Food Research International xxx (2009) xxx–xxx

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Food Research International

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Effect of the structure of monoglyceride–oil–water gels on aroma partition

Sonia Calligaris a,*, Sara Da Pieve a, Gianmichele Arrighetti b, Luisa Barba b

a Dipartimento di Scienze degli Alimenti, Università di Udine, Via Sondrio 2a, 33100 Udine, Italyb Institute of Crystallography, National Council of Research, 34100 Trieste, Italy

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

Article history:Received 24 August 2009Accepted 22 October 2009Available online xxxx

Keywords:Saturated monoglyceridesSelf-assembly structuresGelAroma partition

0963-9969/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.foodres.2009.10.011

* Corresponding author. Tel.: +39 0432558137; faxE-mail address: sonia.calligaris@uniud.it (S. Calliga

Please cite this article in press as: Calligaris, S., e(2009), doi:10.1016/j.foodres.2009.10.011

The aim of the present research was to study the aroma partition properties of saturated monoglyceride–oil–water gels. To this purpose, gels added with lemon oil were prepared mixing water, sunflower oil andcommercial saturated monoglycerides. Three mixtures containing different ratios between the ingredi-ents were considered. The gel characteristics were studied by synchrotron X-ray diffraction (XRD), calo-rimetry, polarized light microscopy and rheological measurements. The aroma partition was studied byusing HSGC analysis.

The gel structure is made of a multilamellar crystalline network, in which monoglycerides form shellsencapsulating oil. Depending on the ratio between ingredients, the gel shows different rheologicalbehaviour. The structured multilamellar phases greatly affect the partition of aroma evidencing thecapability to entrap lipophilic compounds within the monoglyceride/oil shells. Since the aroma entrap-ping capacity showed by monoglyceride structures could greatly affect the perceived aroma intensity ofthe final product, this effect should be carefully taken into account during the development of newformulations.

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1. Introduction

Nowadays one of the new challenges for the food industry is theproduction of ‘‘tailor-made” foods, which are foods able to fulfilspecific consumer needs. In particular, great attention is paid tofoods with peculiar sensorial performance. One of the most prom-ising strategies to produce foods really fulfilling the design charac-teristics is the exploitation of the self-assembly properties of polarlipids to form nano/micro-size structures. The latter consist ofnano-scaled hydrophilic and hydrophobic domains, which are sep-arated by the surfactant membrane. This compartmentalizationcan be used to introduce guest molecules of hydrophilic, lipophilicor amphiphilic nature (Mozafari et al., 2006; Sagalowicz, Leser,Watzke, & Michel, 2006; Weiss, Takhistov, & McClements, 2006).Hydrophilic molecules will be located close to the emulsifier polarhead or in the water domains, while lipophilic molecules will belocalized within the lipophilic domains and amphiphilic in theinterface (Sagalowicz et al., 2006).

Successful application of self-assembly structures as deliverysystems depends on the ability to incorporate the potential guestmolecules within the various phases in appropriate amounts, andto disperse them inside the food matrix without loosing theirexpected functionality (McClements, Decker, & Weiss, 2007;

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: +39 0432598130.ris).

t al. Effect of the structure of mo

Sonoda, Tanaka, Ueno, & Sato, 2006). The physical properties of lip-ids used to obtain these nanostructures greatly affect the chemicalstability as well as the rate of release of the compounds delivered(McClements et al., 2007).

Monoglycerides are among the most promising polar lipid com-pounds able to bring new or improved functionality to food prod-ucts since they can form self-assembly structures in both lipid andaqueous phases (Pernetti, van Malssen, Flöter, & Bot, 2007). In par-ticular, when mixed with water or/and oil under given physico-chemical conditions, commercial monoglycerides can organize bythemselves to obtain lamellar, cubic or hexagonal phases (Larsson,2009; Sagalowicz et al., 2006). Among different self-assemblyphases, the lamellar phase has been indicated as one of the mostpromising for food applications (Batte, Wright, Rush, Idziak, &Marangoni, 2007; Heertje, Roijers, & Hendrickx, 1998; Marangoni,Idizak et al., 2007). This is due to the fact that the lamellar phase,beside the possibility to act as delivery system, could give structureto the product thought the formation of a gel-like network. In par-ticular, a highly hydrated liquid crystalline lamellar phase, calledLa, is formed in the presence of water, anionic co-surfactant andsaturated monoglycerides at temperature above monoglycerideKrafft temperature (Heertje et al., 1998). When the material iscooled below this temperature a highly hydrated mesomorphicphase (named Lb or a-gel) appears, which is metastable and couldeventually transforms to an anhydrous crystalline phase namedcoagel (Krog & Larsson, 1968). If an oil phase is also present, the

