Cyclic fatty acid monomer formation in frying fats. I. Determination and structural study

The formation of monomeric cyclic fatty acids was studied in a model system in which partially hydrogenated soybean oil (PHSO) was heated intermittently for 80 hr of simulated deep fat frying. Oil samples (fresh and heated) and their methyl esters were fractionated according to their molecular size using gel permeation chromatography (GPC). Oils and GPC fractions were analyzed for cyclic monomers by the following steps: (i) preparation of fatty acid methyl esters (FAME); (ii) microhydrogenation of FAME; (iii) urea fractionation of hydrogenated FAME; (iv) analysis by capillary gas liquid chromatography (GLC), and (v) structural characterization of cyclic monomer peaks by mass spectrometry (GC-MS). Under simulated frying conditions the concentration of cyclic monomers increased from 736 ppm (0.07%) in fresh oil to 1803 ppm (0.18%) in heated oil. GC-MS with capillary columns allowed the identification of several C₁₈ α-disubstituted cyclohexane and cyclopentane isomers as hydrogenated methyl esters. Other noncyclic and contaminant compounds eluting within the expected GLC retention range of cyclic monomers also were identified in all the samples and GPC fractions.