Food Chemistry 141 (2013) 3960–3966

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Food Chemistry

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Relative importance of moisture migration and amylopectinretrogradation for pound cake crumb firming

0308-8146/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.foodchem.2013.06.110

Abbreviations: ERO, electrical resistance oven; DSC, differential scanning calo-rimetry; 1H NMR, proton nuclear magnetic resonance; MC, moisture content; PID,proportional–integral-derivative; FID, free induction decay; CPMG, Carr–Purcell–Meiboom–Gill; T2, spin–spin relaxation time.⇑ Corresponding author. Tel.: +32 (0) 16321634 ; fax: +32 (0) 16321997.

E-mail address: annelies.luyts@biw.kuleuven.be (A. Luyts).

A. Luyts a,⇑, E. Wilderjans a, I. Van Haesendonck b, K. Brijs a, C.M. Courtin a, J.A. Delcour a

a Laboratory of Food Chemistry and Biochemistry and Leuven Food Science and Nutrition Research Centre (LFoRCe), KU Leuven, Kasteelpark Arenberg 20, B-3001 Leuven, Belgiumb Puratos, Industrialaan 25, B-1702 Groot-Bijgaarden, Belgium

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 March 2013Received in revised form 21 May 2013Accepted 25 June 2013Available online 4 July 2013

Keywords:Moisture migrationAmylopectin retrogradationOhmic heatingElectrical resistance ovenCake storageNuclear magnetic resonance

Moisture migration largely impacts cake crumb firmness during storage at ambient temperature. Tostudy the importance of phenomena other than crumb to crust moisture migration and to exclude mois-ture and temperature gradients during baking, crustless cakes were baked using an electrical resistanceoven (ERO). Cake crumb firming was evaluated by texture analysis. First, ERO cakes with properties sim-ilar to those baked conventionally were produced. Cake batter moisture content (MC) was adjusted toensure complete starch gelatinisation in the baking process. In cakes baked conventionally, most of theincrease in crumb firmness during storage was caused by moisture migration. Proton nuclear magneticresonance (1H NMR) showed that the population containing protons of crystalline starch grew duringcake storage. These and differential scanning calorimetry (DSC) data pointed to only limited amylopectinretrogradation. The limited increase in amylopectin retrogradation during cake storage cannot solelyaccount for the significant firming of ERO cakes and, hence, other phenomena are involved in cakefirming.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Pound cake, a common cake type in Europe, is essentially pre-pared from a batter containing equal weight portions of flour, su-gar, egg and margarine or butter. Baking transforms the liquidbatter into a solid foam and sets the cake structure (Shepherd &Yoell, 1976, chap. 5). It implies simultaneous heat and moisturetransfer (Sakin, Kaymak-Ertekin, & Ilicali, 2007). During conven-tional baking, convective and radiative heat transfer from the oventowards the cake surface occurs. Combined with conductive heattransfer from the baking pan to the lower and lateral surfaces, thiscauses temperature increases at exposed surfaces (Sakin et al.,2007). Heat transfer from the surfaces to the interior is slow andcontrolled by the differential in temperature between the hot sur-face and the cool interior (Lostie, Peczalski, Andrieu, & Laurent,2002; Megahey, McMinn, & Magee, 2005). Heat transfer stimulatesmoisture transfer. Both occur simultaneously (Sakin et al., 2007).The moisture gradients ensure transfer of liquid water from thecore to the surface. At the same time, the temperature gradients