noglyceride–oil–water gels on aroma partition. Food Research International

Table 1Composition and final concentration by weight of each constituent in gel A–Crespectively.

Ingredient A (% w/w) B (% w/w) C (% w/w)

Monoglyceride/co-surfactant 8.2 7.0 4.7Sunflower oil added with lemon oil 42.9 24.3 55.8Milli-Q water 48.9 68.6 39.5

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Lb phases could encapsulate oil creating a mesomorphic gel withfat-like properties (Batte et al., 2007; Marangoni, Batte, Wright,Rush & Idziak, 2007; Marangoni, Idizak et al., 2007).

A number of applications of monoglyceride self-assembly struc-tures in pharmaceutical industry have already been proposed (Sag-alowicz et al., 2006). In particular, these structures have beenadopted as drug delivery systems able to protect the encapsulatedguest molecules and modulate their release. In food industry,monoglyceride self-assembly structures have been proposed fordifferent applications: (i) as structuring phase in low-fat or low-saturated fat products (Batte et al., 2007; Heertje et al., 1998;Marangoni, Idizak et al., 2007; Rush Jantzi, Dupak, Idziak &Marangoni, 2008); (ii) as systems for controlling the release of vol-atile aroma compounds (Landy et al., 2007; Phan, Liao, Antille, Sag-alowicz, Robert, & Godinot, 2008; Vauthey, Visani, Frossand, Garti,Leser, & Watzke, 2000); (iii) as a reactor for the enhancement offlavour compound formation during heat treatments (Vauthey,Milo, Frossand, Garti, Leser, & Watzke, 2000).

The first two applications could be highly related. Since fats actas solvent of liphophilic aroma compounds, fat reduction/substitu-tion is expected to modify the aroma partition and release from thematrix, dramatically changing the perceived aroma intensity (Rob-erts, Pollien, & Watzke, 2003). An increase in the headspace con-centration of lipophilic aroma compounds when lowering fatcontent has been shown. This is due to the increase of aromaamount in the fat phase (Giroux, Perreaul, & Britten, 2007; RobertsPollien & Watzke, 2003; Schirle-Keller, Reineccius, & Hatchwell,1994; Seuvre, Diaz, & Voilley, 2000). In addition, the substitutionof lipids causing the change of the crystallisation degree (solid/li-quid ratio) could modify the aroma perception as a consequenceof an increase in aroma concentration in the liquid phase sur-rounding fat crystals (Roberts et al., 2003). Finally, changes in foodformulation lead to modifications of the product structure and ofthe interactions between volatile and non-volatile constituents.For these reasons, the effect of the fat substitution in low-fat prod-ucts on the aroma release should be carefully taken into account.

The use of monoglyceride self-assembly structures to controlthe aroma release in low-fat foods has been studied by someauthors. Vauthey, Visani et al. (2000); Vauthey, Milo et al.(2000) have shown that systems composed of 80% unsaturatedmonoglyceride and 20% water, in which the polar lipids organizedin the cubic phase, display weaker aroma retention capabilitythan a water-in-oil emulsion containing 80% of triglycerides. Onthe contrary, Landy et al. (2007) found an increase in aromaretention of lipophilic compounds in a system (30% unsaturatedmonoglycerides; 60% triglycerides; 10% water) containing thelamellar phase of unsaturated monoglycerides. Finally, Phanet al. (2008) studied the aroma retention properties of aromacompounds from the dispersed phase of self-assembly structuresformed by unsaturated monoglycerides in an emulsified watersolution. Beside these data, very few data are available on the ar-oma release properties of systems containing self-assemblymonoglyceride structures.

