Synthesis and physicochemical characterization of mixed diacid triglycerides that contain elaidic acid

Article abstract of DOI:10.1007/s11746-998-0043-6

The synthesis of symmetrical and asymmetrical palmito- and stearo-elaidic triglycerides (PEP, SES, EPP, PEE, ESS, and SEE, in which P = palmitic, S = stearic, and E = elaidic acid) was undertaken to investigate their polymorphism. The chemical pathways and the purification steps, including crystallization and adsorption chromatography, are described. The different chromatographic analyses (gas-liquid chromatography: carbon number profile and fatty acid methyl ester profile, and high-performance liquid chromatography) revealed that the purity of the synthesized products was superior to 99% except for SES (>96%). The thermal behavior, as well as the polymorphism of these triglycerides, has been investigated by means of differential scanning calorimetry and powder X-ray diffraction spectroscopy at variable temperatures. The six compounds crystallize according to a double chainlength packing. The most stable polymorphic form of palmito-elaidic triglycerides belongs to the β′ variety, whereas the stearo-elaidic triglycerides are β stable.

Full text of DOI:10.1007/s11746-998-0043-6

Synthesis and Physicochemical Characterization
of Mixed Diacid Triglycerides That Contain Elaidic Acid
P. Elisabettinia, G. Lognayb, A. Desmedta, C. Culota, N. Istassea, E. Deffensec,1, and F. Duranta,*
aFacultés Universitaires Notre-Dame de la Paix, Laboratoire de Chimie Moléculaire Structurale, B-5000 Namur,
Belgium, bFaculté Universitaire des Sciences Agronomiques, UER de Chimie Générale et Organique,
Gembloux, Belgium, and cFractionnement Tirtiaux s.a., Fleurus, Belgium
ABSTRACT: The synthesis of symmetrical and asymmetrical oil. Because this process raises the melting point, consistency
palmito- and stearo-elaidic triglycerides (PEP, SES, EPP, PEE,
ESS, and SEE, in which P = palmitic, S = stearic, and E = elaidic
acid) was undertaken to investigate their polymorphism. The
chemical pathways and the purification steps, including crystal-
lization and adsorption chromatography, are described. The dif-
ferent chromatographic analyses (gas–liquid chromatography:
carbon number profile and fatty acid methyl ester profile, and
high-performance liquid chromatography) revealed that the pu-
rity of the synthesized products was superior to 99% except for
SES (>96%). The thermal behavior, as well as the polymorphism
of these triglycerides, has been investigated by means of differ-
as well as stability against oxidation of the final product is im-
proved. However, positional isomerization of the cis double
bond takes place and leads to the occurrence of trans isomers
as by-products (3). Such isomers drastically change the
physicochemical characteristics of refined fats.
Because hydrogenated fats are largely used in the food in-
dustry (margarine, shortenings, etc.) and because little is
known about the physicochemical behavior of triglycerides
that contain trans fatty acids, we investigated the polymor-
phism of such triglycerides. We focused on triglycerides that
ential scanning calorimetry and powder X-ray diffraction spec- contain palmitic and stearic acids—the most abundant satu-
troscopy at variable temperatures. The six compounds crystal-
lize according to a double chainlength packing. The most sta-
ble polymorphic form of palmito-elaidic triglycerides belongs
to the β’ variety, whereas the stearo-elaidic triglycerides are β
stable.
rated fatty acids—and elaidic acid, which is the trans ho-
molog of oleic acid. Both symmetrical (PEP and SES, where
P = palmitic, E = elaidic, and S = stearic acids) and asymmet-
rical triglycerides were studied (PEE, EPP, SEE, and ESS).
The symmetrical diacid triglycerides studied are the trans
counterparts of two of the three major constituents of cocoa
butter (POP and SOS, where O = oleic acid).
JAOCS 75, 285–291 (1998).
KEY WORDS: Elaidic acid, mixed diacid triglycerides, poly-
Mixed diacid triglycerides that contain elaidic acid are not
commercially available. Therefore, we first had to synthesize
them. The aim of the present work was to prepare and purify the
six aforementioned mixed diacid triglycerides to establish their
polymorphism (4,5). Particular attention was given to the pu-
rification steps because the polymorphic transitions of triglyc-
erides are known to be strongly influenced by other lipid mate-
rials that are coproduced during the different syntheses (6).
morphism, synthesis.
