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Thermal Properties of Monoglycerides from Nephelium Lappaceum L. Oil, as a Natural Source of Saturated and Monounsaturated Fatty Acids Valentin Romain,†,‡ Adolphe Christian Ngakegni-Limbili,†,‡,§ Zéphirin Mouloungui,*,†,‡ and Jean-Maurille Ouamba§ †

Laboratoire de Chimie Agro-industrielle (LCA), INP-ENSIACET, University of Toulouse, F-31030 Toulouse, France INRA, UMR 1010 CAI, F-31030 Toulouse, France § Unité de Chimie du Végétal et de la Vie, Faculté des Sciences−Université Marien Ngouabi, BP 69, Brazzaville, Congo ‡

S Supporting Information *

ABSTRACT: The present paper studied the transformations of reserve lipids (RLs) of the species Nephelium lappaceum Linn (N. Lappaceum). The triacylglycerols (TAGs) in this species are composed of C18:1, C20:0, and C20:1 fatty acids (FAs). The transition from native triglycerides to pure monoglycerides (MGs) was explored. It has been shown how the physical and physicochemical properties of MGs were influenced by the effect of the FA composition, consisting of saturated and unsaturated fatty acids. In organic process, the first step was lipase-catalyzed hydrolysis of native triglycerides. The second step was the chemically catalyzed esterification of hydrolyzed free fatty acids into MGs. Both steps were carried out in organized media used as microreactors. These transformations increased the melting temperatures from 22 °C to 51 °C and decreased the crystallization temperature from 22 °C to −1 °C, from long-chain TAGs to MGs (C20:0 and C20:1). By monitoring the saturated/unsaturated FA ratio in the lipotransformation into MGs, it was possible to control the composition of binary and ternary mixtures and, therefore, control the physical and physicochemical properties of the systems. This may be very useful for applications in the food, cosmetics, and pharmaceuticals industries.

1. INTRODUCTION Fats and lipids are used as the main constituent of diverse finished food, cosmetic, and pharmaceutical products, as well as matrices in which cosmetics, pharmaceuticals, and chemicals are dispersed.1 Fatty acids (FAs) and glycerides are constituents of fats and lipids, which are classified as long-chain compounds.2 The crystallization of fats and lipids has industrial implications. By controlling crystallization, end products such as chocolate, margarine, and whipping cream can be obtained. On other hand, the crystallization phenomenon of fats can be used to isolate fats from particular natural resources. Vegetable and animal fats mostly contain various molecular species with different chemical and physical properties. There is a growing demand for technologies to obtain high-melting-point fats, and avoid hydrogenation which produces trans-fatty acids as byproducts. Such technology would allow better compliance with new regulations concerning the use of fats for confectionery end products. It would also favor better functionality of physically refined vegetable oils.3 Particularly, fish oils are currently recommended, because of the high content of long-chain and unsaturated fatty acids. However, the production of vegetable oils rich in monounsaturated fatty acids, which are potentially more stable physically and chemically than fish oils, because of the low content in polyunsaturated acids, is potentially valuable. There is no long-chain compound that is not polymorphic, and this property is particularly pronounced in fats and lipids. For example, triacylglycerol (TAG), which is a total fatty acid ester of glycerol, generally has three polymorphs. The © 2013 American Chemical Society

crystallization behavior of TAG (the crystallization rate, crystal size and network, crystal morphology, and crystallinity) is directly influenced by the polymorphism. The polymorphism depends on the molecular structure itself and on several external factors such as temperature, pressure, solvent, the rate of crystallization, and impurities. The conversion of TAGs into partial glycerides or pure monoglycerides (MGs) is an interesting challenge. MGs have many different self-assembling structures, because of their amphiphilic character.4 The physical properties of MGs depend on the fatty acid compositions: saturated or unsaturated acids, short chain acids, even or odd carbon number acids. For example, 1-monostearin (melting temperature = 81 °C) gels are stable at ∼80 °C versus 1monoolein (melting temperature = 35 °C) stable at ∼20 °C.5 It has been shown that MGs are compounds able to bring new or improved functionality to food products, since they can form self-assembly structures in both lipid and aqueous phases.6 A particular self-assembling phase, such as a lamellar phase, was promising in food applications, because of its ability to structure the final oily product through the formation of a gel-like network. The phase could encapsulate oil giving to the new mesomorphic gel fatlike properties.7−9 There are various means and chemical reactions for transforming oils or triglycerides into oleochemical derivatives Received: Revised: Accepted: Published: 14089

June 13, 2013 July 26, 2013 August 24, 2013 August 25, 2013 dx.doi.org/10.1021/ie401875v | Ind. Eng. Chem. Res. 2013, 52, 14089−14098