The aim of the present research was to study the aroma par-tition properties of saturated monoglyceride–oil–water gels. Tothis purpose, monoglyceride–oil–water gels added with lemonoil were prepared mixing water, sunflower oil and saturatedmonoglycerides. Since the structure of the gel phase is crucialto obtain products with the desired structure properties, mix-tures containing different ratios between the ingredients wereconsidered in order to obtain systems with different physicalproperties. The structure characteristics were studied by syn-chrotron X-ray diffraction (XRD), calorimetry, polarized lightmicroscopy and rheological analysis. The aroma release wasevaluated in pseudo-equilibrium conditions by using HSGCanalyses.

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2. Material and methods

2.1. Gel preparation

MyverolTM saturated monoglyceride (MG), kindly supplied byKerry Ingredients and Flavour (Bristol, UK), was mixed with a co-surfactant mixture of palmitic and stearic acid (1:1 w/w) (SigmaAldrich, Milano, Italy) in a ratio of 5:1 (w/w). The water phasewas composed of 1 mM NaOH in deionized milli-Q water (pH6.5) to promote the partial neutralization of the co-surfactant mix-ture and obtain a properly swollen phase (Batte et al., 2007). Thesunflower oil, purchased in a local market, was added with 0.82%(w/w) of commercial lemon oil (LO). The final concentrations ofthe constituents in the A–C systems are shown in Table 1.

The samples were prepared by mixing the water solution andMG/co-surfactant/oil phase previously heated at 70 �C in a waterbath. The samples were homogenized by using a high speedhomogenisator DI25 (Ika-Werke, Staufen, Germany) at20,000 rpm rate for 1 min. Finally, the mixtures were cooled at20 �C in a water bath.

2.2. Sunflower oil added with MG and hydrogel preparation

Sunflower oil added with 5.0% of the mixture of monoglycerideswas prepared by mixing the MG/co-surfactant and oil at 70 �C. Thesamples were then cooled at 20 �C in a water bath. By using thesame methodology, a system containing 5.0% (w/w) of the mixtureof monoglycerides and co-surfactants in 1 mM NaOH water solu-tion was prepared.

2.3. Emulsion containing lecithin preparation

A water/oil emulsion containing 55.8% (w/w) sunflower oiladded with lemon oil (0.82% w/w), 39.5% (w/w) of 1 mM NaOHwater solution containing 4.7% (w/w) lecithin was prepared. Emul-sification was performed by high speed homogenisator DI25 (Ika-Werke, Staufen, Germany) at 20,000 rpm rate for 1 min.

2.4. Analytical determinations

All samples were stored at 20 �C for 24 h before analyses. In ourexperimental conditions, limonene does not undergo oxidationduring storage.

2.5. Fatty acid content

The fatty acid composition (FA) of the MyverolTM was carried outaccording to the European Official Methods of Analysis (1991).

2.6. Static headspace analysis

Aliquots of 3 g of samples were inserted in 10 mL capacity vials,sealed with butyl septa and metallic caps. Samples were equili-brated for 24 h at 20 �C and 0.5 mL of headspace was injected. AHGRC Mega 2 Series gas chromatograph (Fisons Instruments, Mi-lan, Italy) equipped with a headspace sampler (Carlo Erba HS

noglyceride–oil–water gels on aroma partition. Food Research International

Table 2Fatty acid composition of the MyverolTM.

Fatty acid composition %

C14:0 1.4 ± 0.4C16:0 59.9 ± 0.5C18:0 38.7 ± 0.2

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250, Carlo Erba Strumentazioni, Milan, Italy) and a thermal con-ductivity detector (Fisons HWD Control, Fison Instruments, Milan,Italy) was used. The separation was done by using a capillary col-umn CP Wax 52 CB (50 m � 0.32 mm � 0.40 lm film thickness)(Chrompack, Middelburg, Netherlands). The injector temperaturewas set at 200 �C and helium (1.7 mL/min linear speed) was thecarrier gas. The oven temperature was maintained at 60 �C for6 min and programmed to 200 �C at 5 �C/min. The chromatogramswere integrated using Chromcard (Ver. 1.18, 1996, CE Instrument,Milan, Italy) chromatography data system software. The identifica-tion of limonene was done by using the limonene standard (SigmaAldrich, Milano, Italia).

