Phase Equilibrium of Fats and Monoterpenes and How It Affects

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data 2019, 64, 3231−3243

Phase Equilibrium of Fats and Monoterpenes and How It Affects Chocolate Quality Published as part of the Journal of Chemical & Engineering Data Latin America special issue. Renata Costa Di Prinzio,† Paula Virginia de Almeida Pontes,† Mariana Conceiçaõ da Costa,‡ Antonio José de Almeida Meirelles,† Eduardo Augusto Caldas Batista,† and Guilherme José Maximo*,† Downloaded via NOTTINGHAM TRENT UNIV on August 13, 2019 at 07:05:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

EXTRAE, Department of Food Engineering, School of Food Engineering, University of CampinasUNICAMP, Campinas, São Paulo, 13083-862, Brazil ‡ LEF, Department of Process and Product Development, School of Chemical Engineering, University of CampinasUNICAMP, Campinas, São Paulo, 13083-852, Brazil ABSTRACT: Due to triacylglycerol (TAG) polymorphism in cocoa butter, processes conditions or storage, chocolate can present a defect called “fat bloom”, when the fat liquid fraction migrates to the chocolate surface resulting in a whitish and brittle appearance. The addition of essential oils, rich in monoterpenes, has been studied as an alternative to avoid it. Some theories have been considered to explain this phenomenon but no one elucidated what kind of thermodynamic interactions could rule such an effect. Therefore, this work was aimed at describing the solid−liquid equilibrium (SLE) behavior of monoterpenes + TAG mixtures, in order to comprehend how monoterpenes could affect the solid phase of a fat. Experimental SLE data were measured by differential scanning calorimetry and optical microscopy and also modeled by the SLE theory. Results showed that monoterpenes could promote significant changes in the TAG crystalline habit by forming solid solutions (SS). This profile varied with the composition of TAG and monoterpene. The SS increased the initial fat melting temperature which could partially explain why such compounds improve chocolate quality, mitigating the fat bloom, according literature. The SLE was here a powerful tool for evaluating the effects of addictives in food formulation, which is an inspiration for future works in this field. polymorphic forms in chocolate is called “fat bloom”. This defect is defined by the formation of a whitish surface film in the chocolate, compromising its surface gloss and texture.7 It is believed that this phenomenon is due to the migration of the liquid fat to the chocolate’s surface followed by its uncontrolled recrystallization.3,8 This is explained in different ways: (i) an inadequate tempering with formation of unstable crystals with lesser dense network, which increases the mobility of the system;3,8 (ii) temperature fluctuation during storage, promoting changes in the phase equilibrium of the system;7 (iii) microfractures in chocolate due to very fast cooling;7 and (iv) formation of a porous structure, allowing migration of the liquid fat between crystals cavities.9 Taking into account the use of good manufacturing practices, the modification of the crystalline structure of the cocoa butter can be a promising alterative for obtaining chocolates with resistance to the formation and development of fat bloom. Many techniques and additives affect the polymorphism in cocoa butter, changing the molecular

1. INTRODUCTION Cocoa butter is one of the main ingredients for the manufacture of chocolate, being responsible for several attributes of the product’s quality, such as hardness, brittleness, shine, and mouthfeel.1 The ability of cocoa butter to interfere in the chocolate’s sensorial profile is related to its triacylglycerol (TAG) crystallization behavior.1,2 The main fatty acids (FA) found in cocoa butter are palmitic (P), stearic (S), and oleic (O) acids, corresponding to 25, 36, and 35% of total FA content, approximately,1−3 organized in three main TAGs, namely, POP (1,3 dipalmitoyl-2-oleoyl glycerol), POS (1-palmitoyl-2-oleoyl-3-stearoyl glycerol), and SOS (1,3distearoyl-2-oleoyl glycerol).1,4 Cocoa butter’s TAGs can present up to six different polymorphic forms, namely, γ (I), α (II), β′ (III and IV) and β (V and VI), with increasing stability and melting points, respectively 290.45 (I), 296.45 (II), 298.65 (III) and 300.65 (IV), 306.95 (V) and 309.45 (VI) K.1,4 The βV form is the one that confers the desirable sensory and melting characteristics of the chocolate after tempering.4 However, depending on process and storage conditions such forms can be modified altering chocolate’s sensorial profile.5,6 A very common quality defect promoted by the modification of the cocoa butter’s © 2019 American Chemical Society