Industrial & Engineering Chemistry Research

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organized forms appeared: spherical micelles, vesicles, cubic phases, lamellar phases, hexagonal phases, and others. These objects can be exploited as microreactors,39,41 because the reactants are localized and concentrated reversible oleochemical reactions can be driven under experimental conditions, requiring little energy. The efficient transformation of native or free TAGs can be assessed by coupling biocatalytic and chemocatalytic pathways to synthesize pure MGs. In this study, the polymorphic changes of MG mixtures were showed by differential scanning calorimetry (DSC). DSC reveals the energetic phenomena occurring during the heating or cooling of the sample. The overall aim of this study was to show that chemo-catalytic pathways can be used to modify the nonconventional Nephelium lappaceum kernel oil to produce MGs with high melting temperature and low crystallization temperature, and with a high content in C18:0, C20:0 saturated acids and C18:1, C20:1 unsaturated acids.

available to the oleochemical industry. It has been assessed whether some of these oleochemical transformations can be used to exploit the diversity of fatty acid composition of oils from various unconventional species such as Nephelium lappaceum L. The rambutan (Nephelium lappaceum L.) is an exotic fruit originally found in Malaysia. Although little known in many parts of the world, it has the potential for development in regions with agroclimatic conditions favorable for its cultivation,10,11 such as Congo Brazzaville. Industrial processing of the fruit results in residues consisting principally of seeds and rinds which are considered to be waste material. Some studies on seeds have showed that rambutan seed oil can be used for manufacturing candles.12,13 As rambutan seed oil has not been more chemically valuated, the present work studied the lipotransformation of the N. lappaceum L. oil and the physical properties of the monoglycerides obtained. There are various means of enzymatic and chemical reactions for transforming oils or triglycerides into oleochemical derivatives available to the oleochemical industry. Glycerolysis was the more used method to produce monoglycerides from fats and oils.14 The conventional glycerolysis technique requires strong conditions with temperature greater than 220 °C and use of inorganic catalysts.15 Enzyme-catalyzed glycerolysis has been developed16 and assisted by efficient processes such as ultrasounds.17 Organic medium,18−20 solvent-free systems,21 heterogeneous,21 ionic liquids,22,23 and high-pressure media24,25 have been tested. Direct esterification on industrial scale is achieved is catalyzed by homogeneous strong acid or base catalysts such as H2SO4, H3PO4, KOH, NaOH, or Ca(OH)2. The selectivity in monoglycerides was low (40%−50%), because of the formation of diglyceride and triglyceride side products and the undesired formation of soap. Dissolved transition-metal complexes with Lewis acid properties based on Sn are highly active and more selective,26 but the elimination of the transition metal from the monoglyceride product is problematic. Heterogeneous catalysts have been developed and tested under mild reaction conditions.27−36 Generally, the monoglyceride selectivity of the investigated heterogeneous catalysts was not significantly better than in the homogeneous catalytic processes.27 Recently, a tin−organic framework (Sn−EOF) was determined to be the most active catalyst with 98% selectivity in glycerol monooleate and at 40% conversion in catalyzing esterification of oleic acid with glycerol at a low reaction temperature of 150 °C in tert-butanol as solvent.37 In this context, it has been assessed whether some of these oleochemical transformations can be used to exploit the diversity of fatty acid composition of oils from various unconventional species such as Nephelium lappaceum L. In the first step, the lipotransformation of reserve lipids of this species was lipase hydrolysis of the native triglycerides in the presence of Candida rugosa in aqueous media by the patented process of Mouloungui and Mechling.38 This method can be used for reserve lipids of industrial and nonconventional oilseeds. Applied to crude, semicrude, or refined vegetable oils, the method gives repeatable and reproducible results. The second step was to study the chemo-catalytic esterification39 of native fatty acids. The first and second steps were carried out in organized media. These organized forms were composed of native surfactant molecules (phospholipids and membrane proteins40) or were generated in situ (monoglycerides, diglycerides) as their concentration increased.39 Various

