Food Structure
Volume 6 Number 2 Article 7
1987
Physical and Molecular Properties of Lipid Polymorphs - A Review
K. Sato
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Recommended Citation Sato, K. (1987) "Physical and Molecular Properties of Lipid Polymorphs - A Review," Food Structure: Vol. 6 : No. 2 , Article 7. Available at: https://digitalcommons.usu.edu/foodmicrostructure/vol6/iss2/7
This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Food Structure by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. FOOD MICROSTRUCTURE, Vol. 6 (1987), pp. 151 - 159 0730-54 19/87$3 . 00+ . 00 Scanning Microscopy International, Chicago (AMF 0' Hare), I L 60666 USA
PHYSICAL AND MOLECULAR PROPERTIES OF LIPID POLYMORI'HS - A REVIEW -
K. Sa10
Faculty of Applied Biolog ical Sc ience. Hiroshima Un ive rsity Fukuyama 720. Japan Ph one No. 0849- 24- 6211
Abstr:.tct Introduction
The phys ical and molecul ar properties of the polymorphism The physical a nd molecul ar properties of lipid polymorphs of stearic acid. oleic acid and SOS (1. 3-distearoyl-2-olcoyl gly have drawn the attention of many investigators in the fields of cerol) are comparatively di!<~cu~~cd. Temperature depende nce biological sciences and oil chemical technology. This is due to of Gibbs energy (G·T relation) of three polymorphs of stearic the Fact that the pol ymorphi sm or lip ids is highly releva nt in acid: A. 8 and C. revealed close relationships to each oth er. biologica l sys te ms, and also decisive to the phys ical properties The molecular structures subtly differed in these polymorphs. of foods. cosmetics, etc .. which comprise lipids as the main In comrast. three pol ymorphs of oleic acid . a . Band )'. ex co mpound~ of solid fat s hibited remarkably diflerent characteristics. G-T relation showed The polymorphism may be dbcussed in 1c rms of thermody· more diversifi ed ICatures: in pa rti cular. the melling points of namic stability. c rys 1al packing and molecular conformation as a and (3 diffe r by 3°C. An ordcr·disordcr transf( mllation oc· far as the phys ica l aspects arc concerned . The fundamcnlal s of currcd between a ancl -y, as a result of confOrmational dis· the macroscopic ICaturcs of polymorphism, such as morphology. orde ring in the ponion of the oleic acid molecul e from the double solidificati on kinetics and so on. may be explained in terms of bond to the terminal methyl group in a. Finally. five polymorph:-. these phy~ica l a~pccl'i. Each of the above fac to rs is highly dcpen· of SOS we re newly presented, a, "f, pseudo·W . fl1 and 13 t dent on the molecular species which co n ~ titut c 1hc li pid. under X·ray spectra and the rmal behaviors proved thai the above li ve a given set of cx1e rnal condi1 ions. This is easil y seen if one com· fo rms arc the independent polymorphs. The author discu:-.scd pares the melting points of stearic acid (69.6°C) wi th oleic acid the multiple polymorphism of SOS. taking in to accoun1 the (16.2°C of !he higiHnclting polymorph). Obviously. this differ· lame ll ar sorting of stearic/oleic acid!> c hain:. accompanied with e ncc is the re:-. ult of a drastic redut·tion in the chain -packing the change in the c hai n length !.tructure. In relatton to the poly· e ne rgy by the inmxluction of one l'is·doublc bond at the ccn· mo rphism of SOS and othe r 1. 3-disaturatcd - l ·tlleoyl glyce· 1ral posi1ion of the polymcthylcnc chain . rides. the author emphasizes 1he possibili1 y thai th.:: convcr:-.ion Very recently, many investigators have tried 10 e lucidate the from Form V 10 VI in cocoa buller might be caused princi pall y physical and molecular propert ies of some principal fatty acids through the polymorphi c trans!Ormalion from (3~ to (3 1 of the and triglyccridcs. Particul<~r effort has been devoted to the lipids highe r· melling fa1 frac tions of cocoa butter. contai ning unsawrated fa tty ac id !:. as the mai n and fun ctiona ll y active constituents. Accord ingly. thi s paper gives a brief review of the polymorphi sm of stearic acid and oleic acid. being repre· scntati ve of the saturated and un:,aturated fatt y acids. rcspcc· ti ve ly. Furthe rmore. new findi ngs are presented on the pol y· morphism of SOS. 1,3-d istcaroy l-2·oleoyltriglyccride. as rcpre·
scntative of the symmetric S10S1 (51: saturated acid . 0: oleic Initial paper rec eived March 16, 1987 acid) triglyccrides. FirsL we briefly discu:-.s some conceptual Manuscript received November 5, 1987 Direc t inquiries to K. Sato and methodological background:-. Telephone number: 81 -849-24- 6211x322 Polymor phism : T hcr mod)•namic Stabil ity a nd Crysla llogrdph)'
