Surface bloom on improperly tempered chocolate

Pierre LonchamptRichard W. Hartel

University of Wisconsin-Madison,Department of Food Science,Madison, WI, USA

Surface bloom on improperly tempered chocolate

The surface composition, in terms of sugar and fat content, on untempered and over-tempered chocolates was estimated by carefully scraping the surface layer and ana-lyzing fat and sugar melting enthalpies by differential scanning calorimetry. The dullsurface of over-tempered chocolate had a fat and sugar composition similar to theinitial chocolate mass, whereas the surface bloom formed on untempered chocolatewas nearly depleted of fat, containing primarily sugar and cocoa solids. This was con-firmed qualitatively by using polarized light microscopy, where no fat crystals could beobserved in the bloom spots. Bloom on untempered chocolate corresponded to aphase separation between fat, sugar and cocoa solids. In contrast, the grey, dullaspect of the surface of over-tempered chocolate had essentially the same sugar-to-fat ratio as the intact chocolate and was due to a diffuse reflection of light on a roughsurface, most likely induced by large cocoa butter crystals. Bloom on untemperedchocolate developed regardless of the relative humidity of storage (between 0 and75%). However, bloom developed more quickly and to a greater extent at lower relativehumidity. Whiteness was directly related to the number, diameter and growth speed ofthe white bloom spots.

Keywords: Chocolate, bloom, over-tempered, untempered.

1 Introduction

Fat bloom, an undesired phenomenon for the chocolateindustry, typically refers to the whitish haze that formsinitially on a chocolate surface, which then may work itsway through the bulk of the piece. The general termbloom, however, does not represent a single phenom-enon corresponding to a single mechanism, since theorigins and aspects of bloom are numerous [1, 2].

Bloom (or surface changes) can appear quickly after pro-cess problems or, more precisely, after poor tempering.Tempering induces crystallization of cocoa butter into arelatively stable polymorph that protects chocolateagainst bloom. During tempering, bV seeds are formed atsuch a concentration that the chocolate mass crystallizesdirectly into the bV polymorph during subsequent cooling[3–5]. Improper tempering can lead to rapid surfacebloom (untempered) or dulling (over-tempered) due to thesize or polymorphic form of cocoa butter crystals that areformed.

In well-tempered chocolate, fat bloom can occur due tofour main reasons [6].

(a) Bloom can develop during storage depending on thetemperature and fluctuations. High and fluctuating tem-perature leads to more rapid bloom.

(b) Bloom can also occur when the chocolate is exposedto a higher temperature than the melting point of cocoabutter.

(c) Liquid oil that migrates from a center to the chocolatecan lead to bloom.

(d) Bloom due to composition problems is observedwhen two incompatible fats are blended together.Incompatible fats enhance bloom development, depend-ing on the extent and amount of incompatible fat tria-cylglycerols [7].

Tempering is a directed pre-crystallization that consistsof shearing the chocolate mass at controlled tempera-tures to promote proper cocoa butter crystallization [8,9]. Although there are many ways to temper chocolate,the most common method utilizes temperature adjust-ment to promote formation of the desired number ofseed crystals in the proper polymorphic form. In the firststep, any crystalline memory is erased by holding thechocolate, with agitation, above 50 7C for a sufficienttime to completely melt any crystals. The melted choc-olate is then cooled to 26–28 7C to initiate seed for-mation. Nucleation of the fat crystals typically occursinto unstable polymorphs (a and b’) because theirenergy of creation is lower than for more stable poly-morphs. Once nucleation has been completed, thetemperature is increased to just below the bV poly-morph melting point (typically 31–32 7C), where theunstable forms are melted or transformed into thedesired bV form. Specific temperatures are chosen to

Correspondence: Richard W. Hartel, University of Wisconsin-Madison, Department of Food Science, 1605 Linden Drive,Madison, WI 53706, USA. Phone: 11 608 2631965, Fax:11 608 2626872, e-mail: [email protected]

Eur. J. Lipid Sci. Technol. 108 (2006) 159–168 DOI 10.1002/ejlt.200500260 159

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160 P. Lonchampt and R. W. Hartel Eur. J. Lipid Sci. Technol. 108 (2006) 159–168

accommodate crystallization behavior of different choc-olates to control crystal formation and polymorphtransformation.