Natural fats are known for their complex thermal and struc-
tural behavior, which is closely related to their major compo-
nents: triglycerides. Triglycerides are characterized by multi-
ple melting due to polymorphism. Basically, they exhibit
three different polymorphic forms: α, β′ and β, which are eas-
ily pointed out by X-ray measurements. In crude oils and fats,
triglycerides are almost exclusively derived from saturated
and cis unsaturated fatty acids. Their polymorphism has been
widely investigated (1).
To diversify their use, natural fats and oils are industrially
transformed according to different techniques, such as hydro-
genation, fractionation, and interesterification (2). Hydro-
genation aims at reducing the degree of unsaturation of the
EXPERIMENTAL PROCEDURES
Reagents for synthesis. Glycerol used in the synthesis of the
symmetrical triglycerides was purchased from Sigma Chemi-
cal Co. (St. Louis, MO). Isopropylidene glycerol used for the
synthesis of the asymmetrical triglycerides was obtained from
Solvay (Brussels, Belgium). Palmitic, stearic and elaidic
acids, elaidic anhydride, 1,3-dipalmitin, oxalyl chloride, p-
toluene sulfonic acid (pTSA), 4-dimethylaminopyridine, and
pyridine were purchased from Sigma. Sodium acetate and
boric acid were obtained from Janssen Chemica (Beerse, Bel-
gium).
1Present address: Terre l’Oreye 21, B-6032 Charleroi, Belgium.
*To whom correspondence should be addressed at Facultés Universitaires
Notre-Dame de la Paix, Laboratoire de Chimie Moléculaire Structurale, 61
rue de Bruxelles, B-5000 Namur, Belgium.
E-mail: francois.durant@fundp.ac.be.
Thin-layer chromatography (TLC). TLC analyses were
Copyright © 1998 by AOCS Press
285
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286
P. ELISABETTINI ET AL.
matography (HPLC) according to a method previously de-
scribed (10).
performed on 8 × 4 cm silica gel plates (Macherey-Nagel,
Düren, Germany). The eluent, leading to the best achievable
separation of the different lipid classes, consisted of a mixture
of petroleum ether [b.p. = 60–80°C]/diethyl ether/formic acid
(60:40:1.5, vol/vol/vol). After elution, the plates were dried,
sprayed with a 0.2% ethanolic solution of 2,7-dichlorofluores-
cein, and examined under ultraviolet radiation at 254 nm.
Gas–liquid chromatography (GLC). The molar proportion
of the acyl residues in the triglycerides was determined by GLC
analysis of their corresponding fatty acid methyl esters (FAME)
(7) and compared to the “theoretical values.” A Hewlett-
Packard 5880A chromatograph (Palo Alto, CA), fitted with an
on-column injector and a flame-ionization detector (FID) at
250°C, was used. The operating conditions were as follows: 25
m × 0.25 mm CP-WAX 52CB column (Chrompack, Middle-
burg, The Netherlands) with 0.2 µm film thickness; temperature
program from 55 to 150°C at 30°C/min and from 150 to 220°C
at 5°C/min; helium at 50 kPa was used as carrier gas. The pu-
rity of the synthesized triglycerides was checked by establish-
ing the carbon number profile (CNP) (8,9) on a 3 m × 0.32 mm
CP-Sil 5CB column (Chrompack) 0.2 µm film thickness; tem-
perature program from 60 to 340°C at 15°C/min; carrier gas he-
lium at 70 kPa; Carlo Erba MEGA 5160 (Milan, Italy),
equipped with an on-column injector and a FID at 350°C. For
SES and PEP, the proportion of their positional isomers (i.e.,
ESS and EPP) was measured by high-performance liquid chro-
Differential scanning calorimetry (DSC). Determination of
melting and transition points, as well as enthalpies of fusion
(∆H), were performed on a Perkin-Elmer DSC-7 (Norwalk,
CT), assisted by an IBM PS/2 computer and coupled to a TAC
7/3 control instrument (Perkin-Elmer). An intracooler allowed
measurements at temperatures below 0°C. Samples of 3–5 mg
were sealed into aluminum pans; a similar empty pan served as
reference.
