Appropriately chosen blends of fats determine the physical properties of many food products like bakery products and table spreads. Soft natural oils often have to be hardened to give a blend the desired properties. Hydrogenation is a widespread applied hardening treatment with an undesired side effect: the (partial) isomerization of cis double bonds in trans double bonds, for example transformation of oleic acid (cis-9-octadecanoic acid; O) in elaidic acid (trans-9-octadecanoic acid; E). Trans fatty acids, commonly regarded as a health risk [1], supposedly replace their saturated analogues in the solid state but in fact little is known about their actual packing.
  A better understanding of the influence of fatty-acid composition, conformational and packing differences on the polymorphic stability and phase-transition behavior of trans mono-unsaturated triacylglycerols (TAGs), and related saturated ones, can be obtained by combining X-ray powder diffraction (XRPD) techniques. Synchrotron and advanced laboratory time- and temperature-resolved XRPD can reveal the stability and phase-transition behavior of the important the β and β' polymorphs and these results can be related to the underlying crystal structure packing. In absence of single crystals, XRPD turned out to be a realistic but challenging alternative: with direct-space search techniques it was even possible to solve a β' TAG structure with two independent molecules in the asymmetric unit from laboratory diffraction data.
   Results that will be discussed include novel meta-stable β' polymorphs and the structure of one them, methyl end-plane packing analysis in relation to observed melting points for various subgroups of TAGs, the difference in β' to β phase-transition behaviour of symmetric vs. asymmetric TAGs, and the influence of replacing a saturated stearic chain (octadecanoic acid; S) by an trans mono-unsaturated elaidic chain [2,3].    

References:
[1] EFSA Journal (2004) 81, 1-49
[2] Mechelen, J.B. van, Peschar, R. & Schenk, H.  (2008). Acta.Cryst. B64, 240-248.
[3] Mechelen, J.B. van, Peschar, R. & Schenk, H.  (2008). Acta.Cryst. B64, 249-259.

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