Variations in the set temperatures, particularly the lowertemperature, can induce variations in the concentrationand polymorphic habit of cocoa butter seed crystals. Ifthe seed concentration is not high enough or if the seedsare not in the bV polymorph, subsequent cooling of thechocolate causes crystallization into an unstable form,which will consequently bloom very quickly. Under-tem-pering means that the chocolate does not have sufficientbV seed content to completely crystallize the entire mass,and bloom occurs shortly after solidification. Whenchocolate mass has no seed before cooling, the choco-late is called untempered. In untempered chocolates,bloom also forms quickly. Chocolate is over-temperedwhen the mass has either too high a concentration of bVseed after tempering or the seeds are too large. In eachcase, the chocolates bloom or dull very quickly.

Bloom due to incompatible fats, oil migration and storageabuse have been investigated extensively, and mechan-isms have been proposed for each of these types ofbloom [2]. However, surface changes that occur inuntempered and over-tempered chocolates have notbeen extensively studied. Whymper and Bradley [10]hypothesized that the powdery form of bloom on untem-pered chocolate was due to the development of small fatcrystals on the surface of the chocolate. They concludedthat bloom on untempered chocolate was the direct resultof the separation and further crystallization of the high-melting fractions of cocoa butter. Vaeck [11] explainedbloom on under-tempered chocolate as the recrystalliza-tion of the unstable b’ form of cocoa butter into largeb form crystals, thus inducing a sandy structure. This ledto large crevices from which light was diffracted, creatingthe whitish appearance. This theory has been commonlyaccepted [7].

In this study, the primary goal was to characterize thechanges in composition at the surface of improperlytempered chocolates and to investigate factors that affecttheir appearance.

2 Materials and methods

2.1 Preparation of chocolate samples

Three commercial chocolates were used: dark bitter-sweet couverture chocolate with high (64%) cocoa solidcontent (Guittard Chocolate, Burlingame, CA, USA), aregular dark bittersweet chocolate (Blommer Chocolate,Chicago, IL, USA) and milk chocolate (Hershey Foods,

Hershey, PA, USA). The dark bittersweet couverture fromGuittard contained approximately 63% cocoa mass(about 43% cocoa butter) and 37% sugar, in addition to asmall amount of lecithin and vanilla flavoring. The othertwo chocolates contained approximately 45–50% sugarand 30–32% fat. Although the exact composition of thesetwo chocolates is unknown, they represent typical com-mercial chocolates.

Untempered chocolate was obtained by melting thechocolate at 65 7C for 12 h. The mass was poured directlyinto disc-shaped plastic molds of 5 cm diameter and 7 mmthickness. These chocolates were allowed to cool at roomtemperature (about 20–22 7C) with only gentle air flow.

A three-stage tempering protocol was used to obtainover-tempered chocolate [9]. Chocolate was melted in a1-L jacketed tempering beaker (Cole-Palmer, Chicago, IL,USA). A coated paddle stirrer (Cole-Palmer) was used tomix the samples, and a Master Servodyne motor headand controller (Cole-Palmer) governed stirrer speed andmonitored viscosity. Melted chocolate at 70 7C wascooled to 26 7C in the tempering beaker and held thereuntil viscosity approached a constant value. Temperaturewas raised to 32 7C until a viscous paste was obtained.The chocolate was poured into the disc-shaped plasticmolds and cooled at room temperature (22–24 7C) withgentle air flow. Although the degree of temper was notcharacterized, all chocolates processed in this way,including the milk chocolate, exhibited the classic char-acteristics of over-tempering. They were excessively vis-cous, became dull very quickly and were not easilyremoved from the molds (inadequate contraction).

For comparison, well-tempered chocolates were alsoproduced in a similar manner. A cooling-and-heatingmethod was used (as for over-tempered chocolate), butwith temperatures chosen to give chocolates with aglossy surface that were easily released from the molds.

2.2 Melting enthalpies (DH) of sugar and fat

Samples of the chocolate surfaces (bloomed for untem-pered and dull for over-tempered) were collected bycarefully scraping the whitish or dull surface of the choc-olate. Contamination by the interior chocolate wasavoided as much as possible – samples were discardedin case of doubt. Samples (2–7 mg) of bloom scrapingswere sealed in an aluminum pan, and melting enthalpiesof sugar and fat were measured using a Perkin Elmer(Norwalk, CT, USA), Pyris 1, differential scanning calo-rimeter (DSC) monitored with Pyris software. To ensurecomplete standardization of the fat phase, each samplewas melted at 50 7C for 5 min and then cooled to 220 7C


Eur. J. Lipid Sci. Technol. 108 (2006) 159–168 Surface bloom 161

at 15 7C/min. The temperature was then increased at2 7C/ min to 40 7C, and then at a rate of 11 7C/min to200 7C. To calculate the DHSugar/DHFat ratio, the meltingenthalpy of the sugar (peak obtained at about 190 7C) wasdivided by the melting enthalpy of the fat peak (around30 7C). At least three different samples from each choco-late and each tempering condition were analyzed todetermine the melting enthalpies. The mean and standarddeviation were calculated.

