Control of oenological products: discrimination between different botanical sources of L-tartaric acid by isotope ratio mass spectrometry

Natural complex substances: From molecules to the molecular complexes. Analytical and technological advances for their definition and differentiation from the corresponding synthetic substances

Natural complex substances (NCSs) are a heterogeneous family of substances that are notably used as ingredients in several products classified as food supplements, medical devices, cosmetics and traditional medicines, according to the correspondent regulatory framework. The compositions of NCSs vary widely and hundreds to thousands of compounds can be present at the same time. A key concept is that NCSs are much more than the simple sum of the compounds that constitute them, in fact some emerging phenomena are the result of the supramolecular interaction of the constituents of the system. Therefore, close attention should be paid to produce and characterize these systems. Today many natural compounds are produced by chemical synthesis and are intentionally added to NCSs, or to formulated natural products, to enhance their properties, lowering their production costs. Market analysis shows a tendency of people to use products made with NCSs and, currently, products made with ingredients of natural origin only are not conveniently distinguishable from those containing compounds of synthetic origin. Furthermore, the uncertainty of the current European regulatory framework does not allow consumers to correctly differentiate and identify products containing only ingredients of natural origin. The high demand for specific and effective NCSs and their high-cost offer on the market, create the conditions to economically motivated sophistications, characterized by the addition of a cheap material to a more expensive one, just to increase profit. This type of practice can concern both the addition of less valuable natural materials and the addition of pure artificial compounds with the same structure as those naturally present. In this scenario, it becomes essential for producers of natural products to have advanced analytical techniques to evaluate the effective naturalness of NCSs. In fact, synthetically obtained compounds are not identical to their naturally occurring counterparts, due to the isotopic composition or chirality, as well as the presence of different trace metabolites (since pure substances in nature do not exist). For this reason, in this review, the main analytical tests that can be performed to differentiate natural compounds from their synthetic counterparts will be highlighted and the main analytical technologies will be described. At the same time, the main fingerprint techniques useful for characterizing the complexity of the NCSs, also allowing their identification and quali-quantitative evaluation, will be described. Furthermore, NCSs can be produced through different manufacturing processes, not all of which are on the same level of quality. In this review the most suitable technologies for green processes that operate according to physical extraction principles will be presented, as according to the authors they are the ones that come closest to creating more life-cycle compatible NCSs and that are well suited to the European green deal, a strategy with the aim of transforming the EU into a sustainable and resource-efficient society by 2050.

Carbon isotope ratio of organic acids in sake and wine by solid-phase extraction combined with LC/IRMS

We developed an analytical procedure for determining the δ¹³C values of organic acids in sake and wine using solid-phase extraction combined with liquid chromatography/isotope ratio mass spectrometry (LC/IRMS). First, the solid-phase extraction (SPE) procedure was performed and various tests were conducted to extract organic acids from alcoholic beverages using the simulated sake sample. Under the optimal SPE procedure, high recovery rates (96-118%) and good accuracies (≤ 0.7‰) were thus achieved for the simulated sake and wine samples. Next, we determined the δ¹³C of organic acid (tartaric acid, malic acid, lactic acid, succinic acid) in 9 sake and 11 wine samples. Finally, the δ¹³C values of lactic acid in nine sake samples suggested that lactic acid had been added during the brewing process. The high correlation between the δ¹³C values of tartaric acid and malic acid in 11 wine samples was consistent with their common source, grapes. This analytical method may help to identify when organic acids have been added to sake and wine and to elucidate the process of organic acid production therein. Graphical abstract.

Site-specific (13) С/(12) С isotope abundance ratios in dicarboxylic oxyacids as characteristics of their origin

RATIONALE

Recommendations of relevant international organizations controlling the quality of grape wines and beverages specify that only tartaric acids of grape origin can be introduced to achieve the required parameters. The development of methods for determining the origin of tartaric acid in grape wine is of great technological significance.

