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J. of Supercritical Fluids 55 (2011) 1079–1088

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

journa l homepage: www.e lsev ier .com/ locate /supf lu

roduction of microparticles from milk fat products using the Supercriticalelt Micronization (ScMM) process

arta Lubarya,∗, Theo W. de Loosa, Joop H. ter Horsta, Gerard W. Hoflandb

Department of Process & Energy, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The NetherlandsFeyecon Development and Implementation, Rijnkade 17a, 1382 GS Weesp, The Netherlands

r t i c l e i n f o

rticle history:eceived 19 July 2010eceived in revised form 6 October 2010ccepted 8 October 2010

eywords:

a b s t r a c t

The ScMM (Supercritical Melt Micronization) process was applied for the production of microparticlesfrom anhydrous milk fat (AMF) and a diacylglycerol-based modified milk fat (D-AMF). Both fats wereable to dissolve ca. 30 wt% CO2 in the studied pressure and temperature ranges, being the CO2 amountslightly higher for AMF. A melting point depression was observed in both systems in the presence of CO2.Two powder morphologies were obtained (spherical hollow particles and a mass sponge-like broken

upercritical melt micronizationGSSilk fat

hase behavior

particles) depending on the ScMM process conditions. The concentration of CO2 in the fat melt was themain process variable affecting the particle morphology, followed by the temperature of the melt. Thesmall broken particles originated from the breakage of spherical fat particles that solidified before all CO2

could escape from the atomized droplets. While the hollow spheres had a tendency to agglomerate, thebroken microparticles constituted a free-flowing powder as long as they were stored at low temperatures(up to −18 ◦C). Both types of particles have a potential for being incorporated in refrigerated or frozenfood products as a structuring agent.

. Introduction

Fats provide structure and palatability to consumer productsuch as margarine, chocolate and ice cream [1]. In margarines andther spreads, for example, the water–oil emulsion is stabilized bynetwork of small fat crystals, which acts as a structuring agent [2].he plasticity and consistency of margarine depend to a large extentn the amount, size, shape and distribution of the fat crystals [2,3].or food applications, particle sizes up to about 20 �m are desirable,s they can provide structure without being detected in the mouth,hich results in a pleasant, smooth texture [4]. Such particles can

e conveniently produced from edible fats by supercritical melticronization.

The conventional process for structuring fat-based food prod-

cts involves the creation of a fat crystal network by cooling downhe emulsion in a scraped surface heat exchanger [5]. Disadvan-ages of this technology are the high energy input needed for

Abbreviations: AMF, anhydrous milk fat; DAG, diacylglycerol; D-AMF,iacylglycerol-based anhydrous milk fat; MAG, monoacylglycerol; PGSS, Particlesrom Gas Saturated Solutions; ScMM, supercritical melt micronization; SCP, solidlear point; SEM, scanning electron microscope; TAG, triacylglycerol; XRD, X-rayiffraction.∗ Corresponding author. Tel.: +31 15 2786648; fax: +31 15 2786975.

E-mail addresses: m.lubaryfleta@tudelft.nl, marta.lubary@gmail.comM. Lubary).

896-8446/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2010.10.010

© 2010 Elsevier B.V. All rights reserved.

preparing the emulsion (by heating and mixing above the melt-ing point of the structuring agent) and subsequently cooling forpartially crystallizing the fat component of the mixture [6]. In addi-tion, this process limits the range of structuring agents that canbe used to those with a moderate melting point. Too high meltingpoints require temperatures for the emulsion preparation that candegrade heat-sensitive components. On the other hand, if the melt-ing point is too low, the emulsion stability is negatively affected [6].An alternative, advantageous process for the manufacture of thistype of products is based on the incorporation of the structuringagent (the fat crystals) in a liquid water–oil emulsion as microparti-cles of defined properties [6]. Similarly to the conventional process,the fat crystals create a network that stabilizes the emulsion byforming a layer around the water droplets and preventing themfrom coalescing [5].

Cryogenic milling or grinding is the most common method forproducing fine powders from ductile materials, such as fats [5]. Liq-uid nitrogen is used to cool the material below the ductile-brittletransition point, which enables grinding it in a mill. However, thehigh energy input required for cooling down the material pre-vents the use of the technique in large scale. Another common

disadvantage of grinding is the poor control over particle size[7].

Through supercritical fluid-based processes, powders withdefined properties (particle size and distribution, shape, polymor-phism) can be produced. In addition, these processes allow better

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rocess control and flexibility than the conventional ones [7,8].mong them, the supercritical melt micronization (ScMM) pro-ess is particularly useful for producing fine powders from ductileaterials which do not show a high solubility in SCF, but are on

he other hand are able to dissolve considerable amounts of theupercritical fluid.

The ScMM process, also known as PGSS (Particles from Gas Satu-ated Solutions), was patented by Weidner et al. [9]. In this process,he liquid material to be micronized is melted in a high-pressureessel and pressurized by addition of a gas, for example carbonioxide. A gas-saturated melt, solution or suspension is formedy the dissolution of a gas into the liquid phase. Resulting effectsuch as swelling, plasticization, decrease of the viscosity and sur-ace tension, or melting point depression [4,10], which contributeo the sprayability of the material [4]. The gas-saturated liquid ishen expanded through a nozzle to ambient pressure. During theapid expansion of the mixture, the gas breaks down the fluid inmall droplets. Simultaneously, the intensive cooling caused byhe Joule–Thomson effect and the evaporation of the gas causehe solidification of the droplets [10]. The particles obtained bycMM have typically a narrow size distribution, and are free ofny solvents or other additives [11]. Microparticles of differentorphologies, like solid spheres, hollow spheres and distorted or

gglomerated particles can be obtained. Researchers have shownhat it is possible to control the particle size and morphology byariation of process parameters, such as pressure, temperature andhe gas to solid ratio [12,13].