2.3 Polarized light microscopy

Samples of both well-tempered chocolate and thescraped surfaces of improperly tempered chocolateswere observed under a light microscope (Nikon LABO-PHOT-2, Garden City, NY, USA) equipped with a remo-vable polarized lens. Photographs were taken with anOlympus camera (Olympus Corp., Woodbury, NY, USA)attached to the microscope. Sugar crystals were totallydissolved by water injection into the sample (betweenslide and cover slip). Three replicates were observed foreach type of chocolate and bloom.

2.4 Sample storage and monitoring of bloom

Molds of untempered chocolates for storage at differentrelative humidity (RH) were prepared by first melting thechocolate at 65 7C for 12 h and then pouring it directly intoa plastic mold (5 cm diameter and 7 mm depth). Thesamples were stored immediately in controlled RH (0–76%) in desiccators over saturated salt solutions at 20 7C.Duplicate samples of untempered chocolate were pre-pared. Since this chocolate was not tempered prior tocooling, bloom generally occurred rapidly, with whitespots appearing on the surface sometimes within hours(to a day) of sample preparation.

Several methods were used to follow bloom develop-ment. Whiteness is often the most revealing parameter toquantify the color variations at the chocolate surface [12].A Hunterlab Color Quest (model Q 45/0; Hunter Associa-tion, Inc., Reston, VA, USA) colormeter measured the L, a,b values, which were converted to whiteness index (WI)values according to the following equation [12]:

WI = 100 – (100 – L)2 1 a2 1 b2 (1)

The colormeter was calibrated with a black-and-whitestandard plate prior to any sample measurement. Threemeasurements were made for each sample (triplicate).The sample was rotated approximately 307 betweenmeasurements.

To support the whiteness index measurement, the num-ber and the diameter of white spots on each disk weredetermined visually starting from within the first few hoursof molding the chocolate. A growth rate of spot diameter(in cm/day) was estimated as D(spot diameter)/(time). Atleast four spots were used that were not affected byadjacent spots over the duration of the experiment.

Although only semi-quantitative, a nucleation rate oneach disk was estimated based on the number of spotsthat appeared on the surface per day. This rate was cal-culated as D(spot number)/(time).

2.5 Sorption isotherm

Three replicates (3–4 g) of each sample (sugar crystalsand cocoa powder both initially and after coating withlecithin) were placed in desiccators containing saturatedsalt slurries with the following aw values at 20 7C: LiCl,0.11; KC2H3O2, 0.23; MgCl2, 0.31; K2CO3, 0.43; MgNO3,0.54; and NaCl, 0.75. Moisture uptake of samples wascalculated (in percent) based on the gain in weight ofsample per 100 g dry weight of sample.

To study the sorption behavior of chocolate components,sucrose crystals and cocoa powder were sieved to obtaina fraction between 45 and 105 mm particle size. Thisfraction was used either directly as sieved or coated withemulsifier. To prepare the coated samples, 150 g of eachpowder was mixed for 12 h in 500 mL chloroform with asoy lecithin concentration of 37.5 mM, after which it wasfiltered and dried under nitrogen. These lecithin-coatedsamples were used immediately for the determination ofthe sorption isotherm, as described above.

2.6 Moisture content of chocolate

The moisture content of chocolate was determined byKarl Fisher titration at the end of the sorption term(40 days of storage). Chocolate was finely ground anddissolved in anhydrous methanol at 60 7C for 6 h. There-after, 2 mL (in average) of this solution was used to deter-mine the moisture content of chocolate. At least triplicatedeterminations were made for each sample.

3 Results and discussion

3.1 Appearance of the chocolate surface

Typical examples of improperly tempered (both untem-pered and over-tempered) and well-tempered chocolateare shown in Fig. 1. Untempered chocolate is character-ized by a whitish brown sandy color. The bloom grew



Fig. 1. Pictures of well-tempered (unbloomed)chocolate (a), surface dulling of over-temperedchocolate (b) and bloom on untemperedchocolate (c). All samples were stored for1 week at room temperature.

