Follow-up of stable isotope analysis of organic versus conventional milk

Summary

Analysis of the stable isotope ratio of carbon (δ13C) and α-linolenic acid (C18:3ω3) content in milk fat is a useful indicator of organic milk production. Referring to corresponding measurements, further analyses of stable isotope ratios were performed in 120 samples of conventionally and organically produced whole milk collected from German retailers during a period of 18 months.
Conventional milk predominantly exhibited higher δ15N values than organic milk, the latter of which never exceeded a maximum δ15N threshold value of 5.50‰. Measurements of δ34S did not differ significantly between organic and conventional milk. Because δ13C, in general, is related to maize consumption, δ13C in milk protein and δ13C in milk fat were equally suited for authentication of organic milk. Thus, a high correlation (r=0.99) was established between δ13C in milk protein and lipids. Although occurring on different levels in organic and conventional milk, the relatively constant fractionation of carbon isotopes between protein and fat will allow for the advanced detection of adulteration in processed milk products, such as fraudulent combinations of organic milk fat and conventional skim milk. In addition to the strong correlation between C18:3ω3 and δ13Cprotein (r=−0.91), a mutual dependence was identified between both δ13Cprotein and δ15N (r=0.66) and C18:3ω3 and δ15N (r=−0.61).
Thus, multi-variable analyses are useful to increase robustness and reduce the number of exceptions in organic milk authentication. Future work involving multivariate statistical analysis can possibly further improve milk authentication in various respects including differentiating between brands of retail milk.
(C) Springer-Verlag 2010