Quantitative detection of goats’ milk in sheep’s milk by real-time PCR

The need to support food-labeling legislation has provided a driving force for development of analytical techniques for the analysis of food ingredients. In this study, the development of a method for quantification of goats’ milk in sheep’s milk mixtures is described. The technique involves the use of a real time PCR technique, based on the amplification of a fragment of the mitochondrial 12S ribosomal RNA gene (rRNA). The method combines the use of goat-specific primers that amplify a 171 bp fragment from goat DNA, and mammalian-specific primers amplifying a 119 bp fragment from mammalian species DNA, which are used as endogenous control. An internal fluorogenic probe (TaqMan) that hybridizes in the “goat-specific” and also in the “mammalian” DNA fragments is used to monitor the amplification of the target gene. A comparison of the cycle number (Ct) at which mammalian and goat-specific PCR products are first detected, in combination with the use of reference standards of known caprine content, allows the determination of the percentage of goats’ milk in a milk mixture. The assay was used to analyze raw and heat-treated milk binary mixtures (goat/sheep), enabling the quantification of goats’ milk in the range 0.6–10%. The reported PCR assay may represent a rapid and straightforward tool applicable to the authentication of milk and other dairy products.