ADVERTISEMENT RETURN TO ISSUEPREVReviewNEXTPolymorphism of Acylglycerols: A Stereochemical PerspectiveR. John Craven and Robert W. Lencki*View Author Information Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1*Telephone 519-824-4120, ext 54327; e-mail [email protected]Cite this: Chem. Rev. 2013, 113, 10, 7402–7420Publication Date (Web):June 21, 2013Publication History Received8 January 2013Published online21 June 2013Published inissue 9 October 2013https://pubs.acs.org/doi/10.1021/cr400212rhttps://doi.org/10.1021/cr400212rreview-articleACS PublicationsCopyright © 2013 American Chemical SocietyRequest reuse permissionsArticle Views1208Altmetric-Citations30LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Crystals,Lipids,Mixtures,Molecular structure,Space group Get e-Alerts

Negative pressure effects appear to play an important role during crystallization of complex TAG mixtures such as cocoa butter. Crystallization conditions influence the final macroscopic sample density and the resulting gaseous phase void structure. Well-tempered cocoa butter had a relatively smooth surface, higher macroscopic density, and a closed-pore void structure. In contrast, over-tempered samples were rougher, less dense, and contained large continuous gas-filled pores. Under-tempered cocoa butter had properties between these two treatments, with a continuous, yet very fine pore structure. Since cocoa butter is the continuous phase in chocolate, negative pressure phenomena will likely have a significant influence on chocolate density and thus how it de-molds during processing.

Under certain crystallization conditions, beta-tending triacylglyerol (TAG) can form cavities within the crystal network, leading to situations where the macroscopic solid density is less than the melted liquid phase. A similar phenomenon, termed negative pressure effect, has been observed during the crystallization of some synthetic polymers. Crystallization of pure beta-tending trilaurin just below its melting point with seeding created a crystal structure with millimeter-sized star-like crystals and large air-filled pores, resulting in a macroscopic density (887.5 +/- 2.9 kg m(-3)) that was significantly less than the liquid TAG (897.9 +/- 0.6 kg m(-3)) just above its melting point (T-m). A moderate degree of supercooling (11.5 degrees C below T-m) led to a platy crystal habit and a higher macroscopic density (1030 +/- 2.1 kg m(-3)). However, higher degrees of supercooling (>25 degrees C below T-m) created a very fine microporous crystal network with a lower density of 993.2 +/- 2.5 kg m(-3). It is evident from this work that negative pressure effects can have a significant influence on the macroscopic density of beta-tending TAG.

The caption for Fig. 1 should read “Schematic for a β and b β′ unit cells of achiral TAG and racemic mixtures of chiral TAG. Squares symbolize one unit cell, open circles symbolize one stereoisomer, circles containing commas symbolize the opposite stereoisomer and dashed line symbolizes a twinning operation. This scheme indicates the relative proportion (and not the total number) of molecules in the unit cell”.

Unit-cell stereochemistry for triacylglycerols in β′ (beta prime here and throughout) and β forms has been compared using data from studies of phase behavior for enantiomeric mixtures and space group determinations. The β′ form has a unit cell containing one stereoisomer and the β form has a unit cell containing both stereoisomers. As a result, the crystallization and polymorphic behavior for achiral, chiral-racemic, and chiral-enantiopure triacylglycerols is remarkably different. Accordingly, any plausible mechanism for TAG polymorphism must take unit cell stereochemistry into account.

