International Journal of Food Microbiology

Transcription

1 International Journal of Food Miroiology () Contents lists availale at SieneDiret International Journal of Food Miroiology journal homepage: Influene of moisture ontent on inativation of Esherihia oli O:H and Salmonella enteria serovar Typhimurium in powdered red and lak pepper spies y radio-frequeny heating Seul-Gi Jeong, Dong-Hyun Kang Department of Food and Animal Biotehnology, Department of Agriultural Biotehnology, Center for Food and Bioonvergene, Researh Institute for Agriulture and Life Sienes, Seoul National University, Seoul -9, Repuli of Korea artile info astrat Artile history: Reeived Otoer Reeived in revised form January Aepted 9 January Availale online January Keywords: Radio-frequeny heating Moisture ontent Dieletri properties Foodorne pathogen Pepper The influene of moisture ontent during radio-frequeny (RF) heating on heating rate, dieletri properties, and inativation of foodorne pathogens was investigated. The effet of RF heating on the quality of powdered red and lak pepper spies with different moisture ranges was also investigated. Red pepper (.%,.%, 9.%, and.% dry asis, d) and lak pepper (.%,.%,.%, and.% d) inoulated with Esherihia oli O:H and Salmonella enteria serovar Typhimurium were treated in a RF heating system with. MHz. The heating rate of the sample was dependent on moisture ontent up to 9.% (d) of red pepper and.% (d) of lak pepper, ut there was a signifiant derease in the heating rate when the moisture ontent was inreased eyond these levels. The dieletri properties of oth samples inreased with a rise in moisture ontent. As the moisture ontent inreased, treatment time required to redue E. oli O:H and S. Typhimurium y more than log CFU/g (elow the detetion limit, log CFU/g) dereased and then inreased again without affeting produt quality when the moisture ontent exeeded a level orresponding to the peak heating rate. RF treatment signifiantly (P.) redued moisture ontent of oth spies. These results suggest that RF heating an e effetively used to not only ontrol pathogens ut also redue moisture levels in spies and that the effet of inativation is dependent on moisture ontent. Elsevier B.V. All rights reserved.. Introdution Radio-frequeny (RF) heating is a highly appealing tehnology y whih internal heating as a result of moleular frition is rapidly generated in response to an applied alternating eletri field at frequenies etween and MHz (Piyasena et al., ). This tehnology an offer the advantage of more uniform heating than onventional heating eause of the diret interation etween eletromagneti waves and foods (Marra et al., 9). With onventional food heating, externally generated heat is transferred to the food produt y ondution, onvetion, or radiation (Doores, ). Conversely, RF generates heat rapidly within foods due to the osillating movement of polar dieletri moleules and the spae harge displaement aused y an externally applied AC eletri field (Piyasena et al., ; Marra et al., 9). Therefore, it has potential for thermal proessing of solid and semisolid foods whih have low thermal ondutivities while affeting few appearane, texture, or organolepti hanges and simultaneously promoting miroiologial safety and shortening the proessing time Corresponding author at: Department of Food and Animal Biotehnology, Center for Food and Bioonvergene, Researh Institute for Agriulture and Life Sienes, Seoul National University, Seoul -9, Repuli of Korea. Tel.: address: kang8@snu.a.kr (D.-H. Kang). (Luehapattanaporn et al., ). Commerial RF proessing has een developed ased on applied researh of RF heating (Demezky, 9; Houen et al., 99; Zhao et al., 8), and it has een suessfully used in drying, aking and thawing of frozen meat and in meat proessing (Piyasena et al., ). Although onsidered to e a promising food proessing tehnology, RF sterilization is rather limited due to a lak of in-depth tehnial information. Dieletri properties have assumed great importane assoiated with RF heating, sine these properties of food affet their heating rate during RF heating (Tang, ). These properties are affeted y sample moisture ontent, salt ontent, density, temperature, frequeny of the applied alternating field, and a few other fators, though moisture ontent is generally onsidered to e the most ritial fator (Orsat and Raghavan, ; Tang, ). In spie manufaturing and storage, moisture ontent is also important and is monitored ontinuously due to frequent flutuations resulting from hanges in relative humidity. This is eause moisture ontent serves as an indiator not only for storage staility ut also for miroiologial safety of powdered foods inluding spie produts (Shweiggert et al., ). Aordingly, moisture ontent should e taken into aount as a fator in RF heating of spies. Ignorane of dieletri properties relevant to RF sterilization an ause improper heating, leading to old spots or produt urning. To prevent undesirale RF heating, there have een some researh efforts 8-/$ see front matter Elsevier B.V. All rights reserved.

