冻速和终冻温度对冷冻牛角面包品质的影响.pdf

Effects of freezing rate and terminal freezing temperature on frozencroissant dough qualityChoongjin Bana,1,Sangeun Yoond,1,Jungwoo Hana,Sang Oh Kime,Jung Sook Hanf,Seokwon Limg,Young Jin Choia,b,c,*aDepartment of Agricultural Biotechnology,Seoul National University,1 Gwanakro,Gwanakgu,Seoul,151-921South KoreabCenter for Food and Bioconvergence,Seoul National University,1 Gwanakro,Gwanakgu,Seoul,151-921,South KoreacResearch Institute of Life Science and Agriculture,Seoul National University,1 Gwanakro,Gwanakgu,Seoul,151-921,South KoreadCJ Food R&D,CJ Cheiljedang Corporation,Seoul,152-050,South KoreaeLG Electronics,51 Gasan Digital 1 ro,Geumcheongu,Seoul,153-802,South KoreafFood R&D Center,Samyang Group,726 Baekbeom-ro,Seo-Gu,Incheon,404-250,South KoreagDepartment of Food and Biotechnology,Hoseo University,79-20 Hoseoro,Asan,Chungnam,336-795,South Koreaa r t i c l e i n f oArticle history:Received 8 March 2016Received in revised form27 May 2016Accepted 29 May 2016Available online 30 May 2016Keywords:Frozen croissant doughFreezing rateTerminal freezing temperatureIce crystal sizeYeast viabilitya b s t r a c tUsing frozen ready-to-bake dough is a very common practice in the industrial croissant production.However,the freezing process during the preparation frozen croissant dough can deteriorate its quality.In this study,we investigated the effects of the freezing rate(FR)and terminal freezing temperatures onthe volume and firmness of croissants by analyzing frozen dough for yeast viability,thermal propertychanges,and internal microstructure integrity.Croissant dough samples were frozen at rates rangingfrom?0.72 to?3.56?C min?1down to final temperatures of?20,?40,and?55?C.Our results showedthat the ice crystals normally forming in the dough during freezing,causing a lower yeast viability andcroissants quality,were of smaller size when a rapid FR?3.19?C min?1was used.Furthermore,afreezing termination temperature lower than?20?C induced more yeast cell death,thereby deterio-rating croissant quality.Therefore,we suggest that the croissant dough freezing process should beconducted with an appropriate FR down to a suitable terminal temperature.Consequently,our results arehelpful to understand how the freezing procedure affects ice crystal formation and yeast viability in thefrozen dough matrix and our findings can be applied to enhance bread quality in the frozen doughindustry.2016 Elsevier Ltd.All rights reserved.1.IntroductionFrozen bread dough is widely used in the baking industry,mainly due to its convenience.Dough freezing can reduce pro-cessing time and labor intensity,increase products shelf life,improve productivity,and facilitate distribution to distant locations(Chen et al.,2013).Because of these advantages,the freezingtechnology has been used for several viennoiseries,such as crois-sants(Le-Bail,Nicolitch,&Vuillod,2010).However,the freezingprocess used to prepare the frozen dough can induce many nega-tive effects on the quality of baked products compared to thosebaked from unfrozen dough(Ribotta,Le?on,&An?on,2001).Partic-ularly,damaged gluten networks and reduced yeast viability areregarded as the main culprits for quality deterioration in breadprepared from frozen dough(Ribotta,Le?on,&An?on,2003).Inci-dentally,a number of studies have focused on how to preservegluten networks and yeast viability from freezing injuries by sus-taining the yeast gas productivity and the gas retention capacity ofthe gluten networks.Many researchers have suggested that the ice crystal formationin the freezing dough could result in two negative effects.First,theice crystals formed within yeast cells can have cell membranes bedamaged during the freezing procedure and ultimately decreasetheir viability(Muldrew&McGann,1990).Additionally,if the icecrystals only form in the dough matrix(and not in or through thecells),the concentration of salts,sugars,and other molecules canincrease the osmotic pressure as these molecules become more*Corresponding author.Department of Agricultural Biotechnology,Seoul Na-tional University,1 Gwanakro,Gwanakgu,Seoul,151-921,South Korea.E-mail address:choiyjsnu.ac.kr(Y.J.Choi).1These authors contributed equally to this work.Contents lists available at ScienceDirectLWT-Food Science and Technologyjournal homepage: Elsevier Ltd.All rights reserved.LWT-Food Science and Technology 73(2016)219e225concentrated in the water present outside of the cells.This resultsin a water outflow from the yeast cytoplasm and leading to celldeath(Randez-Gil,Sanz,&Prieto,1999).Second,the ice crystalformation and growth can physically damage the gluten networks(Baier-Schenk et al.,2005).Therefore,ice crystal formation shouldbe properlycontrolled to obtain good croissants from frozen dough,which could be accomplished by tightly controlling the freezingconditions(Yi&Kerr,2009a).Notably,the freezing rate(FR)is oneof the parameters used to regulate ice crystal size in the foodfreezing process(Petzold&Aguilera,2009),and