Determination of the oxygen-18/oxygen-16 isotope ratios of sugar, citric acid and water from single strength orange juice

A review of fruit juice authenticity assessments: Targeted and untargeted analyses

Fruit juices are becoming more and more popular in the whole world. However, the increasing fruit juice fraud cases are undermining the healthy development of fruit juice industry. Fruit juice authenticity represents an important food quality and safety parameter. Many techniques have been applied in fruit juices authenticity assessment. The purpose of this review is to provide a research overview of the targeted and untargeted analyses of fruit authentication, and a method selection guide for fruit juice authenticity assessment. Targeted markers, such as stable isotopes, phenolics, carbohydrates, organic acids, volatile components, DNAs, amino acids and proteins, as well as carotenoids, will be discussed. And untargeted techniques, including liquid/gas chromatography-mass spectrometer, nuclear magnetic resonance, infrared spectroscopy, inductively-coupled plasma-mass spectrometry/optical emission spectrometer, fluorescence spectra, electronic sensors and others, will be reviewed. The emerging untargeted for novel targeted marker analysis will be also summarized.

Traceability of fruits and vegetables

Food safety and traceability are nowadays a constant concern for consumers, and indeed for all actors in the food chain, including those involved in the fruit and vegetable sector. For the EU, the principles and legal requirements of traceability are set out in Regulation 178/2002. Currently however the regulation does not describe any analytical traceability tools. Furthermore, traceability systems for fruits and vegetables face increasing competition due to market globalization. The current challenge for actors in this sector is therefore to be sufficiently competitive in terms of price, traceability, quality and safety to avoid scandal and fraud. For all these reasons, new, flexible, cheap and efficient traceability tools, as isotopic analysis, DNA fingerprinting and metabolomic profiling coupled with chemometrics are needed.

Stable isotope ratios of H, C, N and O in Italian citrus juices

Stable isotope ratios (SIRs) of C, N, H and O have been exensively used in fruit juices quality control (ENV and AOAC methods) to detect added sugar and the watering down of concentrated juice, practices prohibited by European legislation (EU Directive 2012/12). The European Fruit Juice Association (AIJN) set some reference guidelines in order to allow the judging of the genuiness of a juice. Moreover, various studies have been carried out to determine the natural variability of SIRs in fruit juices, but none of these has investigated SIRs extensively in authentic citrus juices from Italy. In this work, about 500 citrus juice samples were officially collected in Italy by the Italian Ministry of Agricultural and Forestry Policies from 1998 onwards. (D/H)(I) and (D/H)(II) in ethanol and δ(13) C(ethanol), δ(13) C(pulp), δ(13) C(sugars), δ(18) O(vegetalwater), δ(15) N(pulp), and δ(18) O(pulp) were determined using Site-Specific Natural Isotope Fractionation-Nuclear Magnetic Resonance and Isotope Ratio Mass Spectrometry, respectively. The characteristic ranges of variability in SIRs in genuine Italian citrus juice samples are here presented as well as their relationships and compliance with the limits indicated by the AIJN and others proposed in the literature. In particular, the Italian range of values was found to be not completely in agreement with AIJN guidelines, with the risk that genuine juices could be judged as not genuine. Variety seems not to influence SIRs, whereas harvest year and region of origin have some influence on the different ratios, although their data distribution shows overlapping when principal component analysis is applied.

Elimination of nitrogen interference during online oxygen isotope analysis of nitrogen-doped organics using the "NiCat" nickel reduction system

RATIONALE

Accurate online analysis of the δ(18)O values of nitrogen-bearing organic compounds is of interest to several emergent fields, including ecology, forensics and paleontology. During online analysis, high-temperature conversion (HTC) of nitrogen-bearing organics produces N(2) gas which creates isobaric interference with the isotopic measurement. Specifically, N(2) reacts with trace amounts of oxygen in the mass spectrometer source to form (14)N(16)O (m/z 30), which prevents accurate evaluation of the sample (12)C(18)O peak (m/z 30).

METHODS

We present an alternative system to the conventional HTC, which uses a nickel-catalyzed ("NiCat") reduction furnace to convert HTC-produced CO into CO(2), allowing for δ(18)O measurement using signal intensities at m/z 44 and 46.

RESULTS

This system yields identical δ(18)O values for nitrogen-doped and undoped sucrose and cellulose compounds up to molar yield ratios of N(2):CO  =  0.22. In contrast, our conventional HTC system configured to factory recommendations with the stock gas chromatography (GC) column produced a discrepancy of ~5‰ between nitrogen-doped and undoped samples.

CONCLUSIONS

Because of its ability to eliminate isobaric interference, the NiCat system is a viable alternative to conventional HTC for δ(18)O measurement, and can be constructed from relatively inexpensive and readily available materials. As an additional advantage, the CO(2) analyte produced by NiCat may be cryofocused, to allow for oxygen-isotope determinations on very small amounts of sample substrate.