2 S.-G. Jeong, D.-H. Kang / International Journal of Food Miroiology () to larify the impat of moisture ontent on dieletri properties of solid or semisolid foods (Sailik et al., ; Gao et al., ; Guan et al., ; Roeuk and Goldlith, 9). However, there has een no omprehensive researh on the effet of moisture ontent on heating rate, dieletri properties, and miroial inativation in foods. Also, thus far, the impat of RF heating on produt quality at different moisture levels has not een studied. Therefore, it is neessary to examine the influene of moisture ontent on the heating rate of food produts in order to maximize proess effetiveness as well as ensure uniform ommerial sterilization without affeting produt quality when applying RF heating. Powdered red (Capsium spp.) and lak (Piper nigrum) pepper spies were seleted as a model to investigate the effets of RF heating on a solid food. Beause of their low moisture ontent, spies are nonperishale ommodities, ut they are natural produts and may e urdened with high levels of miroorganisms (Shweiggert et al., ). Sine they are utilized in ready-to-eat foods whih are not sujeted to further ooking, use of ontaminated spies an lead to severe foodorne illnesses (Little et al., ). A large multistate outreak of Salmonella Montevideo infetions assoiated with salami produts ourred in the United States in Novemer 9. The impliated foods were made with ontaminated red and lak pepper powders. During this outreak, a reported persons in states and Washington, DC eame ill (Centers for Disease Control and Prevention, ). In the United States pepper-assoiated outreaks were reported to the Centers for Disease Control and Prevention (CDC) etween 998 and 9. Few deontamination methods exist for reduing the miroial of spies, suh as fumigation with ethylene oxide, irradiation, and steam treatment; however, onstraints on unhealthful residues, poor onsumer aeptane, and quality deterioration still limit widespread use of onventional tehnologies (Shweiggert et al., ; Waje et al., 8). No outreaks involving Esherihia oli O:H on red and lak pepper spies have een reported. However, E. oli O:H ould e a potential risk in spie due to its survivaility in low a w food (Park and Beuhat, ). The ojetives of this study were to examine the impat of moisture ontent on the heating rate of powdered red and lak pepper spies. The effets of RF heating for inativating E. oli O:H and S. Typhimurium in red and lak pepper powders of various moisture ontents, as well as dieletri properties and quality of spies, inluding olor, volatile flavor omponents, and post-treatment moisture ontent, were investigated.. Materials and methods.. Baterial strains All aterial strains, namely, E. oli O:H (ATCC, ATCC 889, ATCC 89), and S. Typhimurium (ATCC 98, ATCC 9, ATCC 8) were otained from the Department of Food and Animal Biotehnology ulture olletion at Seoul National University (Seoul, South Korea). Stok ultures were stored at 8 C in. ml of trypti soy roth (TSB; Difo, Beton, Dikinson, Sparks, MD, USA) and. ml of % glyerol. For all experiments, working ultures were streaked onto trypti soy agar (TSA; Difo), inuated at C for h, and stored at C... Preparation of pathogen inoula Eah strain of E. oli O:H and S. Typhimurium was ultured individually in ml of TSB at C for h, harvested y entrifugation at g for min at C, and washed three times with sterile.% peptone water (PW; Difo). The final pellets were resuspended in ml of.% PW, orresponding to approximately 8 to 9 CFU/ml. This onentration was onfirmed y diret plating method in a preliminary experiment. To inoulate red and lak pepper spies, suspended pellets of all strains of oth pathogens were omined to produe a mixed ulture oktail (six strains). These oktails at a final onentration of a. 9 CFU/ml were used in susequent experiments... Sample preparation and inoulation Commerially dried red and lak pepper powders were purhased from a loal groery store (Seoul, South Korea). The red and lak pepper spies were dried overnight to an initial moisture level of. ±.% (dry asis, d) and.8 ±.% (d), respetively. Moisture ontent was determined using a halogen moisture analyzer (HB-S; Mettler Toledo, Columus, OH). In order to maintain original quality, samples were stored at an amient temperature of ± C in ziplok ags prior to any onditioning. In this study, four moisture levels eah of red pepper (.%,.%, 9.%, and.% d) and lak pepper (.%,.%,.%, and.