in smoked meat andsmoked meat products is 5 mg/kg, while this value is 30 mg/kg forthe sum of B[a]P, B[a]A, B[b]F and Chr (EC, 2011). Although ohmi-cally cooked samples cannot be categorized as smoked meat, thereis no limit for the other meat products. B[a]A and Chr were notdetermined in the ohmically cooked meatball samples, while B[a]Pand B[b]F amounts were 0.09 and 0.19 mg/kg (Table 6). The otherPAHs were also determined in small quantities. These resultsshowed that ohmic cooking procedure is safe in terms of PAH for-mation in meatball.

3.5. Mutagenicity

The results indicates that meatball samples, ohmically cooked at15.26 V/cm voltage gradient and 0 s holding time, exerted no

I.Y. Sengun et al. / LWT - Food Science and Technology 55 (2014) 232e239238

mutagenic activity in the bacterial strains of the testedS. Typhimurium TA98 and TA100.

3.6. Sensory properties of meatball

Ohmic cooking procedure caused pinkish/reddish brown colordistribution on the surface of meatball, while the inner color wasobserved to be slightly pink and slightly yellowish light brown.Surface texture of the meatball samples was evaluated as dry,slightly fatty and slightly moist having a very thin, ignorable crust.On the other hand, the inner surface texture of the samples wasfound as rough, smooth and moderately moist. The overall accep-tance of the samples were evaluated with the score 4 (good) by theassessors.

4. Conclusions

An optimized ohmic cooking conditionwas used to obtain semi-cooked meatballs and some toxicological and mutagenic aspectswere considered. Also high cooking yield, moisture retention andfat retention values in samples were obtained with ohmic cooking.Ohmic cooking conducted at defined condition was sufficient toeliminate Salmonella and S. aureus, which were naturally found inmeatball samples, while it was not effective in the elimination ofL. monocytogenes. For the elimination of this risk, future studiesshould be carried out by using combined cooking methods. Ac-cording to the results of the sensory analysis, overall visual accep-tance of the meatball samples was “good”. Ohmic treatment seemsto be promising cooking method for meatballs in terms of PAHsformation. Additionally, more extensive studies into pathogenicbacteria inactivation should be determined in following studies.

Acknowledgements

The authors are grateful for financial support provided for theproject entitled “Setting up and performance evaluation of ohmic-infrared heating system, and the investigation of its use in meatballcooking by considering its effects on some quality attributes” underproject no: 110O068 by The Scientific and Technological ResearchCouncil of Turkey (TUBITAK), Bati Climate SA, Arçelik SA, BurdurGüçbirli�gi Meat Plant. The authors also thank Prof. Dr. Tomris Altu�gOno�gur for her valuable scientific advices relating to the toxico-logical and sensory parts of the study.

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