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MATERIALS AND METHODS Nephelium lappaceum L. Oil Extraction Method from Kernels. Nephelium lappaceum L. (N. lappaceum) kernels were collected in September and December in the region of Boko (South Congo; latitude 4°47′12″S, longitude 14°37′44″E; altitude 595 m). The samples were depulped in the laboratory, and the kernels were milled with the MF10-IKA grinder. The oil was extracted using an Accelerated Solvent Extractor (ASE 200) equipped with 11 mL cells, an analytical balance, and Dionex vials for collection of extracts (40 mL; P/N 49465), using cyclohexane as solvent. The oil extract was concentrated in a rotary evaporator by distillation at reduced pressure and 40 °C until the solvent was totally removed. Mean kernel content was determined gravimetrically using three different samples replicated three times.42−44 Reagents. Candida rugosa lipase (LIPOLYVE CC) was purchased from Lyven (Nantes, France). Glycerol (99.0% pure) was obtained from Aldrich (Saint-Quentin Fallavier, France). Dodecylbenzene sulfonic acid (DBSA) was purchased from Acros Organics (Geel, Belgium), and extruded 3 Å molecular sieve was obtained from Prolabo (Paris, France). Analysis of Glycerides by Gas Chromatography. Samples were silylated according to the Donike method45 prior analysis by gas chromatography (GC) equipped with a CP Sil 8CB low-bleed MS Varian column (15 m × 0.32 mm × 0.25 m), helium as carrier gas (15 psi column head), and 55 °C for 0.5 min, then 45 °C/min to 80 °C, then 10 °C/min to 360 °C and 360 °C for 26 min. Analysis of Fatty Acid Composition by Gas Chromatography. Samples were transesterified with trimethylsulfonium hydroxide (TMSH)46 and analyzed by GC, using a Varian CP-Select CB 3900 flame ionization gas chromatograph, with a fused-silica capillary column, CP Select CB (50 m × 0.25 mm, 0.25 μm film thickness). The carrier gas was H2 with a flow rate of 1.2 mL min−1; split ratio was 1:100. The initial oven temperature was held at 185 °C for 40 min, increased at a rate of 15 °C min−1 to 250 °C and then held there for 10 min. The detector and injector temperatures were fixed at 250 °C. Liberation of Native Fatty Acids by Enzymatic Hydrolysis.38 Hydrolysis of crude N. Lappaceum oil was performed in a 500-mL glass jacked reactor vessel thermostatted at 37 °C, equipped with a homogenizer (model L4RT, Silverson Machines Ltd. Chesham, England). Aliquots of 25 g of oil (triglycerides) were weighed directly into the reactor and 100 mL of distilled water was added. The reactor temperature 14090

dx.doi.org/10.1021/ie401875v | Ind. Eng. Chem. Res. 2013, 52, 14089−14098


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Figure 1. Diagram of the cascade continuous esterification reaction.

solids percentages, considering that, at −80 °C, samples are 100% solids. Flash Chromatography Purification. To obtain highpurity monoglyceride samples, the products of the esterification of fatty acids of N. lappaceum and glycerol were purified by flash chromatography (Combiflashretrieve, Serlabo, Entraigues sur la Sorgue, France) using a 12-g silica solid phase extraction cartridge (Grace Davison, Templemars, France), and eluted with diethyl ether. Fractions of 3 mL were collected in tubes, the solvent was evaporated and the molecules stored in darkness. The remaining DBSA is highly polar and, therefore, was not eluted but remained in the column; it was subsequently eluted with ethanol. Product purity was determined by GC.

was maintained at 37 °C (optimal temperature for the lipase), and the mixture was stirred at 750 rpm. Candida rugosa lipase, 1% of the mass of triglycerides, was dissolved in 25 mL of water, stirred for 15 min, and added in the reactor under stirring. Then, the reaction mixture was centrifuged at 10000g to separate the upper organic phase (fatty acids) from the aqueous phase containing water, glycerol, and recoverable active lipase. Cascade Continuous Esterification Reaction. The reaction scheme is depicted in Figure 1. At the end of the experiment, the reaction mixture was washed with a saturated NaCl solution (3 × 100 mL) in a separating funnel to remove excess glycerol and DBSA. Determination of the Thermal Properties. A DSC Mettler Toledo DSC1Stare System apparatus (Perkin−Elmer, USA) was used equipped with an IntraCooler cooling system. The purge gas was nitrogen at a rate of 20 mL/min. Indium (with a melting temperature of Tm = 156.6 °C, and a heat of formation of ΔHf = 28.45 J/g) and distilled water (Tm = 0 °C, ΔHf = 334.0 J/g) were used for calibration. Data were analyzed using Pyris software (Perkin−Elmer). Samples of ca. 4−10 mg were weighed into aluminum pans to the nearest 0.1 mg, and covers were hermetically sealed into place with O-rings. An empty, hermetically sealed aluminum pan was used as the reference. The temperature ramps in the calorimeter were as follows: heating from +25 °C to 110 °C, at a rate of 10 °C/min and held for 5 min, cooled to −80 °C at a rate of 10 °C/min, and held for 5 min. The sample was then heated to 110 °C at 10 °C/min. Two cycles were performed. Stare software v.11.0 was used to analyze and plot the thermal data. Melting and Crystallization. Characteristics of each sample in a DSC scan are described by various temperatures: the onset temperature (T0), the offset temperature (Tf) (points where the extrapolated leading edge of the endotherm/ exotherm intersects with baseline), and the various transition peak temperatures (temperature of maximum different heat flow) between T0 and Tf. Solid Fat Content. The amounts of solids in the fats as a function of the temperature were calculated using DSC experiments47 and the associated STARe software. Since the solid fat content (SFC) are dependent on temperature, these values may be presented as a function of temperature. Partial areas of the thermograms were calculated and correlated with