% d) were seleted, overing the possile moisture range where small partiles will not eome agglomerated. Samples were inoulated in suh a way as to simultaneously otain the desired moisture levels. One ml of the mixed ulture oktail (E. oli O:H and S. Typhimurium) and predetermined amounts of sterile distilled water were added to g of eah sample of powdered red and lak pepper spies in polyethylene ags at room temperature ( ± C), and then mixed y hand for min to allow uniform moisture distriution throughout the sample. The final ell onentration was to 8 CFU/g. Inoulated and onditioned spie samples were then immediately sujeted to RF heating... Experimental apparatus The RF heating and dieletri measurement system (Fig. )onsisted of a RF heater (Seoul National University, Seoul, South Korea; Dong Young Engineering Co. Ltd., Gyeongnam, Korea), a temperature signal onditioner (TMI-; FISO Tehnologies In., Quee, Canada), a liquid test fixture (A; Agilent Tehnologies, Palo Alto, CA), and a preision impedane analyzer (9A; Agilent Tehnologies). The RF heater generated a RF eletri field at a frequeny of. MHz and a maximum power of 9 kw. Its avity was omposed of two parallel-plate eletrodes (.. m;. m thik) and the distane etween the two eletrodes was. m. The liquid test fixture onsisted of two parallel-plate eletrodes in ontat with the liquid or powder sample to produe a apaitor harge. The distane etween the parallel plates was. mm and the test fixture was onneted to the preision impedane analyzer y a four-terminal Bayonet Neill-Conelman (BNC) ale (-; Agilent Tehnologies). A preision impedane analyzer measured sample apaitane and equivalent parallel resistane for a frequeny range from Hz to MHz whih was used to alulate dieletri properties of the sample. The temperature signal onditioner and the preision impedane analyzer were onneted to a omputer for ontrol using FISOCommander Control and Analysis Software (FISO Tehnologies In.) and IO Liraries Suite (Agilent), respetively... RF heating treatment For the RF heating treatment, g of inoulated red and lak pepper powders plaed in a polypropylene jar,. m in diameter and.8 m deep, (NALGENE 8-8; Thermo Sientifi, Hudson, NH) was plaed on the enter of the ottom eletrode. In all experiments, the alternating eletri field was fixed at. MHz. RF heating was applied to eah prepared sample and heated to 9 C in order to maximize the effiay of pasteurization while maintaining produt quality... Temperature measurement A fier opti temperature sensor (FOT-L; FISO Tehnologies In., Quee, Canada) onneted to a temperature signal onditioner was used to measure real-time temperatures in samples during RF heating.

3 S.-G. Jeong, D.-H. Kang / International Journal of Food Miroiology () Fig.. Shemati diagram of RF heating and dieletri measurement system at Seoul National University (Seoul, South Korea). The sensor was diretly inserted at the enter of the treated red and lak pepper powders and the temperature was reorded at s intervals. Sine the fier opti sensor was oated with eletri insulating material, it did not interfere with the temperature profile of the treated sample (Wang et al., ). All experiments were repeated three times, and averages and standard deviations of RF treated sample temperatures were ompared to determine the heating rate of samples. The rate of temperature inrease was alulated y dividing the differene in temperature etween the eginning and the end y the treatment time... Dieletri properties measurement Dieletri properties of samples were determined y the parallel plate method in ASTM D. Dieletri measurements of spies were taken at. MHz. The proedure is as follows: the preision impedane analyzer was manually alirated using a BNC ale to onnet it with the liquid test fixture, and then air apaitane of the test fixture was measured. Aout g of sample was plaed into the test fixture and the eletrial data of the samples were measured automatially at room temperature ( ± C). The dieletri properties of red and lak pepper spies were alulated as follows (Eqs. () and ()): ε ¼ C p C where ε is the dieletri onstant, C p is the spie apaitane (pf), and C is the air apaitane (pf). ε ¼ C R p ω where ε is the dieletri loss fator, C is the air apaitane (pf), R p is the equivalent parallel resistane (Ω), and ω is the angular frequeny (πf)..8. Baterial enumeration At seleted time intervals, treated g samples were immediately transferred into sterile stomaher ags (Laphas, In., Sainte-Juilie, Quee, Canada) ontaining ml of.% PW, whih were prehilled in a C refrigerator. After homogenization for min with a stomaher (Easy Mix; AES Chemunex, Rennes, Frane), -ml aliquots of sample were -fold serially diluted in 9-ml lanks of.