induce a water vapour partial pressure gradient which producesa flow of water vapour from the surface to the core (Lostie et al.,2002). That way, water evaporates at the warm end and condensesat the cold end of a pore (Thorvaldsson & Skjoldebrand, 1998). Asthe liquid water flow from core to the surface is lower than thewater vapour flow from the surface to the core, crust starts to form.Such crust is a barrier to both heat and moisture transfer (Lostieet al., 2002; Sakin et al., 2007). During baking, the cake crumbstructure sets as a result of both starch gelatinisation and egg pro-tein coagulation. Further gas expansion results in cell opening. Be-cause of this, water vapour partial pressure is uniform and furthermoisture transfer is limited (Lostie et al., 2002). Temperature andmoisture gradients during baking result in cake crumb which isheterogeneous in terms of moisture content (MC), cell wall sizedistribution, degree of starch gelatinisation and egg protein dena-turation (Wilderjans et al., 2010). The moisture distribution incakes after conventional baking (further referred to as conven-tional cakes) has a large influence on cake crumb firming duringstorage. Indeed, cake crumb firming has been related to the combi-nation of an intrinsic firming process of the cell wall material andmigration of moisture from crumb to crust. The latter can continuefor up to 5 weeks. Once an equilibrium moisture content isreached, no further loss of moisture from the central crumb occurs(Guy, 1983). Even if the starch fraction in cake batter is more di-luted than that in bread dough, it does retrograde during short cakestorage times (Guy & Pithawala, 1981).

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Compared to the firming of bread crumb, that of cake crumb isstill poorly understood and previous studies on the topic have notaddressed the importance of total water loss versus moisture redis-tribution and transfer between the crumb cell wall constituents. Tostudy the relative impact of these phenomena on cake firming, wehere produced crustless cakes baked in an electrical resistance oven(ERO). Temperature gradients during conventional baking inducemoisture gradients in cake (Wilderjans et al., 2010). In an ERO, heatis generated internally by using batter as a conductor between elec-trodes carrying an alternating current. That way, the batter heatsuniformly (Knirsch, dos Santos, Vicente, & Pennaa, 2010; Marra, Zell,Lyng, Morgan, & Cronin, 2009). Shelke, Faubion, and Hoseney (1990)found no temperature gradients when baking cake in an ERO.

Low resolution proton nuclear magnetic resonance (1H NMR),and, more specifically, spin–spin relaxation time (T2) measure-ments, can be used to study water (re)distribution and molecularmobility in food products. We here use 1H NMR for studying themobility of the biopolymers and water in pound cake during stor-age. Comparison of different parameters (firmness, moisture con-tent, retrogradation enthalpy, 1H NMR spectra) of conventionalcakes or cakes baked in the ERO (further referred to as ERO cakes)during storage allowed studying the importance of processes otherthan crumb to crust moisture migration to cake crumb firming. Wehere report on the outcome of this work.

Fig. 1. (A) Schematic drawing of an electrical resistance oven (ERO). Positions atwhich temperature were measured are numbered from 1 to 6. Position 1 is thecentre of the cake, 2 and 3 are at the top (i.e. respectively 1 and 2 cm under the finalcake height), 4 and 5 are at the side (i.e. respectively 1 and 1.5 cm from theelectrode plate), 6 is at 1 cm from the plexiglass wall. (B) Schematic representationof zones at which temperature and moisture content were measured in theconventionally baked cake. Temperature was measured in the centre (i.e. at 3 cmfrom the bottom), at the bottom (i.e. 1 cm from the bottom) and at the top (i.e. 5 cmfrom the bottom).

2. Materials and methods

2.1. Materials

Flour (Halmbloem) [14.0% moisture, 10.2% protein (as is basis)]was from Paniflower (Merksem, Belgium). Margarine (19.3% mois-ture) was from Puratos (Groot-Bijgaarden, Belgium) and sodiumbicarbonate and sodium acid pyrophosphate were from Buden-heim (Budenheim, Germany). Flour MC was determined accordingto Approved Method 44-15.02 (AACC, 1999). Flour protein content(N � 5.7) was determined using an adaptation of the AOAC OfficialMethod (AOAC, 1995), with an automated Dumas protein analysissystem (EAS Vario Max C/N, Elt, Gouda, The Netherlands). Fresheggs and sugar (sucrose) were purchased at a local supermarket.

2.2. Cake batter preparation

The batter for all pound cakes contained 450.0 g of flour, sugar,fresh eggs and margarine, 4.82 g sodium bicarbonate and 6.43 g so-dium acid pyrophosphate. The MC of conventional pound cake bat-ter was 26.9%. To obtain ERO cakes with properties similar to thoseof conventional cakes, the MC of the batter was adapted to 30.0%based on previous experiments. All batters were prepared using amulti stage mixing method as in Luyts et al. (2013).