Tab. 1. Fat and sugar melting enthalpies (DH, J/g), as determined by DSC, of chocolates and theirassociated surfaces, after 1 week at room temperature. Mean and standard deviation of three repli-cates.

DHFat DHSugar RatioDHSugar/DHFat

Dark, bittersweetcouverture chocolate

Well-temperedUntempered bloomOver-tempered surface

19.7 6 2.61.5 6 0.65*

16.5 6 3.2

39.9 6 3.757.1 6 17.2*48.0 6 1.8

2.0 6 0.0838.5 6 7.6*2.91 6 0.37

Dark chocolate Well-temperedUntempered bloomOver-tempered surface

34.7 6 4.0<0*33.2 6 4.7

44.3 6 0.245.0 6 4.249.1 6 2.5

1.29 6 0.15? 1?*1.48 6 0.27

Milk chocolate Well-temperedUntempered bloomOver-tempered surface

34.5 6 4.03.5 6 3.1*

NA§

43.4 6 1.135.0 6 9.5NA

1.26 6 0.1118.5 6 19.1*NA

§ Bloom on over-tempered milk chocolate was not analyzed.* Significantly different (p ,0.05)

either in spots or like a corona surrounding a dark choc-olate center. Bloom on untempered chocolate appearedusually several hours after initial chocolate solidificationand continued to develop for over 30 days.

Over-tempered chocolate showed a uniform thin, dullgray surface. The surface changes occurred during soli-dification and, once it formed, did not seem to vary orvaried only slightly as a function of time.

3.2 Ratio of sugar to fat

The ratios of sugar to fat in the intact chocolates andassociated surface structures after 1 week at room tem-perature were determined by DSC analysis. The resultsfor the ratio DHSugar/DHFat are summarized in Tab. 1. Theratio of fat and sugar enthalpies depended on the choco-late type, which was dependent on its original composi-tion. The couverture chocolate had lower values of fat andsugar melting enthalpies compared to the other choco-lates, which were very similar to each other. The cou-verture chocolate had a sugar/fat enthalpy ratio ofabout 2, whereas the dark and milk chocolates had a ratioaround 1.25.

The scraped surface (whitish or dull) of each chocolatewas analyzed 1 week (at room temperature) after choco-late production. For the over-tempered chocolates, thefat enthalpy of the surface layer was about the same asfor the well-tempered chocolates, whereas sugar enthal-py was slightly greater. Consequently, the fat contentremained similar at the surface of over-tempered choco-late to the interior of the chocolate. That is, no phaseseparation between fat and either sugar or cocoa solidswas evident at the surface of over-tempered chocolate.Consequently, dulling of the surface of these chocolatesmust be due to a rearrangement of the fat crystals withouta separation of solids.

The situation was quite different for untempered choco-lates. The melting peak of fat was almost nonexistent;DHFat was tenfold smaller than that obtained for well-tempered chocolate. It was almost impossible to quantifyany fat peak in the bloom scraped from dark chocolate. Incontrast, no statistical differences were observed be-tween the sugar peaks (DHSugar) of bloom on the untem-pered chocolate and the well-tempered control. Greatervariability in the results was observed with bloom onuntempered chocolate than for the other samples, per-haps related to the heterogeneity of bloom. Each spot



might have had small composition differences caused byheterogeneity of the bloom taken on the surface. How-ever, the apparent decrease in fat content of bloom onuntempered chocolate was clearly evident in all threesamples.

The decrease in fat enthalpy, as measured by DSC melt-ing scan, documented the disappearance of fat in thebloom of untempered chocolate. Sugar and cocoa pow-der (which is not observable by DSC) were the primarysubstances found in this form of bloom. Thus, the pre-vious conjecture that bloom on untempered chocolate iscocoa butter that has recrystallized into a more stableform must be incorrect (although this bloom is still likely tobe caused by that polymorphic recrystallization).

3.3 Polarized light microscopy

The different chocolates and scraped surface sampleswere observed under a polarized light microscope (PLM)to verify the DSC results. Under the PLM, sugar and fatcrystals exhibit polarization, whereas cocoa solids(amorphous) do not. Although not a quantitative meas-urement, the relative amount of polarizing matterobserved for each sample was used to corroborate theDSC results.