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RESULTS AND DISCUSSION Extraction of Oil from N. lappaceum and Its Fatty Acid Composition. The amounts of total reserve lipids extracted via the ASE method from the N. lappaceum kernel samples were 36.8% of the weight of the kernels. These values of oil content are comparable to the results reported by Augustin48 (total lipid content = 37.1%−38.9%), Solis-Fuentes49 (total lipid content = 33.4%), and Winayanuwattikun50 (total lipid content = 39%− 41%). The differences were explained by the process of extraction used. The ASE method was used and allows a high extraction rate at high pressure (100 bar) and high temperature (100 °C). The major advantage lies in the reduction in the time of extraction, 40 min with ASE, versus 6−7 h currently observed with the classical Soxhlet method. As mentioned previously and in Table 1 (presented later in this work), the fatty acid profiles were not modified. Differences may be explained by the different localizations of N. lappaceum trees, as well as the age of maturity of the rambutan fruits. These previous studies report that the major fatty acids were composed of C18:1 (38%−55%), C20:0 (22%−36%), and C20:1 (0%−7.2%). The composition of major fatty acids in our crude oil extracts was close in C18:1 (36.6%), C20:0 (38.1%), and C20:1 (6.8%). In their study, Winayanuwattikun et al.50 noted that N. lappaceum gives a high average productivity yield (2421 kg/ha versus 1690 kg/ha for peanut or 1260 kg/ha for soybean) and constitutes a promising alternative source of the unusual C20:0 and C20:1 fatty acids species. Lipase Hydrolysis. The crude N. lappaceum oil was enzymatically hydrolyzed in a stirred reactor in the presence 14091



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Figure 2. Kinetic of hydrolysis of N. lappaceum oil.

Table 1. Relative Fatty Acid Composition of N. lappaceum Crude Oil and Hydrolyzed N. lappaceum Crude Oila

formula C16:0 C18:0 C18:1 C18:2 C20:0 C20:1 C22:0

name palmitic acid stearic acid oleic acid linoleic acid arachidic acid (eicosanoic acid) gadoleic acid (11eicosenoic acid) behenic acid

N. lappaceum oil 76.8% free fatty acids

herring oil (Clupea harengus)b

unsaturated/saturated = 0.85, monounsaturated/saturated = 0.82

unsaturated/saturated = 3.69, monounsaturated/saturated = 1.54

± ± ± ± ±

0.1 0.18 0.98 0.05 0.97

13.3 1.1 14.7 1.1

6.8 ± 0.18

7.0 ± 0.18

17.9

3.6 ± 0.09

3.4 ± 0.09

N. lappaceum crude oil 4.2 ± 0.1 7.2 ± 0.19 36.6 ± 0.95 1.8 ± 0.05 38.1 ± 1.0

4.1 7.1 37.9 2.0 37.4

a

N. lappaceum crude oil glyceridic profile: 4.6% free fatty acids, 0.26% monoglycerides, 7.57% diglycerides, and 87.25% triglycerides. bData taken from ref 52.

Figure 3. Qualitative description of the esterification reaction of fatty acids (FAs) with glycerol.

of Candida rugosa lipase in an aqueous medium at 37 °C. Hydrosoluble Candida rugosa lipase was active at the water/oil interface and was reusable by recovering the aqueous phase. After 6 h of hydrolysis reaction, the free fatty acid content was 76.8%. The kinetics of the reaction showed that the hydrolysis was complete within 2 h (see Figure 2). The concentrations of reaction intermediates (diglycerides and monoglycerides) remained low: 12%−16% for diglycerides, and 4%−7% for monoglycerides. This is consistent with the observations made by Ibrahim et al.51 regarding the hydrolysis of olive and tallow oils with Candida rugosa lipase. These molecules, which are

present at low concentrations, can act as surfactants and compatibilizers of immiscible reactants in reactions. The relative composition of free fatty acids after hydrolysis is reported in Table 1. The fatty acid profile during the hydrolysis presents two main families of unsaturated and saturated longchain fatty acids: (i) oleic, gadoleic, and linoleic acids; and (ii) arachidic and stearic acids. The products of the hydrolysis of N. lappaceum oil was rich in long-chain saturated fatty acids (37.4% arachidic acid C20H40O2, C20:0), with a constant content in monounsaturated long-chain fatty acids (7.0% eicosenoic or gadoleic acid, C20:1). The fatty acid profiles before and after hydrolysis 14092