The ScMM process has been applied to the micronization of hardats, such as cocoa butter [7,14], hardened rapeseed oil [11], hydro-enated palm oil [15], hydrogenated castor oil [5] and tristearin10]. By micronization of cocoa butter, for example, a free flow-ng powder of polymorph V, the desired form for confectionerypplications, was produced [14].

Milk fat is an attractive natural product with a rich chemicalomposition that can be used for the manufacture of high-valueerivatives [16,17]. Powdered milk fat can be used as an inter-sting low-melting structuring agent in the manufacture of iceream or other frozen foods. For this application, small, free flow-ng particles are desired; but other particle morphologies coulde exploited in new application areas. In this work, ScMM waspplied for producing microparticles from anhydrous milk fatAMF) and from a modified, diacylglycerol-rich milk fat analogD-AMF) [17]. This product has potentially interesting nutritionalroperties, being suitable for its use in substitution of conventionalilk fat or milk fat fractions. The effect of the pre-expansion tem-

erature of the melt and the CO2 concentration in the melt on theorphology, size and crystallinity of the produced particles was

tudied.

. Materials and methods

.1. Materials

Concentrated, 99% anhydrous milk fat (AMF) was obtained fromrieslandCampina (the Netherlands) and stored at 4 ◦C before use.arbon dioxide was obtained from Linde Gas Benelux (Schiedam,he Netherlands).

A diacylglycerol (DAG)-based modified milk fat (D-AMF) wasroduced by lipase-catalyzed modification of AMF [17]. AMF con-ained >98% triacylglycerols (TAG), while the DAG-based AMF

D-AMF) contained roughly 50% DAG, 25% TAG and 25% MAGmonoacylglycerols). D-AMF contained slightly higher amounts ofatty acids C4–C14 and C18:0, and lower amounts of C16 and C18:1.his composition change was due to the specificity of the lipasesed for producing DAG-based AMF [17].

Fluids 55 (2011) 1079–1088

2.2. Experimental methods and equipment

2.2.1. Phase equilibrium measurementsThe phase behavior of systems containing mixtures of fat and

carbon dioxide was studied in a Cailletet apparatus, which hasbeen described elsewhere [5,18]. The Cailletet tube was carefullyobserved with the aid of a microscope installed in front of the tube,in order to detect visually any changes in the system derived fromthe slow manipulation of pressure and temperature. Liquid–vapor(L–V) or liquid–liquid (L–L) curves were measured in a tempera-ture range 25–70 ◦C (298–343 K). The temperature was fixed andthe mixture was brought into a two-phase state by manipulationof the pressure. Then the pressure was slowly increased until thelast bubble of the vapor phase disappeared. The pressure was con-trolled and measured with a dead-weight gauge with an accuracyof 0.025 MPa. Duplicated or triplicated measurements showed avariability in the measured pressures lower than 0.05 MPa.

Solid–liquid equilibrium curves were measured in a pressurerange up to 13 MPa. Because the fats used were composed by amixture of multiple TAG, DAG and MAG with different meltingpoints, they melted over a range of temperatures instead ofhaving a specific melting point. The solid–liquid temperaturesmeasured were those at which the last solid fat was observedto disappear, therefore, the temperatures at which the fats werecompletely liquid. We defined these solid–liquid equilibriumpoints as “solid clear points” (SCP). To detect the SCP, a solid phasewas first allowed to form by decreasing the temperature. Then, thetemperature was slowly increased until the solid phase was notvisible anymore. The temperature of the thermostatic bath wasmeasured using a PT-100 thermometer, with an accuracy of 0.01 K.Repeated measurements showed a variability in the measured SCPtemperatures lower than 0.2 K.

2.2.2. ScMM experimentsMicronization experiments were done batchwise in the set-up

depicted in Fig. 1. The main element of the set-up is the jacketedpressure vessel A (Sitec, T021-320-AFF), with a design pressure of325 bar. The temperature in the vessel was regulated using a HuberCC301 thermostatic oil bath. CO2 was supplied to the system using aLewa ELM-1 pump. The particle collection vessel B and the powderfilter G were home-made. In a typical ScMM experiment, an amountof fat (AMF or D-AMF) was melted and poured into vessel A, whichwas then closed and pressurized by addition of carbon dioxide. CO2was added from the bottom of the vessel by opening valves C andD, until the desired pressure was reached at the set temperature.

The pressure inside the vessel was regulated by a pressure con-trol valve (C). The mixture of fat and CO2 was then stirred forapproximately 30 min. It was assumed that phase equilibrium wasreached after this time, so that the liquid (fat) phase had becomesaturated with CO2 at the corresponding pressure and tempera-ture. A CO2 stream into the particle collection vessel (B) was startedfor cooling it down by effect of CO2 expansion, from ca. 5 MPa toatmospheric pressure. Low temperature in the collection vessel wasnecessary to avoid melting of the fat particles. Once a temperatureof −40 ◦C had been reached, valve D was opened in the directionof the collection vessel. The CO2-expanded fat passed through thenozzle located at the end of the tube (a SK series SprayDry Nozzlefrom Spraying Systems Co., with an orifice diameter of 0.34 mm),creating a spray of fat droplets inside the collection vessel. Simulta-neously, the CO2 supply was shifted from bottom to top by openingvalve E. This allowed maintaining a constant atomization pressure

throughout the experiment.