Crystallographic measurements. Powder x-ray diffraction
(XRD) fraction patterns were recorded on a Philips PW 1710
(Eindhoven, The Netherlands) diffractometer (Cu tube, λ =
1.54178 Å), which was composed of a camera, equipped with
a thermostat unit (Huber HS-60; Offenburg-Elgersweier, Ger-
many; TTK-Anton Paar, Graz, Austria), a temperature control
system (Pt 100 probe) and connected to a heating (20 to 300°C)
and to a cooling device that allowed a minimum temperature
of −40°C. Gaseous nitrogen flow prevented condensation of
water during measurements at low temperatures. The diffrac-
tometer was coupled to a Digital Micro-Vax II system (Digital
Equipment Corporation, Maynard, MA) that allowed data
treatment.
Synthesis and purification of symmetrical triglycerides
(PEP and SES). The chemical synthesis of symmetrical
mixed triglycerides takes place in two steps (Scheme 1). It
SCHEME 1
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SYNTHESIS AND CHARACTERIZATION OF DIACID TRIGLYCERIDES
287
begins with the preparation of 1,3-diglyceride from glycerol
Purification of the triglycerides. A TLC analysis showed
and the corresponding fatty acid in the presence of pTSA as a the presence in the raw product of triglyceride, free fatty acid,
catalyst. We only synthesized 1,3-distearin; 1,3-dipalmitin is 1,3-diglyceride, and traces of ethyl elaidate. This latter is a
commercially available. In the second step, the triglyceride is by-product of the reaction. Its presence is due to traces of
obtained by acylation of 1,3-distearin or 1,3-dipalmitin with ethanol remaining in chloroform (stabilizer). The triglyceride
elaidic anhydride in the presence of 4-dimethylaminopyri- is purified by column chromatography. The glass column (15
dine.
× 1.4 cm) is packed with Machery-Nagel G60 silica gel
Synthesis of 1,3-distearin. In a 250-mL two-necked round- (70–230 mesh) at 5% humidity. n-Hexane/diethyl ether mix-
bottomed flask, topped by a Hartman apparatus, 3.00 g glycerol tures are used for the elution. For the purification of SES, the
(0.033 mol), 18.78 g stearic acid (0.066 mol), and 0.5 g pTSA elution with 85 mL of n-hexane/diethyl ether (9:1, vol/vol)
(0.0029 mol) are dissolved in 100 mL anhydrous chloroform and allows the separation of ethyl elaidate. SES is then eluted with
reflux-heated for 3.5 h. Afterward, the reaction mixture is cooled 85 mL of n-hexane/diethyl ether (8:2, vol/vol). For PEP, 75
and neutralized by 0.50 g sodium acetate (0.006 mol). The con- mL of the same eluents are used. The final step of the purifi-
tent of the round-bottomed flask is transferred into a separatory cation procedure consists in recrystallization in an ethanol/di-
funnel, and the flask is rinsed out with 50 mL chloroform. The ethyl ether mixture (7:3 vol/vol; 100 mL/g of purified triglyc-
solution is then washed several times with deionized water. The eride) to eliminate possible residual colloidal silica. Five syn-
white emulsion at the water/chloroform interface is eliminated theses of SES and PEP were carried out. After purification,
with the aqueous phase. The solution is dried over MgSO4, and we recovered 1.06 g of SES and 0.99 g of PEP. The yield of
chloroform is evaporated at 35°C under reduced pressure. It is the reaction (acylation of 1,3-diglyceride by elaidic anhy-
imperative the reaction takes place in anhydrous conditions to dride) is about 40%. The GLC analyses of SES and PEP as a
prevent hydrolysis of the formed diglyceride.
function of the carbon number reveal purities superior to 99%
Purification of 1,3-distearin. TLC analysis of the raw prod- (99.5% for SES and 99.6% for PEP). The FAME molar ratios
uct revealed the occurrence of 1,3-diglyceride, 1,2-diglyceride, (molar percentage of elaidic acid/molar percentage of stearic
monoglyceride, and free fatty acid. To isolate 1,3-distearin, the or palmitic acid: E/S and E/P), determined by GLC, were
raw product is first purified by crystallization in a 80:20, 0.489 and 0.495 for E/S and E/P, respectively. They are in line
vol/vol petroleum ether [b.p. = 40–65°C]/distilled ethanol mix- with the theoretical ideal value of 0.500. The “isomeric” pu-
ture (15 mL/g of raw product). After a night at 6°C, the precip- rity was checked by HPLC of the epoxidated triglycerides
itate is filtered in a constant-temperature Buchner funnel kept (10), allowing quantitation of the symmetrical and asymmet-
at the same temperature. This operation allows elimination of rical isomers. The purity of PEP was 99.7%, and that of SES
the majority of 1,2-distearin and monostearin. Two or three ad- attained 96.3%. The impurity has not yet been identified.