2.7. Differential scanning calorimetry

Calorimetric analysis were made using a TA4000 differentialscanning calorimeter (Mettler-Toledo, Greifensee, Swiss) con-nected to a GraphWare software TAT72.2/5 (Mettler-Toledo, Grei-fensee, Swiss). Heat flow calibration was achieved using indium(heat of fusion 28.45 J/g). Temperature calibration was carriedout using hexane (m.p �93.5 �C), water (m.p. 0.0 �C) and indium(m.p. 156.6 �C). Samples were prepared by carefully weighing15–20 mg of the sample in 160 lL aluminum DSC pans, closedwithout hermetic sealing. An empty pan was used as a reference.Samples were heated under nitrogen flow (0.5 mL/min) from 20to 80 �C at 5 �C/min. The start of the melting transition was takenas on-set (Ton) points of transition, that are the points at which theextrapolated baseline intersects the extrapolated tangent of thecalorimetric peak in the transition state. Results were normalizedto account for the weight variation of the samples. Total peak en-thalpy was obtained by integration. The program STAR ever. 8.10(Mettler-Toledo, Greifensee, Swiss) was used to plot and analyzedthe thermal data.

2.8. X-ray diffraction

X-ray diffraction patterns were recorded at the X-ray Diffractionbeam-line 5.2 at the Synchrotron Radiation Facility Elettra locatedin Trieste (Italy). The X-ray beam emitted by the wiggler source onthe Elettra 2 GeV electron storage ring was monochromatized by aSi(1 1 1) double crystal monochromator, focused on the sampleand collimated by a double set of slits giving a spot size of0.2 � 0.2 mm. A drop of sample was lodged into a nylon pre-mounted cryoloop 20 micron for crystallographic experiments(0.7–1.0 mm) (Hampton Research HR4-965, Aliso Veijo, CA, USA).Sample temperature was controlled by means of a 700 series cryo-cooler (Oxford Cryosystems, Oxford, UK) with an accuracy of �1 �C.Analysis were performed at 20 �C. Data were collected at a photonenergy of 7.72 keV (k = 1.52 Å), using a 165 mm CCD-based X-rayDetector System (MarCCD) (Marresearch Gmbh, Norderstedt, Ger-many). As the intensity of synchrotron radiation beam decreasesduring the experiment, every diffraction pattern was collectedrotating the sample for 0.02� at variable rotation speed, underthe condition that the dose of photons absorbed by the samplewas the same for every pattern. Several tenth of bidimensional pat-terns collected with the MarCCD were calibrated and integratedusing the software FIT2D (Hammersley, Svensson, Hanfland, Fitch,& Hausermannm, 1996). The analysis of the XRD patterns obtainedby the crystalline phases was performed using the program Win-plotr (Roisnel & Rodriguez-Carvajal, 2000).

2.9. Rheology

Storage module (G0) and loss module (G00) were measured bysmall amplitude dynamic measurements using the rheometerStresstech Rheometer (Reologia Instruments AB, Lund, Sweden)

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with application software Stresstech v.4. The experiment was per-formed in a 40 mm parallel-plate geometry system. The tempera-ture control was obtained by using a circulating coolantconnected to a thermostat. The sample was mounted on the tem-perature-controlled measuring plate. To determine the linear vis-cosity range for the samples, dynamic stress sweep measurementat a frequency of 1 Hz from 0.01 to 100 Pa was conducted at20 �C. G0 and G00 moduli were obtained for a frequency scan from0.1 to 10 Hz using a fixed stress value of 20 Pa.

2.10. Microscopy

One drop of the gel, approximately 20 lm, was placed in themiddle of a glass slide and a glass cover slip was centered abovethe drop. The gels were observed using 1000� objectives with aAxiophot (Zeiss, Okerkochen, Germany) connected with aphotocamera.

2.11. Data analysis

Each sample was analyzed in triplicate. All results are shown asmean and standard deviation. One-way analysis of variance wascarried out, and significance between means was determined usingthe Tukey test (Statistica 6.0, StatSoft Inc., 2001). Linear regressionanalysis by least squares regression was performed by S-PLUS(Statistica 6.0, StatSoft Inc., 2001) and the goodness of fitting eval-uated on the basis of statistical parameters of fitting (R2, p) and theresidual analysis.

