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

Abstract

The suitability of 13 C 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 13 C 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 13 C resonances used in the determination of NLCD, and evaluate the quantitative accuracy of the 13 C NMR analysis. The standardized 13 C NMR methodology allowed the NLCD of the model triacylglycerol mixtures to be determined within ,61%. 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 weightaverage 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 13 C 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. Keywords:13 C NMR / Fats and oils / Mol % / NLCD / Trans analysis / Validation / Weight %

Determining nutritional labeling data for fats and oils by 1H NMR

“…The composite plot of the results for all four NLCD parameters shown in Fig. 3 The Z-regression equation indicates that the overall correspondence between the NMR NLCD estimates and the actual values is close to 1:1, while the overall SD of $AE1.7% is slightly larger than that obtained for the same set of TAG mixtures in our previous 13 C NMR work ($AE1.1%) [4]. This small loss of accuracy relative to 13 C NMR is compensated for to a large degree by the significant advantages of 1 H NMR analysis in terms of analytical time and cost.…”

“…Accordingly, as in our previous 13 C NMR work [4], we developed a standardized spectral preprocessing procedure to minimize the contributions of subjective judgment to the final spectra used for quantification of NLCD. A manual zero-order phase correction was initially applied to the CDCl 3 peak at 7.25 ppm followed by a first-order phase correction based on the TMS peak.…”

“…However, while cisand trans-monounsaturates can both be accurately quantitated from the 13 C NMR spectrum of an oil [6,9], there is no evidence in the literature that 1 H NMR can be employed to distinguish between cis-and trans-unsaturation in fats and oils. As such, our initial work on the NMR determination of NLCD focused on 13 C NMR [4]. That study established a procedure for conversion of the mol% data provided by NMR to wt%, as required for nutrition labeling purposes, and showed that 13 C NMR can serve in lieu of GC as a reference method for the determination of NLCD [4].…”

“…As such, our initial work on the NMR determination of NLCD focused on 13 C NMR [4]. That study established a procedure for conversion of the mol% data provided by NMR to wt%, as required for nutrition labeling purposes, and showed that 13 C NMR can serve in lieu of GC as a reference method for the determination of NLCD [4].…”

“…In a recent paper [4], the suitability of 13 C NMR as an alternative to GC for obtaining reference data was demonstrated by examining model triglycerides as well as preanalyzed samples obtained from the AOCS Laboratory Proficiency Program (AOCS-LPP). The potential use of NMR as a primary reference method was predicated on the fact that NMR spectral responses do not vary across the spectrum, as they do in FTIR spectroscopy and many other spectroscopic methods.…”