ditional recrystallizations at controlled temperature are neces-
Synthesis and purification of asymmetrical triglycerides
sary to isolate pure 1,3-distearin. For that purpose, the mixture (PEE, EPP, SEE, and ESS). The synthesis of asymmetrical
to purify and the crystallization solvent are held at 50°C until mixed triglycerides is also a two-step reaction (Scheme 2).
complete dissolution. It is then cooled at constant rate The first step consists in the preparation of the 1-monoglyc-
(1°C/min). As soon as crystals appear, the temperature of the eride, i.e., 1-monoelaidin, from isopropylidene glycerol and
bath is kept constant, and the solution is filtered at the same elaidic acid; 1-monopalmitin and 1-monostearin are commer-
temperature. Synthesis of the diglycerides is repeated five cially available. In a second step, the triglyceride is obtained
times; 2.55 g of 1,3-distearin was recovered. Its purity, checked by esterification of monoelaidin by stearoyl (for ESS) or
by gas GLC, was greater than 99% but the yield of the reaction palmitoyl chloride (for EPP), or by acylation with the elaidoyl
was only on the order of 12%.
chloride of monostearin (for SEE) or monopalmitin (for
Synthesis of PEP and SES. 1,3-Distearin (0.480 g) or 0.437 PEE). The chlorides have been previously synthesized from
g 1,3-dipalmitin (7.68 · 10−4 mol) and 0.005 g of 4-dimethyl- the corresponding fatty acid and oxalyl chloride.
aminopyridine (4.1 · 10−5 mol) are solubilized in 10 mL of
Synthesis of fatty acid chlorides. Palmitic (5.00 g = 0.019
anhydrous chloroform (ethanol-free) inside a three-necked mol), stearic (0.018 mol), or elaidic acid (0.018 mol) is dis-
round-bottomed flask, which is maintained at 10°C. Elaidic solved in 25 mL of anhydrous benzene. Oxalyl chloride (3.5 g
anhydride (0.250 g = 4.59 · 10−4 mol) dissolved in 3 mL = 0.028 mol) is then added, drop by drop, to this mixture, which
choloroform is added drop by drop under constant stirring is continuously stirred. The reaction takes place at room tem-
with the help of a dropping funnel. When all of the anhydride perature for 3 d. Progress of the synthesis is followed by infrared
has been added, the temperature is raised to 30°C, and the re- spectroscopy: disappearance of the C=O absorption band of the
action is allowed to proceed for 50 min. Afterward, the cata- free fatty acid (1720 cm−1) and appearance of the band charac-
lyst is neutralized with 2 mL HCl (0.5M), and the remaining teristic of the carbonyl group of an acid chloride (1800 cm−1).
anhydride is hydrolyzed by 2 mL water. The solution is trans-
Synthesis of 1-monoelaidin. In a 250-mL two-necked round-
ferred in a separatory funnel, washed three times with 2 mL bottomed flask, topped by a Hartman apparatus, 5.00 g iso-
saturated NaHCO3 solution, and dried over anhydrous propylidene glycerol (0.038 mol) (11), 10.17 g elaidic acid
MgSO4. The solvent is evaporated at 35°C under reduced (0.036 mol), and 0.30 g pTSA (0.0019 mol) are dissolved in 30
pressure.
mL anhydrous chloroform and reflux-heated for 2 h. The mix-
JAOCS, Vol. 75, no. 2 (1998)

288
P. ELISABETTINI ET AL.
SCHEME 2
ture is then cooled, and the excess pTSA is neutralized by by a condenser, 1.00 g 1-monopalmitin (0.0030 mol),
adding 0.3 g sodium acetate (0.0036 mol). The solution is 1-monostearin (0.0028 mol), or 1-monoelaidin (0.0028 mol)
washed with deionized water, the aqueous phase is decanted, and 0.66 g pyridine (8.3 · 10−5 mol) are dissolved in 50 mL
and the rest of the solution is dried over MgSO4; chloroform is anhydrous chloroform. By a separatory funnel, 2.55 g of
then evaporated under reduced pressure. 2-Methoxyethanol (60 elaidoyl (0.007 mol) or stearoyl (0.007 mol) chloride or 1.16
mL) and 20 g boric acid are then added to the product, and this g palmitoyl chloride (0.0045 mol) is then added drop by drop.
