Mixed-milk cheeses (cheese made with a mix of milk from different species) are common on the European market, particularly in Spain. They present a motive to fraudulently increasing the proportion of the cheapest milk species above its maximum legal specification. For example, Iberico cheese must contain a maximum of 50% cow's milk and a minimum of 15% each of goat's and sheep's milk. Verifying the proportions of milk in the final product by quantitative analytical testing is a challenge.
In this study (open access) the authors developed and validated a quantitative LC-MSMS method based on protein markers for each species. They selected their markers using shotgun proteomics of 6 cheeses of known proportions that had been specially made in a pilot plant following the industrial process for manufacturing Iberico-type cheese.
They optimised a quantitative low-resolution LC-MSMS method for these markers and then validated it following AOAC guidelines. They report that the method demonstrated linearity with detection limits less than 1% for all 3 species and showed good repeatability (CV = 8%), reproducibility (CV = 10%) and accuracy (99.6%),.
They applied the method commercial cheeses with diverse compositions and ripening times. They report that measurements were unaffected by either ripening or production process.
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