Carbon and nitrogen stable isotopes in U.S. milk: Insight into production process

Stable isotope analysis (SIA), a potential method of verifying the geographic origin and production method of dairy products, has not been applied to United States (U.S.) dairy samples on a national scale. To determine the potential of carbon and nitrogen SIA in authenticity assessment of U.S. dairy products, we analyzed a geographically representative collection of conventional milk samples to determine isotopic variations with (1) Purchase Location and (2) Macronutrient Content.

A total of 136 milk samples spanning five commercially available varieties (3.25% [i.e., 'whole'], 2%, 1%, 0% [i.e., 'skim'] and 1% chocolate) were collected from randomly selected counties across the U.S. as part of the United States Department of Agriculture's (USDA's) National Food and Nutrient Analysis program. δ¹³ C and δ¹⁵ N values of bulk samples determined via elemental analysis/isotope ratio mass spectrometry (EA/IRMS) were used to assess the contribution of fat content, added sugar content and census-designated region of collection to isotopic variations within the dataset.

There was a negative linear relationship between fat content and δ¹³ C values, with average milk δ¹³ C values decreasing by 0.33‰ for each 8.75% increase in dry weight (1% wet weight) fat content. The average δ¹³ C value of flavored 1% chocolate milk samples, which contain an additional 12 g of added sugar, was 2.05‰ higher than that of 1% unflavored milk (-16.47‰ for chocolate milk vs -18.52‰ for unflavored milk). When controlling for macronutrient content, milk samples collected in West region supermarkets possessed significantly lower δ¹³ C values than samples collected from Midwest, South, and Northeast regions. δ¹⁵ N values did not vary with macronutrient content or region of collection.

Carbon stable isotope ratios in U.S. milk samples varied with macronutrient content and region of purchase, suggesting that SIA can provide insight into production processes within the U.S. dairy industry, with potential applications in national food adulteration and authentication efforts.