Comparison of the oxygen and hydrogen isotopes in the juices of fast-growing vegetables and slow-growing fruits

We have analyzed the oxygen and hydrogen isotopic composition of juices from fruits and vegetables collected from a small orchard in order to investigate the differences in isotopic enrichment and evaporation intensity between fast-growing vegetables and slow-growing fruits grown under the same climatic conditions. The oxygen and hydrogen isotope levels were much higher in the juices of the fruits and vegetables than in the source waters in which they grew because of evaporation effects. According to our data, fast-growing vegetables are subject to greater evaporation than slow-growing fruits. An evaporation experiment using the source water showed that the oxygen and hydrogen isotopic composition of the 60-80% residual fraction was similar to that of the isotopically enriched grape juice, whereas those of the plume and tomato juices were very close to that of the 80-90% residual fraction, thus proving the effect of evaporation.

Stable oxygen isotope composition of plant tissue: a review

With the development of rapid measurement techniques, stable oxygen isotope analysis of plant tissue is poised to become an important tool in plant physiological, ecological, paleoclimatic and forensic studies. Recent advances in mechanistic understanding have led to the improvement of process-based models that accurately predict variability in the oxygen isotope composition of plant organic material (δ¹⁸O_p). δ¹⁸O_p has been shown to reflect the isotope composition of soil water, evaporative enrichment in transpiring leaves, and isotopic exchange between oxygen atoms in organic molecules and local water in the cells in which organic molecules are formed. This review presents current theoretical models describing the influences on δ¹⁸O_p, using recently published experimental work to outline strengths and weaknesses in the models. The potential and realised applications of the technique are described.

Oxygen-18 measurement of Andalusian olive oils by continuous flow pyrolysis/isotope ratio mass spectrometry

We report a method for the determination of delta(18)O isotopic abundance in olive oils. The results obtained by applying the method to various Andalusian oil samples obtained in the 2004/05 and 2005/06 seasons are discussed in relation to olive variety, geographical origin, climate and ripeness index. Application of the method to samples of assured varietal purity exposed the influence of olive variety and origin but not of the ripeness index. The delta(18)O values for the 2005/06 season are higher on average than those obtained in the colder 2004/05 season. Results obtained for samples of the Picual and Hojiblanca varieties in Córdoba and Málaga in the 2005/06 season suggest a correlation between enrichment in heavy isotopes and latitude whereas no clear-cut effect of altitude was observed.

Study of the influence of alcoholic fermentation and distillation on the oxygen-18/oxygen-16 isotope ratio of ethanol

A laboratory procedure for the analysis of the oxygen-18/oxygen-16 isotope ratios of ethanol derived from sugars and fruit juices by pyrolysis-isotope ratio mass spectrometry (IRMS) has been applied to the study of isotopic fractionation induced by the isotope effects of fermentation and distillation. For both processes, an experimental model has been established to describe and explain the observed fractionation phenomena. It is shown that reproducible results can be obtained when appropriate analytical conditions are used. Moreover, the ability of ethanol to act as a reliable indicator of the (18)O/(16)O ratio of sugars in orange juice (and therefore to be used as an internal reference for detecting water addition) is demonstrated both in theory and in practice.

A simple effective method for estimating the [18O] enrichment of water mixtures

Isotopically enriched water, [18O] H(2)O, recovered from the synthesis of [18F] fluorine-labelled PET radiopharmaceuticals, is frequently diluted with natural water, [16O] H(2)O. The extent of isotope dilution limits the usefulness of the recovered water. A simple method for determining the [18O] enrichment of water mixtures could aid PET cyclotron facilities to calculate the [18F] yields to be expected when the recovered [18O] H(2)O is reused. A gravimetric method (repeatability for high enrichment +/-0.3% [18O], detection limit approximately 2% [18O] enrichment) is described.

18O pattern and biosynthesis of natural plant products

Oxygen atoms in plant products originate from CO(2), H(2)O and O(2), precursors with quite different delta18O values. Furthermore their incorporation by different reactions implies isotope effects. On this base the resulting non-statistical 18O distributions in natural compounds are discussed. The delta18O value of cellulose is correlated to that of the leaf water, and the observed 18O enrichment (approximately +27 per thousand) is generally attributed to an equilibrium isotope effect between carbonyl groups and water. However, as soluble and heterotrophically synthesised carbohydrates show other correlations, a non-statistical 18O distribution - originating from individual biosynthetic reactions - is postulated for carbohydrates. Similarly, the delta18O values of organic acids, carbonyl compounds, alcohols and esters indicate water-correlated, but individual 18O abundances (e.g. O from acyl groups approximately +19% above water), depending upon origin and biosyntheses. Alcoholic groups introduced by monooxygenase reactions, e.g. in sterols and phenols, show delta18O values near +5 per thousand, in agreement with an assumed isotope fractionation factor of approximately 1.02 on the reaction with atmospheric oxygen (delta18O=+23.5 per thousand). Correspondingly, a "thermodynamically ordered isotope distribution" is only observed for oxygen in some functional groups correlated to an origin from CO(2) and H(2)O, not from O(2). The individual isotopic increments of functional groups permit the prediction of global delta18O values of natural compounds on the basis of their biosynthesis.