% PW, and. ml of sample or diluent was spread-plated onto eah seletive medium. Soritol MaConkey agar (SMAC; Difo) and xylose lysine desoxyholate agar (XLD; Difo) were used as seletive media for the enumeration of E. oli O:H and S. Typhimurium, respetively. When low levels of surviving ells were antiipated, ml of undiluted ðþ ðþ stomaher ag ontents was equally divided onto four plates of eah medium and spread-plated (detetion limit, log CFU/g). All agar media were inuated at C for h and typial olonies were ounted. To onfirm identity of the pathogens, olonies randomly seleted from the enumeration plates were sujeted to serologial tests. These tests onsisted of the E. oli O:H latex agglutination assay (Oxoid, Ogdenserg, NY) and Salmonella latex agglutination assay (Oxoid) for E. oli O:H and S. Typhimurium, respetively..9. Enumeration of heat-injured ells To enumerate heat-injured ells of E. oli O:H, RF-treated samples were serially diluted and spread-plated onto phenol red agar ase with % soritol (SPRAB; Difo) at time intervals ausing the large temperature hange (Roelle et al., 99). After inuation at C for h, typial white olonies were enumerated. Random olonies were seleted from SPRAB plates and sujeted to serologial onfirmation as E. oli O:H (E. oli O:H latex agglutination assay; Oxoid), sine SPRAB is not a seletive media for enumerating E. oli O:H. The overlay (OV) method was used to enumerate heat-injured ells of S. Typhimurium using TSA as a nonseletive agar and XLD as the seletive agar (Lee and Kang, ). Appropriate dilutions were spreadplated onto TSA medium and inuated at C for h to allow injured ells to reover, and then to 8 ml of XLD was overlaid on the plates. After solidifiation, plates were inuated for an additional h at C, and typial lak olonies were enumerated... Color and post-treatment moisture ontent measurement To measure the effet of RF heating on the olor of powdered red and lak pepper spies of various moisture ontents, a Minolta olorimeter (CR; Minolta Co., Osaka, Japan) was used to measure the olor hanges of RF-treated samples. The values of L, a, and were used to quantify olor attriutes and measurements were taken from treated and untreated uninoulated red and lak pepper taken at random loations. L, a, and values indiate olor lightness, redness, and yellowness of the sample, respetively. After RF heating treatment, the post-treatment moisture ontent was measured immediately with the halogen moisture analyzer desried previously... Volatile flavor omponent measurement Capsaiinoids and piperine were measured as volatile flavor omponents in this study for red and lak pepper powders, respetively. The total apsaiinoid ontent was tested aording to the method desried y Vinent and Ken (98). This method involves apsaiinoid extration, leanup, and separation. Red pepper spie samples ( g) were

4 8 S.-G. Jeong, D.-H. Kang / International Journal of Food Miroiology () mixed with ml of aetonitrile for min with a vortex mixer (WiseMix VM-; Daihan Wisd., Gangwon, South Korea). The apsaiinoids were eluted y passing the sample extrat into a onditioned Sep-pak (WAT9; Waters, Milford, MA) olumn. The eluent was passed through a Teknokroma.-mm-pore-size memrane and sujeted to the following proedure. For apsaiinoid separation, a high-performane liquid hromatography apparatus (HPLC; Waters 9; Waters) equipped with an autosampler and a photodiode array detetor (Waters 99; Waters) was used. The wavelength was set at 8 nm, and a reversed-phase C 8 olumn (-mm partile size,.- mm diameter, -mm length; Young Jin Biohrom Co. Ltd., Gyeonggi, South Korea) where temperature was ontrolled at C was used with these onditions of the moile phase: methanol and tripledistilled water (: [v/v]) at a flow rate of ml/min. A standard aliration urve was otained y using apsaiin (Sigma Chemial Co., St. Louis, MO) and dihydroapsaiin (Sigma Chemial Co.) prepared in aetonitrile. Piperine onentration in lak pepper was also determined using HPLC set at nm. The olumn whih was the same with the one we used to separate the apsaiinoid in red pepper was utilized. The moile phase was prepared aording to the method reported y Chiang (98) and passed through the olumn at. ml/min. Standard piperine was purhased from Sigma Chemial Co. and prepared in methanol. The samples (. g) were homogenized with ml of methanol y vortexing for min in a ml volumetri tue, and methanol was added to the mark. After the solids settled, the supernatant was filtered through a Teknokroma filter and a -μlportionoffiltrate was injeted into the olumn using the HPLC autosampler... Statistial analysis All experiments were repeated three times with dupliate samples. Data were analyzed y the analysis of variane proedure of Statistial Analysis System (SAS Institute, Cary, NC), and mean values were separated using Dunan's multiple-range test. P. was used to determine signifiant differenes in the proessing treatment.. Results.. Temperature urves of powdered red and lak pepper spies with different moisture ontents Average temperatures of red and lak pepper powders of various moisture ontents, respetively, from.% to.% (d), and.% to.% (d) during RF heating at a onstant frequeny of. MHz are shown in Fig.. At the same moisture level, the temperature inreased with inreasing treatment times. Higher heating rates resulting from higher moisture ontent aused heating times to derease. The heating rate of oth spies was dependent on moisture ontent up to 9.% (dry asis, d) for red pepper and.% (d) for lak pepper, ut there was a signifiant derease (P.) when moisture ontent exeeded the aove levels. Red pepper with 9.