n = even) triacylglycerols which are β′-stable, Guinier X-ray powder diffraction photographs were used to determine cell parameters and space groups. The powder diffraction patterns are consistent with a pseudo-orthorhombic unit cell with space group Ic2a. Only one axis is changing as a function of chain lengthening. The experimental results were used to propose a β′-crystal packing for triacylglycerols. In contrast to earlier proposed β′-crystal structures, the acyl chains of this structure are not tilted with respect to the methyl-end group plane. Furthermore, with only one molecule in the asymmetric unit, overall orthogonal chain packing is obtained when the intramolecular acyl zigzag planes are parallel.Paper no. J8979 in JAOCS 76, 603-609 (May 1999)KEY WORDS: Crystal packing, fat, triacylglycerol, X-ray diffraction.Polymorphism of fats and triacylglycerols (TAG) is a permanent subject of research because it may have important implications for the manufacturing of many different food products such as chocolate and margarine. Pure TAG crystallize in the different polymorphic phases α, β, and/or β′ and sometimes even several different β′-and/or β-polymorphs exist (1,2). The different polymorphic phases and their variants can be distinguished by X-ray diffraction through typical reflections in the fingerprint region of the X-ray powder diagram (d-spacings from 3.0 to 6.0 Å) (3,4). Table 1 shows the characteristics of the X-ray patterns of the different phases. Depending on the TAG type the most stable phase is either β′ or β. De Jong and Van Soest (5) showed that TAG of the type C n C n+2 C n and C n C n+4 C n+2 (n = even) are normally β′-stable. Most other types are generally β-stable. TAG mixtures also show differences in phase stability. For example, milk fat is β′-stable (6) while cocoa butter has β as the most stable phase (7,8).The phase behavior of pure TAG as well as TAG mixtures is very complex. Their phase transitions are generally irreversible and depend on experimental circumstances such as temperature and time, and their thermal history as is demonstrated by memory effects (9). Knowledge of this behavior, particularly the β′ → β phase transition is very important, especially for the chocolate industry, since unwanted phase transitions of cocoa butter may cause fat bloom, a white appearance of fat crystals on the surface. Therefore, control of these transitions determine to a large extent the quality of chocolate products.Crystal structure information of the different phases is indispensable to understand the mechanism of these transitions. Unfortunately, so far only crystal structures of β-polymorphs of pure TAG have been reported (10-15), all [β-1,2,3-tridecanoyl-glycerol, or tricaprin, or C 10 C 10 C 10 (β-CCC); β-1,2,3-tridodecanoyl-glycerol, or trilaurin, or C 12 C 12 C 12 (β-LLL); β-1,2,3-trihexadecanoyl-glycerol, or tripalmitin, or C 16 C 16 C 16 (β-PPP); and β-2-11-bromoundecanoyl-1,3-didecanoyl-glycerol, or C 10 C 11 Br C 10 (β-CL Br C)] crystallizing in space group P -1. The molecules are i...