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synthesis of glycerol monooleate. The difference between the works was the source and nature of fatty acids. In our case, the free fatty acid content was lower (76.8% versus 98% for Eychenne et al.39). The nature of the fatty acids was diverse, with a broader distribution in our study (Table 1). In this process, the present study showed that the reactor design did not improve the catalytic and kinetic efficiency but allowed this reaction to be conducted continuously, without any loss of efficiency and selectivity. This is a contribution to the intensification of the process. The reaction medium was composed of a microemulsion containing saturated and monounsaturated long-chain fatty acids, monoglycerides, and diglycerides. It is observed that once the reaction temperature was reached, the reaction mixture became a homogeneous single-phase, although the constituents, glycerol and fatty acids, are immiscible. This was due to the combined action of the catalyst−surfactant DBSA, surfactants generated in situ, and temperature. This microemulsified reaction system behaved as a single phase, allowing optimal contact between reactants, with diffusion phenomena becoming nonlimiting, favoring mass transfer. The water formed during the esterification contributed to this microemulsified system and appeared to be “compartmentalized” and isolated from reagents. The resulting effect is a shift in the equilibrium that favors the production of monoglycerides. Molecular sieve was not needed in our case (see Table 2; 50.5% in MG and a FA decrease of 75.4%) to reach results similar to those of Eychenne et al.39 for the synthesis of glycerol monooleate in a batch reactor, using the same catalytic system but in presence of molecular sieve needed to shift the equilibrium of the reaction catching the produced water. The chemical catalysis assisted by the physicochemical phenomena induced a chemical intensification of the reaction. The fatty acid composition of MGs is detailed in Table 3: 38.1% was glycerol monooleate and 37.7% was glycerol monoarachidate, which was consistent with the fatty acid content of the oil before and after hydrolysis. The esterification reaction showed a good selectivity for MGs, because, when the MG content increased, the triglyceride content remained quite constant and the diglyceride content moderately increased (see Table 2). The FA profile of the final monoglyceride product was very close of the initial FA profile of the hydrolyzed oil. Saturated fatty acids C18:0, C20:0, and C22:0 are conserved and transformed to the corresponding saturated monoglycerides (MG-C18:0, MG-C20:0, and MG-C22:0, respectively), and monounsaturated fatty acids C18:1 and C20:1 are conserved and transformed to the corresponding monounsaturated monoglycerides (MG-C18:1 and MG-C20:1, respec-

was conserved. Very few plant oils present high levels of arachidic acid, except the case of jojoba, which is a mixture of esters of long-chain fatty acids and fatty alcohols. A high 11-eicosenoic acid content, such as the 7% oil content in N. lappaceum, is only found in fish oils: 17.9% C20:1 in herring oil, and 19.0% C20:1 in cod oil.52 Contrary to the oil containing a high monounsaturated/saturated FA ratio (e.g., 1.5 for peanut oil, 1.5 for cod liver oil, 1.45 for sardine, and 1.8 for olive oil),52 N. lappaceum oil contains very few polyunsaturated FAs. Thus, N. lappaceum presents an equilibrated fatty acid composition that is rich in monounsaturated fats, which is consistent with FAO recommendations.53 Monoglyceride Synthesis in a Cascade Continuous Reactor. The catalytic esterification of fatty acids with glycerol in a solvent-free medium was studied in the presence of dodecylbenzene sulfonic acid (DBSA). It is used as surfactant and acid catalyst in this pseudo-homogeneous three-phase system. Fatty acids and glycerol are immiscible, so DBSA was used to decrease the interfacial tension between these noncompatible reagents and allows good contact via emulsification,39 assisted by high shear of the Utraturax apparatus. DBSA can be recyclable. The reaction led to the formation of water and glyceride products and was reversible (see Figure 3). The molar ratios of the reagents determined the selectivity of the reaction. The kinetics of the esterification reaction are shown in Table 2. A high decrease of 75.4% in fatty Table 2. Glyceridic Profile of the Reaction Medium of the Esterification Reaction of N. lappaceum Fatty Acids with Glycerola Relative Composition (%) time (min)

TG

DG

MG

FA

FA decreaseb (% w/w)

0 15 45 60

3.7 2.5 1.6 1.1

13.2 28.1 28.5 32.9

6.3 50.5 51.5 51.6

76.8 18.9 18.4 14.4

75.4 76.0 81.3

a

Legend: TG, triglycerides; DG, diglycerides; MG, monoglycerides, and FA, fatty acids. bFA decrease = (%FAt0 − %FAt)/%FAt0.

acids was reached very quickly within 15 min, before passage into the column reactor containing the 3 Å molecular sieve and reached 81.3% after 60 min of reaction. The compositions of monoglycerides, diglycerides, and triglycerides remained constant within the range of 15−60 min (see Table 2). The MG content was 50.5% after 15 min and 51.6% at the end of the reaction. This result was very similar to that of the MG content obtained in the study of Eychenne et al.39 on the

Table 3. Composition of N. lappaceum MG Fractions Obtained by Flash Chromatography Purificationa Fatty Acid Content (% w/w) MG fraction

C16:0

C18:2

C18:1

C18:0

C20:1

C20:0

saturated content (% w/w)

monounsaturated content (% w/w)

purity (% w/w)

crude monoglyceride MG1 MG2 MG3 MG4 MG5

5.7 4.5 8.3 8.1 5.3

1.2 4.0 1.9 3.2 5.1

16.9 34.9 15.6 34.4 47.2

9.3 5.4 11.0 0.9 5.2

5.2 9.0 6.7 6.6 10.9

59.1 31.5 55.9 46.7 26.3

76.1 46.3 75.6 55.7 36.9

22.1 43.9 22.3 41.0 58.1

51.6 97.4 89.3 99.4 99.9 100

a Crude monoglycerides are composed of monoglycerides obtained after the esterification of N. lappaceum fatty acids with glycerol without purification; MGX (X = 1−5) denotes purified monoglyceride fractions. C16:0, palmitic acid; C18:2, linoleic acid; C18:1, oleic acid; C18:0, stearic acid; C20:1, eicosenoic acid; and C20:0, arachidic acid.