Micrographs of well-tempered couverture chocolate, thewhite spots that appeared on untempered chocolate, andthe surface of over-tempered chocolate are presented inFig. 2 (a–c). Each sample exhibited polarization corre-sponding to the bright white spots on the micrographs.The sample opacity and high proportion of crystals didnot allow determination of whether polarization camefrom sugar or fat. Moreover, differences of polarizationintensity observed between samples were not quantita-tive since thickness and weight of the samples undoubt-edly influenced the appearance.

To determine the origin of polarization, sugar was removedfrom the samples by addition of water to dissolve the sugarcrystals. After washing to dissolve sugar crystals, theremaining polarization was due only to fat crystals. Imagesof the different samples after water treatment are alsoshown in Fig. 2 (a’–c’). Well-tempered couverture choco-late (Fig. 2a’) and the surface scraped from over-temperedchocolate (Fig. 2b’) both exhibited small polarization spotsdue to fat crystals that were dispersed homogeneouslythroughout the sample. However, bloom scraped fromuntempered chocolate (Fig. 2c’) was totally dark; it did notexhibit any polarization. That is, this sample did not haveany fat crystals. Only cocoa powder was observed afterwashing the sugar crystals. Similar results were obtainedwith dark and milk chocolates (images not shown). Themicroscope observations confirmed the results obtainedby DSC. The surface of over-tempered chocolate exhib-ited a slight increase in sugar content, but the averagecomposition was similar to well-tempered chocolate.However, bloom scraped from untempered chocolate wasessentially devoid of fat, as documented by both DSCanalysis and polarized light microscopy.

3.4 Nucleation rate of white spots

Development of white spots on the surface of untem-pered chocolate began at some time after solidification.On the untempered chocolates in this study, sometimesbloom spots generally appeared within a day or so, atwhich time the small black spots appeared, followed by athin white corona around the spot. These spots thendeveloped over time into the large sandy, white spotsapparent on untempered chocolate. Fig. 3 shows theeffect of RH on the number of white spots that developedon disks (at least triplicate disks were analyzed) of 5 cmdiameter. An approximately linear trend was observedbetween the number of white spots and time for the first20 days of storage. Thereafter, it was no longer possibleto count individual spots due to overlap.

Fig. 2. Micrographs of couverture choco-late (a), the scraped surface of over-temperedchocolate (b), and bloom on untemperedchocolate (c). The same samples after sugardissolution by washing with water (a’, b’, c’).



Fig. 3. Effect of RH of storage on the development ofwhite spots on untempered chocolate stored at roomtemperature.

In general, the number of white spots that appeared onthe chocolate disks increased as RH decreased. That is,higher RH led to slower development of bloom and to alesser extent. A nucleation rate was calculated from theslope of each line, as given in Tab. 2. The nucleation rateof bloom spots on chocolate stored below 50% RH wasalmost twice the rate for those stored above 50% RH.

Tab. 2. Effect of RH of storage on nucleation rate of whitespots on bloomed surface of untempered chocolate.

RH [%] Nucleation rate§ R2 #

0 8.9 0.9311 11.6 0.9423 7.5 0.8931 7.1 0.9443 4.2 0.8654 3.6 0.8765 3.8 0.8675 4.1 0.86

§ Changes in number of white spots with time.# Linear regression correlation coefficient.

3.5 Growth rate of white spots

Fig. 4 shows the effects of storage RH on the growth rateof the white spots on disks of untempered chocolate. Ingeneral agreement with the nucleation rate data, thegrowth rate was fastest at 0% RH and the rate generallydecreased as RH increased. Between 10 and 43% RH,the growth rate was intermediate and essentially invar-

Fig. 4. Effect of RH of storage on the diameter growthrate of white spots on untempered chocolate stored atroom temperature.

iant. Growth rate decreased above 50% RH. Higher RH ofstorage significantly (p ,0.05) reduced the growth rate ofwhite spots.

3.6 Whiteness index

Whiteness index based on colormeter measurement wasmonitored for more than 1 month on untempered choco-lates stored at different controlled RH (Fig. 5). Interest-ingly, the whiteness index did not change significantlyuntil about 1 week of storage, despite visual detection ofnumerous white spots on the disks of chocolate within aday of solidification. Comparing the data in Fig. 3, therewere between 15 and 75 white spots visible on the choc-olate disks before the whiteness index began to increaseat day 7. Apparently, the whiteness index method ofcharacterizing bloom in untempered chocolates is not assensitive a method as visual determination. Despite thislimitation of the whiteness index data, the general trend ofbloom increasing as RH of storage decreased is still evi-dent in Fig. 5.

