Fatty acid composition of spanish shortenings with special emphasis on trans unsaturation content as determined by fourier transform infrared spectroscopy and gas chromatography

摘要

A Fourier transform infrared spectroscopic procedure was used to analyze 34 edible fats (22 shortenings and 12 vegetable margarines) as neat fats (IRNF) to determine their total trans fatty acid (TFA) content. The sloping baseline was corrected with a reference spectrum based on a nonprocessed olive oil. The calibration was done using seven partially hydrogenated fats with an individual TFA content previously determined by the combination of gas chromatography (GC) with argentation thin-layer chromatography. Taking into account the different absorptivities of various trans isomers, different correction factors were calculated using the calibration standards (0.83 and 1.71 for single trans bonds in both diethylene and triethylene and for trans, trans-diethylene fatty acids, respectively) and applied to calculate the total TFA of samples. Moreover, the samples were converted to their methyl esters and reanalyzed following the same procedure (IRFAME). Differences in TFA content of fats were not found when a t-test was used to compare the results obtained by IRNF vs. either IRFAME or GC, suggesting that IR of neat fats could be used, thus avoiding the need to prepare sample solutions in organic solvents and to prepare fatty acid methyl esters. The mean TFA content (determined by IRNF) of a representative group of Spanish shortenings (22 samples) that varied widely in terms of fat sources, processes, and purposes (bakery, sandwiches, ice cream, coatings, chocolate coverings) was 6.55±11.40%, although more than 54% contained <3% of TFA. Fatty acid composition of shortenings by direct GC using a 100-m polar cyanopolysiloxane capillary column indicated that the mean trans-18∶2 isomer content was 0.58%, ranging from 0.9 to 3.4%. Small amounts of trans-18∶3 isomers (<0.3%) were observed in 18 of the 22 shortenings studied; the maximal value was <2%. The mean value of the fraction saturated+TFA of shortenings was high (59.95±12.73%), including two values higher than 83%.