2.3. Cake baking

2.3.1. Conventional bakingSix baking pans (internal length 150 mm � internal width

50 mm � internal height 60 mm) were filled with batter (250.0 g)and put in a rotary oven (National Manufacturing Company, Lin-coln, NE, USA) for 55 min at 160 �C.

2.3.2. ERO bakingBatter (400.0 g) was poured between the stainless steel plates

(electrodes) of an ERO (length 150 mm �width 60 mm � height180 mm) (Fig. 1A). From each batter, 3 cakes were baked in 3 dif-ferent EROs. A proportional–integral-derivative (PID) feedbackcontroller (Jumo Automation, Eupen, Belgium) adjusted the voltagesuch that the temperature–time profile was that measured in the

centre of pound cake batter during conventional baking. The PIDcontroller calculates an error value as the difference between theactual batter temperature and the desired set point, i.e. the tem-perature in the centre of pound cake batter during conventionalbaking at any given time. Temperature increased from 25 to100 �C in 37.5 min. The temperature was then held constant at100 �C for 17.5 min. Temperatures were measured in duplicatewith a type T thermocouple (Datapaq, Cambridge, UK) (Fig. 1A).

2.3.3. Differential scanning calorimetry (DSC) measurementsThe ungelatinised starch present in cakes was estimated using

DSC with a DSC Q2000 (TA instruments, New Castle, DE, USA).Freeze–dried cake crumb samples (3.0 mg) of conventional orERO cake were accurately weighed into coated aluminium pans(Perkin Elmer, Waltham, MA, USA). Water was added to obtain adry matter content of 25% (w/w). The hermetically sealed panswere equilibrated at 0 �C before heating from 0 to 120 �C at 4 �C/min (together with an empty reference pan). Calibration was withindium. The TA Universal Analysis software was used to determineenthalpies for starch gelatinisation (expressed in Joules per gramdry matter). DSC measurements were in triplicate and the coeffi-cients of variation (calculated as the ratio of standard deviationsto the mean value and expressed as %) did not exceed 10%. The per-centage of ungelatinised starch after baking is expressed as the ra-tio of the enthalpy for starch gelatinisation measured on cakesamples to those measured on cake batter, multiplied by 100.

2.3.4. Temperature gradientsTemperatures were recorded using a Multipaq 21 datalogger

(Datapaq) at different positions in the ERO (Fig. 1A), i.e. in the cen-

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tre (position 1), at the top (1 and 2 cm under the final cake height,positions 2 and 3, respectively) and at the side (1 and 1.5 cm fromthe stainless steel plates, positions 4 and 5, respectively), and 1 cmfrom the plexiglass wall (position 6) of the batter.

During conventional baking, temperature–time profiles werealso measured at different positions (Fig. 1B), i.e. in the centre(3 cm above the baking pan), at the bottom (1 cm above the bakingpan) and at the top (5 cm above the baking pan) with a Multipaq21 datalogger.

2.3.5. Cake MCAfter 2 h of cooling, cake samples were taken from the crust

(dark brown upper layer), the centre, the top (1 cm under thecrust), and the bottom (1 cm above the bottom) zone in the crumbof conventional cakes (Fig. 1B). For ERO cakes, crumb samples weretaken from the centre, the top and the side (Fig. 1A). All MCs weredetermined in triplicate according to Approved Method 44-15.02(AACC, 1999). An aliquot (about 1.0 g) sample was accuratelyweighed into a moisture dish, and dried for 2 h at 130 �C. MCwas calculated from the moisture loss and coefficients of variation(calculated as the ratio of standard deviations to the mean valueand expressed as %) were smaller than 5%.

2.3.6. FirmnessFirmness measurements were performed on an Instron (Instron

3343, Elancourt, France), with a 50 N load cell. Two hours afterbaking, 2 ERO and 2 conventional cakes were each cut into 4 sliceswith a thickness of 2.5 cm. Cake slices were compressed with acylindrical probe (25 mm) at a speed of 100 mm/min. Firmnesswas defined as the force required for compressing the crumb by25%. In total, 8 firmness measurements were performed, and coef-ficients of variation (calculated as the ratio of standard deviationsto the mean value and expressed as %) did not exceed 10%.