In order to discuss the physical properties of different poly morphic forms of certain fatty acids and triglycerides. a preci~e Ke)·n·ords: Lipid. pol ymorphbm. s1earic acid, oleic acid. 1ri· knowledge of thermodynamic stabili!y is pre requisite. For this glyceride, un saturated fatt y acid . thermodynamical stabi lit y. purpose. measurements on the solubili1ics, melting points and phase transformm ion, differential scanning calorime1ry. cocoa transformation paLhways of all the polymorphs arc most charac· butter. tc ri stic. The less slable polymorphs melt at lower temperatures,
151 K. Sato are more soluble in solvent, and transform to more stable ones As for the thermodynamic stability, the so lubilities of A, B either via solid-state (Verma and Krishna. 1966) or via solution and C were measured independently (Table I) (Beckmann et mediated (Cardew and Davey, 1985) or via melt-med iated tran a\. , 1984) . B has the lowest solubility below 32 oC (Sato ct a\. , si tions (Sato and Kuroda, 1987). The latter two transformations 19 85), whereas Cis the least soluble above that temperature may actually occur if the solid-state transfOrmation is kinetically Form A has a higher solubility than the lowest value at all tem hindered. Many long-chain compounds reveal these transfor peratures. Accordingly, the thermodynamic stability of the A, mations. Know ing the thermal data, one may draw the relation B and C polymorphs of steari c acid may be depicted by the G ship between the thermodynamic stability using a crystal Gibbs T diagram shown in Fig. 3. The G values of 8 and C are the energy (G) and temperature (T) diagram same around 32oC. This is apparentl y contradictory to the The crystallographic aspects of the polymorphism of fatty features of the solid-state transformation. Stenhagen and von acids and triglycerides are reflected in the lateral packing and Sydow (1953) reponed that the transformation temperatures on lamella stacking of the hydrocarbon chains, which are most easi heating from A to C, and from B to C are 54°C and 46°C, ly measured by X-ray diffractometry or by Infrared (IR) and respectively. Another report (Garti eta\., 1980) says that B trans Raman spectroscopy. Indicative of the lateral packing, charac forms to Cat 54°C. All these values are higher than the actual terized by the subcell struct ure, are the X-ray short spacings. crossing points of the Gibbs energies of A, Band C. This is These have so far exhibited three specific subcell s; orthorhom attri buted to a kinetic hindrance of the transformation in the bic perpendicular (0 1 ) , triclinic parallel (Tp) and pseudo solid-state. So, the actual polymorphic transformation in the orthorhombic parallel (0 f) as shown in Fig. I (Abrahamsson crystal may depend on the heating rate, which differs in the above et a!. , IW8). In addition, a hexagonal subcel\ is reported to occur two papers. in highl y metastable states (Abrahamsson et al., 1978) . The satu Structural analyses using single crystals of B and C showed rated aliphatic chains are principally packed in 0 1 and T;,1 ac that the hydrocarbon chains of Care in the all-trans conforma cording to the mode of crystallization and thermodynamic stabili tion (Malta et al. , 1971), whereas the C1 - C3 carbons closest to ty, whereas 0 ',; is reported for the low-temperature polymorph the carboxyl group of Bare in gauche conformation (Goto and of oleic acid (Abrahamsson et al. , 1962). Hence this may be Asada , 1978) one of the subcells characterist ic to the cis-un saturated acyl chain . Oleic Acid The lamellar stacking is indicated by the X-ray long spaci ng spectrum which equal s the inter-lamellar distance between the Three polymorphs of oleic acid. a, {3 and -y, were recently terminal CH3 groups of the lipid lamellae. The long spacing confirmed by means of DSC, X-ray diffraction (S uzuki eta\., also can be a measure of the chain length structure, in particular, 1985), IR and Raman spectroscopy (Kobayashi et a\., 1986) . of triglycerides (S mall , 19 86). Normal triglyceridcs, such as The transformation circuit of the three polymorphs is depicted monosaturated acids, reveal a double chain length structure in Fig. 4 (Sato and Suzuki , 1986). a is crystallized by chilling where the long spacing equals the length of two fatty acids and the melt, although it is thermodynamically metastable. The one glycerol group. A change from the double to triple chain preferred crystalli zati on and metastability for a resemble those length structures occurs when the fatty acid moieti es become of a of glycerides. a and)' undergo a reversible transformation mixed: i.e., large differences in the numbers o f carbon atoms. in the solid-state at - 2.2°C on heating. 