Characterisation of volatile compounds generated in milk by high intensity ultrasound

Although high intensity ultrasound (US) has the potential to simultaneously homogenize milk and to reduce its microbial load, the treatment may give rise to off-odours under certain conditions. This study is a preliminary examination of the chemical nature and possible genesis of volatiles implicated in this process. An ultrasonic processor fitted with an ultrasonic horn (diameter 22 mm) set at a maximum acoustic power output of 400 W (frequency 24 kHz) was used to sonicate 200 mL milk samples (1.5% fat). Solid phase microextraction headspace analysis was used in combination with gas chromatography/mass spectrometry to analyse the products evolved in the process during treatment times of 2.5, 5, 10, 15 and 20 min. Volatiles generated by US treatment, which included benzene, toluene, 1,3–butadiene, 5-methyl-1,3-cyclopentadiene and a series of aliphatic 1-alkenes, were predominantly hydrocarbons and believed to be of pyrolytic origin, possibly generated by high localized temperatures associated with cavitation phenomena.