The physical properties of foods containing fat are often dependent on the polymorphism of the constituent triacylglycerols (TAG). This is illustrated by the favourable physical and sensory properties associated with the beta' form for margarine and butter and the b(V) form for chocolate. Recent investigations have revealed that the stereochemistry of TAG molecules has a profound influence on their polymorphism. For instance, a pure enantiomer of TAG (sn-10: 0-10: 0-16: 0) was beta'-tending while the corresponding racemic mixture (rac-10: 0-10: 0-16: 0) was b-tending. In addition, the binary phase diagram for mixtures of the two enantiomers, sn-10: 0-10: 0-16: 0 and sn-16: 0-10: 0-10: 0, showed the formation of a eutectic (metastable beta'-form conglomerate) and a molecular compound (stable b-form racemic compound). At heart, these differences in polymorph and crystalline tendency stem from differences in the stereochemistry of the unit cell -i.e. both enantiomers in the b unit cell, one enantiomer in the beta' unit cell. Information on the relative stereochemical arrangement of molecules within the unit cell is also available from the crystallographic space group. This information (determined by X-ray diffraction) is available for a number of beta-and beta'-tending, chiral and achiral TAG systems. Like crystalline tendency (discussed previously), space group data indicates that the unit cell for TAG in the beta' polymorph contains only one stereoisomer whereas the unit cell for TAG in the b polymorph contains both stereoisomers (conformers in achiral and enantiomers in chiral systems). Therefore, based on the current data, the stereochemical arrangement of TAG molecules in the unit cell is associated with the polymorphic form of the solid-both stereoisomers in the b form and one stereoisomer in the beta' form. This perspective clearly explains the observed differences in polymorphic behavior for enantiopure and racemic TAG including the beta'-stability of enantiopure systems. As a result, the current descriptive mechanism for TAG polymorphism is vastly improved when the stereochemical orientation of the constituent TAG molecules is considered.

Crystallization and melting behavior, small-angle X-ray scattering, X-ray powder diffraction and infra-red absorbance were measured for nine 1,3-acyl-palmitoyl-rac-glycerols (1,3-acetoyl-, -butyroyl-, -hexanoyl-, -octanoyl-, -decanoyl-, -lauroyl, -myristoyl- and -oleoyl-palmitoyl-rac-glycerol and 1,3-dipalmitoyl-glycerol). All but one of the prepared 1,3-diacylglycerols (1,3-DAG) were β-stable with 1,3-acetoyl-palmitoyl-rac-glycerol being the exception (β′-stable). Small-angle X-ray scattering indicates that molecules in β-tending diacid 1,3-DAG adopt a herringbone-type configuration similar to monoacid 1,3-DAG. In this configuration acyl chains of the same length associate and regular chain-end matching between terminal methyl groups delineate lamellae. In contrast, molecules in crystalline 1,3-acetoyl-palmitoyl-rac-glycerol are oriented similar to those of 1(3)-monoacylglycerol. Interestingly, DSC curves indicate five of the nine diacid compounds have meta-stable forms—suggesting these forms are quite common for diacid 1,3-DAG. Meta-stable forms are observed in the melting curve when the difference in length between acyl chains is large (1,3-acetoyl-, -butyroyl- and -hexanoyl-palmitoyl-rac-glycerol), and in the crystallization curve when the difference is moderate (1,3-decanoyl- and -lauroyl-palmitoyl-rac-glycerol).

Fifteen phase diagrams were prepared using data from differential scanning calorimetry analysis of binary blends of representative diacid 1,3-DAG. The behavior observed in binary phase diagrams is related to the difference in T m (ΔT m) between system components—eutectic for ΔT m < 26 °C and monotectic for ΔT m > 30 °C. Binary blends were prepared using six diacid 1,3-DAG: 1,3-hexanoyl-lauroyl-rac-glycerol, 1,3-hexanoyl-palmitoyl-rac-glycerol, 1,3-hexanoyl-oleoyl-rac-glycerol, 1,3-lauroyl-palmitoyl-rac-glycerol, 1,3-lauroyl-oleoyl-rac-glycerol and 1,3-palmitoyl-oleoyl-rac-glycerol. Diacid 1,3-DAG were synthesized using representative FA: hexanoic (6:0)—short-chain FA; lauric (12:0)—medium-chain FA; palmitic (16:0)—long-chain FA; and oleic (18:1)—mono-unsaturated FA. In addition to the aforementioned phase diagrams, the physical chemistry of 1,3-hexanoyl-lauroyl-rac-glycerol, 1,3-hexanoyl-oleoyl-rac-glycerol and 1,3-lauroyl-oleoyl-rac-glycerol is reported.

