Supercritical fluids extraction and on-line methylation of long chain saturated/unsaturated fatty acids

Supercritical fluid extraction procedures for fatty acids have been investigated. The fast mass transfer rate, excellent solvating power and mild extraction conditions of supercritical fluids, using XAD-4 resin as the matrix, gives quantitative extraction of fatty acid methyl esters (chain length from 14 to 18). The extraction of 50 ppm level linoleic acid methyl ester was complete within 60 minutes at 300 atm and 70$\sp\circ$C with a recovery of 98% and a SD less than 5%. The extraction of free fatty acids by supercritical carbon dioxide requires higher temperature and higher pressure than that of methyl esters because of their polarity and hydrogen bonding. Using 5% hydrochloric acid in pure methanol to serve as a catalyst/methylation reagent on solid matrices of XAD-4 resin, the on-line methylation reaction of fatty acids with supercritical fluids was complete within 20 minutes at 70$\sp\circ$C. Applying the same mild conditions used for methyl esters, the methylation/extraction of fatty acids, both saturated and unsaturated, can be completed within 80 minutes with an overall recovery of 95% and a SD less than 7%. The fatty acids used for this study were myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid and linoleic acid. The effect of temperature on the extraction/methylation of fatty acids and the effect of static extraction time on the percentage recovery of fatty acids were studied. A comparison between supercritical carbon dioxide and nitrous oxide for on-line methylation of fatty acids was made. Both fluids give satisfactory results under identical conditions. The effect of methanol, hexane and methylene chloride on the extraction and methylation at different temperatures was studied. It was found that higher temperature (70$\sp\circ$C) facilitates the extraction. The effect of water on the extraction and methylation reaction was investigated and it was found that while trace amount of water (1-2 $\mu$l/ml) did not interfere the methylation, large amount of water ($>$2 $\mu$l/ml) interfered with the extraction of fatty acid methyl esters.