mixture is reflux-heated for 40 min. After reaction, the mixture The mixture is reflux-heated for 4 h. After reaction, the mix-
is dissolved in 300 mL diethyl ether, rinsed out with deionized ture is dissolved into 500 mL petroleum ether [b.p. =
water, and dried over MgSO4. The solvent is finally evaporated 40–65°C]/diethyl ether (50:50, vol/vol). The excess chloride
at 40°C under reduced pressure.
is neutralized by adding 150 mL distilled water, and pyridine
Purification of 1-monoelaidin. According to TLC analy- is neutralized by a 1% aqueous hydrochloric acid solution.
sis, the raw product is a mixture of monoglycerides, diglyc- The organic phase is decanted and dried over anhydrous
erides, and elaidic acid. Two crystallizations, in a diethyl MgSO4. The solvent is finally evaporated under reduced pres-
ether/hexane mixture (80:20, vol/vol) are necessary to isolate sure at 40°C.
1-monoelaidin. The first one is performed at −18°C for 2 h
Purification of PEE, SEE, EPP, and ESS. TLC analyses re-
with 20 mL solvent/g product, while the second is made vealed that the raw product is a mixture of triglycerides,
overnight at 6°C in 15 mL solvent/g product. About 1.5 g of diglycerides, monoglycerides, and free fatty acids. Two pu-
1-monoelaidin were obtained per synthesis; the yield of the rification steps are necessary to isolate the triglycerides. A
reaction was approximately 11%.
first crystallization, which allows elimination of the residual
Synthesis of PEE, SEE, EPP, and ESS. Triglyceride PEE fatty acids, is performed in an ethanol/diethyl ether mixture
(or SEE) is obtained by reaction between 1-monopalmitin (or (80:20, vol/vol; 100 mL/g of raw product). The triglyceride is
1-monostearin) and elaidoyl chloride. EPP (or ESS) is syn- then purified by adsorption chromatography. The glass col-
thesized from 1-monoelaidin and palmitoyl (or stearoyl) chlo- umn (15 × 1.4 cm) is packed with G60 silica gel (70–230
ride. In a 1000-mL two-necked round-bottomed flask, topped mesh) at 5% humidity. A benzene/hexane mixture (50:50,
JAOCS, Vol. 75, no. 2 (1998)

SYNTHESIS AND CHARACTERIZATION OF DIACID TRIGLYCERIDES
289
TABLE 1
vol/vol) is used as eluent. This solvent mixture (100 mL) is
Crystallographic Data [short (Å) and long (Å) spacings]
used to elute the fatty acids, mono- and diglycerides. After-
ward, benzene (300 mL) is used to elute the triglycerides. The
recovered product is finally recrystallized in an ethanol/di-
ethyl ether mixture as specified above. Yields of 0.185 g PEE
(or 0.289 g SEE) and 0.250 g EPP (or 0.281 g of ESS) are ob-
tained per synthesis after purification. Although the global
yields of synthesis are low (around 10%), highly purified
triglycerides are obtained. Indeed, the CNP recorded for the
different molecules indicate purities superior to 99%, while
the FAME mole% ratios are close to the theoretical values:
2.000 [for SEE (2.014) and PEE (1.984)] and 0.500 [for ESS
(0.503) and EPP (0.501)]. For ESS and SEE these values are
presented as mole% methyl elaidate/mole% methyl stearate;
for EPP and PEE, mole% methyl elaidate/mole% methyl
palmitate.
of PEP, EPP, and PEE Obtained After a Dynamic Processa
α-2
β′ -2
β′ -2
1
2
SS
4.2 (strong)
4.2 (very strong)
3.9 (medium)
4.4 (medium)
4.2 (very strong)
4.1 (medium)
3.9 (strong)
PEP
EPP
PEE
LSb 48
47
44
SS
4.2 (very strong)
4.3 (very strong)
3.9 (medium)
4.4 (weak)
4.3 (strong)
4.2 (weak)
3.9 (strong)
LSb 48
SS
46
44
4.2 (very strong)
4.2 (strong)
3.9 (medium)
4.4 (weak)
4.2 (strong)
4.1 (weak)
3.9 (strong)
LSb 49
47
45
aSS = short spacings; LS = long spacings.
bMean reticular distance calculated from odd diffraction orders n = 1 and
n = 3. P, palmitic; E, elaidic.