Triacylglycerols (TAG) are the main component in fats and oils and a major component in many food and consumer products. These compounds are always asymmetric (about the sn-2position), while many diacid and all triacid TAG are chiral. To understand what effect this has on their crystallization behavior, model enantiopure (1,2-bisdecanoyl-3-palmitoyl-sn-glycerol) and racemic (bisdecanoyl-1(3)-palmitoyl-rac-glycerol) TAG were prepared and characterized. In addition, a binary phase diagram was prepared to investigate their phase behavior and the racemate’s crystalline tendency. For the subject compounds, infrared spectroscopy and X-ray powder diffraction data indicate the enantiopure TAG is β′-stable, whereas the racemic mixture is β-stable. In addition, based on the phase diagram, the high-melting form of the racemic mixture is a racemic compound (with a unit cell containing equal quantities of both enantiomers). Racemic (and near-racemic) mixtures also crystallize in a lower-melting metastable conglomerate...

A complete methodology (including synthesis, purification and analysis) for the preparation of 1,3-DAG is described. For a successful synthesis project, the strengths and weaknesses of each particular process should be taken into account and measures taken to offset or balance potential weaknesses. To this end, we describe some of the challenges associated with: chemically and enzymatically catalyzed acylglycerol syntheses; recrystallization and flash chromatography for purification of partial acylglycerols; and thin-layer chromatography (TLC) separation of DAG. For this work, 1-MAG intermediates and subsequent diacid 1,3-DAG were prepared using non-enzymatic methods, whereas, monoacid 1,3-DAG were prepared by enzymatic methods. It was not always possible to obtain pure samples of target compounds—in recrystallizations this is due to solid solution formation and co-crystallization and in chromatographic separations it is due to co-elution of components with similar Rf. Furthermore, TLC Rf of DAG is determined by two main factors: acyl chain length and positional isomerism. Interestingly, while the role of positional isomerism is well-known, the role of acyl chain length in these separations has only recently come to light.

The addition of phospholipids (PL), either in the form of the milk fat globule membrane (MFGM) or soy lecithin, had a significant influence on butterfat crystal morphology. At low concentrations, PL addition increased spherulite size, but as PL levels reached 2 wt%, spherulite formation was inhibited and the microstructure consisted of well dispersed individual crystals. This change in crystal structure made the butter harder, less grainy at low temperatures, and less prone to oiling-off above room temperature. PL addition was also shown to affect the relative concentrations of the various species created during crystallization. Of the two 2L β′ species that form at crystallization onset, the presence of PL shifted the balance towards the first species at the expense of the second. PL also facilitated the polymorphic transition of the 3L α species to a 2L β′ structure. It is unclear, however, whether the inhibition of spherulite formation was due directly to the inhibition of secondary nucleation by PL at the crystal surface, or indirectly by the reduction of supercooling resulting from a more rapid polymorphic transition. Evidently, PL concentration must be properly controlled during manufacturing in order to optimize butter functional properties.

Depending on the production method, the fat component in butter can either be in a continuous phase or entrapped by milk fat globule membranes (MFGM). Phospholipids like those present in the MFGM are known to affect milk fat crystallization behavior. This study examines the effect of MFGM phospholipid concentration on butter crystal structure. Regular raw cream was first concentrated to 85% fat via centrifugation to produce a “plastic” cream, and was then blended with anhydrous milk fat (AMF) and skim milk in order to maintain the total fat-to-water ratio while altering the total amount of MFGM in the butter product. Whereas mixtures of AMF and skim milk contained large spherulites that had finer structures as the degree of supercooling was increased, the addition of globular fat (GF) broke up these structures, eventually producing smaller individual needle-like crystals. However, high levels of GF also lead to the coalescence of the aqueous phase, creating large water pockets that adversely affect sensory properties. Thus, the phospholipid concentration in the final butter product must be controlled in order to obtain optimal crystal structure and product quality.