1' is the form on which (Koda li et al. , \984), saturated/unsaturated mixed acids. (Lut the structural determination was done (Abrahamsson et a\., ton. 1972) etc. The triple chain length struct ure is caused by 1962). {3 , the most stable polymorph , crystallizes with very slow a sorting of one chain acid from the other two chain acids (Fig. rates, both from solution and the melt. There is no solid-state 2). Thus. the long spacing equals the sum of the lengths of three transformation from a (or )') to {3 in th e melt-grown crystal due fatty acids and two glycerol groups. Even a six chain length to a ste ri c hindrance. Instead , the solution mediates the con structure is proposed (Fahey et al.. 1985), consisting of two triple version. Lutton's high-melting form (Lutton , 1946) is equivalent chain length lamellae in a polytypic relation (Vem1a and Krishna, to i3 in both the melting poi nt and X-ray diffraction spectra 1966) . a, however, is contradictory to his X-ray data on the low-melting Last, molecular information about the hydrocarbon portions fOrm , although the melting point is the same. of the molecules can be elucidated with IR , Raman, high-reso Table 2 summarizes the melting points and the enthalpy and lution NMR, etc. The data on chain pack ing within the lamellar entropy of fu sion , dissolution and transformation. Solubility plane, inter-lamellar end packing, configuration and conforma measurement (Sato and Suzuki, 1986) made it possible to depict tion of the carbon chains and hydrogen bonding, etc., obtained the G-T relationship as shown in Fig. 5. The G values of)' and with these spectroscopic methods are not only complimentary {3 arc parallel to each other, whereas those off] and a come to those from X-ray diffraction but also diagnostic of subtle struc close together with increasing temperature. Far below their tural properties of the polymorphism on a molecular level. crossing point, they melt. This multiple melting is characteristic of oleic acid, and is not observed in saturated fatty acids. The morphology and X-ray diffraction patterns of the three Stearic Acid forms are shown in Figs. 6 and 7. All the crystals were of a tabular shape with a well-developed basal surface in a slightly Three typical polymorphs of stearic ac id , A , Band C, have supersaturated solution. a reveals a slender hexagonal shape, been known for 30 years (von Sydow, 1956). The fourth form , while {3 shows a truncated lozenge shape. The truncation occurs E, was found later by the spectroscopic methods (Holland and nonnal to the bi sectrix of 55°. )' reveals a rectangular shape which Nielsen, 1962) . Form A is triclinic, and B, C and E arc mono is consistent with the subcel\ structure of 0 ' ;,1 (Abrahamsson clinic. The subcell structures of B and C are reported to be 01 . el a!.. 1962). The morphology of the three !arms changes
152 Physical and Mol ecu Iar l'roperlies of L'•1,. 'd PoiJ'morphs CD -). \\P 8 (i)
o, •• • E 0 II' fl cFig.hain I.s. Subcell sl ruclures ofOl , T il and ,II Vfa\ -----~ @fJ CD 0 ,. ";•'""' II // )0 (jf
!~ i g. 4. A transition circuit ~ among the polymorphs of IJ and 'Y and a, acid and Smelt of oleic (Sat~ uzuki, 1986).
Fig. 6. Cryslal h (S a I o and. Suzsuki apes, 1986) of a. ' fJ a nd "Y polymorphs of oleic acid
(a ) Fig. 2. Double ( (b) in triglyceride~ . a), and triple, (b), chain lengl h s I rucrures
_i~l Fig. 7. X-ray diffrac ~~~~ spectra of a, {3 . 1' polymorphs of oleiC acid (S uzuki al., 1985). el 10 20 30 ljJJL . T I'Cl 40
Fog. 3. Relalionsl · ture l~5 4 045 0.40 0.30 0 d/nm TableA. 8 a ndI. ECnlhal I PY (ll H) and enlropy (llS . Cl al. ' 19841'". ymorphs of slearic acid .m Jecanc of dossolulion (Beckmann of llH (k.J/mol) llS (J/moi/K) 10 Fig. 5. Relalionshi T I'Cl 20 ·c A c A B c (T) of a, bel ween Gibbs energy (G) and ~~~· ~ a o~ 65.7 69.0 64.4 193.4 205.0 189.9 boh ly dala (S a Io and1' Spolymorphsuzuki , 1986). of olei c aCid. , fromtempe sord-lu- 153 K . Sato Table 2. Enthalpy and entrop) nf fusion , dissolution and solid-slate transformation of a, B ~md "( ))()1 · morphs of oleic acid (S uzuki rt al.: 1985, S:.1to and Suzuki: 1986). fu sion dissolution tnmsitim polymorph yl 13' "b (3b -y-a T (OC) 13.3 \6.2 - 2.2 IIH (klima\) 39.6 5 1.9 \00 100 59.4 76.0 8.71 LIS (J /mo l/K) 138.4 179 .3 360 352 222.9 279.7 32.' a. dccanc. b. acetonitrile. dmstically to needle shape when the supersaturation of solution (a) or supercooling is increased. The short spacing spectra of 'Y corresponds to the subcell ofO', . The other two forms reveal remarkably different patterns, implyi ng different subcell struc tures. The long spacings are 4.34 nm (a), 4.12 nm ({i) and 4.19 nm (-y). Vibr.uional spectroscopic studies on a, {3 and 'Y forms of oleic y acid have resulted in thefollowing (Kobayashi et al.. 1986): (a) the "(-a transformation is of an order ("y)-disordcr (a) type which is accompanied by a conformational disorderi ng in the ponion of al iphatic chain between the double