Received: December 13, 2018 Accepted: June 19, 2019 Published: July 5, 2019 3231

DOI: 10.1021/acs.jced.8b01200 J. Chem. Eng. Data 2019, 64, 3231−3243

Journal of Chemical & Engineering Data

Article

Table 1. Sources, Purities, and Information of the Compounds Used in This Study

a

Declared by the supplier.

hend how these biocompounds can affect the crystalline behavior of the solid phase of fats, possibly explaining why they improve chocolate’s quality by mitigating fat bloom.

mobility and the crystal lattice of the fat. The addition of monoterpenes (or essential oils), such as D-limonene, is one of the techniques evaluated in the literature.4 According to Miyasaki et al.10 the addition of D-limonene accelerates the crystallization of the fat after tempering. The authors attributed this effect to the low melting point of such a terpene, which could increase the system’s mobility allowing a fast structural rearrangement. On the other hand, Rigolle et al.1 argued that D-limonene could act as reducing the number of unstable crystals at low temperatures, contributing to the formation of more stable polymorphic forms at high temperatures. Nevertheless, despite all these studies, the complete mechanisms that explain why monoterpenes could mitigate fat bloom are not fully elucidated. In this context, the solid− liquid phase equilibrium theory can be a powerful tool. A SLE diagram characterizes the solubility of a compound in a mixture or the behavior of its crystallization in the mixture.11 Therefore, this work was aimed at determining the complete SLE phase diagrams of binary mixtures of some representative monoterpenes (L(−)-menthol and thymol) and TAGs (trimyristin, tripalmitin, and tristearin), in order to compre-

2. EXPERIMENTAL SECTION 2.1. Chemicals. Four binary SLE phase diagrams composed of L(−)-menthol, thymol, trimyristin, tripalmitin, and tristearin were evaluated in this study. Information on the pure compounds used to formulate the systems is presented in Table 1. A differential scanning calorimeter (DSC) was calibrated using indium (CAS number 7440-74-6, purity ≥0.999 mass fraction, certified by PerkinElmer) and n-decane (CAS number 124-18-5, purity ≥0.998 mass fraction, Sigma-Aldrich), comprising the melting temperature range of the compounds used in this work. 2.2. Methods. Preparation of the Binary Mixtures. Binary mixtures of monoterpenes and triacylglycerols were prepared to cover the molar composition range of the phase diagram, from 0 to 1, in steps of 0.1 molar fraction. The mixtures studied were L(−)-menthol + trimyristin, L(−)-menthol + tripalmitin, L (−)-menthol + tristearin, and thymol + 3232



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Table 2. Melting Temperatures and Enthalpies for Pure Compounds at p = 94.6 kPaa ΔHimelt/kJ·mol−1

Ti,melt/K component L(−)-menthol

thymol trimyristin tripalmitin tristearin

this study 314.40 321.90 329.40 336.80 343.80

literature 24

25

this study 26

316.15 315.90 315.68 324.1527322.8025323.5026 330.2028330.2522330.9015 338.1529337.4030334.6531338.8220 344.3533345.7620345.5534

11.93 13.03 144.48 158.67 184.17

literature 24

11.90 12.832512.8926 17.542519.6526 141.9715152.4928146.8022 165.8815165.0029162.6032160.8233 190.0033194.6034

a

Pressure = 94.6 kPa. Standard uncertainties for temperature u(T) = 1.16 K, enthalpy ur(H) = 0.08, and pressure u(P) = 0.5 kPa.

0.02 to 0.25). The mean value was reported as the relative standard uncertainty for melting enthalpy: ur(H) = 0.08. Temperature Controlled Optical Microscopy. To evaluate the melting profile of the systems, the formation of solid solutions, or other crystallographic behavior, samples were analyzed by optical microscopy using a Leica DM2700 microscope (Germany) equipped with a temperature controller (Linkam LTS420, Linkam Scientific Instruments Ltd., United Kingdom). Systems were cooled and heated at 1 K· min−1, following the DSC methodology. Thermal events were recorded with a sequence of photos and evaluated with the DSC data. Thermodynamic Modeling. The SLE can be defined by eq 1, as described elsewhere:21,15