% (d) and lak pepper with.% (d) moisture ontent inreased from. C to 9. C when exposed to RF energy for s and s, respetively. For the same treatment time, the temperature of oth spies did not exeed C at the lowest moisture ontent... Influene of moisture ontent on dieletri properties of powdered red and lak pepper spies The dieletri properties of onditioned powdered red and lak pepper spies at eah of the four moisture levels and at a fixed frequeny of. MHz are listed in Tale. Both dieletri onstants and dieletri loss fators of red pepper signifiantly (P.) inreased with inreasing moisture ontent from.% to.% (d). Similar patterns of the effet of moisture ontent were otained for lak pepper in the A Temperature ( o C) B Temperature ( o C) 9 8.8%.% 9.%.% %.%.%.% Fig.. Temperature urves of red (A) and lak (B) pepper powders at ontrolled moisture levels during RF heating. All moisture ontents are expressed on a dry asis. The results are means from three experiments, and error ars indiate standard deviations. range of.% to.% (d), however, higher dependene on moisture ontent was shown in not only dieletri onstants ut also dieletri loss fators of the samples ompared to those of red pepper. In all of the experiments, dieletri properties of the sample varied slightly at the eginning of moisture-onditioning, while those greatly inreased as moisture ontent inreased... Relationships etween rate of temperature inrease, dieletri loss fator, and moisture ontent of powdered red and lak pepper spies The results of the rate of temperature inrease, dieletri loss fator, and moisture ontents of powdered red and lak pepper were arranged for analysis of tripartite relationships (Fig. ). In oth samples, the dieletri loss fator was proportional to moisture ontent as listed in Tale. The rate of temperature inrease was signifiantly (P.) dependent on moisture ontent up to a. 9.% (d) for red pepper and.% (d) for lak pepper as shown in Fig.. The patterns of relationship etween the rate of temperature inrease and the dieletri loss fator were similar to those etween the rate of temperature inrease and moisture ontent. The rate of temperature inrease diminished aove a.. and. of dieletri loss fator in red and lak pepper powders, respetively.

5 S.-G. Jeong, D.-H. Kang / International Journal of Food Miroiology () 9 Tale Dieletri properties of red and lak pepper powders of varying moisture ontent at. MHz. a Moisture ontent (%, d) Red pepper Moisture ontent (%, d) Blak pepper ε ε ε ε.. ±. d. ±. d.. ±. d. ±. d.. ±.. ±.9.. ±.. ± ±.8. ±... ±..8 ±...8 ±.8 a 8.9 ±.9 a.. ±. a. ±.8 a a Means ± standard deviations from three repliations. Values followed y different letters within the olumn are signifiantly different (P.). ε is the dieletri onstant and ε is the dieletri loss fator... Influene of moisture ontent on inativation of foodorne pathogens in powdered red and lak pepper spies Populations (log CFU/g) of E. oli O:H and S. Typhimurium in red pepper during RF heating are depited in Fig.. As the moisture ontent inreased from.% to 9.% (d), surviving populations of oth pathogens dereased more effetively. However, treatment time required to redue E. oli O:H and S. Typhimurium to elow the detetion limit ( log CFU/g) inreased again when the moisture ontent exeeded 9.% (d). The levels of surviving ells of oth pathogens were redued to elow the detetion limit within s at a moisture ontent of 9.% (d). At.% (d), levels of E. oli O:H experiened a signifiant redution of. log CFU/g after s and a N.-log redution to elow the detetion limit after s of treatment. Cell numers of S. Typhimurium were redued y. log CFU/g after s and to elow the detetion limit after s of treatment. At.% (d), levels of oth foodorne pathogens were redued to elow the detetion limit after s of treatment. The numers of E. oli O:H and S. Typhimurium in red pepper were greatly redued to undetetale levels after 8 s at.% (d). Fig. shows the survival of E. oli O:H and S. Typhimurium of lak pepper treated with RF heating. The overall redution patterns of oth pathogens in lak pepper were similar to those in red pepper. A Rate of temperature inrease ( o C/s) Rate of temperature inrease ( o C/s) A B d Moisture ontent (%, d) a.... Moisture ontent (%, d) a 9 8 Log CFU/g B Log CFU/g 8 Detetion limit (.) 8 9 Detetion limit (.) 8 9 Fig.. Relationship etween rate of temperature inrease and dieletri loss fator of red (A) and lak (B) pepper powder at ontrolled moisture levels during RF heating. The results are means from three experiments, and error ars indiate standard deviations., rate of temperature inrease;, dieletrilossfator. Fig.. Survival urves for Esherihia oli O:H (A) and Salmonella Typhimurium (B) on red pepper powder with moisture ontents of.% ( ),.% ( ), 9.% ( ), and.% ( ) during RF heating. All moisture ontents are expressed on a dry asis. The results are means from three experiments, and error ars indiate standard deviations.