bond and the terminal methyl group (methyl-sided chain). The most conspicuous spec y tral change is seen in the low-frequency Rama n spectrum (Fig. Sa). All the sharp bands of 'Y collapse to a broad band in o: due 60 •o 20 0 to a loss of translat ional sy mmetry. This originates from a dis wave number/cm- 1 ordered structure. Additionally, a peculiarity, indicating the same conformational disordering, appeared in two strong bands due Fig. 8. Raman spectra of a and 'Y 1>olymorphs o· oleic acid, to a C-C stretching mode. This mode is reflected in a single (a) low-frequency bands, (b) C-C stretching bmds (Koba band for saturated acid s. After the "(-a transition , the 11 25 yashi et al., 1986). cm- t band of melhyl-sided chain drasticall y decreased in in tensity. whereas the 1095 cm ~ t band due to the carboxyl-s ided chain remained unchanged. (Fig. 8b). Thu ~. introduction of one of crystalliza tion are different. The polymorphs o stearic acid cis-double bond at the central positi on of the alkyl chain in crystallize in a different manner. depending on sovent. super duces an increase in the chain mobility. rc:-.uhing in a new type saturation and temperature. The quantitative diffeenccs in the of transformation of an interfacial melting. Conformational dis nucleation rate (Sato and Boi stclle. 1984) and the cystal growth ordering of this kind was not detectable in fj. (b) The confor (Beckmann and Boistelle. 1985) are up to approxmately 50% mation of the polymethylene chai ns of 'Y :tnd {3 is all -trans. It under normal conditions of crysta ll ization. In the :ase of oleic is likely that gauche conformations occur in the disorde red acid. however. the rate of cryst I 54 Physical and Molecular Properties of Lipid Polymorphs Table 3. Lo ng spacing (LS, nm), melting 1>o int (T111 , °C) and cnthalpJ' of fusion (.6.Hr: kJ/mol) of tin! J>O lymorphs of SOS obtained in the ()resent study and corresponding pol)'morphs in the literature. JJresent stud)' literature form purity LS Tm llHr (a) (b) (c) (d) (e) (I) 9 1% 5.05 28* IV IV 99% 5.05 23.5 9 1% 7.37 37* II {3' sub {3 {3" sub{J 99% 7.35 35.4 98.5 91% 7.06 38* pse udo-ff ' II {3' {3' 99 % 7.00 36.5 104 .8 9 1% 6.50 42* /l {3 {3 {3 99% 6.50 4 1.0 143.0 91 % 6.50 43* {3 {3 99% 6.50 43.0 151.0 (a) Daubert and Clarke. 1944). (b) Filer ct al. , ( 1946). (c) Lullon and Jackson (1950). (d) Malkin and Wil son, 1949). (c) Lavery ( 1958), (f) Landmann et al.. ( 1960). (*) Exami ned by viewing transparency temperature of sampl e with eyes has been we ll establi shed. as far as the thermodynamic proper characteristic X-ray diffraction patterns, DSC data. and related ti es a rc concerned . In the literature, however. results arc rather previous work . This paper presents a summary of new results contradictory for mixed satu rated/unsaturated ac id triglyccrides. of SOS and POP (Sato et al. . submitted to J. Am. Oil Chem. despite the importance of these compounds in confectionery fa ts Soc.) and other StOSt compounds (Wa ng et al. , 1987). For the same compound. inconsistent results are repor1ed for Five polymorphs of SOS we re obtained at ambient tempera the number and nomenclature of polymorphs. structu res of the tures (above I5°C): a, "f, pseudo-ff'. !32 and /3 1• all of which subcell and chain length. melting po ints and thennal behaviors. occurred both in higher- and lowe r-purity samples. Addition etc. This is show n in Table 3 which summarizes the nomencla ally, the lower-purity samples conta in another intermediate ture and the long spacing data of the polymorphs of SOS. Al polymorph having X-ray short spacing spectra, e.g., three peaks though not presented here. there is a wide variat ion in the melt of 0.435 nm, 0.419 nm, and 0.393 nm , sim ilar to ff1' of tr is ing point. This confu sion might be attribured to rhe puriry of tearin (S impson and Hagemann , 1982). This form , however. sample empl oyed. or experimental methods and in struments. did not appear in the higher-purity samples a nd should be dis The authors have recently studied the polymorphism of a regarded as a polymorph of pure SOS seri es of StOSt triglycerides (S t: C 16 , POP; C", SOS: C,o, Each polymorph has the following characteristics: (a) DSC AOA. and Cn. BOB). Particu lar concern was given to purity (2 °C/mi n) reveals a single melting peak except for a whose of the sa mples and tec hniques for identification of indi vidual melting peak (around 23 °C) was followed by solidification of polymofllhs. Two sa mples, lower-purity (91%) and higher-purity )'. (b) Thermal treatments ex hibited successive irreversible (99.0%). were examined for each compound using the same ther transformations in the solid-state. (c) The X- ray short spacings mal treatments to reduce the effect of purity. Polymorphs