tripalmitin. These mixtures allowed an evaluation of the effect of different TAGs and different monoterpenes in the solid− liquid equilibrium of a monoterpene + TAG system. TAGs were chosen based on the FA profile of cocoa butter (rich in palmitic and stearic acids), but also because cocoa butter’s TAGs were not available as pure compounds in the market. On the other hand, trimyristin, tripalmitin, and tristearin are representative TAGs for simulating other fats. The monoterpenes were chosen due to their large presence in essential oils. Systems (1 g) were formulated in a glass vessel by using an analytic balance (XP205, Mettler Toledo, precision = 0.2 mg). To complete the mixture of the compounds, the systems were melted under magnetic stirring on a heating plate at approximately 10 K above the melting temperature of the compounds until they formed a homogeneous liquid phase. Samples were then stored at 273.15 K. The uncertainty for molar fraction was calculated by error propagation, considering the precision of the equipment, and set as u(x) = 0.005, being the maximum value found. Differential Scanning Calorimetry. The melting temperatures and enthalpies data were evaluated on a differential scanning calorimeter (DSC 8500 PerkinElmer, USA) equipped with a gas nitrogen (purity >0.9999 mass fraction) cooling system. Samples (2−5 mg) of each system were weighed into aluminum pans in a microanalytical balance (AD6, PerkinElmer, USA, precision = 0.002 mg) and put in sealed aluminum pans. Analysis followed an adaptation of the method described by Costa et al.12 specially designed for lipidic compounds. Each sample was submitted to a first heating ramp to 15 K above the higher melting temperature among the components of the mixture, in order to erase the thermal history of the solid phase of the sample, followed by an isothermal treatment for 20 min. Samples were then cooled to 213.15 K at a rate of 1 K· min−1 and stayed at this temperature for 30 min. After this pretreatment, melting data were collected in a heating ramp at 1 K·min−1 and evaluated in the PerkinElmer Universal Analysis software (PerkinElmer, USA). The peak top temperature of the last endothermic event was defined as the melting temperature of the mixture, due to the existence of broad and overlapped peaks. Other thermal events were also measured at the peak top temperatures. Transition enthalpies were obtained through the area of the thermal event. These procedures were extensively evaluated in previous works on the SLE of lipidic substances.13−20 The standard uncertainties (0.68 level of confidence) for the melting temperatures of the compounds were determined by evaluating the limits of the data set composed of experimental and literature data. The mean value was considered as the standard uncertainty for systems’ melting temperatures: u(T) = 1.16 K. Considering the high differences in magnitude, the relative standard uncertainties for melting enthalpies were calculated (ur(H) =

melt S y ji ziγ zy ΔHi(Ti) ij Ti ,melt jj − 1zzzz + lnjjjj iL zzzz = j j xiγ z RTi ,melt k T { k i {

n

∑ tr = 1

Δtr H ijj 1 1 yz − zzz jj j R k Ttr T z{

(1) −1

−1

where R is the universal gas constant (8.314 J·mol ·K ), T (K) is the melting temperature of the mixture, Ti,melt (K) is the melting temperature of the pure compound, ΔHi(Ti)melt (J· mol−1) is the melting enthalpy of the pure compound, x and z are the molar fraction of the compound in the liquid (L) and solid (S) phases, γi is the activity coefficient of the compound in each phase, ΔHtr (J·mol−1) and Ttr (K) are the polymorphic transition enthalpies and temperatures of the n forms found. Each term related to each of the n polymorphic forms is only considered at temperatures below the respective transition. Equation 1 is a simplification of the complete SLE theory, considering that the variation of the heat capacity (Δcpi) of the compounds in the solid and liquid states is low enough to be omitted in the equation. More details of this simplification are presented elsewhere.15,21 The melting temperatures and enthalpies of the pure compounds used in eq 1 were those experimentally measured by DSC in this work. The values for the enthalpies and temperatures of the polymorphic transitions were found in the literature.22 The description of the complete SLE phase diagram followed the procedure proposed in a previous work.15 The method is based on a “solid−liquid isoenthalpic flash algorithm”, such that for a given x, the values for T and z are calculated. The three-suffix Margules equation was used for the calculation of the activity coefficients of compounds in both liquid and solid phases, and its parameters were adjusted by using the experimental data. The set of parameters adjusted is that which minimized the deviation between experimental and theoretical melting data. However, considering the number of adjustable parameters and the nonlinearity of the SLE equation, there is not a unique set of adjustable parameters that answers the problem. This problem is explained in 3233



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Table 3. Experimental Solid−Liquid Equilibrium Data of the Systems at p = 94.6 kPa. Molar Fraction (x1), Solid Phase (SP), Solid−Solid Transitions (Tsolid), Eutectic (Teut), and Melting (Tmelt) Temperaturea x1