6 S.-G. Jeong, D.-H. Kang / International Journal of Food Miroiology () A Log CFU/g B Log CFU/g Detetion limit (.) Detetion limit (.) Fig.. Survival urves for Esherihia oli O:H (A) and Salmonella Typhimurium (B) on lak pepper powder with moisture ontents of.% ( ),.% ( ),.% ( ), and.% ( ) during RF heating. All moisture ontents are expressed on a dry asis. The results are means from three experiments, and error ars indiate standard deviations. RF heating at.% (d) moisture ontent redued oth pathogens to elow the detetion limit after s. At.% and.% (d), E. oli O:H and S. Typhimurium were redued to elow detetale levels after s and s of treatment, respetively. At the lowest moisture ontent, the treatment time required to redue oth pathogens to elow the detetion limit in lak pepper was less than that in red pepper. Populations of E. oli O:H and S. Typhimurium dereased y.9 log CFU/g and. log CFU/g after s, respetively... Reovery of heat-injured ells Surviving ells and heat-injured ells of E. oli O:H and S. Typhimurium from red and lak pepper powders following RF heating were ompared (data not shown). When inoulated red and lak pepper powders were treated with RF heating, slightly higher numers of oth pathogens were deteted on the agar for reovery (SPRAB and OV-XLD) than on the seletive agar (SMAC and XLD). However, during the entire treatment time, no signifiant (P N.) differenes in the levels of ells enumerated etween the agar for reovery and the seletive agar were oserved in powdered red and lak pepper spies with different moisture ranges... Effet of RF heating within different moisture range on produt quality The olor and volatile flavor omponents (apsaiinoids for red pepper and piperine for lak pepper) of spie samples of various moisture ontents after RF heating for time intervals required to redue E. oli O:H and S. Typhimurium to elow the detetion limit ( log CFU/g) are summarized in Tales and. Within all tested levels of moisture ontent, L, a, and values of RF-treated samples were not signifiantly (P N.) different from those of untreated samples. There were also no signifiant (P N.) differenes in volatile flavor omponents, apsaiinoids and piperine, etween untreated and treated red and lak pepper powders. Although they varied slightly in aordane with RF heating treatment at several moisture levels, statistially signifiant differenes were not deteted etween any of the tested samples (Tales and ). Thus, RF heating did not affet the olor or amount of volatile flavor omponents of powdered red and lak pepper spies of differing moisture ontent (P N.). Tale shows the moisture ontent of powdered red and lak pepper following RF treatment. There were statistially signifiant (P.) differenes etween untreated and treated samples. The moisture ontent of red and lak pepper powders sujeted to RF heating was redued y % to 8% and % to % (Tale ), respetively. Exept for red pepper of.% d, all samples of different moisture levels experiened signifiantly drying (P.).. Disussion The ojetives of the present study were to examine the influene of moisture ontent of red and lak pepper powders during RF heating on heating rate, dieletri properties, and inativation of foodorne pathogens. The impat of RF heating on the quality of red and lak pepper was also investigated. Inreasing the moisture ontent of oth spies not only had an effet on the heating rate ut also inreased the dieletri properties. These results imply that RF heating is influened y dieletri properties of the produt whih depend on its moisture ontent. The dieletri properties of food materials determine how they reat to an external eletromagneti wave and indiate permeaility, permittivity and eletrial ondutivity (Kuang and Nelson, 998). Of these, the permittivity whih determines the dieletri onstant and the dieletri loss fator affets RF heating (Marra et al., 9). The dieletri onstant is a harateristi of the aility of a material to store eletri energy and a measure of the polarizing effet from an external eletri field; in other words, how easily the medium is polarized. The dieletri loss fator reflets the amount of eletri energy lost as heat, whih is related to how the energy from an applied eletri field is asored and onverted to heat (Gao et al., ; Zhang and Datta, ). Based on the theory, there has een a need for knowledge of the dieletri properties of foods in order to develop effetive RF heating proesses. Speifi fators suh as frequeny of the eletromagneti waves, temperature, moisture ontent, and salt ontent of food materials affet the related dieletri properties and the resultant RF heating (Galema, 99). Moisture ontent an e one of the key parameters for rapid heating y inreasing the dieletri loss fator of