|Chang, S - ARS RETIRED|
Submitted to: Journal of American Chemists Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 27, 1995
Publication Date: N/A
Interpretive Summary: Durability in plastics and texture and quality in processed and refrigerated food products often depend on thermal history. In crop plants, temperature induced responses can cause crop failure, or they can improve seed germination and lead ultimately to frost resistance and heat tolerance. Enormous waste and compromised productivity must be tolerated because such behavior remains complex and poorly understood. In part, limited durability and changes in texture and quality result from the different ways that molecules interact with each other at different temperatures. They are polymorphic. They can melt at one temperature then resolidify in different forms at higher temperatures, or as solids they deform and expand or contract as temperatures change. This research discovered that compounds derived from seed oil of Limnanthes alba (Meadowfoam), a new crop for the Pacific Northwest, exhibit unusual polymorphic behavior. They can occur in two different well-defined forms depending on temperatures to which they are cooled after freezing. This behavior adds to novel properties that already make Meadowfoam fatty acids valuable. The discovery also provides scientists a simple and responsive system for analysis of factors that control low-temperature polymorphic transitions.
Technical Abstract: Methyl esters of cis-5-eicosenoic (5-EAME) and cis-11-eicosenoic (11-EAME) acids from the seed oil of Limnanthes alba (Meadowfoam) exhibit a degree of thermotropic polymorphism unobserved with shorter and longer chain length monoenoic methyl esters. 5-EAME freezes at 264**K and melts at 266**K if cooled no lower than 215**K. 11-EAME freezes at 239**K and melts at 255**K if cooled no lower than 240**K. Solids cooled to lower temperatures undergo phase transformation to higher- melting polymorphs (274**K, 5-EAME; 262**K, 11-EAME), and samples often exhibit double melting endotherms. Quantities of each high-melting phase vary with time at temperatures below characteristic initiation temperatures. Highly temperature- sensitivity phase conversions suggest low temperature nucleation followed by crystal growth and conversion as reheating allows molecular motion. Once formed, both high-melting phases melt with essentially the same TriSm. Thermodynamic and kinetic analyses imply that differences exhibited by the isomeric esters derive from aliphatic configuration distal to the double bond.