Tsolid/K

Teut/K

Tmelt/K

0.107 0.203 0.303 0.395 0.501 0.602 0.697 0.799 0.900 0.106 0.211 0.298 0.401 0.493 0.602 0.701 0.799 0.900

+ Trimyristin 328.65 311.49 328.70 305.99 311.53 327.99 306.88 311.58 327.21 306.42 311.34 326.33 311.75 325.11 306.98 312.15 322.50 306.76 312.38 320.62 305.99 312.49 316.75 L(−)-Menthol + Tristearin 344.07 343.48 343.32 343.39 341.41 340.64 306.41 314.61 338.98 306.83 314.24 336.34 306.02 314.23 331.78

x1

SP

L(−)-Menthol

Tsolid/K

Teut/K

Tmelt/K

2 2 2 2 2 2 2 2 2

+ + + + + + + + +

1 1 1 1 1 1 1 1 1

0.126 0.204 0.304 0.411 0.503 0.599 0.740 0.800 0.900

2 2 2 2 2 2 2 2 2

+ + + + + + + + +

1 1 1 1 1 1 1 1 1

0.099 0.198 0.301 0.407 0.497 0.593 0.699 0.800 0.898

306.70 306.26 306.78 306.66 307.15 307.14

+ Tripalmitin 337.57 336.59 335.74 335.46 313.66 334.03 313.72 332.70 313.59 331.34 313.94 329.40 314.26 325.84 Thymol + Tripalmitin 337.53 336.42 335.66 334.06 332.63 330.92 312.72 325.55 312.47 316.54 313.44

SP

L(−)-Menthol

2 2 2 2 2 2 2 2 2

+ + + + + + + + +

1 1 1 1 1 1 1 1 1

2 2 2 2 2 2 2 2 1

+ + + + + + + + +

1 1 1 1 1 1 1 1 2

a

Standard uncertainties for temperature u(T) = 1.16 K, pressure u(P)= 0.5 kPa, and molar fraction u(x) = 0.005.

Figure 1. Solid−liquid phase diagram (left) and DSC thermograms (right) for L(−)-menthol + trimyristin: (○) melting temperature; (△) eutectic transition; (×) solid−solid transition; (dashed line) ideal profile; (continuous line) eq 1.

detailed in the literature.23 Therefore, the set of parameters here presented is only one of the solutions of the problem.

Solid−Liquid Phase Equilibrium Diagrams. Table 3 presents the experimental data obtained by DSC: melting temperature (Tmelt), eutectic temperature (Teut), and other transitions in the solid phase (Tsolid), as well as the type of solid phase found in each composition. Figures 1 to 4 show the SLE phase diagrams and the thermograms of each system. The phase diagrams were plotted in a way that the monoterpene is the one at the right-hand side of the diagram. Symbols are for experimental data obtained by DSC and microscopy, and continuous lines are for the modeled phase diagram by using eq 1. Moreover, the ideal SLE thermodynamic behavior of the systems was also plotted. For this it was considered that the activity coefficients of the compounds in the liquid phase γiL =

3. RESULTS AND DISCUSSION Melting temperatures and enthalpies of each pure compound are presented in Table 2 as are some data found in the literature. Experimental data were in good agreement with those found in the literature, with a mean relative deviation of 0.37% for the melting temperature and 9.38% for the melting enthalpy. Deviations could be attributed to differences in methodology among literature data as well as the broad melting profile of of such biocompounds. 3234



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Figure 2. Solid−liquid phase diagram (left) and DSC thermograms (right) for L(−)-menthol + tripalmitin: (○) melting temperature; (△) eutectic transition; (×) solid−solid transition; (dashed line) ideal profile; (continuous line) eq 1.

Figure 3. Solid−liquid phase diagram (left) and DSC thermograms (right) for L(−)-menthol + tristearin: (○) melting temperature; (△) eutectic transition; (×) solid−solid transition; (dashed line) ideal profile; (continuous line) eq 1.

