Biologie  |  Umwelt

 

Benjamin Gartner, 2003 | Rüfenach, AG

 

This study investigates the correlation of aqueous and organic isotopic signatures of oxygen (d18O), carbon (d13C), and nitrogen (d15N) in milk with those of soil, grass, and rainwater to check the possibility of determining the origin of milk. Samples were collected from conventionally operated and Demeter-labelled farms all over Switzerland, the latter to minimize the influence of non-local fodder. Statistical analyses revealed significant variances in isotopic ratios among different geographic origins, surpassing variations within cows from one farm. High correlations were observed between d18O in rainwater and milk, as well as between d15N in grass and milk, particularly in Demeter farms. Moreover, d15N correlations were stronger in Demeter farms, suggesting the influence of local nitrogen-rich fertilizers in conventional farms. Notably, 66% of conventional milk exhibited unexpected 13C enrichment, potentially indicating non-local fodder usage. While d18O and d15N could aid in confirming or refuting the claimed origin of milk, their precision remains insufficient for accurate geographical localization. Conversely, d13C and d15N could serve to verify Demeter farms’ adherence to feeding guidelines and fertilizer utilization.

Fragestellung

(I) How strong are the isotopic signatures of O, C and N in the organic and aqueous part of the milk related to each other and to those of soil, grass and rainwater (II) Is it possible to confirm the geographic origin of the milk? (III) Does the isotope signature of milk from conventionally operated and Demeter-labelled farms differ?

Methodik

Three samples of soil, grass and milk were taken from 17 farms (8 Demeter, 9 conventional) all over Switzerland. Demeter farms require that 80% of the fodder must origin from local production and hence the effect of non-local fodder is minimized. Water from the soil and grass samples was extracted using cryogenic vacuum distillation, while the dried portions were ground. The milk samples were separated into organic and aqueous parts using lyophilization and centrifugation with chloroform. The O, C and N isotope signatures were determined in the organic samples (1mg) and the oxygen signature in the aqueous samples (1ml) using mass spectrometry. The data was statistically evaluated using Pearson correlation analysis and compared with literature data on precipitation (rainwater).

Ergebnisse

The isotope ratios of O, C and N all had a significant higher variance when comparing data from different geographic origins as opposed to different cows from the same farm. The highest correlations were found between the d18O literature data of rainwater and the aqueous portion of milk , and between the d15N values of the organic portion of the gras and milk. All d18O correlations obtained between rain, soil or gras and milk were stronger in Demeter milk, as were the d15N correlations. The d13C measurements yielded no relevant correlations. Milk from Demeter farms was 13C depleted in comparison to the gras, in contrast to conventional milk.

Diskussion

Observed variances in milk from many cows from one farm are lower compared to variations observed in samples from geographically separated farms. Therefore, the effect of cow metabolism on isotopic composition is weaker than geographic origin. Stable isotope ratios in milk can therefore be used to differentiate between farms. The correlation between d18O values of rainwater and milk were high as expected. The expected correlations between soil and milk, and gras and milk only occurred in milk from Demeter farms. The average of measured d18O values was consistent with the known altitude effect, but single measured values deviated too much from predicted ones to make them a consistent marker of origin. The absence of a significant difference in average d18O values between Demeter and conventional milk suggests that d18O values are mainly determined by geographic location and not by cultivation type. As the d13C values in the milk are mainly determined by the fodder, the absence of a correlation between d13C values of soil and milk as well as gras and milk was expected. We also found d13C to indicate rather the fodder than the location, and feeding of non-local fodder, and d15N to indicate the origin of Demeter milk.

Schlussfolgerungen

d13C and d15N could be used to verify the adherence of Demeter farms to the guidelines of feeding local food and utilization of fertilizers. d18O and d15N could be used to confirm or at least disprove the claimed origin. d13C values depend too much on the fodder plants to allow for geographic localization.

 

 

Würdigung durch den Experten

Dr. Marco Lehmann

Benjamin Gartner hat mit grossem Interesse und Engagement Proben von Kuhmilch, Futter und Umwelt aus landwirtschaftlichen Höfen in verschiedenen Teilen der Schweiz gesammelt, im Labor aufbereitet und die Isotopenverhältnisse unterschiedlicher Elemente gemessen. Er hat die Fragestellung, Umsetzung und Methodik aufeinander abgestimmt sowie die Komplexität der Anwendungsmöglichkeiten und Limitierungen der Isotopenanalyse verdeutlicht. Die umfangreichen Ergebnisse sind klar herausgearbeitet und bilden eine ausgezeichnete Grundlage für weitere Forschungsarbeiten zum Thema.

Prädikat:

sehr gut

 

 

 

Kantonsschule Wettingen
Lehrer: Stefan Ibold