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Figure 4. N. lappaceum crude oil thermogram: () first cycle and (- - -) second cycle.

Figure 5. Crystallization and melting curves of MG1, MG2, MG3, MG4, and MG5 monoglyceride fraction at 10 °C/min. MGX (X = 1−5). Purified monoglyceride fractions. 14094



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Table 4. Values Obtained from DSC Melting and Crystallization Curvesa Melting (°C)

Crystallization (°C)

Temperature Range (°C)

Total ΔH (J/g)

Transition Temperatures (°C)

compound

T0

Tf

T0

Tf

melting

crystallization

supercooling temperature (°C)

N. lappaceum oil MG1 MG2 MG3

−34

24

22

−45

−69

72

2

58

67

−31, 6, 11, 21

−43, 4, 14, 19

−5 −5 3

49 51 37

6 1 −1

−30 −47 −33

−51 −52 −39

58 53 35

43 50 38

54 55 33

36 48 32

16, 24, 32 16, 24, 29, 42 15, 21, 30, 40

MG4

0

49

−1

−35

−51

39

50

49

34

MG5

5

40

5

−15

−40

46

34

35

20

15, 22, 30, 39, 47, 69 15, 23, 32

1, −7 −0.5, −3, −12 −3, −6, −7, −14, −22 30, −6, −8, −10, −14, −24 −1, 1.37

a

melting

crystallization

melting

crystallization

Legend: MGX (X = 1−5), purified monoglyceride fractions; T0, onset temperature; Tf, offset temperature.

our samples did not show the higher-temperature peak of crystallization at 30.8 °C. The MG1 crystallization curve (Figure 5) had one exothermic peak at 1 °C with a shoulder at −7 °C. The crystallization temperature range was 36 °C and ΔH = 58 J/g. MG2 displays two major peaks at −3 °C and −12 °C, with a shoulder at −0.5 °C. The crystallization temperature range was higher (48 °C) and the ΔH value was 53 J/g. The MG3 crystallization curve shows four peaks at −22, −14, −7, and −6 °C and a shoulder at −3 °C. Total ΔH and the temperature range of crystallization were 35 J/g and 32 °C, respectively. MG4 displayed six exothermic peaks, with those at −6, −8, −10, and −24 °C attributed to crystallization, and the peak at 30 °C attributed to the crystallization of a liquid crystal. The total ΔH value and the temperature range of crystallization were 39 J/g and 34 °C. The MG5 crystallization curve has one peak at 1 °C and a shoulder at −1 °C; the total ΔH value was 46 J/g, and the temperature range of crystallization was 20 °C. Melting. Figure 5 shows the melting profile of N. lappaceum crude oil. The phase transition corresponds to the melting of TAGs with different fatty acid compositions. There are three major peaks, at −31, 6, and 21 °C, with a shoulder at 11 °C. The phase change was complete at 30 °C. This differs from the thermal analysis of N. lappaceum oil from Mexico,49 for which the melting curve profile had more endothermic peaks and a higher final melting temperature (50.8 °C). The total heat of melting (ΔH) was −69 J/g, and the temperature range of our crude oil sample was 58 °C, versus values of 124.3 J/g and 66.4 °C, respectively, for the Mexican N. lappaceum oil. This difference of thermal behavior could be due to the fact that Solis-Fuentes et al.18 did not analyze crude oil as in our case, but, instead, purified oil via an adaptation of the Wesson method. This purified oil consisted of only neutral triglycerides and was free of free fatty acids, phosphates, impurities, and perhaps partial glycerides.57 Figure 5 shows the melting curves for pure MG samples, as well as MG1, MG2, MG3, MG4, and MG5. The MG1 melting curve displays two major endothermic peaks, at 16 and 24 °C, with a shoulder at 24 °C. The melting temperature range was 54 °C and the ΔH value was −51 J/g. The MG2 melting curve shows four peaks, at 16, 24, 32, and 42 °C. The melting temperature range and total ΔH values (55 °C and −52 J/g, respectively) are similar to those for MG1. The MG3 melting curve shows four peaks, at 15, 21, 30, and 40 °C. The total ΔH value is −39 J/g, and the temperature range of melting is 33 °C. The MG4 melting curve, similar to the crystallization curve, displays six endothermic peaks, at 15, 22, 30, 39, 47, and 69 °C.