METHODS

Organic dicarboxylic oxyacids were extracted from wines as barium salts. Carbon dioxide, which included all the carbon atoms of the acids, was used to determine the carbon isotope ratios by Isotope Ratio Mass Spectrometry. The alkyl part of the oxyacids was burned at 560°C in the presence of air; BaCO3 containing the carboxyl carbon was left. This carbonate was used to measure the carbon isotope ratios in the carboxyl part of the acid. The carbon isotope ratios of the alkyl part of tartaric acid were found by isotope mass balance.

RESULTS

The carbon isotope composition of carboxyl groups (δ(13) С values) in tartaric acid of grape (biogenic origin) had a higher (13) С content than the carbon in the alkyl part of the molecule. Tartaric acid produced by chemical synthesis (abiogenic origin) was noted to have a different (13) С/(12) С distribution: the carboxyl group of tartaric acid produced by chemical synthesis contained a smaller than or equal amount of (13) С to the alkyl part.

CONCLUSIONS

This is the first determination of the site-specific distribution of the (13) С/(12) С isotopes in tartaric acids as evidence of their biogenic and abiogenic origins. The presented method for determining the origin of tartaric acid can be used for efficient control of the quality of grape wines and beverages.

Botanical traceability of commercial tannins using the mineral profile and stable isotopes

Commercial tannins are natural polyphenolic compounds extracted from different plant tissues such as gall, the wood of different species and fruit. In the food industry they are mainly used as flavourings and food ingredients, whereas in winemaking they are classified as clarification agents for wine protein stabilisation, although colour stabilisation, metal removal, unpleasant thiol removal and rheological correction are also well-known and desired effects. Due to their particular technical properties and very different costs, the possibility of correct identification of the real botanical origin of tannins can be considered a primary target in oenology research and in fulfilling the technical and economic requirements of the wine industry. For some categories of tannins encouraging results have already been achieved by considering sugar or polyphenolic composition. For the first time this work verifies the possibility of determining the botanical origin of tannins on the basis of the mineral element profile and analysis of the (13) C/(12) C isotopic ratio. One hundred two commercial tannins originating from 10 different botanical sources (grapes, oak, gall, chestnut, fruit trees, quebracho, tea, acacia, officinal plants and tara) were analysed to determine 57 elements and the (13) C/(12) C isotopic ratio, using inductively coupled plasma mass spectrometry and isotope-ratio mass spectrometry, respectively. Forward stepwise discriminant analysis provided good discrimination between the 8 most abundant groups, with 100% correct re-classification. The model was then validated five times on subsets of 10% of the overall samples, randomly extracted, achieving satisfactory results. With a similar approach it was also possible to distinguish toasted and untoasted oak tannins as well as tannins from grape skin and grape seeds.

Authentication of farmed and wild turbot (Psetta maxima) by fatty acid and isotopic analyses combined with chemometrics

Fatty acid composition and stable isotope ratios of carbon (delta(13)C) and nitrogen (delta(15)N) were determined in muscle tissue of turbot (Psetta maxima). The multivariate analysis of the data was performed to evaluate their utility in discriminating wild and farmed fish. Wild (n=30) and farmed (n=30) turbot of different geographical origins (Denmark, The Netherlands, and Spain) were sampled from March 2006 to February 2007. The application of linear discriminant analysis (LDA) and soft independent modeling of class analogy (SIMCA) to analytical data demonstrated the combination of fatty acids and isotopic measurements to be a promising method to discriminate between wild and farmed fish and between wild fish of different geographical origin. In particular, IRMS (Isotope Ratio Mass Spectrometry) alone did not permit us to separate completely farmed from wild samples, resulting in some overlaps between Danish wild and Spanish farmed turbot. On the other hand, fatty acids alone differentiated between farmed and wild samples by 18:2n-6 but were not able to distinguish between the two groups of wild turbot. When applying LDA isotope ratios, 18:2n-6, 18:3n-3, and 20:4n-6 fatty acids were decisive to distinguish farmed from wild turbot of different geographical origin, while delta(15)N, 18:2n-6, and 20:1n-11 were chosen to classify wild samples from different fishing zones. In both cases, 18:2n-6 and delta(15)N were determinant for classification purposes. We would like to emphasize that IRMS produces rapid results and could be the most promising technique to distinguish wild fish of different origin. Similarly, fatty acid composition could be used to easily distinguish farmed from wild samples.