Abstract: A gas chromatography-mass spectrometric (GC-MS) method was established for the determination of β-sitosterol in raw milk. Theoretical studies had shown that the fat in raw milk belonged to animal fat, fresh milk should not contain plant oils or rarely contain phytosterols,especially β-sitosterol. Based on the analysis of the content of β-sitosterol in raw milk from different area,the identical method of mixing plant oil in raw milk had been given. Using the mass ratio of KOH aqueous solution of 1.25 of the 25 mL saponificate sample,100 mL vitamin C ethanol solution as stabilizer, liquid saponification with petroleum ether and diethyl ether,turn solvent to toluene,centrifugal inlet mass spectrometry. Selecting 60 m DB-5 capillary column with high column efficiency,the injection volume was 1 μL,programmed raising temperature to achieve a good separation of β-sitosterol and other alcohols and impurities within 35 minutes. Selected ion 414 quantitative,the detection limit was 0.10 mg/L. The linear correlation coefficient r was 0.9991. Using this method to detect β-sitosterol content in fresh milk from 6 different regions,the highest content was 0.86 mg/L,the content of β-sitosterol common plant oil were 1 to 3 g/L. In conclusion,using plant fat replace dairy fat,and each one percent of plant fat would produce β-sitosterol as 12 times as the milk without plant fat at least. Thus,mixing plant fat could be identified in raw milk.

Key words: dairy fat adulteration; β-sitosterol; GC-MS