many studiesexplored the relationship between the FR and baked productsquality(Le-Bail et al.,2010).Moreover,the storage temperature iswell known for its important influence on ice crystal growth duringfrozen storage and on bread quality after baking(Phimolsiripol,Siripatrawan,Tulyathan,&Cleland,2008).A slower freezing process can result in larger ice crystals in thedough than rapid freezing,and larger ice crystals are moredisruptive for the gluten networks.Damaged gluten networksprevent the proofing dough from properly retaining the CO2gasreleased by the yeast cells,which can reduce the overall loaf vol-ume of bread prepared from frozen dough.Contrastively,theslower freezing process better preserves yeast viability in thedough(Selomulyo&Zhou,2007).Therefore,to produce goodbreads,the optimal FR should be determined empirically from acomplex consideration of both yeast viability and gluten networkintegrity.The FR is a very significant factor affecting the quality of breadbaked from frozen dough,as referred to earlier.Nevertheless,as faras we know,there was no study for frozen croissant dough focusingon determining the optimal dough FR.Moreover,even in manystudies for freezing process of general frozen bread doughs,theresearchers have only focused on the suitable temperature for longtime storage not mentioned the appropriate terminal temperaturefor freezing process(Yi&Kerr,2009a,2009b).Despite the fact that frozendough ice crystals nucleateand growduring the freezing stage,analyses using cooling curves underdifferential scanning calorimetry(DSC)were rarely used in studiesassessing ice crystal formation relative to the FR.Actually,manyresearchers thoroughly studied the frozen dough ice crystal for-mation process using DSC heating curves(Bot,2003;Chen,Jansson,Lustrup,&Swenson,2012;Kontogiorgos,Goff,&Kasapis,2008).Consequently,in this study,the freezing process was controlled bythe FR and terminal freezing temperature of the croissant dough.Lastly,we investigated the ice crystal formation phenomenon inintact dough using DSC thermograms.Additionally,we analysedthe internal structure of the dough using SEM micrographs duringthe cooling part of the freezing procedure to evaluate the beneficialeffects of a controlled freezing method on croissant quality.2.Material and methods2.1.Samples preparationTwo kinds of commercial flours and white sugar were gifted bySamyang Genex Co.,Korea;flour 1 was composed of 72.5%carbo-hydrates,12.5%protein,0.6%fat,14%water,and 0.4%ashes and flour2 was composed of 75%carbohydrates,9.5%protein,0.6%fat,14.5%water,and 0.4%ashes.Fresh compressed yeast(Ottugi Fresh YeastGold,Ottugi Co.,Ltd.,Korea),eggs,unsalted butter(Samyang GenexCo.,Korea),butter for rolling(Pastry Sheet Gold,Samyang GenexCo.,Korea),and refined salt(Beksul,CJ Cheiljedang Co.,Korea)wereobtained from the local market.A dough was prepared using thefollowing recipe:275 g of flour 1,150 g of flour 2,40 g of whitesugar,6.25 g of refined salt,20 g of freshyeast,100 g of eggs,162.5 gof water,12.5 g of unsalted butter,and 250 g of cold butter forrolling.The flours,sugar,salt,and yeast were blended for 2 minusing a mixer at speed 2(Kenwood titanium major kitchen ma-chine,KM020,UK).Then,water and eggs were added,and themixture was blended for 2.5 min at speed 1 before adding butterand the dough was kneaded for 3 min at speed 1.Then,the doughwas placed in a 4?C refrigerator for the first 30 min of resting.Fordough layering,the cold butter spread was placed on the dough,shaped in a 30 cm by 30 cm square after resting,and wrapped withthe pressed out part of the dough.The dough with the cold butterwas folded three times and allowed to rest in a refrigerator for5 min(second resting).After the second resting,the dough foldingprocedure was repeated twice,with resting steps of 15 and 30 min.After the fourth resting,the dough was rolled and cut into isoscelestriangle shaped(base,10 cm;height,15 cm;and thickness,3 mm)and each triangle was rolled until the dough overlaps three times.Freezing system was composed of two parts:a freezer(FD-170-SF,Unique Daesung Co.,Ltd.,Gyeonggi,Korea)and an adiabatic box.The styrofoam adiabatic box had two large holes on opposite sides,one of which was occupied by an electric fan,and a sample loadingrack at center of the box.The FR of the dough was controlled by acombination of two means,namely the temperature of the cold airof the freezer,and the convection induced by the electric fan.Additionally,the terminal dough freezing temperature was definedas the point when the temperature at the center of the doughreached either?20,?40,or?55?C.During the freezing process,the temperature change at the center of the dough was monitoredusing a data logger(Agilent 34970A,Agilent Technologies Inc.,Santa Clara,CA,USA).The FR of the samples was calculated on thebasis of the freezing profiles showed in the Fig.1,using the defi-nition provided by the International Institute of Refrigeration(Bgh-Srensen,2006):FR Tt?Ti=twhere,Ttis