2.3.7. 1H NMRProton relaxation measurements were performed on a Bruker

(Rheinstetten, Germany) minispec Mq20 low resolution spectrom-eter operating at a resonance frequency of 20 MHz. The tempera-ture of the probe head was 25.0 �C. Crumb samples were takenfrom ERO cake and from the centre of conventional cake. Samples(ffi0.3 g, accurately weighed) were transferred into 10 mm diame-ter NMR tubes. The tubes were sealed and Free Induction Decay(FID) single pulse and Carr–Purcell–Meiboom–Gill (CPMG) mea-surements were performed as in Luyts et al. (2013). Relaxationcurves were fitted to a continuous distribution of exponentialsusing the ‘CONTIN’ algorithm of Provencher (Provencher, 1982)(Bruker Software). From the continuous distributions of exponen-tials, T2 and peak areas were derived. T2 is closely related withmolecular mobility, and peak areas of the proton populations areproportional to their relative quantities and expressed in arbitraryunits (a.u.) normalised per gram of sample. 1H NMR measurementswere performed in triplicate and coefficients of variation (calcu-lated as the ratio of standard deviations to the mean value and ex-pressed as %) of T2 and peak areas did not exceed 10%.

2.4. Cake storage

All cakes were stored in sealed plastic bags for up to 12 days at23 �C. Moisture loss by evaporation was limited since the totalweight loss of the cakes upon storage was less than 0.6% of the to-tal cake weight. At different times (i.e. at days 1, 3, 5, 8, 10 and 12)2 cakes of each baking mode were analysed. During storage, MC,firmness, DSC and 1H NMR measurements were performed as de-scribed above. For DSC measurements, samples were defatted be-fore analysis. Hereto, samples (1.0 g) were shaken with 10.0 mlhexane in a 30 ml test tube for 60 min. Then, hexane was removed

and the extraction repeated. Finally, samples were dried under astream of nitrogen. Enthalpies for amylopectin retrogradation weredetermined and expressed in Joules per gram dry matter sample.Coefficients of variation (calculated as the ratio of standard devia-tions to the mean value and expressed as %) of MC, firmness, amy-lopectin crystal melting enthalpy, T2 and peak areas did not exceed10%. Measured values of MC and firmness were analysed by Tu-key’s tests (P < 0.05) using the statistical analysis system software9.3 (SAS Institute, Cary, NC).

3. Results and discussion

3.1. Pound cake baking with ERO

The crumb of ERO cakes was compared with that of the crumbin the centre of conventional cakes. The crumb of ERO cakes frombatters with 26.9% moisture was very dry and crumbly. DSC mea-surements showed that, after baking, the centre of these cakes stillcontained 8.0% ungelatinised starch, while all starch was gelatin-ised in the centre of crumb of conventional cake. The crust formedduring conventional baking can indeed prevent moisture loss dueto evaporation. The absence of crust in ERO cakes allows moistureto evaporate from the cake surface during the entire baking pro-cess, resulting in the need to increase the batter MC to 30.0% to al-low full starch gelatinisation. After ERO baking, no ungelatinisedstarch was found in cake samples from either the centre or the sideof the cakes.

3.2. Temperature gradients during conventional and ERO baking

Fig. 2A shows the temperature–time profiles at the positionsshown in Fig. 1A during ERO baking. Only very small temperaturegradients existed during ERO baking. The temperature was onlylower at positions up to approximately 1 cm distance from theplexiglass wall (Fig. 2A). Here, heat loss at the surface caused asmall decrease in temperature.

During conventional baking, the temperature was monitored inthe centre and the top and bottom zones of the batter (Fig. 1B).Temperature increased very fast at the top zone of the cake, andreached a plateau at about 100 �C after 17 min (Fig. 2B). In the cen-tre, the temperature increased more progressively, and the plateauvalue of 100 �C was reached later (Fig. 2B). In the centre and topzones of the batter, the high level of water did not allow the inter-nal temperature to rise much above 100 �C. In the bottom zone,temperature increased faster than in the centre, but slower thanat the top. Wilderjans et al. (2010) also measured temperature gra-dients during pound cake baking in a similar baking procedure andalso found large temperature gradients between the top, and thecentre and bottom zones during baking.