3. Results and discussion

3.1. Gel structure characterization

The main fatty acid of MyverolTM is the palmitic acid, followed bythe stearic acid (Table 2). Low quantities of the myristic acid arealso present. The melting temperature of the product, determinedby calorimetric analysis, is 68.5 ± 0.5 �C.

Although each sample appears as a white gel at a macroscopiclevel, differences in the microstructures are observed. Fig. 1 showsthe microstructure of the monoglyceride–oil–water gels consid-ered. As well explained by Batte et al. (2007) and Marangoni, Idizaket al. (2007), monoglyceride–oil–water gel is an oil-in-water emul-sion in which monoglycerides self-assemble to form walls sur-rounding oil droplets. The walls are formed of lamellar MGmultilayers, in which saturated MG are crystallized.

The gel microstructure could be related to the ratio betweenMG and oil: as the MG/oil ratio increases (from 0.08 of gel C to0.19 of gel A and finally to 0.29 of gel B) the number of oil dropletsalso increases as well as their size decreases. This is attributable tothe fact that a higher amount of the MG is available respect to theoil quantity for the stabilization of the increasing surface area asso-ciated with smaller oil droplets.

Differences among samples are also evidenced in the rheologi-cal behaviour. In Table 3 the rheological parameters of gels are pre-sented: G0 is in all cases higher than G00 confirming the gel-likebehaviour of the systems. As reported by Batte et al. (2007), theelasticity of the system is determined by the network of crystal-

noglyceride–oil–water gels on aroma partition. Food Research International

Fig. 1. Microscographs of the gels A–C.

Table 3G0 , G00 at a frequency of 1 Hz of gel A–C.

Gel G0 � 104 G00 � 104

(Pa) (Pa)

A 4.52 ± 0.03a 2.79 ± 0.06a

B 1.30 ± 0.02b 0.67 ± 0.01b

C 1.09 ± 0.03c 0.48 ± 0.03c

a,b,c: Means with the common letter in the same column are not significantly dif-ferent (P < 0.05).

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lized monoglyceride vesicles containing oil. The rheological param-eters of gel A are dramatically higher than that of other gels. Thismeans that the highest MG content induces the formation of agel network stronger than that of the other gels. As the MG contentdecreases, G0 and G00 also decrease. However, it should be notedthat the rheological parameters are not correlated to the MG con-tent. This is probably attributable to the complex interactionsamong the phases.

Synchrotron X-ray diffraction (XRD) analyses are performed todescribe and compare the lamellar structures of gels. Fig. 2 showsthe patterns recorded during the diffraction experiment performedat 20 �C. The feature of X-ray diffraction patterns is similar to thosereported in literature for other monoglyceride–oil–water gels(Batte et al., 2007; Marangoni, Idizak et al., 2007).

All samples show broad diffraction peaks at about 4.50 and21.27 Å. These bumps are associated with the short-range organi-zation of the triacylglycerol molecules of the liquid sunflower oil.These results are in agreement with data reported by Calligaris,Arrighetti, Barba, and Nicoli (2008) on the phase transition behav-iour of bulk sunflower oil. These authors observed two broad peaksat similar interplanar distances associated to the diffraction of theliquid oil.

Beside the bumps, the patterns have a wide angle diffractionpeaks at 4.52 and 4.11 Å which can be associated to the in-planechain ordering of monoglyceride crystals in the Lb phase, as clearlyevidence by Marangoni, Idizak et al., 2007. The XRD results of amonoglyceride–water gel confirm this hypothesis. As shown inthe inset of Fig. 2, the wide angle diffraction peak at 4.52 and4.11 Å are characteristic of the pattern of MG lamellas indicatingthe formation of the Lb phase. Since there are not significant differ-ences between the d spacing of the MG–oil–watergels in wide an-gle region, it is likely that the gels show the same in-planemolecule ordering.

On the contrary, differences in the diffraction patterns havebeen evidenced in the small angle reflection region. In fact, gel Ashows one single diffraction peak at 47.96 Å, gel B one at 62.83 Åand gel C both two diffraction peaks. The other peaks observed athigher q values correspond to higher order reflection of the samerepeating distance.

