827 Korean J. Food Sci. Ani. Resour. Vol. 31, No. 6, pp. 827~833(2011) DOI http://dx.do.org/10.5851/kosfa.2011.31.6.827 Quantitative Detection of Cow Milk in Goat Milk Mixtures by Real-Time PCR Yu-Kyung Jung, Deok-Young Jhon, Kanghwa Kim, and Youn-Ho Hong* Department of Food and Nutrition, and Human Ecology Research Institute, Chonnam National University, Gwangju 500-757, Korea Abstract The objective of this study was to develop a fluorogenic real-time PCR-based assay for detecting and quantifying amounts of cow milk in cow/goat milk mixtures or goat milk products. In order to quantify the exact amount of cow milk in cow/goat raw milk mixtures and commercial goat milk products, it was necessary to achieve quantitative extraction of total genomic DNA from the raw milk matrix. Both mammalian-specific PCR and cow-specific PCR were performed. A cow-specific 252 bp band obtained from the raw cow milk and raw goat milk mixtures, commercial goat milk, and two goat milk powders was identified, along with the relationship between the cow milk amount and band intensity of the electrophoresis image. The detection threshold was found to be 0.1%. The expression of cow's 12S rRNA in the cow/goat milk mixtures, commer- cial goat milk, and two goat milk powders was identified. The expression quantity of the milk 12S rRNA increased with increasing ratios of the cow/goat milk mixtures. Using these calibrated relative expression levels as a standard curve in the cow/goat raw milk mixtures, the contents of cow milk were 1.8% in the commercial goat milk, 9.6% in goat milk powder A, and 11.6% in goat milk powder C. However, cow milk was not detected in goat milk powder B. Key words: real-time polymerase chain reaction, quantitative detection, goat milk, cow milk Introduction It is well recognized that bovine (cow) milk and dairy products are nutritionally almost perfect animal foods. However, some bovine milk products are well known as allergy-inducing substances (Kotowicz et al., 2007). Goat milk has been used as a health functional food in many countries. The chemical compositions of bovine milk and goat milk are different; goat milk contains less α s1 -casein, which cannot be digested by babies, and β- lactoglobulin, which is a known allegen for some people. Goat milk has more nucleotides, immunoglobulins, poly- amines, and taurine than bovine milk (McCane et al., 2007). Nutrient contents in goat milk are slightly lower than those in bovine milk, but the size of fat globules is much smaller than in bovine milk, which means better digestion. Furthermore, goat milk has more chlorine, bro- mine, and silicone than other animal milks (McCane et al., 2007). In many countries, goat milk is much more expensive than bovine milk, and this could be a reason why people may fraudulently mix bovine milk with goat milk. Some cheeses have been made with bovine milk, sheep milk, and goat milk by mixing a minimum of 10%, respectively (Mayer et al., 1997). In such cases, consumers do not trust products and some could suffer from allergy symp- toms caused by bovine milk protein. Therefore, consumer demands for detailed information about food composition and ingredients have been increased (Mayer, 2005). The protein in milk and dairy products has been ana- lyzed by capillary electrophoresis (Molina et al. 1999), two-dimensional electrophoresis (Chianese et al., 1990), isoelectric focussing (Moio et al., 1990), HPLC (Ferreira and Cacote, 2003), ELISA (Anguita et al., 1997; Hurley, 2004), and so on. However, these methods are not opti- mal, but rather limiting because the proteins can be lost or degraded by heating, high pressure, and other operations during food processing (Bottero et al., 2003). DNA in milk somatic cells is known to be very stable and therefore it is possible for species identification of milk using the polymerase-chain reaction (PCR) (Mafra et al., 2004). Furthermore, real-time PCR is useful to quantify milk *Corresponding author: Youn-Ho Hong, Department of Food and Nutrition, and Human Ecology Research Institute, Chonnam National University, Gwangju 500-757, Korea. Tel: 82-62-530-1333, Fax: 82-62-530-1339, E-mail: yhhong@ chonnam.ac.kr ARTICLE