Temperature gradients during ERO baking were much smallerthan during conventional baking. For ERO cakes, the lowest tem-peratures were reached at the surfaces, since heat is lost duringbaking at the cooler surfaces (plexiglass walls and stainless steelelectrode plates). In contrast, the temperature at the surfaces ofconventional cakes was the highest, and temperature decreasedprogressively towards the centre. The very small temperature gra-dients during ERO baking are thus opposite to those in conven-tional baking.

3.3. Cake properties

3.3.1. Moisture gradientsMC was determined for the centre, side and top zone of ERO

cake (Fig. 1A). The MC was respectively 19.8%, 23.0% and 23.1%(Fig. 3A) and thus lower in the centre than at the sides. This small

A B

Fig. 2. (A) Temperature evolution during baking in the electrical resistance oven (ERO). Temperature (�C) was measured at position 1 (__), 2 (_ _ _), 3 (__), 4 (_.._), 5 (__ __) and6 (. . ..) in the batter in function of the baking time (min). (B) Temperature gradients during conventional baking. Temperature (�C) was measured in the centre (__) at the top(__ __) and at the bottom (_ _ _) of the batter in function of the baking time (min).

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moisture gradient was caused by the small temperature gradientbetween the centre and the surfaces, as described above. The MCof conventional cakes ranged from 26.2% in the centre, to 24.2%in the top zone, 23.1% in the bottom zone and 6.6% in the crust.

Much smaller moisture gradients occurred in ERO cake than inconventional cake. During ERO baking, temperature gradients werevery small and opposite to those during conventional baking. In theERO, the batter is surrounded by cooler surfaces (the electrodeplates and plexiglass walls) at which water condenses. Therefore,MCs were higher at the surfaces. In conventional cake, moistcrumb in the centre of the cake is surrounded by drier crumb,and a very dry crust. As a result of the above, moisture gradientsin ERO cakes are opposite to those during conventional baking.

The MC of crumb from the centre of ERO cake was significantlylower than that of crumb from the centre of conventional cake(Fig. 3A and B). The crust formed during conventional but not dur-ing ERO baking is a barrier to moisture loss.

3.3.2. FirmnessThe initial crumb firmness readings after 2 h of cooling of ERO

and conventional cakes were similar, i.e. 2.5 N (Fig. 3C), althoughthe MC of ERO cakes was significantly lower than that in the centreof crumb of conventional cake.

3.3.3. 1H NMRTransverse relaxation curves were measured with a FID single

pulse (for T2 from 7 to 500 ls) and a CPMG pulse (for T2 from 0.2to 1000 ms) sequence. The FID and CPMG mode measurements al-low studying proton populations of low and high molecular mobil-ity, respectively. For the spectra of conventional cake, peaks wereassigned as: (A) non-exchanging protons of crystalline starch andproteins (both gluten and egg proteins), (B) and (C) CH protonsof amorphous starch and gluten in little contact with water, (D)exchanging protons of water, starch, proteins and sucrose andnon-exchanging protons of sucrose, (E) non-exchanging protonsof margarine and egg yolk lipids (Fig. 4) (Luyts et al., 2013). Protonpopulations A and B are measured in the FID mode. The other pro-ton populations (C–E) are measured in the CPMG spectrum (Fig. 4).For ERO cakes, the software could not differentiate between protonpopulations A and B in the FID spectrum of day 1 (Table 1 andFig. 4). Proton population C was not completely measured in theCPMG spectrum. At day 1, ERO cakes had lower T2 and peak areafor proton population D than conventional cakes (Table 1 andFig. 4). As described above, mainly exchanging protons of watercontribute to proton population of D. Lower T2 indicates lower

proton mobility. This can be explained by the differences in initialMC, i.e. 19.8% for the centre crumb of ERO cakes instead of 26.2%for the centre crumb of conventional cakes (Fig. 3A and B).