1, and the compounds in the solid phase are immiscible, such that xiSγiS = 1. In this case, the polymorphism effect was also not considered. This was presented by dashed lines. The ideal equilibrium profile neglects the formation of solid solutions and considers that the system behaves as a simple eutectic mixture. All systems evaluated in this work exhibited a similar solid− liquid equilibrium phase profile. The DSC thermograms showed that mixtures exhibited up to three thermal transitions. The transition at the highest temperature could be attributed to the melting of the mixture. Below the melting transition, up to two transitions were observed. Both of them were temperature-invariant transitions. Therefore, they could be attributed to the eutectic transition (that is a temperatureinvariant transition), and solid−solid transitions. Some mixtures, especially at high concentration of TAG, did not

exhibit eutectic transitions, proposing that the systems formed solid solutions, which will be discussed below. On the basis of such observations, the phase diagrams were organized in five SLE regions: one liquid phase above the liquidus line (the melting temperature of the mixture), one solid phase composed of a solid solution rich in TAG at the left-hand side of the diagram, one biphasic region composed of the solid solution + liquid phase (at the left-hand side of the eutectic point), one biphasic region composed of solid monoterpene + liquid phase (at the right-hand side of the eutectic point− except for the tristearin system, that will be discussed below) and one solid−solid domain below the eutectic transition. Such a consideration will be discussed point to point below. For the L(−)-menthol + tristearin system (Figure 3), the eutectic point (minimum melting temperature of the system) is almost close to pure monoterpene, probably due to the large 3235



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Figure 4. Solid−liquid phase diagram (left) and DSC thermograms (right) for thymol + tripalmitin: (○) melting temperature; (△) eutectic transition; (×) solid−solid transition; (dashed line) ideal profile; (continuous line) eq 1.

Figure 5. Tammann plots of the eutectic transition: (■) DSC experimental data. (A) L(−)-menthol + trimyristin; (B) L(−)-menthol + tripalmitin; (C) L(−)-menthol + tristearin; and (D) thymol + tripalmtin. Continuous lines are linear regressions of experimental data and dashed lines are hypothetical profiles.

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Figure 6. Optical micrographs of the L(−)-menthol + trimyristin system: xmenthol ≅ 0.1 (1A−D); xmenthol ≅ 0.5 (2A−D); and xmenthol ≅ 0.9 (3A−D).

events were observed, although this was not so visible in the figure due to magnifying effects. The peak below the melting event could be attributed to the eutectic transition since it is temperature-invariant and its intensity increases close to the eutectic point, which is typical for a eutectic transition. Therefore, one could consider that a solid solution was formed at concentrations up to xmenthol ≅ 0.2. Some temperatureinvariant peaks were also observed below the eutectic transition. In the case of the system of Figure 1 (L(−)menthol + trimyristin system), this occurred from xmenthol ≅ 0.4. These peaks could be associated with some thermal transition of the solid phase. In this case, since either menthol or thymol do not present polymorphic forms,26 such invariant transitions were probably due to the TAG polymorphism. The same occurred for the other systems. For the L(−)menthol + tripalmitin system (Figure 2), the eutectic event was observed from xmenthol ≅ 0.5 and the solid−solid transitions from xmenthol ≅ 0.4. For the L(−)-menthol + tristearin mixture (Figure 3) the eutectic transition was observed from xterpene ≅ 0.7. In the case of the thymol + tripalmitin mixture (Figure 4), the eutectic transition was also observed from xthymol ≅ 0.7. If one considers that when no eutectic transition is observed a solid-solution was formed, one could assume that the larger are

difference in the melting temperature of both compounds. It means that the eutectic temperature of the system is close to the melting temperature of pure menthol. Also, the solid− liquid domain at the right-hand side of the diagram, composed of solid terpene + liquid phase was difficult to experimentally define. This also happened for the L(−)-menthol + tripalmitin system (Figure 2). On the other hand, the L(−)-menthol + trimyristin (Figure 1), and the L(−)-thymol + tripalmitin (Figure 4) presented a well-defined eutectic point, close to xterpene = 0.9. This means that the mixture could establish a minimum temperature lower than the melting temperatures of both compounds. The DSC thermograms showed that in some cases only the melting transition was observed, with no eutectic or solid− solid transition. For the L(−)-menthol + trimyristin system (Figure 1), for example, only one peak was seen at xmenthol ≅ 0.1. Since no eutectic transition was observed it is believed that a solid solution at this region of the diagram was formed. This occurs because when a solid-solution is formed the compounds are miscible in the solid phase. Therefore, if the solid phase is a miscible system, compounds do not melt separately, as occurs in case of immiscibility and thus in the case of a eutectic system. However, from xmenthol ≅ 0.2 at least two other thermal 3237



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Figure 7. Optical micrographs of the L(−)-menthol + tripalmitin system: xmenthol ≅ 0.1 (1A−D); xmenthol ≅ 0.5 (2A−D); and xmenthol ≅ 0.9 (3A−D).