Since the interplanar distance in small angle region representsthe width of the monoglyceride bilayers (Marangoni, Idizak et al.,2007), the results obtained seems to indicate that the lamellarphases formed in our systems could have two different widthsdepending on system composition. In particular, the system withthe highest MG content is the one with the lowest bilayer width.Since the width of the lamellar phase is associated to the swollenof the bilayer with water, it is likely that the lamellas in Gel Aare the less swollen and have similar characteristics than thoseform by MGs in water (see inset Fig. 2). On the contrary, the gelB, containing the highest ration between MGs and oil (0.29), showsonly the formation of lamellas with a width of 62.83 Å, evidencingthat the layers incorporate more water. Finally, gel C having thelowest MG/oil ratio (0.08) presents both bilayer structures.

Differential scanning calorimetry measurements are also car-ried out on samples to identify the melting temperature and theenthalpy of the transition. As shown in Table 4, there were no sig-nificant differences between sample melting temperature(P > 0.05). These results could be associated to the fact that gelshave similar lamellar structure, as previously shown by the resultsof XRD analysis. On the contrary, as expected, the melting enthalpyresulted well correlated to the MG content (R2 = 0.983; P < 0.05). Aswell known, the transition enthalpy is associated with the quantityof crystallised matrix.

On the basis on these results, it can be concluded that, even ifthe lamellar structure of the gels is similar, the structure propertiesof systems with different MG–oil–water ratios are greatly different.

noglyceride–oil–water gels on aroma partition. Food Research International

0

10000

20000

30000

40000

50000

60000

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

q (A-1)

Inte

nsity

(arb

itrar

y un

its)

4.11 Å MG4.50 Å

(oil)

21,27 Å(oil)

62,83 Å

47,89 Å

4.52 Å MG

0

5000

10000

15000

20000

25000

30000

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0q (A-1)

Inte

nsity

(arb

itrar

y un

its) 47,89 Å

4,50 Å4,11 Å

Fig. 2. Synchrotron X-ray diffraction scans of monoglyceride–sunflower oil–water gel A–C stored at 20 �C. Inset: Synchrotron X-ray diffraction scans of monoglyceride–watergel containing 5% (w/w) of monoglycerides stored at 20 �C.

Table 4Onset temperature (Ton) and enthalpy (DH) associated with the melting of the gelA–C.

Gel Ton (�C) DH (J/g)

A 53.9 ± 0.5a 4.14 ± 0.15a

B 53.7 ± 0.3a 3.30 ± 0.10b

C 52.4 ± 0.8a 0.36 ± 0.09c

a,b,c: Means with the common letter in the same column are not significantly dif-ferent (P < 0.05).

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Increasing the MG concentration, the gel network becomes stron-ger. However, the prediction of the gel structure is not easy, prob-ably due to the complex interactions between the phases. Thus, theevaluation of the physical properties of gels during the productdevelopment phase becomes essential to obtain the desired struc-ture properties of the food product.

3.2. Aroma release

In order to study the effect of the structure on the aroma re-lease, the gel A–C were added with the same concentration of lem-on oil in the lipid phase. Lemon oil has been considered since it isthe most common ingredient used to give lemon flavour to foodproducts. The headspace of lemon oil is rich in aroma compounds.The most abundant and characteristic is the terpene derivativelimonene (Schieberle & Grosch, 1988). For this reason, limonene

Please cite this article in press as: Calligaris, S., et al. Effect of the structure of mo(2009), doi:10.1016/j.foodres.2009.10.011

has been selected as target aroma compound. The headspace con-centration of limonene was detected by using HSGC analysis. Table5 shows the headspace peak area, obtained in pseudo-equilibriumconditions, of limonene in gel A–C. In addition, the limonene con-centration (%) relative to sunflower oil (100% reference) is alsoreported.