3.4. Cake storage

During storage of conventional cakes, moisture migrates fromcrumb to crust (Fig. 3B). This was not the case for ERO cake. There-fore, it was possible to study the contribution of changes otherthan crumb to crust migration onto cake crumb firming.

3.4.1. FirmnessDuring storage, cake crumb firms (Fig. 3C). Crumb from both

ERO and conventional cake extensively firmed during the first 5–8 days to remain rather constant thereafter. Crumb firmness in-creased from 2.5 to 6.7 N for ERO cakes and from 2.5 to 9.9 N forconventional cakes during the first 12 days of storage (Fig. 3C).The increase in firmness is thus higher for conventional (7.4 N)than for ERO (4.2 N) cake. A strong correlation was found betweenthe firmness and the MC of conventional cakes (R2 = 0.93), whichindicates that extensive moisture migration from crumb to crustof conventional cakes contributes to crumb firming. Half of thefirming occurred in the first 3 days for conventional cakes, whereasfor ERO cakes, even more than half of the firming (i.e. 60%) oc-curred in the first 3 days. These results are in agreement with thefindings of Guy (1983). He performed firmness measurements onwhole and crustless Madeira cake samples. For both samples, firm-ness increased quickly during 5–6 days. Thereafter, cake firmnessincreased at a much lower rate for up to 30 days. The increase infirmness of crustless Madeira cakes was smaller than that forwhole cake (Guy, 1983).

The increase in firmness of ERO cakes during storage is morethan half that of conventional cakes (Fig. 3C). The very limitedmoisture migration during storage of ERO cakes (Fig. 3A) probablycannot account for that. Processes other than moisture migrationalso contribute to the firming of cake during storage.

3.4.2. DSCThe melting temperature of amylopectin crystals was ca. 51 �C

in excess water (Fig. 5), in line with literature data (Delcour &Hoseney, 2010). During storage, the amylopectin crystal meltingenthalpy increased from 0 to 0.6 J/g dry matter and from 0 to1.1 J/g dry matter for ERO (Fig. 5A) and conventional (Fig. 5B)cakes, respectively. In both cakes, only small levels of retrogradedamylopectin were found. In bread, significant amylopectin crystal-

A

B

C

Fig. 3. Moisture content (MC) (%) in function of storage time of (A) cake samplesfrom the centre, top and bottom of electrical resistance oven (ERO) cakes and of (B)crumb and crust samples of conventional cakes and (C) Firmness (N) in function ofstorage time of the crumb of cake baked in an electrical resistance oven (ERO) orconventionally. For each sample type, values with a different letter are significantlydifferent from each other (P < 0.05).

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lisation occurs during storage. The increase in cake firmness duringstorage is highly dependent on moisture migration, as shownabove. Besides moisture migration, amylopectin crystal meltingenthalpy also correlates well with cake crumb firmness for bothconventional and ERO cakes (R2 = 0.87).

3.4.3. MCAfter 2 h of cooling, MCs in the centre of ERO and conventional

cakes were 19.8% and 26.2% respectively. For conventional cake,moist crumb was surrounded by crumb with a lower moisture con-tent and a very dry crust layer. In ERO cakes, only small moisturegradients were found. Those moisture gradients were opposite tothose in conventional cakes.

Fig. 3A shows the MCs of ERO cake crumb samples of the top,the centre and the side zones during storage. Initially, the MC inthe centre was slightly, lower than that of the top and side zones.After 8 days, there were no significant differences in MC in thethree zones [Tukey’s test (P < 0.05)] and the MC in the three zonesremained constant at about 21.5%.

Crumb and crust MC were determined for conventional cakes(Fig. 3B). During storage, moisture migrated from the moist centre(MC = 26.2%) to the dry crust (MC = 6.6%). During 8 days, moisturemigration occurred very fast, and, thereafter, more slowly (Fig. 3B).At day 8, the MCs of the crumb and crust were 20.2% and 15.9%,respectively. After 12 days, the corresponding readings were20.9% and 16.9%, respectively. These results are in agreement withthose of Guy (1983) for Madeira cake. He reported that moisturemigration continues for periods exceeding 21 days at temperaturesup to 21 �C. Crumb to crust moisture migration ended when theMC of the crumb and crust was identical, i.e. 21%.