this is a very interesting result because TAGs and monoterpenes do not present similar molecular structures, which is classically attributed to the ability of a mixture to form solid solutions. However, they present similar crystalline cells, which will be commented on below. Additionally, the eutectic points could be also confirmed by the Tammann plots at xmenthol ≅ 0.95 for L(−)-menthol + trimyristin, xmenthol ≅ 0.99 for L(−)-menthol + tripalmitin, xmenthol ≅ 0.99 for L(−)menthol + tristearin, and xthymol ≅ 0.90 for thymol + tripalmitin. In all cases, the eutectic point was very close to that of pure monoterpene, and therefore the region at the right-hand side of the eutectic point was very narrow. In this case, the eutectic and melting transitions overlapped in the thermograms, avoiding obtaining correct eutectic enthalpies and temperatures. Therefore, hypothetical linear profiles for this side were plotted for visual purposes, demonstrating that at the right-hand side of the diagram enthalpy tended to zero, showing probably no formation of solid solution close to pure monoterpene. Microscopy. Optical microscopy was used to confirm and identify the equilibrium regions observed in the DSC thermograms. Evaluations were performed for all molar concentrations. Figures 6−9 show the micrographs for systems at the both sides of the diagrams (xterpene ≅ 0.1 and 0.9) and at

the TAG carbon chains, the larger is the solid solution region formed. Also, when thymol was replaced by L(−)-menthol a larger solid solution region occurred. Tammann Plots. The formation of the solid−solid solution could be confirmed by the Tammann plots (or Tammann triangles) of the eutectic transition. This plot relates the enthalpy of the eutectic transition as a function of the mixture composition. When the mixture concentration comes close to the eutectic composition, the enthalpy should increase, confirming the eutectic point.12,16,20,35,36 Therefore, this plot also allows a determination of the regions of solid phase miscibility. When complete solid phase immiscibility occurs, the enthalpy values of the eutectic transition tend to zero when mixture concentration tends to the pure components.18 On the other hand, when a solid-solution is formed, the eutectic enthalpy tends to zero at a different concentration. The Tammann plots are presented in Figure 5. According to Figure 5, at the left-hand side of the diagram, the enthalpy of the eutectic transition tended to zero at xmenthol ≅ 0.20 for L(−)-menthol + trimyristin, xmenthol ≅ 0.50 for L(−)menthol + tripalmitin, xmenthol ≅ 0.60 for L(−)-menthol + tristearin, and xthymol ≅ 0.60 for thymol + tripalmitin. These values could establish the limit in which the solid solution was formed, corroborating to what was observed in DSC. In fact, 3238



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Figure 8. Optical micrographs of the L(−)-menthol + tristearin system: xmenthol ≅ 0.1 (1A−D); xmenthol ≅ 0.5 (2A−D); and xmenthol ≅ 0.9 (3A−D).

the same behavior was seen, such that the eutectic transition could be observed. These sequences confirmed the occurrence of the solid solution region at the left-hand side of the diagram as well as the eutectic transition. Notably, the initial melting temperature at xmenthol ≅ 0.1 was significantly higher than the eutectic temperature. This could mean that even at a small composition, menthol could strongly affect the TAG crystalline structure. This could be also the reason that the peak top temperature observed in the DSC thermograms at this region is slightly higher than the pure TAG (Table 3), when one should expect that it should be theoretically lower than the pure compound melting temperature. On the other hand, this behavior could also be attributed to the fact that the melting event of these complex mixtures was broad, that is, occurs at a temperature range and not in a single temperature. Figure 7 also shows three image-sequences for the L(−)menthol + tripalmitin system. Similarly to what occurred at the previous system, in the first image sequence (1A−D), xmenthol ≅ 0.1, one could observe that the initial melting occurred only at 338.55 K, very close to the melting temperature of the system. In the second image sequence (2A−D), for xmenthol ≅ 0.5, the melting occurred at a lower temperature (314.65 K), close to what was observed for the eutectic event. In the last