The fat particles were removed from the collection vessel andstored at a maximum temperature of −18 ◦C before analysis.

We carried out series of ScMM experiments at temperaturesTmelt of 306, 315, 325 and 334 K, and dissolution pressures Pdiss

M. Lubary et al. / J. of Supercritical Fluids 55 (2011) 1079–1088 1081

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L–V (or L–L) equilibrium pressures increased both with the tem-perature and with the CO2 concentration in the mixture. The slopeof the curves was larger the higher the CO2 concentration, partic-ularly for the L–L curve for 30 wt% CO2. In the liquid–vapor curvescorresponding to 5 wt% and 7.5 wt% CO2, some of the L–V points

Fig. 1. Set-up fo

etween 7 and 20 MPa. Pressures lower than 7 MPa were found toe not suitable for spraying, as they resulted in a too low flowratehrough the nozzle. As a result the fat crystallized inside andlocked the nozzle orifice.

The combination of dissolution pressure and temperatureesulted in a specific concentration of CO2 on the fat phase (wCO2 ).he applied atomization pressure, Patom, was mostly the same as theissolution pressure Pdiss. Previous research showed little effect ofhe atomization pressure on the particle properties [11]. One exper-ment was dedicated to confirm this effect: after equilibration ofhe fat melt and the carbon dioxide at the desired Pdiss and Tmelt,n extra amount of CO2 was added right before atomization, forncreasing the atomization pressure above Pdiss.

.3. Particle analysis

Particle morphology and size. The particle size and morphol-gy were visualized on scanning electron microscope (SEM)icrographs. The scanning electron microscope used was a JEOL-

SM5400, with a resolution of 4 nm. The particle samples to becanned were sputtered with gold. To prevent samples from melt-ng, the entire system (sample preparation and the sample stage)

ere cooled with liquid nitrogen to a temperature of −170 ◦C.icrographs were taken at magnifications between 200× and

500×.Polymorphism. XRD analyses of powder samples were carried

ut in a D8 Discover X-ray Diffractometer from Bruker AXS. Theeasurements were carried out in duplicate, at a controlled tem-

erature of 5 ◦C. The powder samples were stored prior to XRDnalyses at a maximum temperature of −18 ◦C. Polymorphic formsere identified from the short spacing diffraction pattern.

Bulk density. The bulk density of the particles was measured byeighting a volumetric glass of 500 mL filled with the powders

mmediately after spraying.

. Results and discussion

First, phase equilibrium measurements were done on theystems AMF:CO2 and D-AMF:CO2 for determining the pres-ure/temperature operating window for ScMM. Micronizationxperiments were then carried out at known concentrations ofO2 in the fat melt, as determined by the phase equilibrium data.

M experiments.

The morphology, size and polymorphism of the produced particleswere examined in a relevant window of the micronization processconditions.

3.1. Phase equilibrium of the system AMF:CO2

Phase equilibria of the system AMF:CO2 was determined bymeasurement of bubble point curves for an overall CO2 concen-tration (wCO2 ) range between 5 wt% and 30 wt%. Most of the datareflect liquid–vapor equilibrium (L–V). The equilibrium curve for30 wt% CO2, however, corresponds to liquid–liquid (L–L) equilib-rium between a AMF-rich liquid phase and a CO2-rich liquid phase,since this equilibrium curve lies above the bubble point curve forpure CO2 (L–V, 100%). Solid clear points (SCP) were measured atCO2 concentrations wCO2 between 0% and 30%. L–V (or L–L) curvesand SCP curves are plotted in Fig. 2 as a function of the temperature,T.

Fig. 2. Liquid–vapor (L–V) or liquid–liquid (L–L) equilibrium curves and solid clearpoint (SCP) curves for the system AMF:CO2, for different concentrations of CO2 in thefat–CO2 mixture. The liquid–vapor equilibrium curve for pure carbon dioxide (L–V,100%) is also shown for comparison. The L–V or L–L curves represent the boundaryat which two phases (L+V) become a single L or V phase. The SCP curves representthe boundary at which two phases (L+S) become a single L phase.

1082 M. Lubary et al. / J. of Supercritical Fluids 55 (2011) 1079–1088

Fig. 3. (a) (up): P-wCO2 cross-sections of the L–V or L–L equilibrium points (30 wt%CO ) of the system AMF:CO , for a range of temperatures between 303 and 343 K.Dsb

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Fig. 4. Liquid–vapor (L–V) or liquid–liquid (L–L) equilibrium curves and solid clearpoint (SCP) curves for the system D-AMF:CO , for different CO concentrations in

2 2

ata points surrounded by circles were extrapolated. (b) (down): T-wCO2 cross-ections of the melting point data of the system AMF:CO2, for a range of pressuresetween 2 and 12 MPa.

ere measured at temperatures below the SCP point. This meanshat, at those conditions and in equilibrium, a solid phase shouldave been present in the mixture as well. The absence of this solidhase during the measurement of L–V points indicates that a higheregree of undercooling (i.e. higher supersaturation) was necessaryor the formation of the first fat crystals. Therefore, the liquid fat-ich phase was at these conditions in metastable equilibrium. Thisituation of metastability did not seem to affect the slope of the L–Vurves.

