13C NMR as a primary method for determining saturates, cis- and trans-monounsaturates and polyunsaturates in fats and oils for nutritional labeling purposes

The suitability of C-13 NMR as a primary method for the analysis of lipids to obtain nutritional labeling compositional data (NLCD), i.e. the percentages of saturated, cis-monounsaturated, trans-monounsaturated, and cis-polyunsaturated fat, was assessed. The C-13 NMR methodology was developed by using mixtures of pure triglycerides as model lipids to optimize and standardize scan conditions and spectral pre-processing procedures, establish fixed integration limits for measurement of the C-13 resonances used in the determination of NLCD, and evaluate the quantitative accuracy of the C-13 NMR analysis. The standardized C-13 NMR methodology allowed the NLCD of the model triacylglycerol mixtures to be determined within similar to +/- 1%. To further evaluate the methodology, two sets of validation samples, consisting of ten unhydrogenated oils from the American Oil Chemists' Society Laboratory Proficiency Program (AOCS-LPP) and two trans-containing AOCS-LPP samples combined with three samples from a hydrogenation process, were analyzed. Good overall agreement between the NMR-determined NLCD (in units of mol-%) and the mol-% NLCD calculated for these samples from fatty acid compositional data obtained by gas chromatography was found, including good tracking of the trans content in the second validation set. Given that the NLCD must be expressed on a wt-% basis to be of practical utility, a means of mol-% to wt-% conversion was developed assuming all unsaturates to be C-18 and obtaining the weight-average molecular weight of the saturated fatty acid contributions from the NMR data. This conversion was shown to be especially effective for oil blends, where errors become significant if unit conversion is not done. This work indicates that C-13 NMR can provide excellent primary NLCD data, even in wt-% terms, which can be used for calibrating simpler and automatable instrumental methods such as FTIR spectrometers to determine or screen for NLCD for fats and oils or lipids extracted from food on a routine basis.