A reversed-phase high performance liquid chromatography (RP-HPLC) method was established for the determination of 6S-5-methyltetrahydrofolate calcium in milk powder. After protein precipitation with trichloroacetic acid solution, samples were extracted with ascorbic acid solution in a hot water bath, and the extract was separated on a C18 reversed-phase column by gradient elution using a mobile phase composed of 0.1% trifluoroacetic acid solution and methanol, and the detection wavelength was set at 280 nm. Quantitative analysis was carried out by the single standard method. The results showed that the limit of quantification and detection of 6S-5-methyltetrahydrofolate calcium in milk powder samples were 150 and 50 μg/100 g, respectively. The recoveries for samples spiked at levels ranging from 150 to 600 μg/100 g were 95.0%–109.1% with relative standard deviations (RSDs) between 1.26% and 4.04%. This method is simple, accurate, with high recovery and reproducibility, and suitable for the determination of 6S-5-methyltetrahydrofolate calcium in milk powder.

A reversed-phase high performance liquid chromatographic (HPLC) method was established for the determination of erythrosine in fermented milk. After enzymatic hydrolysis, the sample was extracted with anhydrous ethanol, and purified by solid phase extraction using an HLB column under alkaline conditions. The separation was performed on a C18 reversephase column by gradient elution using a mobile phase comprising 20 mmol/L ammonium acetate buffer solution (pH 6.5) and methanol. The detection wavelength was set at 520 nm, and quantification was performed by the external standard method. The results showed that the linear range for erythrosine was 0.05–20.00 μg/mL. The limit of quantitation (LOQ) and the limit of detection (LOD) of erythrosine in fermented milk samples were 0.2 and 0.1 mg/kg, respectively. The recoveries from fermented milk samples spiked at levels of 0.2–2.0 mg/kg were 96.5%–105.6%, and the relative standard deviations (RSD) were 1.87%–2.21%. The method is simple and accurate, has high recovery and good repeatability, and is suitable for the determination of erythrosine in fermented milk.

The measurement uncertainty was estimated for the determination of 1-oleic-2-palmitic-3-linoleic acid triglyceride (OPL) content in milk powder by gas chromatography (GC). According to the Measurement and Expression of Uncertainty in Measurement (JJF 1059.1—2012) and the Guidance on Quantifying Uncertainty in Chemical Analysis (CNAS-GL 006: 2019), a mathematical model was established to analyze the sources of measurement uncertainty. The major factors affecting the measurement uncertainty were evaluated. Results showed that the average OPL content of six milk powder samples was measured to be 1.172 g/100 g. The relative expanded uncertainty (urel) was 0.023 6 with a coverage probability of approximately 95%. The standard uncertainty arising from sample preparation was the main source of uncertainty.

A high performance liquid chromatographic (HPLC) method was developed for the determination of naringin in milk and dairy products. Samples were extracted with methanol, and the analytes were separated on a C18 column using a mobile phase consisting of 0.1% acetic acid aqueous solution and acetonitrile (75:25, V/V), detected by a diode array detector at 284 nm and quantified by an external standard method. The results showed that the linearity of this method was good in the concentration range of 5–500 μg/mL with a correlation coefficient of 0.999 9. The recoveries of powdered and liquid milk power and yogurt spiked at 0.02, 0.04, and 0.08 g/100 g were 96.2%–103.2%, 96.3%–103.7%, and 99.7%–104.1%, respectively, with relative standard deviations (n = 6) of 1.12%–2.31%, 1.25%–1.85%, and 1.24%–1.66%, respectively. The reported method is simple, rapid, accurate, repeatable, and suitable for the determination of naringin in milk and dairy products.

