A wide variety of substances have been proposed for use as fat substitutes in food compositions.  The chemical structures of such substances are selected such that they are more resistant to breakdown by the metabolic processes of the human digestive system which normally occur upon ingestion of conventional triglyceride lipids.  Because of their increased resistance to digestion and absorption, the number of calories per gram available from the fat substitutes is considerably reduced as compared to common vegetable oils, animal fats, and other lipids.  The use of such substances thus enables the preparation of reduced calorie food compositions useful in the control of body weight. 
U.S. Pat.  No.  4,861,613 (incorporated herein by reference in its entirety) describes one class of particularly useful fat substitutes wherein a polyol such as glycerin is alkoxylated with an epoxide such as propylene oxide and then esterified with any of a number of fatty acids or fatty acid derivatives to form an esterified alkoxylated polyol.  These substances have the physical and organoleptic properties of conventional triglyceride lipids, yet are significantly lower in available calories than edible oils owing to their pronounced resistance towards absorption and pancreatic lipase enzymatic hydrolysis.  The thermal and oxidative stability of the esterified alkoxylated polyols renders them especially suitable for use in the preparation of reduced calorie food compositions requiring exposure to high temperatures such as fried or baked foods. 
Unfortunately, as a consequence of their hydrolytic stability and low digestibility, the esterified alkoxylated polyols described in U.S. Pat.  No.  4,861,613 may tend to cause certain undesirable gastrointestinal side effects when consumed at high levels in the diet.  That is, since such esterified alkoxylated polyols are not readily broken down into simpler substances upon ingestion, they largely retain their oily, fat-like character and pass through the digestive tract in substantially unaltered form.  Non-digestible fat substitutes in general often function as laxatives in much the same manner as mineral oil.  Problems with diarrhea, leakage of the fat substitute through the anal sphincter, separation of the fat substitute as an oil from the excreted fecal matter, and shortened bowel transition times resulting in gastrointestinal discomfort can occur as a result of the non-digestibility of the fat substitutes.  Other fat substitutes which are similarly resistant towards digestion are known to produce such gastrointestinal side effects.  Examples include sucrose polyester which is esterified with up to 8 fatty acid groups; see U.S. Pat.  Nos.  3,954,976, 4,005,195, 4,005,196, and 5,006,360.  Obviously, such problems will greatly limit the maximum usage level of these substances which can be tolerated in various food compositions, thereby constraining the amount of conventional triglyceride and the number of calories which can be removed from certain foods. 
One solution to this problem is provided in copending application Ser.  No.  07/886,538, filed May 20, 1992, and entitled "Esterified Propoxylated Glycerin Fat Substitute Compositions Resistant to Gastrointestinal Side Effects" (incorporated herein by reference in its entirety).  The copending application describes a fatty acid-esterified propoxylated glycerin composition useful as a reduced calorie fat substitute resistant to gastrointestinal side effects having an average number of oxypropylene units per equivalent of glycerin of from 3 to 20, a fatty acid acyl group content such that at least 40 mole percent of the fatty acid acyl groups in the composition are derived from a C.sub.20 -C.sub.24 saturated linear fatty acid, and a solid fat index at 27.degree.  C. as measured by dilatometry of at least 30.  The utilization of such a composition in combination with a conventional fully digestible fatty acid triglyceride fat or oil in a food composition normally containing a fatty component is also described.  The copending application suggests that these fatty acid-esterified propoxylated glycerin compositions may be obtained by first propoxylating glycerin with the desired number of equivalents of propylene oxide and then esterifying with a fatty acid or a fatty acid equivalent such as a fatty acid ester, or fatty acid halide, or a fatty acid anhydride. 
The use of fatty acid esters in such an esterification step is described in copending application Ser.  No.  07/227,048, filed Aug. 1, 1988, now U.S. Pat.  No.  5,175,323, entitled "Preparation of Esterified Propoxylated Glycerin by Transesterification" (incorporated herein by reference in its entirety).  The fatty acid esters employed in this process are C.sub.1 to C.sub.4 alkyl esters of saturated or unsaturated C.sub.10 to C.sub.24 fatty acids.  The esterification reaction is readily driven to completion by removing the C.sub.1 to C.sub.4 alkyl alcohol generated during the transesterification reaction by distillation or similar means.  Although this approach works well on a laboratory scale and affords a high yield of esterified alkoxylated polyol with minimal by-products or color formation, it suffers from the practical disadvantage that the required C.sub.1 to C.sub.4 alkyl esters are relatively expensive as compared to the corresponding free fatty acids.  In addition, great care must be taken to ensure that all of the residual C.sub.1 -C.sub.4 alkyl alcohol formed is removed from the product prior to use in a food composition since certain alcohols of this type (methanol, for example) are considered harmful when ingested. 