Each chocolate bloomed, regardless of the RH, with atrend in whiteness that had a common shape consistingof three parts. At the beginning, an induction phase wascharacterized by a constant whiteness value. There wereeither no white spots on the surface or the number wasnot sufficient to affect the measured whiteness index. Asnoted before, visual counting of white spots indicated thatthe colormeter was not very sensitive when the number ofspots was too low (less than 50–75 spots). Once therewas a sufficient number of spots to affect the whitenessindex, there was an exponential increase of whiteness.The number and diameter of white spots increased



Fig. 5. Changes in whiteness index, asmeasured by colormeter, of untemperedchocolate stored at different RH and roomtemperature.

sharply. Finally, a plateau was observed where the sur-face was completely covered with bloom and the white-ness remained at a maximal value.

The chocolates stored between 0 and 43% RH all gen-erally showed identical behavior, with a short inductiontime and high final whiteness index. However, above 50%RH, the induction time increased to about twice theinduction time at lower RH and the maximal whitenesswas also lower. This decrease was greater as the RHincreased.

To be sure that the RH itself did not affect the perceptionor intensity of bloom, the whiteness variations of a sugar/cocoa powder blend (with a composition of 40 : 60 wt/wt,similar to bloom) was monitored as a function of RH ofstorage (data not shown). Whiteness remained constantfor more than 6 weeks, except for the sample stored at75% RH, where whiteness decreased slightly (threepoints) at the 6-week point. Thus, the simple effect ofmoisture on whiteness perception was not sufficient toexplain the difference in whiteness observed betweenchocolates stored at the highest and lowest RH (Fig. 5).

3.7 Sorption isotherms

The sorption isotherm of untempered chocolate is pre-sented in Fig. 6. Moisture content was below 1% until RHexceeded 45%, at which point it then increased quickly to2.2% at RH above 60%. The result was similar to valuesobtained previously with well-tempered chocolate [13,14]. Thus, the moisture uptake was not affected by thechocolate structure or type; the disorganized structure of

Fig. 6. Sorption isotherm of untempered chocolate atroom temperature.

untempered chocolate or the tight structure of well-tem-pered chocolate apparently have similar sorption behav-ior. Thus, a coating of fat on sugar and cocoa particles,which should prevent the sorption of water, was not alimiting factor for the moisture uptake of chocolate.

Hypothetically, the moisture mediator in chocolate couldbe cocoa butter, emulsifier, cocoa solids or sugar. In ageneral sense, fat is a good moisture barrier, with fatscomposed of saturated and longer fatty acids being moreimpervious to moisture. Landmann et al. [15] found amoisture permeability constant of 0 for paraffin, 1610212

for hydrogenated cottonseed oil and 36610212 for cocoabutter in its most stable form. Thus, cocoa butter is nottotally impervious to moisture; it has a very low perme-ability due to a certain liquid fat content and the presenceof minor compounds with emulsifying properties. Emulsi-



fiers are well known to interact with moisture. The lecithinin chocolate (0.3% of the total mass in average) adsorbsprimarily on sugar particles, as confirmed by Johanssonand Bergenstahl [16], who compared the adsorption ofemulsifiers on fat or sugar crystals [17]. Emulsifiersadsorbed in multilayers on sugar crystals, whereas theyadsorbed as mono- or bilayers on fat crystals. Moisturesorption in chocolate is almost certainly mediated byemulsifiers. However, Ogunmoyela et al. [18] showed thathigher content of lecithin had only a slight effect on themoisture content of chocolate. In our results, no differ-ence in moisture sorption was observed until RH wasabove 50% and the differences were very small even at75% RH.

