Seasonal variation in the C, N and S stable isotope composition of retail organic and conventional Irish beef

The objective of this study was to investigate seasonal variation in the C, N and S stable isotope composition of retail organic and conventional Irish beef. A total of 242 beef samples (127 organic, 115 conventional) was collected in a one-year survey and analysed by isotope ratio mass spectrometry. The delta C-13 time series in conventional beef was significantly non-random, with a pronounced seasonal positive shift of >2 parts per thousand between December and June, whilst delta C-13 in organic beef was less variable and significantly more negative. In conventional beef, delta N-15 was remarkably invariant (remaining close to 7 parts per thousand) throughout the year, while organic beef was more variable and also significantly lower in delta(15) N. The S isotope composition (delta S-34) exhibited a complex seasonal pattern in both types of beef. These results show that seasonal patterns can occur in the isotopic composition of beef, probably reflecting seasonality in animal feeding practices modulated by tissue turnover rates. Such seasonal variation needs to be considered in the isotopic authentication of beef and other animal-derived products. (C) 2007 Elsevier Ltd. All rights reserved.