RESULTS AND DISCUSSION
Physicochemical characterization of the synthesized triglyc-
erides. The thermal behavior as well as the polymorphism of
the pure synthesized compounds (i.e., the ability to crystallize
in different crystal forms) was studied by means of DSC and
powder XRD at variable temperature. To point out both stable
and metastable polymorphic forms, the samples were condi-
tioned according to a thermal treatment called dynamic
process. The dynamic process consisted of a three-step proce-
dure: complete melting of the triglycerides at temperatures at
least 20°C above their melting point, quenching to −40°C at
50°C/min, and heating at constant rate (5°C/min) until com-
plete fusion.
Description of palmito-elaidic triglycerides (PEP, EPP, and
PEE). After complete melting and rapid cooling, PEP crystal-
lizes in the α form. During heating at constant rate, this form
transforms in β2′ form and finally melts under β1′ form. For EPP,
the less stable form identified, according to the same thermal
treatment, is the α form as well. However, during heating, the
α form melts and recrystallizes in the β′2 form, which quickly
evolves toward the β′ form. As far as PEE is concerned, the
1
same three polymorphic forms are indicated. All palmito-
elaidic triglycerides studied present three polymorphic forms:
α, β2′ and β1′, which crystallize in double chainlength packing.
Crystallographic data of both symmetrical and asymmetrical
triglycerides, PEP, EPP and PEE, are summarized in Table 1.
In addition, at low temperature, the diffraction line correspond-
ing to the α form of each triglyceride is broadened, suggesting
the presence of a sub-α form. A β form was never isolated, nei-
ther by dynamic process nor after long stabilization times of
the β1′ form. The β′ → β transition is probably hindered by an
important interpenetration of the palmitic and elaidic hydrocar-
bon chains at the methyl-end group planes. Therefore, the β′
form is the thermodynamically stable form of all those triglyc1-
erides. The DSC curves provide different melting and transi-
tion temperatures as well as the enthalpy of fusion of the poly-
morphic forms (Fig. 1). A symmetric arrangement of the hy-
FIG. 1. Differential scanning calorimetry curves of PEP, EPP, and PEE
(P = palmitic, E = elaidic) recorded according to the dynamic process.
Melting and transition temperatures: °C—Enthalpies of fusion (∆H):
drocarbon chains around the glycerol moiety induces better kcal/mol.
JAOCS, Vol. 75, no. 2 (1998)

290
P. ELISABETTINI ET AL.
TABLE 2
Crystallographic Data [short (Å) and long (Å) spacings] of SES, ESS, and SEE Obtained After a Dynamic Process
sub-α-2
α-2
4.2 (very strong)
β′ -2
β′ -2
β-2
4.6 (strong)
3.9 (medium)
3.8 (medium)
2
1
SS
4.1 (strong)
4.25 (medium)
4.0 (weak)
SES
ESS
SEE
LSb
SS
56
55
4.2 (very strong)
54
48
4.1 (strong)
4.3 (weak)c
4.4 (very weak)
5.3 (medium)
4.6 (very strong)
3.9 (strong)
3.7 (strong)
45
4.0 (medium)c
4.3 (very strong)
4.1 (very weak)
3.9 (strong)
LS
SS
51
52
4.2 (very strong)
47
47
4.1 (strong)
5.3 (weak)
4.6 (very strong)
3.9 (strong)
3.6 (strong)
LS
46
49
44
aSS = short spacings; LS = long spacings.
bMean reticular distance calculated from odd diffraction orders n = 1 and n = 3.
cObtained by cooling from the melt at 42°C.
stability of the β′ crystalline lattice than an asymmetric one
1
(higher melting temperature and ∆H of fusion). Moreover, the
α → β′ transition of PEP takes place in the solid state, while
this transition is preceeded by melting of the α form (liquid α
= Lα) for EPP and PEE.
