A Purge&Trap Concentrator (Hewlett-Packard, Palo Alto, CA) coupled to the gas chromatograph by means of a fused silica transfer line was used for volatile analysis; a 25 ml non-fritted sparger, a trap containing a mixture of Carbosieve-SIII and Carbopack-B 60/80) as well as a liquid nitrogen cryofocusing unit were used. 2hexanone (Analytical Standards, Polyscience Co., Niles, IL) was added as internal standard to the milk in the sparger. Helium was the carrier gas. Operating conditions were as follows: purge ¯ow (vent), 35 ml/min; sample temperature, 45C; purge time, 15 min; desorption, 5 min at 220C; cryofocus temperature, ÿ100C; injection within 2 min from ÿ100C to 200C. A blank was run after every sample.