The P/wCO2 cross-sections of the L–V and L–L equilibrium datarom Fig. 2 are shown in Fig. 3(a). Equilibrium pressures areere plotted as a function of CO2 concentration, for temperaturesetween 303 and 343 K. The graph was constructed by fitting the/T data to a quadratic polynomial. The pressures correspondingo temperatures of 323, 333 and 343 K at a CO2 concentration of0 wt% were too high to be measured experimentally, so these dataoints were extrapolated. In general, it can be observed in Fig. 3(a)hat the difference between the bubble pressures measured at dif-erent temperatures was larger the higher the CO2 concentration,

CO2 , in the melt.The P/wCO2 cross-sections reflect a reasonable correlation

etween the equilibrium points measured for different CO2 con-entrations. Deviations are likely to occur when measuring phasequilibria of natural compounds, such as the fats used in this work.his is a consequence of slight difference in the molecular compo-

ition of the fat samples used in each measurement series.

Fig. 2 also shows the solid–liquid curves for the AMF:CO2 sys-ems at different pressures. The melting temperatures (measuredere as SCP – solid clear point temperatures, which are those at

2 2

the fat–CO2 mixture. The L–V or L–L curves represent the boundary at which twophases (L+V) become a single L or V phase. The SCP curves represent the boundaryat which two phases (L+S) become a single L phase.

which the visually detectable solid phase disappeared) increasedslightly with pressure for any given CO2 concentration. Meltingtemperatures decreased with the concentration of CO2, as it canbe observed from the shift of the SCP curves to the left at increas-ing CO2 concentrations, which can be explained as melting pointdepression. The level of depression is comparable to those observedby other researchers in gas-saturated fat systems [7,18,19].

The melting point depression of our system can be clearly visual-ized in Fig. 3(b), where the T-wCO2 cross-sections of the measuredSCP curves are shown. The melting point decrease was consider-able: at 12 MPa, for example, the melting point was reduced from312.4 K to 299.3 K (13.1 K) by incorporating 30 wt% of CO2 in the fat.

3.2. Phase equilibrium of the system D-AMF:CO2

The liquid–vapor (or liquid–liquid) and SCP curves of the systemD-AMF:CO2 are plotted in Fig. 4 for a range of CO2 concentra-tions, wCO2 . Similarly to the AMF:CO2 system, the bubble pressureP increased with the temperature T and with the CO2 concentra-tion, wCO2 . For a concentration of 20 wt% CO2, the equilibrium curvemeasured was liquid–liquid.

The L–V and L–L equilibrium pressures measured for eachgiven (mass) concentration of CO2, wCO2 , were higher in the D-AMF:CO2 system than in the AMF:CO2 system. This differencecan be observed clearly by comparing the P/wCO2 cross-sectionsof the L–V and L–L equilibrium data for each system, shown inFigs. 3(a) and 5(a) respectively. For example, at a CO2 concentra-tion of 20 wt%, a single homogeneous phase was created in theD-AMF:CO2 system at 14 MPa, while the system AMF:CO2 becamehomogeneous at 12 MPa (value interpolated from the fitted poly-nomial curve). D-AMF is rich in partial acylglycerols, which containpolar hydroxyl groups. These polar groups could decrease thecapacity of the fat for dissolving CO2.

A L–V equilibrium point at a temperature below the SCP was alsomeasured in this system, in the series at 10 wt% CO2. It seems thatthe metastability of the liquid phase could have affected the slopeof the bubble-point curve, as the L–V point was found at a pressureslightly below what could be expected from the extrapolation ofthe rest of the curve. The possible presence of a small amount of fatcrystals could have been responsible for lowering the solubility ofCO2 in the fat.

Melting point depression was again observed, as indicated in

Fig. 4 by the shift to the left of the SCP curves at increasing CO2concentrations, similarly to the AMF:CO2 system. Fig. 5(b) showsthe T-wCO2 cross-sections of the clear point curves of the sys-tem D-AMF:CO2. The melting point decreased from 318.1 K to

M. Lubary et al. / J. of Supercritical Fluids 55 (2011) 1079–1088 1083

Fig. 5. (a) (up): P-wCO2 cross-sections of the L–V and L–L data of the systemDrma

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Table 1Summary of experimental conditions in ScMM experiments performed at differentCO2 concentrations for each fixed temperature. CO2 concentration values (wCO2 )were calculated from the measured L–V data. Extrapolated wCO2 values are indicatedwith an asterisk.

Experiment Material Tmelt [K] Pdiss [MPa] Patom [MPa] wCO2 [wt%]

A-1 AMF 334 7 7 9.4A-2 AMF 334 10 10 15.3A-3 AMF 334 15 15 26.9*A-4 AMF 325 7 7 9.9A-5 AMF 325 10 10 18.0A-6 AMF 325 15 15 32.3*A-7 AMF 315 7 7 11.4A-8 AMF 315 7 15 11.4A-9 AMF 315 10 10 23.0A-10 AMF 315 15 15 >35*A-11 AMF 315 20 20 >35*A-12 AMF 306 7 7 15.0A-13 AMF 306 10 10 32.0A-14 AMF 306 13 13 >35*A-15 AMF 306 15 15 >35*D-16 D-AMF 333 10 10 15.5

-AMF:CO2, for a range of temperatures between 303 and 343 K. Data points sur-ounded by circles were extrapolated. (b) (down): T-wCO2 cross-sections of theelting point data of the system D-AMF:CO2, for a range of pressures between 2

nd 12 MPa.