we re reveal unique patterns for all polymorphs as shown in Fig. 9. identified by their characteristic X- ray diffraction pattern after Hig her- and lower-purity samples gave identical short spacing obtai ning a si ngle melting or soli dification peak. In taki ng these spectra and there was no doubt in discriminating between spe data , two thermal treatments were used; transformation from c ifi c spectra of a,)', pseudo-/) ' . This is also true for long spac the polymorphs whi ch we re directly solidified from the melt ings. melting points and enthalpies of fus ion . .6. Hr (Tabl e 3). at va rious temperatures and the mel t-mediated transformation. The di stinction between ff2 and t3 1 is subtle. yet it has an im The latter tra nsformation is a re-solid ification from the melt portant releva ncy. The short spacings. melting points and .6.H r which was formed by rapidly raising the temperature to just are distinclly different but the long spacings have the sa me value. above the melting poi nt of a less stable form (Sato and Kuroda , For short spacings. a strong peak for 0.458 nm is common, but 1987). The X- ray and DSC measurements were carried o ut the intensity ratios of other peaks show a clear contrast. Fur simultaneously for each sample during the above thermal treat thermore. the two peaks in /32 denoted by arrows (0.400 nm and ments. Polymorphs were named by taking into account the 0.390 nm) are split into two in /3, and a peak in i12 denoted by 155 K. Sato a filled triangle (0.375 nm) disappeared in {3 1• The difference in 82 and Bt is also manifest in the me hing curves examined by DSC. Figure 10 depicts DSC curves of the five polymorphs of SOS recorded while heating (2°C/min). When a sampl e revealing the superimposed X-ray short spacing spectra of ff2 and 13 t was heated , double melting peaks we re detected. Sub X' sequent results confirmed that the higher-melting peak increased in size as the durati on of tempering increased. II Typical the rmal treatments for obtaining the fi ve polymorphs of SOS are as follows . a is formed by chilling the me lt below ' .. J ~'·'" 23°C. 'Y is formed either by transformation from a (e.g., over 12 hat l7 °C) or by the solidification from the melt in a tempera ture rangeof24-28°C. The melt-mediated transformalion from 'JirJ~ P, a also yielded 'Y in the same temperature range. 'Y transformed to pseudo-8' by tempering over I h at 30°C. The melt also solidified into pseudo-ff' around 29-36°C. although very slowly. The transformations of pseudo-.8' to .82 and IJ2 to f3t occurred P'..m-p' 'J~'·" ove r 10 hat 35°C and II days at 40°C, respecti ve ly. These dam confirm the five indiv idual polymorphs of SOS. _____}" vvl_' The chain length structure of a is double, Fig. 2, but in con trast triple chain length structures are revealed in the other fou r fo rms. T his means that a conversion in the chain length struc Fig. 9. X-ra)' short spacing spectra of a, "f, J>seudo-{3' . f3z ture occurs during the transformation in the solid-state. a nd f3 t of SOS. After I he transformation, two peaks of 132 Analysis of other StOSt compounds show that AOA and BOB denoted by arrows split into two, a nd a peak denoted by a possess the same five polymorphs as SOS with regard to X-ray filled triangle disappeared. (unit = nm) short and long spacings (Wang et al .. 1987). POP. however, dis pl ayed unusual behavior. a, "f, {32 and 13t are found as in al\ the StOSt compounds, but three more intermediate forms we re also obtained. They were named 0, pseudo-{32 and pscudo /31. The latter two forms show the X-ray short spac ing spectra si milar to pseudo-/3' of SOS, but subtle differences were also observed. Furthermore, their chain length structure was dou ble chain-length . Pseudo-l32' and pseudo-13 1 ' we re formed either via the transformation from 'Y or via the melt solid ifi ca tion and differed in melting point by about 2.5°C. Pure 0 oc curred during the me lt solidification in a narrow range of the solid ification temperature. Consequently a total of seven poly morphs were obtained in POP. The description of the polymorphism of the StOSt com fig. 10. DSC curves (on heating: 2°C/min) of a -melting/"( pounds is concluded with justifications for the nomenclature solidification, melting of pseudo-,8' , {3 2, {3 1 and the sample of the different polymorphs. a corresponds to what is commonly which was formed during the transformation from {3 2 to {1 1• seen in glycerides, being characterized by a sing le short spacing around 0.42 nm . 