tively). All analysis were carried out by GC and identification of molecules was performed by using standards. Physicochemical Characteristics of C18:1-, C20:0-, and C20:1-rich N. lappaceum Monoglycerides. Monoglycerides were purified from the products of esterification. Five fractions of monoglycerides were obtained: MG1, MG2, MG3, MG4, and MG5. The purity and fatty acid compositions of these fractions of monoglycerides are reported in Table 3. MG2, MG1, MG3, and MG4 present a high purityfrom 89.3% to 99.9%, respectively, determined via gas chromatography. Other compounds were exclusively diglycerides. MG5 was found pure at 100%. MG1 and MG3 are constituted by highly saturated fatty acid. MG5 contained less saturated fatty acid and more C18:1 and C20:1 unsaturated fatty acids (47.2% and 10.9%, respectively) than the other fractions of monoglycerides. Melting and crystallization require the intake or release of thermal enthalpy, and they are commonly used to characterize thermal behavior of oil and MG samples. Thermal curves for the MG fractions were determined by DSC (Figures 4 and 5), and the crystallization curves and melting curves (Figure 5) were obtained. The melting curves present complex features that are not easy to interpret, such as shoulders not separable from peaks, reflecting the complex nature of oil and MG samples. This is a consequence of the polymorphism of natural oils and fats. Unlike pure TAGs and MGs, the polymorphic form of mixtures of TAGs in N. lappaceum oil and in the derived MGs cannot be established unequivocally by DSC.54 This can only be achieved by X-ray diffraction analysis. Therefore, polymorphic transformations in samples were not determined in this study. Because of the complexity of the thermal events recorded, all melting and crystallization points were read at the maximum/minimum of either the endotherm or exotherm peaks. The designation of these transition temperatures, total ΔH, and supercooling temperature for melting and crystallization curves are indicated in Table 4. The peaks appearing at higher temperatures for each sample are characteristic of the most stable form of the molecule in the solid state.55 They generally have values very close to the melting temperatures.56 Crystallization. N. lappaceum oil displayed three exothermic regions (recall Figure 4). The higher-temperature region corresponds to the crystallization of saturated TAGs (for example, the stearin fraction), and the lower temperature region corresponds to the crystallization of unsaturated TAGs, including the olein fraction. Our crystallization curves are generally similar to those reported by Solis-Fuentes et al.49 in a calorimetric study of N. lappaceum oil from Mexico, except that 14095



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Figure 6. Solid fat content profile of N. lappaceum oil and monoglyceride fractions MG1, MG2, MG3, MG4, and MG5. MGX (X = 1−5) represents purified monoglyceride fractions.

The peak at 69 °C could the attributed to the transition of a liquid crystal. The total ΔH value is −51 J/g, and the temperature range of melting is 49 °C. The MG5 melting curve shows three peaks, at 15, 23, and 32 °C, and the total ΔH and temperature range of melting are 40 J/g and 35 °C. The thermal characteristics of various MG samples, as assessed by DSC melting and crystallization curves, can be characterized by various transition temperatures. However, comparison of these transition temperatures is difficult, because of their complex features. A more practical approach to differentiating the MGs is to consider variables such as Tf for melting curves, T0 for crystallization curves, temperature range (the difference between T0 and Tf), and supercooling temperature. The melting point is usually defined as the point at which a material changes from a solid to a liquid; however, natural fats do not have a true melting point. Pure compounds have sharp and well-defined melting points, but complex mixtures of compounds soften progressively with increasing temperature before becoming completely liquid. This melting procedure is further complicated by the fact that crystals can adopt several polymorphic modifications, depending on the composition and the temperature/time of pretreatment (tempering) of the sample. The various crystal forms are often stable enough to exhibit distinctive melting points; therefore, instead of a melting point, it is more appropriate to use melting range or melting interval. To determine the melting point, one point within the melting range must be selected by a defined method. Rigorous and specific definitions of the conditions of the pretreatment and the test procedure are required to determine the melting point. Many methods have been devised to determine the melting point or a point close to it; some involve direct observation, and some involve indirect and objective processes.58 The advantage of most melting point methods is their relative simplicity, but the dependence of the melting point on the sample pretreatment and on the method used are disadvantages. Several melting-point procedures for fats and oils have been standardized by the American Oil Chemists’ Society (AOCS) and other associations. These melting point methods differ considerably with regard to the end-point determination, conditioning of the sample, degree of automation, time requirements, attention required, and degree of melt, as well as in other ways. The melting point obtained by DSC is generally considered as the onset temperature, the inflection point of the melting curve and the solid line. In their work, de Man et al.54