molar fractions close to 0.5. In the micrographs the solid phase was represented by black (dark) coloration while the liquid phase acquired a white coloration, allowing the visualization of the phase change. At xterpene ≅ 0.1 the solid phase, with a high concentration of TAG, showed smaller and agglomerated crystals. Otherwise, the solid phase at xterpene ≅ 0.9 also showed clear needle-like structures, related to the high concentration of monoterpenes (menthol or thymol). The initial melting could be easily observed by the rounding of the crystals’ edges as well as by the whitening of the samples. In fact, the DSC thermograms cannot provide the correct temperature in which crystals starts to melt due to the lower sensibility of the technique. Figure 6 shows 3 image-sequences for the L(−)-menthol + trimyristin system. The first image sequence (1A−D), xmenthol ≅ 0.1, at 304.55 K, shows that the sample was completely solid. With the gradual increase in temperature no change was observed up to 325 K, approximately, when crystals started to melt. The second image sequence (2A−D), xmenthol ≅ 0.5, at 305.55 K, shows that the sample was also completely solid, but from 311.85 K crystals started to melt. This temperature is close to that observed in DSC for the eutectic transition of this mixture. In the third image sequence (3A−D), xmenthol ≅ 0.9, 3239



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Figure 9. Optical micrographs of the thymol + tripalmitin system: xthymol ≅ 0.1(1A−D); xthymol ≅ 0.5(2A−D); and xthymol ≅ 0.9 (3A−D).

sequence of images (3A−D), xterpene ≅ 0.9, the melting also started at the eutectic temperature. These sequences also confirmed the occurrence of the solid solution region at the left-hand side of the diagram as well as the eutectic transition. Figure 8 shows the same image sequences for the L(−)menthol + tristearin system. The first image sequence (1A− D), xmenthol ≅ 0.1 shows that the system also started to melt at a very high temperature, close to 345.05 K. The second image sequence (2A−D) xmenthol ≅ 0.5, shows that the system also started at a temperature very close to the final melting temperature observed in the DSC thermogram, close to 344.75 K. Only in the last image sequence (3A−D), xmenthol ≅ 0.9, it is noted that the melting started at a low temperature, 321.50 K, close to the eutectic transition. In this case, the tristearin system presented a larger solid solution region. The melting behavior of the thymol + tripalmitin system is presented in Figure 9. The first image sequence (1A−D), xthymol ≅ 0.1, shows that the system started to melt at a temperature very close to the DSC melting temperature. In the second image sequence, (2A−D), xthymol ≅ 0.5, the same behavior occurred. However, at the second image sequence (3A−D), xthymol ≅ 0.9, it is noted that the system started to melt at 312.55 K, approximately, close to the eutectic transition

observed in DSC. Therefore, thymol could improve the solid solution formation, when compared to (L)-menthol. Thermodynamic Modeling. Table 4 reports the adjusted parameters of the three-suffix Margules equation, as well as the mean absolute deviation between calculated and experimental melting temperature data (liquidus line), considering both real Table 4. Adjusted Parameters (Aij and Aji) for Margules Equation, and Mean Absolute Deviation between Calculated and Experimental Melting Temperature Data three-suffix Margules/kJ·mol−1

systems L(−)-menthol

+ trimyristin L(−)-menthol + tripalmitin L(−)-menthol + tristearin thymol + tripalmitin 3240

solid phase

liquid phase

deviation (K)

Aij

Aji

Aij

Aji

ideal

three-suffix Margules

8.2

5.0

−0.3

−1.0

0.84

0.28

6.0

6.0

−0.1

−1.5

1.62

0.49

6.0

6.0

−1.5

−5.0

1.22

0.44

3.0

6.0

−10.0

−9.0

3.32

1.12



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4. CONCLUSIONS The formation of solid solutions is expected when compounds present similar molecular structures, and so similar crystalline profiles. However, TAG and monoterpenes clearly do not present similar molecular structures or similar crystalline habits. Therefore, one would expect that the SLE of TAG + monoterpenes system would present a simple eutectic profile, when compounds are immiscible in the solid phase. However, this work showed that when menthol or thymol were mixed to triacylglycerols, large solid solution regions could be promoted. The tendency of solid solution formation was higher when the molar mass of the TAG increased, providing this sequence: trimyristin < tripalmitin < tristearin. Considering the same TAG, thymol could induce a larger solid solution region when compared to L(−)-menthol. These results propose that when a monoterpene is added in a fatty system, which is the case of cocoa butter, the crystalline habit of the TAG is altered, contributing to the formation of solid solutions. This modification can occur even at low concentrations of monoterpene. This conclusion also corroborates with the results presented by other authors on the effect of adding essential oils in cocoa butter during chocolate production. The formation of a solid solution increases the temperature in which the crystal starts to melt. Therefore, considering the temperature fluctuation during storage, which is the main cause of fat bloom in chocolates, the monoterpene can decrease the tendence of the fat to form a liquid phase. With less liquid phase, the migration of the liquid phase to the surface of the chocolate could decrease, decreasing the whitish appearance of the chocolate. This work showed that the solid−liquid phase equilibrium can be used as a complete and robust tool for the evaluation of the effect of adding compounds in food formulations, inspiring new studies in this field. In this specific case, it was possible to evaluate the use of monoterpenes, or other additives, during chocolate manufacture, which can influence texture, color, stability, and shelf life.

