Journal of Food Engineering - Washington State University

1 Quality and mold control of enriched white bread by combined radio frequencyand hot air treatmentYanhong Liua, Juming Tangb, , Zhihuai Maoa, Jae-Hyung Mahc, Shunshan Jiaob, Shaojin WangbaCollege of Engineering , China Agricultural University , Beijing 100083, ChinabDepartment of Biological Systems Engineering , Washington State University , 213 Smith Hall, Pullman, WA 99164-6120, USAcDepartment of food and Biotechnology, Korea University , Jochiwon, Chungnam 339-700, Republic of Koreaarticle infoArticle history:Received 18 August 2010 Received in revised form 5 November 2010 Accepted 19 November 2010 Available online 17 December 2010 Keywords:BreadQualityMold controlRadio frequencyHot air treatmentabstractThis study explored the application of radio frequency (RF) energy in conjunction with conventional hot airtreatment to provide uniform heating for control of mold in pre-packaged bread loaf.

2 A 6 kW, MHz RFsystem was used to develop treatment protocols. The treatment parameters were selected based on min-imum time temperature conditions that were required for 4-log reduction ofPenicillium citrinumsporeswhile yieldingacceptable bread combined RFand hot airtreatments, thecoreand peripheryof the bread loaf were heated together with almost the same heating rate. The maximum temperature dif-ference within one bread slice was less than 5 C. The moisture contents and water activities of RF treatedsamples first increased and then decreased compared to those of untreated samples, while firmnessincreasedduringthestorageforboth heattreatedanduntreatedsamples,yettheove ralldifferencesinsam-ple qualities between RF treated bread samples and control were not significant.

3 Because of better heatinguniformity, with combined RF and hot air treatment as compared to conventional heating alone. Heatingbreadto58 life at room temperature (23 C) was extended by 28 2 days for the treated white bread. 2010 Elsevier Ltd. All rights IntroductionBread is prone to rapid microbial spoilage, particularly moldgrowth, due to post-baking contamination during cooling, slicingand wrapping, which greatly limits its shelf life. Thus methods ofmold control are of great importance to the bakery industry (Smithet al., 2004). Radio frequency (RF) heating has been studied as ameans to control mold growth and extend shelf life of finished bak-ery products (Cathcart et al.)

4 , 1947; Bartholomew et al., 1948; Zhaoet al., 1999; Piyasena et al., 2003; Tang et al., 2005). Effects of RFtreatments on bread quality are also considered in those et al. (1947)reported that heating wrapped white breadto 60 C by means of RF energy could control mold growth within10-day storage without causing reduction of et al. (1948)found that the storage life of inoculated Bostonbrown bread was extended by 12 days after heated by RF to66 C. The RF treatment resulted in 4-log reduction ofPenicilliumandAspergillusspores. Yet according toCathcart et al. (1947) andBartholomew et al. (1948), severe condensation appeared fromthe surface of bread loaves after RF treatment, which resulted fromtemperature difference between bread and the surrounding air; airhas a relative loss factor of zero and cannot be heated by RF , in this study, we used surface hot air to maintain a relativelyhigh surface temperature of bread loaves while using RF for volu-metric heating.

5 In order to design an effective combined RF andhot air treatment, lethal thermal conditions for bread molds, to-gether with mold growth and quality changes of treated bread dur-ing storage, must be spoilage mainly results from mold contamination, espe-ciallyPenicilliumandAspergillus(Smi th et al., 2004; Guynot et al.,2005; Pateras, 2007). Lethal thermal condition forPenicilliumandAspergillushas been reported to be 68 70 C for 20 min with con-ventional heating method (Olsen, 1965), while brining bread to57 and 60 C by RF heating resulted in no mold development with-in 4 and 10-day storage period, respectively, and their untreatedcontrol grew molds on the 4th storage day as a contrast (Cathcartet al.)

6 , 1947). Similar findings were reported byBartholomew et al.(1948)that mold control can be achieved at lower temperatureswith shorter holding time when using RF heating compared withconventional heating. But all the above researches were conductedonly using either RF heating or conventional surface heating meth-ods, data about lethal effect of combined RF and hot air treatmenton bread mold control is is also a lack of information in the literature on moldgrowth and bread quality changes during storage after combined0260-8774/$ - see front matter 2010 Elsevier Ltd. All rights Corresponding author. Tel.: +1 509 335 2140; fax: +1 509 335 Tang). Journal of food Engineering 104 (2011) 492 498 Contents lists available atScienceDirectJournal of food Engineeringjournal homepage: and hot air treatments.

7 The objectives of this study were to: (1)determine the lethal thermal condition for mold isolated from mol-dy bread using combined RF and hot air treatments. We selectedtarget sample temperatures of 53, 58, 63 and 68 C and determinedminimum conditions to achieve 4-log reduction of bread moldspores; and (2) study mold growth and quality changes in the trea-ted bread during storage. The quality indexes included moisturecontent, water activity and bread Materials and Sample preparationSliced enriched white breads (Oven Joy White Enriched Breadmade by Lucerne Foods, Inc., Pleasanton, CA, USA) were purchasedfrom a local grocery store in Pullman, WA, USA. The ingredients ofthe bread are shown inTable 1, among which ash content (AOAC,2000a) and porosity (Rahman, 1995, 2005) were measured in thelaboratory and the others were based on the Mold spore suspensionTwo white bread loaves were stored at room temperature forthe incubation of molds.

8 Individual organisms in pure culture werethen isolated from moldy breads using streak plate method andinoculated onto the surface of potato dextrose agar medium, whichwere incubated afterward for the growth of mold colony at 25 Cinan electric constant temperature incubator (Precision EconomyIncubator, Jouan, Inc., Winchester, VA, USA) for 5 days. Mold sporeswere gathered and made into spore suspension with a concentra-tion level of 107 CFU/mL for future Inoculated bread samplesBread columns with a diameter of 20 mm and thickness mm (1 slice) in Petri dishes were exposed to ultraviolet lightsfor 30 min, and then inoculated with 1 106 CFU/g of pure moldstrain isolated from moldy bread.

9 The bread columns inoculatedwith mold spores were then placed under a sterile hood for themoisture content to equilibrate for 12 min and kept in sterileLow-Density Polyethylene (LDPE) Ziploc bags for future loaves treated with ultraviolet lamp were inoculated 106 CFU/g of pure mold strain isolated from moldy suspensions were evenly squirted to the cold spot of breadslices predetermined by preliminary tests. The bread slices werethen placed under a sterile hood for moisture equilibration for12 min and put back into original packages for future Combined RF and hot air treatmentA 6 kW, MHz RF system (COMBI 6-S, Strayfield Interna-tional, Wokingham, ) was used in this research, with an areaof 750 mm 550 mm for the top RF plate electrode.

10 The gap be-tween the top electrode and bottom electrode was adjusted be-tween 130 and 240 mm (Fig. 1) to regulate coupling of RF energyto the bread samples for obtaining desired heating rate in conveyor belt with changeable moving direction and speedwas equipped to assure different moving styles and residence timeof food products in RF field. An auxiliary hot air system was in-stalled to increase air temperature in RF field for maintaining prod-uct surface temperature (Fig. 1). The ambient air entered from theinlet to a kW electric heater, heated to target temperature andblown upwards by a fan through air distribution box and air holeson the bottom provide stable hot air condition, 1 h was allowed for the hotair to circulate in the empty RF cavity prior to the combined RF andhot air treatment.