03.6 K (14.5 K) at 12 MPa, by dissolving 20 wt% CO2 in the fat. Thelopes of the melting point vs. CO2 concentration curves shown inig. 5(b) are more pronounced than the corresponding curves forhe AMF:CO2 system, presented in Fig. 3(b), showing that meltingoint depression was stronger for the modified, diacylglycerol-ased D-AMF than for AMF. This effect can be attributed to differentpecific chemical interactions between each of the fats and CO2,iven the high content of D-AMF of polar hydroxyl groups.

.3. Micronization of AMF and D-AMF

Table 1 summarizes the ScMM experiments carried out onMF and D-AMF at different temperature of the melt, Tmelt, CO2issolution pressure, Pdiss, and atomization pressure Patom. The

iquid–vapor (or liquid–liquid) equilibrium curves measured at dif-erent CO2 concentrations were used to estimate the concentrationf CO2 (wCO2 ) in the fat phase at each combination of pressure (Pdiss)nd temperature (Tmelt) applied. Some combinations of Pdiss andmelt used for ScMM exceeded the conditions measured in the Cail-etet tube. For those combinations the equilibrium concentration ofO2 was extrapolated from the data available, and should be con-idered as just indicative. These values are indicated in Table 1 withn asterisk. Several additional experiments, not listed in Table 1,ere carried out as duplicates at selected process conditions to

heck the reproducibility of the obtained particles at given condi-

ions. Practically identical particle properties were obtained fromhe duplicated ScMM experiments, demonstrating a high experi-

ental reproducibility.Two different particle morphologies, and mixtures thereof, were

dentified in the fat powders obtained by ScMM: hollow spheres

D-17 D-AMF 333 13 13 20.1D-18 D-AMF 324 7 7 11.9D-19 D-AMF 323 10 10 17.1

in an approximate size range 10–40 �m (Fig. 6(a)), and a mass ofsmall broken particles with a sponge-like appearance (Fig. 6(c)).Despite the agglomerated appearance of the sponge-like particlemorphology on the SEM images, the powders were observed to befree-flowing at a macroscopic level, provided that they were keptat subzero temperatures. Mixtures of smaller hollow spheres andbroken microparticles were obtained in some of the micronizationruns (Fig. 6(b)). In the following paragraphs, the effect of the ScMMprocess conditions (Tmelt, Pdiss, Patom, and wCO2 ) on the morphologyof the particles obtained is discussed.

3.3.1. Effect of pressure (Patom and Pdiss)In the ScMM experiments, as described in Section 2, we loaded

an amount of molten fat in the pressure vessel and added CO2 sub-sequently until a certain pressure (Pdiss) was reached. The pressurein the vessel during atomization, Patom, was the same as the disso-lution pressure Pdiss. The concentration of CO2 in the melt, wCO2 ,was a function of both the temperature of the melt Tmelt and thedissolution pressure Pdiss.

Fig. 7 shows the obtained particle morphology (hollow spheres,sponge-like mass and mixtures of both) as a function of pressureand concentration of CO2 in the melt. The hollow sphere morphol-ogy (circles) was obtained only at the lower end of pressures/CO2concentrations, while sponge-like particles (squares) were alwaysproduced above 20 wt% CO2, regardless the temperature or pres-sure applied. These results seem to indicate that the concentrationof CO2, wCO2 , was the most relevant parameter influencing the par-ticle morphology, and that the effect of pressure is mostly limitedto its influence on wCO2 .

According to the observations of other researchers, the atomiza-tion pressure is not a relevant variable influencing the properties ofthe produced particles [11]. In order to discard a possible effect ofthe atomization pressure Patom on the particle morphology, a pairof ScMM experiments was performed at identical dissolution pres-sure Pdiss and CO2 concentrations wCO2 , but different atomizationpressures Patom (experiments A-7 and A-8 in Table 1 and Fig. 7). A-7 was the “reference” experiment, while the atomization pressurewas increased in A-8 immediately before spraying. The powders

obtained in the two experiments had a very similar appearance:hollow spheres of 10–30 �m, surrounded by sponge-like mass wereproduced in both cases. The measured bulk density of the powderswas practically the same (85 and 89 kg/m3). These results, in line

1084 M. Lubary et al. / J. of Supercritical Fluids 55 (2011) 1079–1088

Fig. 6. Micrographs of different powder morphologies produced by ScMMmicronization of AMF. (a) Exp. A-1 (9.4 wt% CO2, 334 K): hollow spheres. (b) Experi-ment A-7 (11.4 wt% CO2, 315 K): mixture. (c) Experiment A-10 (>35 wt% CO2, 315 K):sponge-like mass.

Fig. 7. Particle morphology as a function of pressure and CO2 concentration wCO2 .

Fig. 8. Micrographs of AMF powders with hollow sphere morphology, produced byScMM at varying CO2 concentrations. (a) Exp. A-1 (9.4 wt% CO2). (b) Experiment A-5(18.0 wt% CO2).

with the observations of Munuklu and Jansens [11], indicate thatpressure has by itself little effect on the morphology and qualityof the powders produced by ScMM. The effect of pressure on thepowder properties is mostly connected to the amount of CO2 thatis dissolved in the fat (wCO2 ).