'Y has a strong short spacing line at 0.472 nm, regard. a possible variation may be a change within the saturated which is not seen in any polymorphs of saturated fatt y acids or unsaturated lame ll ae which a re separated into the different and glycerides. The refore names of {3' (Filer et al. , 1946), {3' ' layers in the triple chain length structure. Since the polymor (Malkin and Wil son , 1949), and sub-{3 (Lutton and Jackson , phic behaviors of the un saturated and saturated acid s arc quite 1950, Lavery, 1958) may be inadeq uate. This peak was observed different , it is highl y fea sible that mu lti ple polymorphic forms onlyin"(ofoleicacid(Fig. 7) . Thcnameofpscudo·.B' is used of S10S1 may occur due to independent or cooperative mole because the short spacing spectra are similar to palterns of .B' cular changes in the two different lamell ae. Vibrational spectro for StStSt, and because Gibon et al. (1986) gave the same name scopic measurements and X-ray structure anal yses usi ng single to the polymorph of POP having features equi valent in the pre crystals should give decisive data. Work is in progress. sent study. Bt a nd {32 have the X-ray short spacings similar to .8 of StStSt with increasing subscripts denoting decreasing melt Cocoa Butter Polymorphs ing point. Use of Roman numerals (1 , II , etc .. ) (Daubert and Clarke, 1944 ; Landmann et al. , 1960), or of A. B, etc., for POP The peaks of medium strength in the X-ray patterns of {32 and (Lovegre n et al. , 1971) is disregarded since they are far from ,8 1 are very similar to those of Forms V and VI. respectively. the nomenclature traditionall y employed in glycerides. of cocoa butter (Fig. II) (Wille and Lutton: 1966, Garti et al. , At present. the structures of the subcell and unit cell, and 1986). Furthermore, the difference in the melting point is of the molecular conformations of the StOSt polymorphs a re un the same order of magnitude: 2 .5°( between Forms V and VI known . Due to subtle differences in X-ray diffraction patterns, of cocoa butter (Wille and Lutton, 1966) and 2.2°C between slight variations in molecular structure are possible. In this 132 and {3 1 (Table 3) . 156 Physical and Molecular Properties of Lipid Polymorphs To verify these simi larities. the polymorphism of a mixture cocoo butter of POP/ POS/SOS (wt % rati o. 18.2/47.8/34.0: % purity: POP, 99.2: SOS. 99.0: POS, 98.3) was examined usi ng the thermal treatments previously described (Sato et al .. unpublished). The V-form same DSC and X ~ ray short spacing patterns were obtained as 0 . 458 those of {jz and {3 1 of SOS and POP. The me lt ing po ints of {jz and {3 1 of the mi xtu re were 33.2°C and 35.3°C. respectively. In addition, mix tures of POS/SOS wi th a lower conten t of POS. Fig. 11. X-ra)' short or without POS. also revealed essemially the same results. From s pacing s pectra of this it is concluded thai the {jz and {3 polymorphs are charac ~ 1 Forms V and VI of tcristic forms in pure StOSt triglycerides and their mixtures. and cocoa buller (re that the transformation of Forms V - VI in cocoa butter is the drawn from data of result of a pol ymorphic transformation of the StOSt fractions Wille and Lulton: from {3, to {3 • Wille and Lutotn (1966) compared the X-ray data 1 1966). (unit = nm). of a mixture of SOS (25%). POS (50%) and SOO (25%). and reported the similarity of its pauern to the {3 polymorph of SOS VI-form and POP. No distinction WdS made, howeve r. between "two rr 0 . 4 59 forms of the two triglycerides. 0 . 370 As for the mechanism of the V - VI transition in cocoa but ~ ter, the present consideration agrees bener wi th ideas based on 0 . 386 the ~o lid -statc tmnsfonnation (Gani et al.. 1986). yet contradicts those which assume that the V - VI change is caw)cd by the separation of a portion rich in high-mehing fat from a lower me lting portion of cocoa butter (Man ning and Dimick. 1985). Obviously. the actual process of the V ----. VI transfo rmation in cocoa butter is rather compl icated. At elevated temperatures (around 30°C). the hi gher-mehing fractions of cocoa buuer. mainly POP/ POSISOS, coexist with liquid oil whi ch consists of the lower-melting fractions. Therefore, transformatio ns to reduce the effects of impurities. Then. the complicated poly mediated in liqu id oil may also take place: di ssolution of {32 morphism of lipids in real systems which consist of vary ing fats (Fo rm V) in oil. and re-crystallization of fJ 1 (Form VI). The and fa tty ac ids can be analyzed using the data of pure substances. transformat ions mediated via oi l (solution)-mediated and via Furthermore, multiple techniques for identification of the in solid ~s tate may concu rrently take pl ace. In both processes, how dividual polyrnorphs mu st be applied to the polymorph is m of ever, the basic phys ical features may be regulated by two differ complicated triglycerides, since the differences in thermal and ent polymorphs of the StOSt triglyceridcs. structural behaviors between the polymorphs