The large moisture gradient in conventional cakes resulted insubstantial moisture migration during storage. Only after extendedstorage times, crumb and crust MCs were similar and moisturemigration ended. In contrast, the small moisture gradient in EROcakes resulted in limited moisture migration over a short storagetime and in a direction opposite to that noted in conventionalcakes.

3.4.4. 1H NMRTable 1 shows the changes in T2 and in corresponding areas of

the 5 proton populations in both cake types during storage.Proton population A mainly contains CH protons of crystalline

starch and proteins. For both cake types, the peak area of protonpopulation A increased during storage (Table 1). The increase be-tween day 1 and 12 was more pronounced for conventional thanfor ERO cakes, i.e. respectively 3455 and 955 a.u. For ERO cakes,moisture migration was negligible. The increase in area of protonpopulation A can thus be attributed to phenomena other thanmoisture migration from crumb to crust. Several authors earlierobserved an increase with time in the peak area of the peak withthe lowest mobility in the FID spectra of different starch gels (Far-hat, Blanshard, & Mitchell, 1999; Teo & Seow, 1992). This was ex-plained by the reduced mobility of protons when incorporated inamylopectin crystals during retrogradation. For conventional cakecrumb, the increase in area of proton population A was much morepronounced than that for ERO cakes. This can partly be explainedby the different MC of the samples. MC of the crumb of conven-tional cakes decreased from 26.2% to 21.0% in 12 days, implyingthat the dry matter content increased from 73.2% to 79.0%, and,thus, that the portion of non-exchanging protons as present in pro-ton population A, increased. The effect of differences in MC was ex-cluded by calculating this peak area per gram dry matter (Table 1).Taking into account the dry matter content, the increase in area Aupon 12 days of storage is 1335 and 3850 a.u. for ERO cakes andconventional cakes, respectively (Table 1). The peak areas of protonpopulation A expressed in g dry matter cake correlated well withamylopectin crystal melting enthalpy measured with DSC(R2 = 0.75 and 0.80 for ERO and conventional cake, respectively).

Proton populations B and C contain non-exchanging protons ofamorphous starch and gluten in little contact with water. Changesin non-exchanging protons of amorphous starch and gluten duringstorage can only be measured in proton population B for EROcakes. During storage, the area of proton population B decreasedfor both cake types. For conventional cakes, the area of proton pop-ulation C decreased as well. According to Choi and Kerr (2003),these results can be explained by the formation of amylopectincrystals. They observed a decrease in the population of water pro-tons inside granule remnants due to retrogradation. In this study,these protons are present in population B. For ERO cakes, the areas

Fig. 4. Free induction decay (FID) (left) and Carr–Purcell–Meiboom–Gill (CPMG) (right) spectra of pound cake baked in the electrical resistance oven (ERO) (—) and poundcake baked in the conventional oven (__) measured at day 1. The different proton populations are indicated with capital letters in order of increasing mobility.

Table 1Spin–spin relaxation times (T2) and corresponding peak areas and moisture content (MC) (%) during storage of cakes baked in the electrical resistance oven (ERO) and theconventional oven.

FID CPMG

T2A (ls) Area A Area A/g dm T2B (ls) Area B T2C (ms) Area C T2D (ms) Area D T2E (ms) Area E MC (%)

Cake baked in ERODay 1 15.6 8375 10,440 0.3 265 2.97 6515 66 3640 19.8Day 3 13.2 7950 10,025 37 730 0.3 235 3.62 6830 71 3650 20.7Day 5 12.8 8355 10,360 40 505 0.3 375 3.93 6660 66 3755 21.4Day 8 12.6 8350 10,705 40 565 0.4 235 3.91 6830 65 3800 22.0Day 10 12.6 8605 10,975 23 425 0.3 180 3.72 6590 65 3900 21.6Day 12 12.8 8905 11,360 40 375 0.3 255 3.77 6930 63 4055 21.6