and ideal profile. Modeled curves are presented in the phase diagrams of Figures 1−4. The DSC thermograms, Tammann plots, and microscopy could show that all systems presented a large region of solid solution formation. This reveals that the systems presented significant deviations from ideality and, therefore, could not be considered as ideal mixtures. In fact, Table 4 shows that, except for the thymol + tripalmitin system, the values of the deviations of the ideal assumption were low, close to 1 K. However, the values of the deviations decreased when the three-suffix Margules equation was considered and, therefore, the description of the SLE of the systems could be slightly improved. The adjusted parameters of the equation revealed that the activity coefficients of the compounds in the liquid phase were, in general, close but slightly lower than 1. This revealed favorable enthalpic and entropic interactions in the liquid phase. Figures 1−4 showed that the three-suffix Margules equation was able to show the solid solution formation, but with a significant deviation compared to the real profile. It means that except for the L(−)-menthol + trimyristin system, the concentration range in which the solid solution was observed in DSC, in the Tammann plots, and in the microscopies could not be well described by the model. In fact, all excess Gibbs energy based activity coefficient models are not accurate when strong deviations from the ideal behavior occur.37 This means that the model was only accurate for the L(−)-menthol + trimyristin system, which presented the smallest solid solution region. Its ability decreased as the solid solution region increased. The values of the activity coefficients of the compounds in the solid phase given by the Margules equation could show some relevant information on the nonideal behavior of the mixtures. If systems are immiscible in the solid phase, that is, presenting a simple eutectic profile, the activity coefficients of the compounds in the solid phase tends to infinite. On the other hand, when a solid solution occurs, the activity coefficients of the compounds in the solid phase decrease. For all cases, systems were partially miscible in the solid phase at a region close to pure TAG. In this case, the model showed that monoterpenes presented slightly positive deviations (γ > 1) and the triacylglycerols presented slightly negative deviations (γ < 1). These values revealed that the interaction forces of the compounds, but also their size and shape could induce the formation of a partial solubility in the solid phase. The crystallographic data in the literature shows that L(−)menthol presents a hexagonal form P1(2) in the solid phase,38 which is the same crystalline cell for the β-triacylglycerol polymorphic form.39 Otherwise, thymol presents a rhombohedral form R(3) in the solid phase,40 which is within the same hexagonal crystal family of the α-tiacylglycerol polymorphic form. This probably facilitated the formation of solid solutions during the crystallization of the system. Also, as the concentration of monoterpenes increased, their activity coefficient also increased, indicating the appearance of repulsion forces and a more unfavorable entropic situation. This corroborated with the appearance of the eutectic transition at the right-hand side of the diagram (solid phase separation). These results suggested that the crystalline structure of the TAGs could act as a host for the monoterpenes molecules forming stable solid solutions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +55 (19) 3521-0063. ORCID

Paula Virginia de Almeida Pontes: 0000-0002-0237-8643 Eduardo Augusto Caldas Batista: 0000-0002-8102-9424 Guilherme José Maximo: 0000-0002-3252-3004 Funding

The authors wish to acknowledge Prof. Dr. Priscilla Efraim, School of Food Engineering, UNICAMP, for important support and suggestions, and the national funding agencies: National Council for Scientific and Technological Development (CNPq), Brazil, Grant No. 305870/2014-9, Grant No. 406963/2016-9, Grant No. 308924/2017-7, and Grant No. 140702/2017-2; The São Paulo Research Foundation (FAPESP), Brazil, Grant No. 2014/21252-0 and Grant No. 2016/08566-1; and The Teaching, Research and Extension Support Fund (FAEPEX-UNICAMP), Brazil, Grant No. 125/ 16 for financial support and scholarships. This study was also financed in part by the Coordenaçaõ de Aperfeiçoamento de ́ Superior, Brasil (CAPES) - Finance Code Pessoal de Nivel 001. Notes

The authors declare no competing financial interest. 3241



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Phase Equilibrium of Fats and Monoterpenes and How It Affects

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