the sample terminaltemperature,Tiis the sample initialtemperature,andDt is the time difference to reach Ttfrom Ti.Whenthe dough reached its pre-defined Tt,a frozen sample was packagedin a polyethylene bag and stored for a day in a freezer set at?18?C.Time(min)020406080100120Temperature(degree Celsius)-60-40-2002040Fig.1.Temperature profiling at the center of the croissant dough during the freezingprocedure and sample coding names.The samples were coded as follows:FR1,?0.72?C min?1(open circle;?0.24?C min1in zone of maximum ice crystal forma-tion);FR2,?1.43?C min?1(open triangle;?0.50?C min?1in the zone);FR3,?1.50?C min?1(open inverted triangle;?0.52?C min?1in the zone);FR4,?1.84?C min?1(open square;?0.61?C min?1in the zone);FR5,?3.19?C min?1(opendiamond;?1.53?C min?1in the zone);and FR6,?3.56?C min?1(open star;?2.01?C min?1in the zone).C.Ban et al./LWT-Food Science and Technology 73(2016)219e225220The frozen croissant dough samples were thawed at 25?C for90 min and proofed for 60 min in a chamber kept at 35?C with 85%relative humidity.The proofed dough samples were baked using anelectric oven(MA921SBT,LG DIOS,Seoul,Korea)at 180?C for20 min.After baking the croissants,theywere cooled for 1 h prior tosubsequent analyses.2.2.Determination of croissant qualityThe croissant volume was measured using a laser-based VolscanProfiler 600(Stable Micro Systems,UK).On the basis of loaf volumeand weight of the croissants,a specific volume value wascalculated.A compression test was conducted to determine the croissantfirmness using a texture analyser(TA-XT2i,Texture Technologies,Scarsdale,NY,USA)based on a method modified from AACC 74-09(AACC,1986).The croissant samples were placed at the center ofthe base in the texture analyser.The measurement was performedusing an aluminum cylindrical probe(diameter,50 mm)to pressthe sample until reaching 40%of deformation at a speed of1.7 mm s?1.The maximum force obtained from the force-deformation curve was used as the firmness value.2.3.Croissant dough yeast viability measurementYeast viability was defined as the number of viable yeasts in thedough,and measured using a modified version of the AACC 42e50(AACC,2000).The frozen dough was taken out of the storagefreezer and thawed at 25?C for 90 min.The thawed dough wasplaced in a Filter-bag(Labplas,Canada),diluted tenfold withpeptone water(2 mg mL?1),and homogenised for 2 min using astomacher WS-400(Shanghai Zhisun Equipment Co.Ltd.,Shanghai,China).Then,the samples were appropriately re-diluted withpeptonewater(2 mg mL?1).One milliliterof the diluted suspensionwas cultured on Sabouraud Dextrose Agar(Difco,Detroit,MI,USA)and incubated for 48 h in a chamber at 30?C before the yeastviability of the frozen dough was calculated.2.4.Thermal characterisation of croissant dough and yeastThe thermal characteristics of the croissant dough and com-pressed yeast were determined by DSC(Diamond DSCPerkinElmer,Waltham,MA,USA).Prior to the freezing procedure,doughsamples weighing between 14.8 and 24.2 mg were collected fromthe central part of the dough and from the fresh yeast,respectively.Samples were placed in a hermetic aluminum pan,which was thensealed.An empty pan was used as the reference.The DSC scanningof the dough immediately started at 25?C and were conductedusing six FR(?0.72,e1.43,e1.50,e1.84,e3.19,and?3.56?C min?1;referred to as FR16,respectively)until?20?C was reached.On thebasis of the thermograms,the values for the onset(Tonset),peak(Tpeak),and end(Tend)temperatures were determined and the timevalues between each onset(tonset)and end(tend)exothermic peakwere extracted using the Pyris 7 DSC data processing software(Perkin Elmer,Waltham,MA,USA)to characterise the formation ofice crystals in the dough.In addition,a DSC scan of the yeast wasperformed from 25 to?30?C with a FR6.2.5.Observation of the croissant dough microstructureThe internal microstructure of the frozen croissant dough wasobserved using a SEM S-3500N(Hitachi Science Systems Ltd.,Hitachinaka,Japan).Dough fragments were obtained from thecenter of the frozen dough using a hammer.These steps wereperformed in the freezing room to avoid thawing.Observationswere conducted at an accelerating voltage of 20 keV with a Rob-inson backscattered electron detector,which was maintained at?20?C using a Deben Coolstage Peltier Stage(Deben UK Ltd.,Suf-folk,UK).2.6.Statistical analysesAll results were analysed using a Tukeys significant differencetest performed with the IBM SPSS Statistics software version 21.0(IBM Co.,Armonk,NY,USA).All data represent an average of at leastthree independent experiments or measurements.3.Results and discussion3.1.Freezing procedure for frozen croissant dough preparationBread dough prepared with wheat flour,fat,sugar,salt,yeast,and water would freeze at subzero temperatures due to thefreezing point depression.The actual freezing temperature slightlydiffers according to changes in the ingredient contents and thedegree of dough fermentation(Sharadanant&Khan,2003).Addi-