may be rather subtle. In this regard. to get the molec ular properties. s pec ~ Conclusion troscopic methods are very convincing. The polymorphism of stearic acid , oleic acid and SOS has References been discussed. It was shown that , physical and molecular characteristics in the polymorphism is quite different between Abrahamsson S. Ryderstadt-nahringbauer I (1962). The crys stearic and oleic acids. This difference is caused by introduct ion tal struc tu re of the low-melting form of oleic acid . Acta Crys of one cis-double bond at the center of the aliphatic chai n in tall ogr. 15: 1261- 1268. oleic acid. Therefore, one may expect that the molecular natures Abmhamsson S, Dahlen B. Lofgren H, Paschcr I. (1978) . Late of the polymorphs of various unsaturated fany acids may be more ral packing of hydrocarbon chains. Prog. C hern . Fats Other complicated. depending on the number. position and configura Lipids. 16: 125- 143. tion of the double bond . Extensive work should be devoted to Beckmann W, Boistelle R, Sato K. (1984). Solubility of the the study of these unsaturated fatt y acids, si nce data are quite A, B and C polymorphs of stearic acid in decane, methanol lacking. and butanone. J. Chern. Eng. Data . 29: 2 11 - 214. Kn ow ledge about the polymorphism in mixed saturated/un Beckmann W, Boistelle R. (1985). Growth kinetics of the (110) saturated ac id triglycerides is mther scarce or contmdictory. This faces of stearic acid from butanone solutions-pure and in the shoul d be overcome, since many naturally- important trigly presence of an emul sifier. J. Crystal Growth. 72 : 621- 630. cerides consist of saturated and un saturated acyl chain s, e.g., Cardew PT, Davey RJ. (1985) . The kinetics of solvent-mediat cocoa butter. Examples presented in this review. POP and SOS. ed phase transformations. Proc. Roya l Soc. London A398: indicate that the numbers of individual polymorphs are increased 415-428. and their molecular natures become more complicated in com Dauben BF. Clarke TH . (1944). Unsatu rated synthetic gly pari son to monoacid triglycerides. More detailed analysis for cerides. VI. J. Am. Chern. Soc. 66: 690- 69 1. the crystal and molecular structures of all polymorphs of SrOSt Fahey DA , Smal l DM , Kodali DR. Atkinson D, Redgrave TG. or other mixed triglyce ridcs must be necessary. (1985). Structure and polymorphism of 1,1-dioleoyl-3-acyl-:m In these researches, one may have to employ pure sample to glycerols, three and six-layered structures. Biochemistry. 24: get essential features of the polymorphism of each compound 3757- 3764. 157 K. Sato File r U . Sidhu SS, Daubert BF, Lo ngenecker HE (1946). Small DM . (1986) . Physical C hemistry of Lipids. In Hand X· ray investigation of glycerides. Ill. J. Am . Chem. Soc. 68: book of Lipid Research. vol. 4. (ed.) Hanahan DJ. Plenun Press, 167- 171. New York . Chapte r 10, p. 345. Gart i N. Wellne r E. Sarig S. (1980). Stearic acid polymorphs Ste nhagen E. Sydow Evon. (1953). On the phase traJsitions in correlation with crystallization conditions and solve nts. Krist in normal chain carboxylic acid s with 12 up to 20 and ir.cluding Techn. IS : 1303- 13 10. 29 carbon atoms between 30°C and the melting poi nt ~. Arkiv GartiN, Schlichter J, Sarig S. (1986). Effects of food emuls i Kemi. 6 : 309-316. fi e rs on polymorphic transitions of cocoa buller. J. Am. Oil Suzuki M , Ogaki T, Sato K. (1985). Crystallization ar.d tmns Chcm. Soc. 63: 230- 236. formation mechanism of a, {3 and "Y polymorphs of ul:ra-pure Gibon V, Durant F, Deroanne Cl. (1986) . Polymorphism and oleic acid. J. Am. Oil Chem. Soc. 62: 1600- 1604. ErratlUll; ibid , intersolubility of some palmitic, stearic and ole ic triglyccrides: 63: 553 (1986) . PPP. PSP and POP. J. Am. Oil Chern . Soc. 63: 1047- 1055. Sydow E von . (1956). The no rmal fatt y acids in solid state Goto M. Asada E . (1978) . The crystal structure of the B form Arkiv Kemi . 9 : 231- 254. of stearic acid . Bull . Chern. Soc. Japan. 51 : 2456- 2459. Ve rma AR, Krishna P. (1966). Pol ymorphism and Polytypism Holland RF, Nie lsen JR. (1962). Infrared spectra of single in Crystal s. John Wiley & Sons, New York . p. 7. crystals. II . four forms of octadecanoic acid . J. Mol. Spectrosc. Wang ZH, Sato K. Sagi N. Izumi T, Mori H . (198"). Poly 9 : 436-460. morphism of 1,3-disaturated-2-oleoyl triglycerides: POP. SOS, Kobayashi M , Kaneko F, Sato K, Suzuki M . (1986). Vibra AOA and BOB. J. Japan. Oil Chem. Soc. 36. 671- 619. tional spectroscopic study on polymorphism and order-disorder Willie RL, Lutton ES. (1966). Polymorphism of cocoa but phase transition in oleic acid . J. Phys. Chem. 90: 6371- 6378. te r. J. Am. Oil Chem. Soc. 43: 491-496. Kodali DR, Atkinson D. Redgrave TG, Small DM. (1984). Synthesis and poly morphism of 1.2-dipalmitoyl-3-acyi-SII -gly Discussion with Reviewers cerols. J. Am. Oil Che rn . Soc. 61 : 1078- 1084. Landmann W. Feuge RO, Lovegre n NV. (1960). Melting and j.