However, if the C.sub.20 -C.sub.24 saturated linear acyl groups in the esterified propoxylated glycerin compositions of copending application Ser.  No.  07/886,538 are introduced using the corresponding free fatty acids rather than the C.sub.1 -C.sub.4 alkyl esters in order to reduce the overall cost of the esterification, certain other processing problems are encountered.  In particular, a direct esterification process must generally be run at a higher temperature than a transesterification process, especially when the only catalytic effect is from the excess fatty acid present.  Additionally, a fairly large excess (10-20% molar excess) of fatty acid relative to the initial hydroxyl concentration must be utilized in order to self-catalyze the reaction and to accomplish complete or near-complete esterification of the propoxylated glycerin.  As a consequence, the excess fatty acid which remains at the completion of the esterification must be removed prior to formulation of the fat substitute into a food composition, as the excess fatty acid may cause severe taste, odor, and stability problems.  A convenient way to remove the excess fatty acid is by vacuum steam stripping the acids away from the esterified propoxylated glycerin composition.  This procedure is quite difficult to accomplish when C.sub.20 -C.sub.24 saturated linear fatty acids are being employed since such acids are relatively high melting (typically, over 74.degree.  C.) and consequently readily form troublesome plugs in commercial processing equipment.  At times, particularly in vacuum equipment, even steam tracing is not an effective solution due to temperature-lowering effects in the vacuum eductor.  As a result, it is often nearly impossible to carry out a large scale non-catalyzed direct esterification of a propoxylated glycerin intermediate with C.sub.20 -C.sub.24 saturated linear fatty acids without having to frequently shut down to remove plugs of unreacted fatty acid.  If a transition metal esterification catalyst such as a zinc, titanium, or tin compound is utilized so as to permit the use of a stoichiometric amount of fatty acid relative to propoxylated glycerin, quantitative removal of the metal catalyst following esterification is often quite difficult to achieve.  To be useable as a reduced calorie fat substitutes in food compositions, however, the esterified alkoxylated polyol must be essentially free of such metallic impurities. 
Copending application Ser.  No.  07/886,538 suggests another method by which the desired C.sub.20 -C.sub.24 acyl groups may be introduced into an esterified propoxylated glycerin composition.  Unsaturated linear fatty acids containing 20 to 24 carbon atoms may be used to esterify a propoxylated glycerin intermediate and the resulting esterified propoxylated glycerin composition may be hydrogenated so as to convert the long chain unsaturated acyl groups to long chain saturated acyl groups.  This approach has the advantage of avoiding the use of free fatty acids which are high melting, since C.sub.20 -C.sub.24 unsaturated fatty acids melt at significantly lower temperatures than their saturated analogues.  For example, erucic acid (a C.sub.22 monounsaturated fatty acid) melts at 33.degree.-35.degree.  C. while behenic acid (the corresponding C.sub.22 saturated fatty acid) melts at 80.degree.-82.degree.  C.
However, because the ingestion of lipids containing erucic acid residues has been associated with adverse physiological effects, it will generally be necessary to carry out the subsequent hydrogenation of the esterified propoxylated glycerin under conditions effective to accomplish substantially complete saturation of any acyl groups derived from erucic acid.  Such conditions will generally also result in complete hydrogenation of any other monounsaturated acyl groups in the composition as well as at least partial hydrogenation of any di-or polyunsaturated acyl group.  It will consequently be difficult to prepare an esterified propoxylated glycerin by this method wherein both C.sub.20 -C.sub.24 saturated linear acyl groups and C.sub.6 -C.sub.19 unsaturated acyl groups are connected via oxyalkylene units to the same glyceryl residue since possible harmful levels of erucic acid residues will likely also be present, thus limiting the usefulness of the composition as a fat substitute in food compositions.  It would be highly desirable to develop a method whereby an esterified propoxylated glycerin containing both C.sub.20 -C.sub.24 saturated linear acyl groups and C.sub.6 -C.sub.19 unsaturated acyl groups, but essentially no C.sub.22 monounsaturated acyl groups derived from erucic acid may be prepared.  Such substances may have unique and advantageous properties (solid fat index, hardness, melting point, smoke point, flash point, plasticity, thermal and oxidative stability, etc.)  rendering them extremely valuable as fully functional reduced calorie substitutes for conventional triglycerides.