Combination of ¹³C and ²H SNIF‐NMR isotopic fingerprints of vanillin to control its precursors

Abstract

Synthetic vanillin is the flavouring most used in agro‐food industries, but more and more frequently consumers are now turning to natural origin. Vanilla planifolia (Andrews) is the most commercialized species of pods in the world and is mainly produced in Madagascar. However, periodically the production of V. planifolia suffers from poor climatic conditions, with the resulting degradation of both the quality and the quantity of the pods. Consequently, the price of vanilla pods rose to record in 2017 and the risk of fraud has increased in response. Analytical methods based on isotopic compositions have already proven their ability to ensure the authenticity of natural vanillin. In 2006, 2H SNIF‐NMR (site‐specific natural isotopic fractionation by nuclear magnetic resonance) was approved as official method by the Association of Official Analytical Chemists (AOAC 2006.05). However, this method is time consuming and the extraction of 1 g is cumbersome for the finished products. The information brought by combined 2H and 13C SNIF‐NMR profiles was compared using chemometric tools to determine the best routine tool to improve both the global analysis time and the potential of detection. As a result of this work the ability of the SNIF‐NMR method to verify the authenticity of vanillin has been improved, in particular by providing the first means to discern the geographical origin of vanilla pods. Furthermore, 13C NMR using pulse sequences such as INEPT (insensitive nuclei enhanced by polarization transfer) offers the possibility to improve the sensitivity of the analysis, with a reduced quantity of product (less than 50 mg) and a shorter analysis time, which will facilitate the study of the finished matrices as well as clearly discriminating the origins of the vanilla flavourings.

Isotopic characterization of vanillin ex glucose by GC-IRMS - New challenge for natural vanilla flavour authentication?

“…A cheap source of vanillin enriched in 13 C will make it easier to simulate the  13 CBulk value of vanillin from vanilla pods by mixing vanillin from the respective sources, which can increase the risk of fraud being perpetrated while remaining unnoticed by the analysis of  13 CBulk values. However, several studies on purified vanillin have demonstrated that additional information about the  2 H values as well as positionspecific isotope analysis of stable hydrogen and carbon enables the distinction of vanillin obtained from different precursors even if the  13 CBulk ranges overlap (Geißler et al, 2017;Tenailleau, Lancelin, Robins, & Akoka, 2004;Guyader et al, 2019).…”

“…Another analytical technique for the authentication of vanillin is the determination of 13 C and 2 H-SNIF-NMR isotopic fingerprints. Recently, vanillin ex ferulic acid (corn) has also been included in a database where vanillin from different precursors (ferulic acid (rice), eugenol, cucurmin, lignin, guaiacol, vanilla pods) were investigated (Guyader et al, 2019). The 13 C isotopic profiles allowed a promising distinction of all of the precursor groups.…”

“…This is in accordance with the GC-IRMS results where the  13 CBulk and  13 CMethoxy of vanillin ex ferulic acid (corn) and ex vanilla pods are overlapping, while a separation can be observed when comparing the values for  2 HBulk and  2 HMethoxy of both groups. One of the aims of the SNIF-NMR analysis is to reduce the needed sample amount which might open the possibility to analyse vanillin isolated from food products by SNIF-NMR (Guyader et al, 2019). In contrast to that, only a small amount of vanillin is needed for the analysis by GC-IRMS in combination with the Zeisel method.…”

“…Position-specific analysis in combination with multivariate data analysis has been applied successfully to distinguish vanillin from different sources, even when the  13 CBulk values are overlapping (Geißler et al, 2017). SNIF-NMR methods (site-specific isotopic fractionation by nuclear magnetic resonance) were applied, however, large sample amounts and long analysis times are required (Guyader et al, 2019). GC-IRMS, which requires much smaller sample amounts delivers the isotopic ratio of the vanillin bulk value and, by applying the 'Zeisel reaction', also provides the stable carbon and hydrogen isotope signature of the methoxy group (Greule et al, 2010;Krueger & Krueger, 1983).…”

The Application of NMR Spectroscopy and Chemometrics in Authentication of Spices

“…Due to magnetization transfer from the sensitive proton to the coupled carbon an improvement in the sensitivity was achieved. Furthermore, by means of INEPT the precision of the order of 1‰ was able to be reached [ 82 , 85 ].…”