\V. Hagemann: Saturated monoacid triglyccrides of even dilatomctric behavior of 2-oleopalmitostearin and 2-oleodis chainlength less than 16 carbo ns exhibit a third intermediate tearin . J. Am. Oil Chern. Soc. 37: 638- 643. melting polymorphic fo rm . Us ing this analogy. do you pe rceive Larsson K. (1966) . Classification of glyceride crystal forms. that the three new inte rmedi ate forms of POP refl ect an effect Acta Che m Scand. 20: 2255- 2260. due to the shorte r c hainlength acids? Since the new forms are Lave ry H. (1958) . Differential thermal analysis of fat s. Ill. double chainlength structures, is it possible that chain reorder J. Am. Oil Che m . Soc. 35: 418-422 ing is occurring between the carboxyl group and double bond? Lovcgrcn NV, Gray MS, Feuge RO. (197 1) . Prope rties of 2- Author: Complex intermediate polymorphs were found only oleodipalmitin, 2-elaidodipalmitin and some of their mi xtures. in POP. Therefore. we think that this complexit y is caused by J. Am. O il Che m . Soc. 48: 116- 120. cenain interaction between sho ner length of saturated acy l c hain Lunon ES. (1 946). Diffraction patterns of two crystalline forms with oleic acid . Among three inte rmediate forms of POP, of oleic ac id . J. Am . Oil Chem. Soc. 23: 265- 266. pseudo-{31 and psc u do -~2 a re double chai nle ngth but h is of Lutton ES. Jackson FL. (1 950). T he polymorphis m of syn tripl e chai nl ength . although a ll of their X-ray ~ h on spacing thetic and natural 2-oleoyldi palmitin. J. Am. Chcm. Soc. 71. : spectra are sim ilar to that of pseudo-{3' of SOS. There are two 3254- 3257. transition circu its in the crystal after melt crystallization: a - Luuon ES. (1972). Lipid structures. J. Am. O il Che m. Soc. 'Y - pseudo-{3:! - pscudo-{3; - ~'! - {31 . and 0 - pse ud o-~ I 49 : 1- 9. - fl1 -+ {3 1. Both unde rgo conve rsions in the chainlength Malkin T. Wilson BR . (1949). An X-ray and thermal examina structure; (double -+) triple double -+ tripl e. Presumably ti on of the glycerides. X. J. Chem. Soc.: 369- 372. these conversions may be accompanied with reorde ring of oleic Malta V, Cell otto G. Zenetti R, Martelli A (1971). Crystal F. and palmitic acyl chains. Thus, it is possible that, in forms of structure of the C form of stearic acid . J. Chcm. Soc.: 548-553. POP, specific lateral packings consisting of oleic and palmitic Manning DM , Dimick PS . (1984). Crystal morphology of acyl chains may exist in the same l;.1me ll a . cocoa bulle r. Food Microstructure. 4 : 249- 265 Sato K, Boistelle R. (1984). Stability and occurre nce of poly K. Larsson: The name o- wform in paraffins, simple esters and morphic modifications of stearic acid in polar and nonpolar solu glycerides corresponds to crystals with a hexagonal (or pseudo tions. J. Crystal Growth. 66: 441 - 450. hexagonal) subccll . The a-form of oleic acid seems to have the Sato K, Suzuki K, Okada M, GartiN. (1985). Solvent effects triclinic chain packing. Thus the type of disorder is different on kinetics of solution-mediated transition of steari c 158 Physical and Molecular Properties of Lipid Polymorphs that some confusion might arise. e.g .. between a of glycerides and a of oleic acid as you poi nted out. No similarity is seen both in molecular conformation and suOCell structure between these two a-forms. although solidification behaviors look alike. I hope, hov.revcr. that th is confusion will be solved in accordance with progress in research for varying unsaturated fatty acids. I would note that the a-form was observed in a few unsaturated fa ny acids, being characterized by an interfacial melting as brief ly mentioned in the tex t. In addition, it was found that th e 'Y form also exists in erucic and palmitoleic acids. exhibiting the same molecul ar conforma ti on and subcell structure as -y of oleic ac id. K. Larsson: Your proposal of independent molecular change in the 1\VO differen t lamellae of StOSt is interesting. Do you think it would be possible that the unsaturated chain layer even could '" melt"' below the saturated chain laye r, corresponding to a liquid crystal formation? Author: In each polymorph of SOS, no significant change was detected in X-ray short spacing spectra taken at 5°C and just below its melting point. This means that the lareral packings of stearic and oleic lame ll ae are uniquely fixed in each po ly morph . However. we have no co nvmc ing information on oleic acyl chai ns in any form s. e.g .. whether in crystalline state or in liqu id-crystalline st • ordered in {32 and {3 1 Further syste matic study is needed to solve thi s important problem. / 59