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.

In this study, headspace gas chromatography-mass spectrometry (HS-GC-MS) was used to accurately identify and quantify ethylene oxide and 2-chloroethanol in milk and dairy products (liquid milk, fermented milk, infant formula milk powder, and ice cream). The sample was diluted to a constant volume in a headspace bottle and balanced at 90 ℃ for 20 min, and the extract was separated on a DB-624 capillary column (60 m × 0.32 mm, 1.8 μm) with temperature programming. Ethylene oxide and 2-chloroethanol were detected using an electrospray ionization (ESI) source in the selected ion monitoring (SIM) mode. Quantitation was carried out using matrix-matched calibration curves. Good linearity was obtained in the range of 0.1–10.0 mg/L (R2 ≥ 0.99). The limits of detection of ethylene oxide and 2-chloropropanol in milk, fermented milk and ice cream were 0.12 and 0.60 mg/kg, respectively. At three spiked levels, the average recoveries of the two compounds were between 75.9% and 101.6%, with relative standard deviations (RSD) less than 10%. This method is precise and accurate, and can meet the requirements for the detection of milk and dairy products with a complex matrix.

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.

Lactulose, also known as isomerized lactose, is a reducing disaccharide made from the dairy industry by-product lactose. Its concentrate is an important over-the-counter (OTC) drug that has a significant therapeutic effect on acute or chronic constipation and hepatic encephalopathy. The methods that have been developed to produce lactulose include the chemical method, β-galactosidase method and epimerase method. In recent years, the epimerase method has aroused extensive attention due to the high conversion rate it yields, which makes it a promising candidate for lactulose preparation although currently there exist some bottlenecks like the high proportion of epilactose as a by-product and the rather low enzyme activity. This article reviews the history and current status of research on the methods for lactulose production, in the hope of providing reference for basic theoretical research and industrialization in this field.