It can be observed that the limonene concentration in the head-space above gels resulted significant lower than that above thesunflower oil. In addition, the headspace concentration of limo-nene above the gels is similar evidencing that the gels, even ifshowing different in structure, affect the aroma partition in thesame manner. Results are in agreement with those reported byLandy et al. (2007), who found aroma retention in emulsions con-taining lamellar phases of unsaturated monoglycerides. The de-crease in the headspace concentration of aroma compounds ofgelled systems was also found in gels containing other hydrocol-loids such as proteins, pectins and polysaccharides (Karaiskous,Blekas, & Paraskevopoulou, 2008; Monge, Negri, Giacomazza, & Bu-lone, 2008; Renard, van de Velde, & Visschers, 2006). The mecha-nism of the reduction in flavour perception in these systems isnot well understood. Two main mechanisms have been proposed:(i) the increase of the viscosity of the product by lowering the masstransfer; (ii) the binding of flavour to hydrocolloids. In our experi-mental conditions, the changes in gel rheological properties seemto not affect the aroma partition. In fact, even if gel A has the high-est values of the rheological parameters, it shows similar aromapartition respect to other samples.

noglyceride–oil–water gels on aroma partition. Food Research International

Table 5Headspace peak area of limonene, relevant standard deviation and limonene partition(%) in gel A–C, the emulsion containing lecithin and oil added with MG relative tosunflower oil.

System Headspace peakarea

Standarddeviation

Aromapartition (%)

Sunflower oil(reference)

379244a 28,271 100

Gel A 204332b,c 14,945 53.9Gel B 173818b 13,861 45.8Gel C 245336c 15,665 66.8Sunflower oil added

with MG456745d 16,785 120.4

Emulsion with lecithin 345387a 35,170 91.5

a,b,c,d: Means with the common letter in the same column are not significantlydifferent (P < 0.05).

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In order to evaluate if the self-assembly structures of monogly-cerides create specific interactions with limonene, sunflower oiladded with MG in the same concentration used for the preparationof gel C has been considered. The MG crystals in oil also tend toorganize giving a solid-like aspect to the sunflower oil (Pernettiet al., 2007).

As shown in Table 5, the percentage of aroma in the headspacewas higher than that found in sunflower oil. This is probably due tothe increase in the limonene concentration in the liquid oil afterthe formation of self-assemble MG crystals. This effect of lipid crys-tallization has been previously described in other systems with dif-ferent level of crystallization (Roberts et al., 2003). Results allowsupposing that no specific interactions between MG and limoneneare involved in aroma retention.

According to McNulty and Karel (1973), the release of hydro-phobic compounds in the headspace is performed in two steps:transfer from oil to water and then from water to the air phase.To evaluate the influence of the water phase on aroma partition,the headspace concentration of limonene above an emulsion con-taining the same concentration of sunflower oil and water thangel C, but lecithin as emulsifier was measured. Results reportedin Table 5 shows that the partition percentage is not significant dif-ferent (P < 0.05) from that of the bulk oil. Limonene is free to dif-fuse in the water phase and then in air. These results suggestthat the bilayers formed by MG around oil droplets play a roleon aroma partition completely different to that of the interfaciallayer of lecithin. It can be hypothesized that the self-assemblystructures formed by polar lipids separate the hydrophilic domainfrom the hydrophobic one hindering the diffusion of lipophiliccompounds solubilized in the lipid matrix. In other words, theMG lamellas, which expose their lipophilic domains to the oilphase and their hydrophilic domains to the water phase, slowdown the rate of aroma partition creating a diffusion limitingstructure. The similar limonene release of the gels consideredcould be attributed to the fact that the lamellar structure is compa-rable as shown by the XRD and DSC analysis.

4. Conclusion

Results obtained provide information useful in the task of appli-cation of monoglyceride gels in food formulation. The gelled struc-ture greatly affects the aroma partition of hydrophobiccompounds. The release results associated to the formation of li-quid-crystalline lamellar bilayers as a consequence of self-assem-bly of monoglycerides. In particular, the MG lamellar structureaffects the rate of diffusion of limonene from the oil to the waterphase by creating a diffusion limited structure. On the contrary,the rheological properties of the systems appear not so importantin determining the flavour release in the headspace. Gels with dif-

Please cite this article in press as: Calligaris, S., et al. Effect of the structure of mo(2009), doi:10.1016/j.foodres.2009.10.011

ferent visual and texture properties could have a similar flavour.This means that the aroma entrapping capacity showed by MGlamellas could greatly affect the perceived aroma intensity of thefinal product. This effect should be carefully taken into accountduring the development of new formulations containing MG–oil–water gels.

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