Description of stearo-elaidic triglycerides (SES, ESS, and
SEE). After melting and quenching, SES, ESS and SEE crys-
tallize in a sub-α form (X-ray pattern characterized by a
broadened diffraction line at 4.1Å). During constant heating,
this form successively transforms into α, β′ and β forms.
However, the β′ form is only observed for SES and ESS. In-
deed, it was not possible to isolate the β′ variety for SEE as
for EEE (12). All polymorphic forms of the three triglycerides
adopt a double chainlength packing. The crystallographic
data are presented in Table 2. Thermal data, i.e., melting and
transition temperatures, as well as enthalpies of fusion, are
summarized in Figure 2. It appears that (like palmito-elaidic
compounds) a better stability of the most stable form (in this
case β) is obtained when the arrangement of the hydrocarbon
chains around the glycerol is symmetric (higher melting
points and ∆H of fusion). Moreover, as for EPP, the stability
of the β form of ESS is low. A particularity of the β′ → β tran-
sition of ESS is that it is preceded by melting of the β′ form
(liquid state called Lβ′). As mentioned for EPP and PEE, the
α → β′ transition of the three stearo-elaidic compounds takes
place via a liquid state called Lα. For SEE, this unstable liq-
uid phase directly transforms in a β form.
In conclusion, the polymorphic behavior of high-purity
palmito- and stearo-elaidic triglycerides can be summarized
as follows:
PEP:
α-2 → β2′-2 → β1′-2
[1]
EPP and PEE: α-2 → Lα → β2′-2 → β1′-2
[2]
[3]
SES:
ESS:
SEE:
sub-α-2 → α-2 → Lα → β2′-2 → β-2
sub-α-2 → α-2 → Lα → β2′-2 → β1′-2 → β-2 [4]
sub-α-2 → α-2 → Lα → β-2
FIG. 2. Differential scanning calorimetry curves of SES, ESS, and SEE
(S = stearic, E = elacidic) recorded according to the dynamic process.
Melting and transition temperatures: °C—enthalpies of fusion
(∆H):kcal/mol.
[5]
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SYNTHESIS AND CHARACTERIZATION OF DIACID TRIGLYCERIDES
291
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de la Paix, Namur, Belgium, 1994.
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Mori, Polymorphism of POP and SOS. I. Occurrence and Poly-
morphic Transformation, J. Am. Oil Chem. Soc. 66:664–674
(1989).
All palmito-elaidic compounds are stable in a β′ polymorphic
form, whereas stearo-elaidic triglycerides are β-stable. The
six triglycerides studied crystallize according to a double
chainlength longitudinal packing. For these palmito- and
stearo-elaidic series, a symmetric arrangement of the hydro-
carbon chains around the glycerol induces better stability of
the stable polymorphic form than an asymmetric one. Except
for PEP, all α → β′ (or α → β for SEE) transitions take place
via a liquid state called Lα.
7. IUPAC, Standard Methods for the Analysis of Oils, Fats and
Derivatives, 2.301—Preparation of Fatty Acid Methyl Esters,
International Union of Pure and Applied Chemistry, Pergamon
Press, Oxford, United Kingdom, 1979.
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Processed Soy Oil by Gas Chromatography, J. Am. Oil Chem.
Soc. 58:215–227 (1981).
ACKNOWLEDGMENTS
Paola Elisabettini and Christine Culot are indebted to I.R.S.I.A. (In-
stitut pour l’Encouragement de la Recherche Scientifique dans l’In-
dustrie et l’Agriculture) for financial support.
9. Christie, W.W., Gas Chromatography of Lipids—A Practical
Guide, The Oily Press, Ayr, 1989.
10. Deffense, E., Nouvelle méthode d’analyse pour séparer, via
HPLC, les isomères de position 1-2 et 1-3 des triglycérides
mono-insaturés des graisses végétales, Rev. Franç. Corps Gras
n°1/2:33–39 (1993).
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Am. Oil Chem. Soc. 44:381–393 (1967).
12. Elisabettini, P., A. Desmedt, V. Gibon, and F. Durant, Effect of
Sorbitan Tristearate on the Thermal and Structural Properties of
Monoacid Triglycerides—Influence of a “Cis” or “Trans” Dou-
ble Bond, Fat Sci. Technol. 97:65–69 (1995).
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[Received November 14, 1996; accepted April 21, 1997]
JAOCS, Vol. 75, no. 2 (1998)