To develop a better understanding of moisture behavior inchocolate, the sorption isotherms of sugar and cocoapowder, either bare or coated with emulsifiers, weredetermined. Coating of particles with emulsifier wasaccomplished by soaking sugar and cocoa powder in a37.5 mM soy lecithin solution (in chloroform as solvent).The extent of lecithin adsorption was quantified by an in-direct method, corresponding to the difference of phos-pholipid concentration in the soaking solution before andafter adsorption. It was found that 8.6% of the initialphospholipids were adsorbed on sugar, with phosphati-dylcholine (61%), phosphatidylinositol (35%) and phos-phatiylacid (3.5%) as the main adsorbants. A rough cal-culation of phospholipid adsorption based on an averagesugar crystal size of 75 mm and coverage of 1.5 mmol/m2

for a phospholipid monolayer [16] gave four phospholipidlayers adsorbed on each sugar particle. With cocoapowder, it was not possible to quantify the phospholipidvariations after coating. In fact, the concentrations ofphosphatidylcholine and phosphatidylinositol in solutionincreased by 50% after the adsorption step compared tothe initial soaking solution. Apparently, phospholipidswere desorbed from cocoa powder.

The moisture sorption isotherms of sugar crystal andcocoa powder, coated with emulsifieror not, are presentedin Fig. 7. The sugar crystal and cocoa powder moisturesorption isotherms were quite different. Cocoa powderhad 4% moisture content when stored at 10% RH and thatincreased to 12% at 75% RH. The differences betweenbare and coated sample were very small. This is certainlydue to the initial presence of phospholipids on the cocoasolids. In contrast, sugar crystals had a very low hygro-scopicity. Bare sugar had a moisture content below 0.05%even when stored at high RH. Coating with emulsifierincreased the extent of moisture uptake when RH .60%.In this case, moisture uptake increased substantially;however, moisture content was always very low, below1%. Thus, moisture content in chocolate seemed to bemainly due to the moisture uptake of cocoa powder.

Fig. 7. Moisture sorption isotherm of cocoa powder (a)and sugar (b), bare (r) and coated with phospho-lipids (h).

3.8 Correlation between chocolate moistureand bloom development

The final moisture content of chocolate after 40 days ofstorage at different RH was compared to the differentparameters associated with bloom development. Moist-ure content of chocolate showed a good linear correlationwith the whiteness value (y = –8.12x 1 52.5, r2 = 0.93) aswell as with the growth rate of the white spots(y = 0.009x 1 0.038, r2 = 0.76) (Fig. 8a, c). The higher themoisture content, the lower the final whiteness of thechocolate as well as the slower the growth rate of thewhite spots. The effect of chocolate moisture content onnucleation rate seemed to be less directly related(Fig. 8b). It appeared that nucleation rate was greatlyaffected by RH for values lower than 40% RH, but above40% RH, the nucleation rate was low and not affected byfurther increases in RH.

4 Conclusions

The surface characteristics of poorly tempered choco-lates were characterized. Untempered chocolate quicklyforms a surface bloom, within a few hours to a few days,



Fig. 8. Correlation between moisture content of choco-late (after 40 days of storage at different RH) and finalwhiteness index of untempered chocolate (a), nucleationrate of white spots (b), and growth rate of white spots (c).

that appears as numerous white spots surrounded bydark chocolate. These white spots consist of sugar crys-tals and cocoa solids, and are essentially devoid of fat.Recrystallization, and subsequent contraction, of thecocoa butter into more stable polymorphic forms isthought to be responsible for this type of bloom forma-tion.

In contrast, the surface of over-tempered chocolatequickly, within minutes to hours, becomes dull, even dur-ing solidification, but this dull surface has essentially thesame composition of fat as the original (and well-tem-pered) chocolate. Thus, surface dulling of over-tempered

chocolate is apparently due to growth of cocoa buttercrystals at the surface that influence light reflection andgive the dull appearance, as suggested by Musser [19].

The rate and extent of bloom formation on untemperedchocolate was found to be dependent on the RH of stor-age, with bloom occurring more readily at very low RH.We speculate that the mechanism of this type of bloomformation is related to the volume contraction that occursduring the polymorphic transformation from unstableb’ crystals to the more stable bV and bVI polymorphs. Asthe cocoa butter molecules rearrange into more compactcrystals, the gaps between crystals are regions wherecocoa solids and sugar crystals become entrapped. Atlower RH, formation of these white spots is enhancedcompared to storage at higher RH. This suggests thateither the particulate matter is less able to move whenmoisture content is higher or the transition of cocoa butterfrom less stable to more stable polymorphic forms is in-hibited.

Acknowledgments

Financial support for this work was provided by the Col-lege of Agricultural and Life Sciences at the University ofWisconsin-Madison. Discussions and review with Mr. EdSeguine of Guittard Chocolate are greatly appreciated.

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[Received: September 7, 2005; accepted: November 7, 2005]


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Surface bloom on improperly tempered chocolate