2001 Volume 50 Issue 11 Pages 905-911
The heat decomposition of soybean lecithin was studied using saturated hydrocarbon. On heating with isooctane for 18 hrs, phospholipid composition did not change, but solution color went to yellowish brown. The main coloring substances were separated by silica gel column chromatography and were found to be negative to Dittmer-Lester’s reagent. These substances were mixed polyene compounds possessing a small amount of carbonyl compounds. No exact determination was made of their structures. These substances were also found present in pre-heated soybean lecithin and their amounts increase on heating. The coloring of soybean lecithin on heating under 100°C is thus not due to decomposition of phospholipids, but the presence of polyene substances. On heating with octane for 9 hrs, the phospholipids decomposed but each at a different rate with residual amounts as follows : phosphatidylcholine (PC), 98% ; phosphatidylinositol (PI), 80% ; phosphatidic acid (PA), 65% ; phosphatidylethanolamine (PE), 0%. By heating with decane, PI, PA, and PE decomposed within 1 hr, while PC did so slowly, with 3.5% remaining at 9 hrs. With octane solution color quickly became deep brown. In the isolation of these browning substances, attention was directed to substances having absorption maxima at 350 nm. On heating soybean lecithin, substances having absorption maxima at 240 nm and 280 nm have previously been noted but not those with absorption maxima at 350 nm. The separation and purification of these new products will appear in a future paper.