The addition of Congo red (CR) dye to diluted raw skim milk resulted in a red shift indicative of the presence of fibril-like structures. Thioflavin T (ThT) is another dye that very specifically binds to protein fibrils, and when added to undiluted raw skim milk, the classic 485 nm fluorescence peak of a ThT-fibril complex was observed. Repeating these experiments with various raw milk components showed that the CR red shift and ThT fluorescence peak were due to the presence of casein micelles, and to a lesser extent, sodium caseinate. Fluorescent peaks were also observed when ThT was added to solutions of purified alpha(S)- and kappa-casein, but not beta-casein, in 0.5 M HEPES buffer (pH = 6.8). The addition of 25 mM Ca(2+) had no effect on beta-casein fluorescence, and significantly reduced the kappacasein peak. However, adding 25 mM Ca(2+) to alpha(S)-casein produced a turbid solution and a 6-fold increase in fluorescence, indicating that the aggregates formed contain fibril-like structure. Casein micelle images obtained by transmission electron microscopy showed the presence of short (7 to 10 nm) fibers cross-linked by dense aggregate junction zones. The observed fibers closely resemble protofibrils, intermediate structures that are observed during the formation of amyloid fibrils.

Combining ultrafiltration and precipitation was studied to improve the productivity of xanthan separation. Changes in solution properties and the association behavior of xanthan were modified by salt (KCI) and solvent (isopropanol) addition, and were assessed using scanning electron microscopy (SEM). The concentration polarization layer produced by xanthan dissolved in buffer solution had a very high specific flux resistance that was enhanced by the addition of either KCI or isopropanol. SEM analysis revealed that the fine macromolecular structure observed with xanthan in buffer was further stabilized in the presence of KCl. Isopropanol addition induced phase separation and the formation of closely packed soft aggregate particles that significantly increased fouling layer specific resistance. The simultaneous salt and organic solvent addition resulted in surface morphologies containing aggregates that associated to create gels with varying porosities. When at least 1% (w/v) KCl and >30% (v/v) isopropyl alcohol was added to 2.5% (w/v) xanthan solution, a very porous fouling layer was created, which dramatically increased membrane flux. Membrane-assisted precipitation reduced the amount of precipitating solutes used while greatly increasing membrane flux, resulting in a large improvement in separation productivity. (C) 2007 Elsevier B.V. All rights reserved.

Mushrooms (Agaricus Bisporus cv. U3 Sylvan 381), broccoli (Brassica oleracea L. cv. Acadi) and mature-green tomatoes (Lycopersicon esculentum cv. Trust) were packaged in Modified Atmosphere (MA) containers and steady-state atmospheres of 5% O2–10% CO2, 3% O2–8% CO2, and 5% O2–5% CO2 were maintained a 4, 3 and 13°C, respectively. The packages were then subjected to a sequence of temperature fluctuations (ΔT=10°C) during 12, 30 and 35 days for mushrooms, broccoli and tomatoes respectively to simulate storage and transport conditions. Temperature, relative humidity and atmospheric composition were followed throughout storage and quality attributes were evaluated at the end of the storage period. Temperature fluctuations had a major impact on the composition of the package atmospheres and on product quality. CO2 concentrations increased rapidly, reaching maxima of 16%, 15.5% and 11% for mushrooms, broccoli and tomatoes, respectively. O2 concentrations decreased to less than 1.5% for the three products. The quality of the products stored under the temperature fluctuating regime was severely affected as indicated by extensive browning, loss of firmness, weight loss increase, the level of ethanol in the plant tissue, and infection due to physiological damage and excessive condensation, compared to products stored at constant temperature. It was clear that temperature fluctuation, even if it should occur only once, can seriously compromise the benefits of modified atmosphere packaging and safety of the packaged produce. Major problems caused by temperature fluctuation must therefore, be addressed to improve the usefulness and reliability of modified atmosphere packaging technology.