The SEM micrographs suggest as well that the average size ofthe hollow spheres decreased at increasing concentrations of CO2in the fat melt (Fig. 8). A larger amount of small hollow particles canbe for example observed in Fig. 8(b) (18.0 wt% CO2), as compared toFig. 8(a) (9.4 wt% CO2). The decreased viscosity and surface tensionof the melt at high CO2 concentrations could explain the formationof smaller fat droplets during the atomization process.

3.3.2. Temperature of the melt (Tmelt)In Fig. 9, the morphology of the micronized fat powders is plot-

ted vs. the temperature of the melt Tmelt and the concentration ofCO2, wCO2 . A certain effect of the temperature, next to the alreadydiscussed effect of the CO2 concentration, can be identified at rel-atively low wCO2 . As shown previously in Fig. 7, hollow spheres(circles) and the mixed morphology (stars) were only obtained atCO2 concentrations below 20 wt%. The data in Fig. 9 shows thatcombination of relatively low CO2 concentrations (below 16 wt%)and relatively high temperatures of the melt (above 315 K) ledto the hollow sphere morphology (circles). A mixture of spheresand sponge-like powder was obtained below 20 wt% CO2 whenthe temperature of the melt was lower than 315 K (stars). At CO2

concentrations above 20 wt%, sponge-like powders (squares) wereobtained, regardless the temperature of the melt.

The temperature of 306 K was slightly below the solid clear point(SCP) of the mixture for all pressures tested, therefore a small pro-portion of solid fat could have been present in the pressure vessel

M. Lubary et al. / J. of Supercritical

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ig. 9. Particle morphologies obtained by ScMM of AMF in a Tmelt. (temperaturef the melt) vs. wCO2 (CO2 concentration) plot. The labels indicate the dissolutionressure, Pdiss (MPa) at each experiment.

efore spraying. The similarity between the powders produced at06 and 315 K at all CO2 concentrations indicates that the possibleresence of solid fat at 306 K before spraying did not influence theroperties of the produced powder.

A micronization mechanism for ScMM was proposed byunuklu [5,11]. The mechanism, schematically represented in

ig. 10, explains the formation of completely solid spherical, hollowpherical and distorted, sponge-like particles depending on wCO2nd Tmelt. The particle morphology is the result of a competitionetween the solidification rate of the formed fat droplets and thescape rate of CO2. Both temperature and CO2 concentration mayave an additional effect on the particle formation process throughheir impact on relevant physical properties of the melt (viscositynd surface tension). Thus, at high CO2 concentration and temper-ture of the melt, the decrease in viscosity and surface tensionould facilitate the disintegration of the melt into droplets uponxpansion, leading to the formation of finer particles.

The proposed model can be qualitatively described as follows:

1) Atomization: in the first step, the fat melt saturated with CO2 is

broken into droplets upon expansion through the atomizationnozzle, under the influence of shear forces.

2) Surface solidification: the fat droplets cool down due to thetemperature decrease caused due to the expansion and the

Fig. 10. Different particles obtained by the ScMM technique, at increasing CO2

Fluids 55 (2011) 1079–1088 1085

evaporation CO2, and start to solidify from the surface. A solidcrust is formed on the particle surface.

(3) Three cases, leading to three main particle morphologies, canbe distinguished depending on both the (pre-expansion) con-centration of CO2 in the melt and the temperature of the melt.(a) In the absence of CO2, completely solid particles are formed,

at any Tmelt.(b) In the presence of a moderate CO2 concentration and not too

low Tmelt, the particles are inflated due to the evaporation ofCO2, leading to the formation of hollow spherical particleswith a solid fat crust. At high (pre-expansion) Tmelt, solid-ification takes a longer time, which may promote particleagglomeration, since the partially liquid droplets are likelyto stick together in case of collision.

(c) In the presence of high CO2 concentrations and rela-tively low Tmelt, solidification begins immediately after thedroplets are formed, and a solid crust is formed before all thedissolved CO2 can escape from the fat droplet. Under suchconditions, and depending on the permeability and the flex-ibility of the crust, the droplets will be deformed or evendisintegrated, leading to distorted spheres or fragmentedsmall particles. These fragments may have an agglomeratedappearance, leading to the sponge like morphology.

According to the above described particle formation mech-anism, the sponge-like broken particles are the result of thedisintegration of spherical hollow particles formed during atom-ization. The initial temperature of the melt determines thesolidification time of the droplet, and therefore the time availablefor CO2 to escape from the fat droplet without causing its breakage.Below 20 wt% CO2 in the melt, the hollow sphere morphology wasobtained as a result of the inflation of the particles while a solidcrust had not been yet formed. In this wCO2 range, the mixed mor-phology was obtained at the lowest temperatures due to partialbreakage of the particles. Above 20 wt% CO2, only broken particleswere found at any temperature, because the concentration of CO2was too high for evaporating before the fat solidified.

3.3.3. Micronization of D-AMFScMM experiments were done using D-AMF, at temperatures

Tmelt between 323 and 333 K and dissolution pressures Pdissbetween 7 and 13 MPa. The corresponding CO2 concentration was

concentrations, wCO2 and/or decreasing temperatures of the melt, Tmelt.