red and lak pepper. The eletrial power transferred to the food as heat (P) is given y the equation P =πfv ε ε, wheref is the frequeny, V is the eletri field strength, ε is the dieletri onstant of a vauum onsidered equal to 8.8 F/m, and ε is the dieletri loss fator of the sample. From this equation, the heat generated is proportional to the frequeny, the eletri field strength, and the dieletri loss fator. In the present study, frequeny and eletri field strength were fixed at. MHz and. kv/m, respetively, leaving only the impat of the dieletri loss fator to e examined, whih depends on moisture ontent during RF heating. Our results agree with the equation; more heat was generated as a result of a higher dieletri loss fator of powdered red and lak pepper spies resulting from higher moisture levels. However, aove

7 S.-G. Jeong, D.-H. Kang / International Journal of Food Miroiology () Tale Color values and apsaiinoid ontent of treated and untreated red pepper powder of varying moisture levels sujeted to RF heating. a Moisture ontent (%) Treatment time (s) Color Capsaiinoids (mg/ g) L a Capsaiin Dihydroapsaiin Total d.. ±.9 a. ±. a.8 ±. a.8 ±. a. ±.9 a.9 ±. a 8. ±. a. ±. a. ±.8 a. ±.8 a.9 ±. a.9 ±. a.. ±. a.9 ±. a. ±. a. ±. a. ±. a. ±. a.9 ±. a.9 ±. a. ±.9 a.88 ±. a.8 ±. a.8 ±. a 9..8 ±.9 a. ±. a.9 ±. a. ±. a. ±.9 a. ±. a. ±. a. ±. a. ±. a.9 ±.8 a. ±. a. ±. a. 9. ±.9 a 8. ±. a.8 ±. a. ±. a.9 ±. a 9. ±. a 9. ±. a 8. ±. a. ±. a.9 ±. a. ±. a 9. ±. a a Means ± standard deviations from three repliations. Values followed y the same letters within the olumn per moisture ontent are not signifiantly different (P N.). d All moisture ontents are expressed on a dry asis. Color parameters are L (lightness), a (redness), (yellowness). Total apsaiinoids; apsaiin + dihydroapsaiin. threshold dieletri loss fators whih are. at 9.% (d) for red pepper and. at.% (d) for lak pepper, the heating rate dereased. This result is in agreement with earlier researh y Orfeuil (98) whih suggested that if the dieletri loss fator was too high, urrent leakage took plae through the material. Conversely, if it was too low, heating took plae slowly and it eame diffiult to reah the desired temperature. It has een proposed that the dieletri loss fator should e within proper range for suessful RF heating (Birla et al., 8). Therefore, it is important to onsider thoroughly the relationships etween moisture ontent, dieletri loss fator, and heating rates of foods to e heated in order to maximize the effetiveness of RF heating. Although the numer of researh studies on RF heating inativation of foodorne pathogens has grown in reent years (Ha et al., ; Kim et al., ; Byrne et al., ; Uemura et al., ; Shlisselerg et al., ), to date, there is no pulished information onerning the inativation of E. oli O:H and S. Typhimurium in spies of varying moisture ontent using RF heating. In the present study, moisture ontent of powdered red and lak pepper had a profound effet on inativating E. oli O:H and S. Typhimurium. At a moisture ontent of.% (d), 8 s was required to redue the levels of E. oli O:H and S. Typhimurium in red pepper to undetetale levels, ut only s was required at 9.% (d). As for lak pepper, and s were required to redue oth pathogens to elow the detetion limit ( log/cfu) at.% (d) and.% (d), respetively. As with the relationship etween moisture ontent and heating rate, there was a threshold at whih inreasing moisture ontent led to an extension of RF treatment time required to redue E. oli O:H and S. Typhimurium to elow detetale levels; this was true for oth red and lak pepper spies. Following heating treatment, su-lethally injured foodorne pathogens are potentially as dangerous as their uninjured ounterparts (MCleer and Rowe, 99; Lee and Kang, ). This is eause heat-injured ells ould undergo resusitation and eome normal. Therefore, the ell numers enumerated on seletive agar are not good enough to represent the total surviving populations in the samples. After RF heating within different moisture range, there were no signifiant (P N.) differenes etween injured and uninjured ells in powdered red and lak pepper spies. This suggests that RF heating effetively inativated E. oli O:H and S. Typhimurium in red and lak pepper powders without generating heat-injured ells whih ould reover. In addition, after the maximum treatment applied for inativation of foodorne pathogens, olor values (L, a, and ) and volatile flavor omponent values of samples of varying moisture ontent were not signifiantly (P N.) different from those of the ontrol. Other researhers also reported that RF heating treatment resulted in food produts of superior quality. Ha et al. () found that the RF heated peanut utter rakers maintained