A stable isotope approach to assessing water loss in fruits and vegetables during storage

Comparison of Carbon Isotope Ratio Measurement of the Vanillin Methoxy Group by GC-IRMS and 13C-qNMR

Site-specific carbon isotope ratio measurements by quantitative 13C NMR (13C-qNMR), Orbitrap-MS, and GC-IRMS offer a new dimension to conventional bulk carbon isotope ratio measurements used in food provenance, forensics, and a number of other applications. While the site-specific measurements of carbon isotope ratios in vanillin by 13C-qNMR or Orbitrap-MS are powerful new tools in food analysis, there are a limited number of studies regarding the validity of these measurement results. Here we present carbon site-specific measurements of vanillin by GC-IRMS and 13C-qNMR for methoxy carbon. Carbon isotope delta (δ13C) values obtained by these different measurement approaches demonstrate remarkable agreement; in five vanillin samples whose bulk δ13C values ranged from -31‰ to -26‰, their δ13C values of the methoxy carbon ranged from -62.4‰ to -30.6‰, yet the difference between the results of the two analytical approaches was within ±0.6‰. While the GC-IRMS approach afforded up to 9-fold lower uncertainties and required 100-fold less sample compared to the 13C-qNMR, the 13C-qNMR is able to assign δ13C values to all carbon atoms in the molecule, not just the cleavable methoxy group.

Variations in lignin monomer contents and stable hydrogen isotope ratios in methoxy groups during the biodegradation of garden biomass

Lignin, a highly polymerized organic component of plant cells, is one of the most difficult aromatic substances to degrade. Selective biodegradation under mild conditions is a promising method, but the dynamic variations in lignin monomers during the biodegradation of lignocellulose are not fully understood. In this study, we evaluated the differences in lignin degradation under different microbial inoculation based on the lignin monomer content, monomer ratio, and stable hydrogen isotope ratio of lignin methoxy groups (δ2HLM). The weight loss during degradation and the net loss of lignocellulosic components improved dramatically with fungal inoculation. Syringyl monolignol (S-lignin), which contains two methoxy groups, was more difficult to degrade than guaiacyl (G-lignin), which contains only one methoxy group. The co-culture of Pseudomonas mandelii and Aspergillus fumigatus produced the greatest decrease in the G/S ratio, but δ2HLM values did not differ significantly among the three biodegradation experiments, although the enrichment was done within the fungal inoculation. The fluctuation of δ2HLM values during the initial phase of biodegradation may be related to the loss of pectic polysaccharides (another methoxy donor), which mainly originate from fallen leaves. Overall, the relative δ2HLM signals were preserved despite decreasing G/S ratios in the three degradation systems. Nevertheless, some details of lignin δ2HLM as a biomarker for biogeochemical cycles need to be explored further.

Isotope chemometrics determines farming methods and geographical origin of vegetables from Yangtze River Delta Region, China

Shanghai city has encountered possible food fraud regarding the geographical mislabeling of vegetables for economic gain. A combination of δ¹³C, δ¹⁵N, δ²H and δ¹⁸O values and partial least squares discrimination analysis and support vector machine (SVM) methods were used for the first time to assess farming methods and determine the origin of vegetables from Shanghai city, Anhui and Zhejiang provinces. The results showed that 65.8% of Shanghai vegetables, 38.2% of Anhui vegetables and 23.6% of Zhejiang vegetables appeared to be grown using green or organic farming methods. The optimal discriminant model was obtained using SVM with a predictive accuracy of 100% for Shanghai vegetables. Zhejiang vegetables had a predictive accuracy of 91.7%, while it was difficult to distinguish Anhui vegetables from Shanghai or Zhejiang vegetables. Therefore, this study provided a useful method to identify vegetable farming methods and discriminate vegetables from Shanghai and Zhejiang.

Methyl sulfates as methoxy isotopic reference materials for δ13 C and δ2 H measurements

RATIONALE

Stable hydrogen and carbon isotope ratios of methoxy groups (OCH₃ ) of plant organic matter have many potential applications in biogeochemical, atmospheric and food research. So far, most of the analyses of plant methoxy groups by isotope ratio mass spectrometry have employed liquid iodomethane (CH₃ I) as the reference material to normalise stable isotope measurements of these moieties to isotope-δ scales. However, comparisons of measurements of stable hydrogen and carbon isotopes of plant methoxy groups are still hindered by the lack of suitable reference materials.

METHODS

We have investigated two methyl sulfate salts (HUBG1 and HUBG2), which exclusively contain carbon and hydrogen from one methoxy group, for their suitability as methoxy reference materials. Firstly, the stable hydrogen and carbon isotope values of the bulk compounds were calibrated against international reference substances by high-temperature conversion- and elemental analyser isotope ratio mass spectrometry (HTC- and EA-IRMS). In a second step these values were compared with values obtained by measurements using gas chromatography/isotope ratio mass spectrometry (GC/IRMS) where prior to analysis the methoxy groups were converted into gaseous iodomethane.

RESULTS

The ² H- and ¹³ C isotopic abundances of HUBG1 measured by HTC- and EA-IRMS and expressed as δ-values on the usual international scales are -144.5 ± 1.2 mUr (n = 30) and -50.31 ± 0.16 mUr (n = 14), respectively. For HUBG2 we obtained -102.0 ± 1.3 mUr (n = 32) and +1.60 ± 0.12 mUr (n = 16). Furthermore, the values obtained by GC/IRMS were in good agreement with the HTC- and EA-IRMS values.

CONCLUSIONS

We suggest that both methyl sulfates are suitable reference materials for normalisation of isotope measurements of carbon of plant methoxy groups to isotope-δ scales and for inter-laboratory calibration. For stable hydrogen isotope measurements, we suggest that in addition to HUBG1 and HUBG2 additional reference materials are required to cover the full range of plant methoxy groups reported so far.

Application of carbon and hydrogen stable isotope analyses to detect exogenous citric acid in Japanese apricot liqueur

Japanese apricot liqueur manufacturers are required to control the quality and authenticity of their liqueur products. Citric acid made from corn is the main acidulant used in commercial liqueurs. In this study, we conducted spiking experiments and carbon and hydrogen stable isotope analyses to detect exogenous citric acid used as an acidulant in Japanese apricot liqueurs. Our results showed that the δ¹³C values detected exogenous citric acid originating from C₄ plants but not from C₃ plants. The δ²H values of citric acid decreased as the amount of citric acid added increased, whether the citric acid originated from C₃ or C₄ plants. Commercial liqueurs with declared added acidulant provided higher δ¹³C values and lower δ²H values than did authentic liqueurs and commercial liqueurs with no declared added acidulant. Carbon and hydrogen stable isotope analyses are suitable as routine methods for detecting exogenous citric acid in Japanese apricot liqueur.