1086 M. Lubary et al. / J. of Supercritical

Fvw

boli

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0.380 and 0.420 nm indicate that the metastable ˇ polymorph waspredominant in all AMF powders [20]. The considerable width ofthe peak around 0.420 nm possibly indicates the additional pres-ence of the ˛ polymorph, which shows a single peak at 0.415 nm.A small shoulder at d=0.460 nm, characteristic of the ˇ polymorph,

ig. 11. Micrographs of the D-AMF powder produced by SCMM micronization atarying CO2 concentrations and between 323 and 333 K. (a) Experiment D-18 (lowCO2 ). (b) Experiment D-17 (high wCO2 ).

etween 11.9 and 20.1 wt%. Similar observations as in the casef AMF were done: at relatively low CO2 concentrations, hol-ow spheres were formed, while a sponge-like mass appeared atncreasing CO2 concentrations, as shown in Fig. 11.

Some differences were observed between the D-AMF and AMFowders generated at similar conditions. In general, the D-AMFarticles were smaller in size than their AMF counterparts, andlso seemed to be more sensitive to disruption. For example,rom the pair D-16 (D-AMF) and A-2 (AMF), both generated at33 K and a wCO2 around 15.5 wt%, the AMF powder consisted ofhole spheres of about 30 �m, while the D-AMF powder con-

ained partially disrupted spheres with a diameter up to 20 �m.possible reason for this effect could be the faster solidifica-

ion rate of D-AMF as compared to AMF, derived from its higherCP.

Another interesting observation is the fact that D-AMF particleshowed a higher number of holes in the surface than AMF particles.hese punctured D-AMF particles can be observed in both Fig. 11(a)nd (b). This could be an indication that more CO2 bubbles wereormed inside the D-AMF fat droplets upon expansion, as comparedo the AMF droplets, meaning that the driving force for CO2 releaserom the fat was higher for D-AMF than for AMF. This can be relatedo a lower (mass) solubility of the supercritical gas in D-AMF, whichs in agreement with the phase equilibrium data discussed in therevious section. For identical concentrations of CO2 in D-AMF andMF melts, the relative gas supersaturation is higher for the first

han for the last.

In conclusion, the concentration of CO2 in the melt before

xpansion (wCO2 ) was the main factor influencing the size and mor-hology of the particles produced by ScMM. The temperature ofhe melt (Tmelt) had a secondary effect, while the pressure (Patom)

Fluids 55 (2011) 1079–1088

was shown to have no effect on the particle morphology beyond itsinfluence on the CO2 concentration in the fat.

3.3.4. Properties of the powders3.3.4.1. Temperature stability. Both AMF and D-AMF powders wereobserved to be temperature-sensitive and tended to loose theirstructure and become sticky and agglomerated when exposed toambient temperature. This heat sensitivity is due to the low melt-ing temperature of AMF and D-AMF, which show a melting rangebetween −40 ◦C and 40 ◦C roughly. The properties of the powderswere well preserved for months at a storage temperature up to-18 ◦C, indicating that the percentage of liquid fat up to this tem-perature is too low for interfering with the structure of the particles.

3.3.4.2. Polymorphism. Three main polymorphs are known for fats:˛, ˇ′ and ˇ in order of increasing stability [21]. The most stable poly-morphs are usually formed at slow crystallization rates, and viceversa [22]. The metastable ˇ′ is the desired polymorph in crystal-lized milk fat products, because it forms small crystals providing apleasant mouthfeel [21,23]. This polymorph is commonly obtainedby controlled crystallization of the fat melt.

Powders of different morphologies (small broken particles andhollow spheres) were analyzed using X-ray diffraction (XRD), forelucidating if different polymorphs could have been formed dur-ing ScMM. For comparison, reference samples of AMF and D-AMF,crystallized in a cooling bath at a controlled cooling rate (1 ◦C/min)were analyzed by XRD as well. The results are shown in Fig. 12. TheAMF powders (Fig. 12(a)) showed an identical XRD pattern to thereference, cool-crystallyzed sample. The short spacing (d) peaks at

′

Fig. 12. (a) XRD pattern of AMF powders of different morphologies (A-10, sponge-like broken particles and A-2, hollow spheres). All powders showed a practicallyidentical pattern as the reference sample. (b) XRD of D-AMF powders of differ-ent morphologies (D-17, mixture of sponge and broken spheres, and D-18, hollowspheres).

M. Lubary et al. / J. of Supercritical

Fb

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ig. 13. Bulk density of the AMF powder (�) vs. CO2 concentration in the fat phaseefore spraying (wCO2 ).

ndicates that traces of ˇ were also present in the samples. It is pos-ible that the unstable ˛ polymorph nucleated initially in the ScMMowders due to the very fast solidification rate, and recrystallizeduring the sample storage (at a maximum temperature of −18 ◦C)o produce the detected mixture of polymorphs.

The XRD pattern of D-AMF samples (Fig. 12(b)) showed shortpacing peaks at 0.380, 0.415 and 0.455 nm. The peaks at 0.380 and.455 nm were more pronounced for the cool-crystallized, refer-nce sample (D-AMF, 1 ◦C/min). The interpretation of the XRD datas in this case more complex, since D-AMF was a mixture of TAG,AG and MAG (predominantly DAG) containing a wide variety of

aturated and unsaturated fatty acids. The predominant polymorpheems in this case to be an ˛ (d=0.415 nm), with some presencef ˇ or ˇ′ (0.380, 0.455 nm). A higher fraction of the ˇ and/or ˇ′

olymorph seems to be present in the melt-crystallized sample, asndicated by the higher intensity of the shoulder at 0.455 nm forhis sample. This was probably a consequence of the slower crys-allization rates at which the melt-crystallized was produced, asompared to the micronized samples.