olor, flavor, texture, and overall aeptaility. Geveke et al. () reported that the RF heating method produed no loss in asori aid ontent and aused little enzymati rowning in orange juie. Based on our results, no signifiant quality differenes were oserved etween untreated and RF-treated red and lak pepper powders of various moisture ontent, ut treatment time required to redue oth pathogens to elow the detetion limit was different depending on moisture ontent. Besides pasteurization and sterilization, RF heating has an exellent drying effet and it has een used in some drying appliations suh as post-aking drying of ookies, rakers, and snak foods (Piyasena et al., ). This is eause the eletromagneti energy of RF eletri field tends to at on water and aqueous ions (Orfeuil, 98). In the present study, the drying performane during RF heating of powdered red and lak pepper spies was evaluated in terms of the final moisture ontent. A statistially signifiant (P.) drying effet was shown in all tested samples exept for.% (d) red pepper. Although most samples sujeted to RF heating did not meet the miroiologially safe moisture level (around %) for spie produts, drying effiieny Tale Color values and piperine ontent of treated and untreated lak pepper powder of varying moisture levels sujeted to RF heating. a Moisture ontent (%) Treatment time (s) Color Piperine (mg/ g) L a.. ±. a. ±. a. ±. a 9. ±. a.8 ±. a.9 ±. a. ±.8 a.9 ±.8 a.. ±. a. ±. a. ±. a.8 ±.8 a. ±. a. ±.9 a. ±.8 a. ±.9 a. 9. ±. a. ±. a.9 ±. a.9 ±. a.9 ±. a. ±. a. ±. a 8.9 ±. a.. ±.8 a.9 ±.8 a.9 ±.8 a. ±. a.8 ±.9 a. ±. a. ±.8 a. ±. a a Means ± standard deviations from three repliations. Values followed y the same letters within the olumn per moisture ontent are not signifiantly different (P N.). All moisture ontents are expressed on a dry asis. Color parameters are L (lightness), a (redness), (yellowness).

8 S.-G. Jeong, D.-H. Kang / International Journal of Food Miroiology () Tale Moisture ontent of red and lak pepper powders efore and after RF heating. a Produt Moisture ontent (%, d) Control RF treated Red pepper. ±. a. ±. a. ±. a. ±. 9. ±.8 a. ±.. ±. a. ±. Blak pepper. ±. a. ±.. ±. a. ±.. ±. a. ±.. ±. a. ±. a Means ± standard deviations from three repliations. Values followed y different letters within the row are signifiantly different (P.). ould e improved y using a srew onveyor in onjuntion with an industrial RF heater (Waje et al., ). Therefore, RF heating is suitale for drying as well as sterilization of spies. Industrial sale RF heating for ontrolling foodorne pathogens and reduing moisture in dried ommodities an e performed using a ontinuous system. Powdered red and lak pepper spies moving in a srew onveyor ould e exposed to RF energy from two eletrodes oriented horizontally on oth sides of the onveyor. Treatment time ould e seleted y adjusting the flow rate. There is a need to model the effet of moisture ontent on inativation of foodorne pathogens in oth spies y RF heating. This an assist the industry in developing appropriate treatment times at ertain moisture ontents of spie produts. Optimization of this proess is essential to prevent insuffiient heating or overheating whih ould permit survival of miroorganisms or result in quality deterioration of food produts. Therefore, in order to simultaneously guarantee produt safety and quality, further studies involving modeling inativation kinetis are required. Our results indiate that RF heating leads to effetive inativation of E. oli O:H and S. Typhimurium in powdered red and lak pepper of varying moisture ontent, as well as produing spies of superior quality and simultaneously having an ideal drying effet. The results of this study are fundamental in order to understand and model the response of spies to the RF eletromagneti field at ertain moisture ontents, and y extension, to apply ommerial RF sterilization. With a fuller understanding of the influene of moisture ontent on heating rates in red and lak pepper, RF heating ould e a very promising alternative tehnology to ontrol miroiologial ontamination in spies. Aknowledgments This study was arried out with the support of Cooperative Researh Program for Agriultural Siene & Tehnology Development (Projet No. PJ9), National Aademy of Agriultural Siene, Rural Development Administration, Repuli of Korea. This researh was also supported y the Puli Welfare & Safety researh program through the National Researh Foundation of Korea (NRF) funded y the Ministry of Siene, ICT and Future Planning (NRF-MAA9). Referenes Birla, S.L., Wang, S., Tang, J., 8. Computer simulation of radio frequeny heating of model fruit immersed in water. J. Food Eng. 8, 8. 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