Cake baked in the conventional ovenDay 1 11.6 5825 7895 41 1165 0.4 620 6.25 7610 69 3470 26.2Day 3 11.4 7020 9285 40 1030 0.4 465 5.40 7405 71 3505 24.4Day 5 11.3 7890 10,155 40 940 0.3 435 4.52 7040 71 3600 22.3Day 8 11.9 8370 10,490 40 765 0.3 165 3.85 6900 67 3690 20.2Day 10 11.8 9320 11,785 33 765 0.4 265 3.08 6540 70 3695 20.9Day 12 11.9 9280 11,745 37 635 0.3 140 3.29 6720 70 3395 21.0

Fig. 5. Differential scanning calorimetry (DSC) thermograms of defatted cake crumb samples of (A) cakes baked in the electrical resistance oven (ERO) and (B) the centre ofcakes baked in the conventional oven at different times during storage.

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3966 A. Luyts et al. / Food Chemistry 141 (2013) 3960–3966

of population B correlated well with amylopectin crystal meltingenthalpy (R2 = 0.61). For conventional cakes, an even stronger cor-relation was found between the areas of population B and amylo-pectin crystal melting enthalpy (R2 = 0.79).

Proton population D contains exchanging protons of water,starch, gluten, egg proteins and sucrose. During storage of EROcakes, no clear changes were observed for this proton populationbecause only limited moisture migration takes place. During stor-age of conventional cakes, both T2 and the peak area of proton pop-ulation D decreased (Table 1) due to water loss from crumb tocrust. After 8 days, the area and T2 of proton population D weresimilar to those of ERO cakes. The MC of conventional cakes was,at that time, 20–21% and very similar to that of ERO cakes (Table 1).The T2 of proton population D is highly dependent on cake MC. Forbread, different authors (Chen, Long, Ruan, & Labuza, 1997; Sereno,Hill, Mitchell, Scharf, & Farhat, 2007) reported a decrease in T2 ofthe proton population containing the protons of mobile water. Inour study, the peak area and T2 of proton population D correlatedwell with the MC of cakes (R2 = 0.80 and 0.91, respectively).Changes in the spectra of cake crumb during storage are dominatedby changes in MC, and thus by moisture migration during storage,as also found for bread (Sereno et al., 2007). Retrogradation leadsto much more pronounced changes in the NMR spectra of starchsystems and in bread than in cake, since the starch fraction is muchmore diluted in cakes. Moisture migration between components,such as starch and protein, is not clearly measured with onedimensional 1H NMR experiments. Two dimensional 1H NMRexperiments might be useful to study moisture migration on amolecular scale, since Luyts et al. (2013) found that additionalinformation on protons of flour components can be derived fromtwo dimensional spectra.

4. Conclusion

Cake baking in ERO allows making cakes with only limited (ifany) moisture gradients and, thus, allows studying processes otherthan moisture migration that contribute to cake crumb firming.Moisture migration significantly contributes to firming of conven-tional cakes, since the increase in firmness of such cakes duringstorage is much higher than that for ERO cakes. The contributionof amylopectin retrogradation is rather small for cakes bakedeither by ERO or conventional technology, as measured with DSCand 1H NMR. Moisture migration from crumb to crust stronglyinfluences the 1H NMR spectra. For conventional cakes, changesin the spectra caused by amylopectin retrogradation are domi-nated by such moisture migration. In spite of the large contributionof moisture migration to crumb firming, amylopectin retrograda-tion and moisture migration cannot completely explain the in-crease in firmness during storage. Thus, other phenomena areinvolved in cake firming as well. Two dimensional 1H NMR exper-iments may well offer added value to study the contribution ofwater migration between different constituents on cake crumbfirming.

Acknowledgements

This work is part of the Methusalem programme ‘Food for theFuture’ at the KU Leuven. Puratos (Groot-Bijgaarden, Belgium) isthanked for financial support of KU Leuven’s cake research. K. Brijsacknowledges the Industrial Research Fund (KU Leuven, Leuven,Belgium) for a position as Industrial Research Manager. J.A. Delcouris W.K. Kellogg Chair of Cereal Science and Nutrition at KU Leuven.

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