An ultra-high performance liquid chromatography (HPLC) method was established for the determination of the content of citric acid in dairy products. The samples were precipitated by K3Fe(CN)6 and (CH3COO)2Zn, extracted ultrasonically, and cryogenically centrifuged to precipitate protein and fat. The chromatographic separation was accomplished on a Poroshell 120 Aq-C18 column (4.6 mm × 100 mm, 2.7 μm) using gradient elution with a mobile phase consisting of a mixture of methanol and ammonium dihydrogen phosphate solution. The analyte was detected by an ultraviolet (UV) detector and quantified by the external standard method. The results showed that the calibration curve was linear with a correlation coefficient above 0.999 9. The average recovery from spiked samples was 95.3%–104.8%, the detection limit was 100 mg/kg, and the relative standard deviation (RSD) for precision was 0.16%–2.41%. The method is simple and rapid and has good separation effect. It is suitable for the quantitative analysis of citric acid in milk and dairy products.

A liquid chromatographic (LC) method was established for the determination of α-casein, β-casein, κ-casein, α-lactalbumin and β-lactoglobulin in ultra high temperature (UHT) milk. The proteins in milk were denatured by adding a buffer solution containing bis[tris(hydroxymethyl)aminomethane (BisTris), guanidine hydrochloride, sodium citrate, dithiothreitol, etc. The analytes were detected by an ultraviolet (UV) detector and quantified by the external standard method. The results showed that the calibration curve of each protein had good linearity with a correlation coefficient above 0.995. The recoveries of spiked samples were between 95.2% and 105.0%, the detection limit was 0.01–0.02 g/100 mL, and the relative standard deviation was 0.78%–2.70%. The method is characterized by good separation effect, good repeatability and simple operation, and is suitable for quantitative analysis of α-casein, β-casein, κ-casein, α-lactalbumin and β-lactoglobulin in UHT milk.