As observed for the AMF powders, the same polymorphic formsn very similar proportion are found in D-AMF powders of differ-nt particle morphologies. The fact that the particle morphology isot associated to a certain polymorph does not contradict the pro-osed model for particle formation, according to which both typesf particles have the same origin.

.3.4.3. Bulk density. The bulk density � of the powders generatedn the ScMM experiments ranged between 40 and 90 kg/m3 or allMF and D-AMF powders. A certain correlation, although weak,as observed between the bulk density � and the concentration

f CO2 in the melt, wCO2 (Fig. 13). In general, the bulk densityeems to be lower the higher the CO2 concentration. The sponge-ike powders had in general a lower bulk density than the hollowpheres (40–75 kg/m3 vs. 60–120 kg/m3). This observation can beelated to the fact that the sponge-like morphology resulted in free-owing powders, while the hollow spheres, especially when mixedith sponge-like particles, had a tendency to agglomerate. This

gglomeration tendency can be explained considering that the hol-ow sphere morphology was generated at conditions such that theolidification rate of the fat droplets was slower than in the case ofponge-like particles. The generated particles could have attachedo each other due to collisions during the solidification process.

.4. Applications of the ScMM particles

The small, broken particle morphology with a sponge-likeppearance produced from AMF constitute a free-flowing powderf small particle sizes (<10 �m) of mostly ˇ′ and some ˛ polymorph.herefore they are suitable for its use as structuring agent in (refrig-

Fluids 55 (2011) 1079–1088 1087

erated or frozen) food products, considering both their particle sizeand polymorphism. They can also be dispersed in liquid foods as ameans of adjusting the viscosity of the liquid or creamy product [4].

D-AMF particles were probably produced as a mixture of ˛ and ˇpolymorphs. The desired polymorph for milk fat products is usuallyˇ′ but this need comes from the fact that ˇ′ has the optimal char-acteristics in terms of crystal size and melting behavior. By ScMM,D-AMF powders can nevertheless be produced in a small enoughparticle size range as to provide food products with a smooth tex-ture. The measured melting point (clear point) of D-AMF was about5 ◦C higher than that of AMF, which offers some versatility for theuse of one or other type of fat depending on the application require-ments.

As pointed out by Weidner [4], small fat particles melt ordissolve rapidly, due to their high surface-to-volume ratio. Thisproperty represents a technical advantage for the manufacture offoods in which a solid fat needs to be incorporated into a mix-ture, for example in the preparation of baked products or (milk)chocolate. In addition, from the point of view of the consumer appli-cation, rapid melting of fat particles in the mouth causes rapid flavorrelease which can be a desirable organoleptic feature [4].

The possibility of producing hollow spheres from AMF and D-AMF can be exploited, for example, for the generation of compositeparticles in which AMF or D-AMF are the coating (shell) materialthat encapsulates another substance. This can be for example anactive compound, such as caffeine [24] or theophylline [25]. Thelipid shell plays in this case the role of protecting the active sub-stance and facilitating its controlled delivery. Synthesis of suchcomposite particles can be achieved by a simple modification ofthe ScMM process described in this paper. The substance to beencapsulated, typically a hydrophilic liquid, is simply dispersedin the molten fat inside the pressure vessel, in which CO2 is thenincorporated. Upon expansion, the mixture is disintegrated in smallemulsified droplets, where the fat rapidly solidifies immobilizingthe substance inside [4,24,26]. This technique has also been appliedfor encapsulating seasonings, liquors, sauces and honey into fatshells [4].

4. Conclusions

Microparticles of anhydrous milk fat (AMF) and a DAG-basedmodified milk fat (D-AMF) were produced using the ScMM pro-cess. Both fats were able to dissolve considerable amounts of CO2,up to 30 wt% at 13 MPa. Melting point depression was observed inthe mixtures of each fat with CO2. ScMM was an effective tech-nique for producing microparticles of both AMF and D-AMF. ScMMexperiments were carried out varying the temperature of the melt(306–334 K) and the dissolution pressure (7–20 MPa). Two dif-ferent particle morphologies (spherical hollow and sponge-likeparticles) were obtained, depending on the CO2 concentration inthe fat melt in the first place, and on the (pre-expansion) tem-perature of the melt in the second place. Sponge-like particles,despite their agglomerated appearance on the SEM micrographs,were free-flowing at low temperatures (−18 ◦C) and had a lowerbulk density than hollow spheres, which showed a certain tendencyto agglomerate. The AMF particles were constituted of mainly the ˇ′

polymorph, regardless of their morphology. D-AMF seemed to crys-tallize predominantly as the ˛ polymorph. Possible applications ofthe produced powders are as a structuring agent in refrigeratedfood products or in partially solid dispersions or emulsions (partic-

ularly the sponge-like powders, due to their smaller particle size).The hollow particle morphology could be exploited for producingcomposite particles in which the fat solidifies as a shell around a liq-uid core material, which would be dispersed in the fat melt beforeexpansion.

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cknowledgements

The authors thank Eugene Straver (Delft University ofechnology, the Netherlands) and Gerben-Jan Boland (Feyeconevelopment & Implementation, the Netherlands) for the exper-

mental support, Michel van den Brink (Delft University ofechnology) for acquiring the SEM micrographs, and Ben NorderDelft University of Technology) and Patricia Heussen (Unilever&D, Vlaardingen, The Netherlands) for their assistance with XRDeasurements. SenterNovem (Dutch Ministry of Economic Affairs,

he Netherlands) is gratefully acknowledged for the financial sup-ort of this work.

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