considered the melting point to be the peak temperature calculated from a DSC melting curve. The true melting point had been considered to be the temperature measured at the end of the melting curve obtained by DSC, where the melting phenomenon finishes, that is when all crystals in the solid state became liquid. Siew et al.59 concluded that the melting point is essentially determined by the major peak. Chaiseri et al.,60 in a study of crystallization of cocoa butter, used DSC to measure the melting point of low and high melting fractions as the onset melting temperature. In a study of the effect of varying cooling and heating rates, Cebula et al.61 measured the melting point of pure triglycerides, and similarly considered the onset temperature to be the melting point. Tf corresponds to the end of the melting curve, and T0 corresponds to the beginning of the crystallization curve and characteristic of the apparition of solid crystals in the liquid (recall Table 4). These temperatures were taken as characteristic measures of melting and crystallization.62 Calorimetric behavior was evaluated after the thermal history of the samples was erased. Our N. lappaceum oil from Congo differed from the N. lappaceum oil of other origins, such as Mexico:18 although the fatty acid composition of the TAGs was similar, the melting temperature (Tf melting) was lower, even for our samples (recall Table 4). Lipochemical transformation of TAG into MGs changed the thermal properties. The melting points increased from 24 °C for N. lappaceum oil to 37−51 °C for MG fractions. The melting temperature range was lower for MGs with values ranging from 54 °C to 33 °C (the melting temperature range for N. lappaceum oil was 58 °C). Crystallization began at lower temperatures for MG fractions and occurred over a shorter temperature range: 67 °C for N. lappaceum oil versus 20 °C for MG5 or 48 °C for MG2. The absolute values of the total enthalpies of melting and crystallization were lower for all MG fractions than for N. lappaceum oil. N. lappaceum oil and MG fatty acid compositions were dominated by the ternary mixture of oleic acid/arachidic acid/11-eicosenoic acid. The length and the nature of the lipophilic fatty chains were presumably constant, because all samples were of the same origin and had very similar fatty acid compositions; therefore, this calorimetric study highlighted the influence of the hydrophilic part of the molecules. The three ester functions in TAG, and the one ester function and two free hydroxyls in MGs, played a major role in the organization of these lipid systems. The degree of esterification of the glycerol controls the physicochemical properties. 14096


Industrial & Engineering Chemistry Research Solid Fat Content. Figure 6 shows the solids profile of N. lappaceum oil and the MG fractions. It clearly appears that all MG fractions present a greater consistency than the N. lappaceum oil. While their FA compositions were different, the SFC profiles of the MG fractions were very close. Their solid fat profiles show that MG fractions have a greater consistency than tallow, lard, and palm oil, a close consistency to cocoa butter, and a lower consistency than hardened oils.63

CONCLUSION Enzymatic hydrolysis of Nephelium lappaceum (N. lappaceum) crude oil revealed that the ratio between the unsaturated fatty acid (FA) and saturated FA contents was 0.82. This oil, which is rich in essential fatty acids, is suitable for foods and has high chemical and physical stability. During the esterification of FAs released from N. lappaceum oil by enzymatic hydrolysis, the yield in C20:0 and C20:1 monoglycerides (MGs) reflected its FA composition. The reaction was performed in a complex system. Contact between the reagents and catalyst was favored by the organization of the medium in emulsions and microemulsions used as microreactors. The presence of long-chain compounds (C20:0 and C20:1) at high concentration in the mixture may affect the physical properties: in particular, they appear to result in melting temperatures that are high (51 °C) and crystallization temperatures that are low (−1 °C), as shown by the differential scanning calorimetry (DSC) technique. Controlling the ratio of unsaturated/saturated FAs in the synthesis of long-chain MG mixtures allows the structures of the resulting mixtures to be controlled. Therefore, the physicochemical properties can be of industrial interest, in particular, the thermal properties such as solid fat content (SFC), may be useful for applications in the food, cosmetics, and pharmaceuticals industries. This analysis of the lipochemistry of the N. lappaceum seed crude oil reveals the influence of pure isolated fatty acids FAs from vegetable oils on the synthesis of MGs at high purity. They present a complex mixture of crystalline states. N. lappaceum produces oil with a higher arachidic acid content than any other plant (>Jojoba), and it has a monounsaturated acid content (eicosenoic acid) comparable to that of herring oil; however, it is thermally more stable to oxidation. A method for lipotransformation of the N. lappaceum crude oil was described here. This is an alternative to TAG hydrogenation for producing oils with high melting temperature and higher SFC, similar to that of cocoa butter.64 This work also initiates a model system for studying the transition from native triglycerides to homogeneous triglycerides.

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REFERENCES

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S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

Thanks to the France/Congo Cooperation Commission Campus France, the “Agence Universitaire de la Francophonie”, Dr. P. HUET from ESPA Company (INPT Contract No. 20080071) and the French Agency EGIDE for financial support. The authors thank Muriel CERNY for her highly efficient technical assistance.

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*Tel.:+33 (0)5 34 32 35 20. Fax: +33 (0)5 34 32 35 38. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 14097



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