Potentials and caveats with oxygen and sulfur stable isotope analyses in authenticity and origin checks of food and food commodities

Abstract

Analyses of stable isotope ratios are officially accepted methods in food authenticity and origin determination. They are routinely practiced by empirical comparison of the unknown samples' δ values with those of authentic material. However, as the isotope characteristics of food are influenced by many parameters, it is desirable to also study and use causal correlations of isotope fractionations for the interpretation of experimental data. Corresponding potentials and limits are outlined for oxygen and sulfur stable isotopes.In the natural water cycle, plant leaf and animal cell water are the most important sources for food integrated water and organically bound oxygen. The way from sea water to fruit juice water and to organic matter and the integrated isotope fractionations are shown and the possibilities and limits for the assignment of juices and wine to their geographical origin, history and authenticity are deduced. The oxygen flux and isotopic balance in animals and the sources and drains of animal body water are outlined and the problems and limits for its suitability as a bioindicator for origin assignments of animal food products are discussed. The potential of the δ18O value of organically bound oxygen is demonstrated.The sulfate reduction in plants is accompanied by isotope fractionation but as normally no sulfur is excreted, the δ34S value of bulk plant matter is identical to that of the primary local source. Small differences in the δ34S values of plant compartments are often due to differences in their abundance of main S-containing ingredients, as residual sulfate and cysteine or methionine containing proteins. This is similar with animal tissues and products. Therefore, the sulfur isotope analysis of the bulk matter or of defined fractions of plant and animal samples is an ideal and reliable tool for food origin and authenticity proof and for archaeological and animal migration research.