A gas chromatographic (GC) method for the determination of 1-oleic-2-palmitic-3-linoleic-triglyceride (OPL) in milk powder was established. The samples were extracted with ether and petroleum ether, and the extract was concentrated to near dryness, re-dissolved in n-heptane, separated on a CP-TAP CB elastic quartz capillary column, detected by a hydrogen flame ionization detector, and quantified by an external standard method. The column temperature was programmed to rise. The inlet temperature was set at 330 ℃. The samples (1.0 μL) were injected using the split mode (1:10). The detector temperature was set at 360 ℃. Nitrogen was used as the carrier gas. The makeup flow rate (nitrogen) was 25 mL/min. The recovery of OPL from spiked milk powder at 1–6 g/100 g ranged from 95.2% to 101.9%, and the relative standard deviations (RSDs) for precision ranged from 1.289% to 1.998%. The limit of detection (LOD) and limit of quantitation (LOQ) of the proposed method were 0.2 and 1.0 g/100 g, respectively. This method, therefore, is simple, accurate, repeatable, and suitable for the determination of OPL in milk powder.

A gas chromatographic (GC) method to determine the contents of 16 trans fatty acids in ice cream was established. The 2.0 g samples were pretreated sequentially with 8 mL of hydrochloric acid and 10 mL of anhydrous ethanol, separated on an SP-2560 capillary column, detected by GC with a flame ionization detector (FID) and quantified by the peak area normalization method. Under optimized chromatographic conditions, the limit of detection and limit of quantification of the proposed method were 0.012% and 0.024%, respectively, and it allowed effective separation of trans fatty acid methyl esters from cis fatty acid methyl esters and saturated fatty acid methyl esters.

A method for the determination of α-lactalbumin and β-lactoglobulin in pasteurized milk by high performance liquid chromatography (HPLC) was developed. The sample pretreatment procedure involved sequential water dilution, pH value adjustment to 4.6 for precipitation of casein, and filtration. The subsequent analysis was performed on an Xbridge Protein BEH C4 column using gradient elution with a mobile phase made up of 0.1% (V/V) aqueous trifluoroacetic acid and 0.09% (V/V) aqueous trifluoroacetic acid-acetonitrile. The analytes were detected by an ultraviolet (UV) detector and quantified by an external standard method. The results showed that a good linearity was observed for α-lactalbumin in the concentration range of 10–500 mg/L, with a correlation coefficient of 0.999 87. The recovery of α-lactalbumin spiked at levels of 200 to 1 032 mg/L in milk ranged from 97.9% to 104.0%, and the relative standard deviation (RSD) for precision was 0.00%–1.09%. The linear range for β-lactoglobulin was 20–900 mg/L, with a correlation coefficient of 0.999 96. The recovery of β-lactoglobulin spiked at levels of 600–4 727 mg/L in milk was 96.1%–103.0%, and the precision RSD was 0.00%–2.23%. This method has good separation effect and repeatability and is useful for accurate quantitation of α-lactalbumin and β-lactoglobulin in pasteurized milk.

Iodine is an important index of the nutritional quality of milk. Milk and dairy products are important dietary sources of iodine for humans, which has always aroused extensive concern. With the continuous exploration of nutrients in foods, research on iodine in milk has been increasingly intensified and refined. This paper reviews the recent literature on the factors that affects the iodine content of milk and the analytical methods that have been applied to determine it, which is expected to provide a reference for researchers in this industry and also provide a theoretical basis for the labelling of iodine in infant formula preparation.

In order to develop a new method to determine nitrite and nitrate in milk and milk products, some improvements were made on the ion chromatography method specified in the national standard GB 5009.33-2016 Determination of Nitrite and Nitrate in Food. Samples were extracted with ultra-pure water and added with acetic acid to precipitate proteins before being chromatographed on AS11-HC column and detected with a conductivity detector. Under the above conditions, the calibration curves for nitrite and nitrate were linear with correlation coefficients greater than 0.99. The limit of quantitation (LOQ) for nitrite and nitrate was 0.25 and 0.8 mg/kg in milk, and 1 and 5 mg/kg in milk powder, respectively. The recoveries for spiked samples of milk powder and milk were from 80% to 110%. The improved method was simple to operate and cheap with ideal recovery and high precision. It could avoid potential problems encountered in the use of the traditional method and allow for more rapid and accurate detection of low contents of nitrite and nitrate in milk and milk products.

A method for the determination of zeaxanthin in milk powder was developed using high performance liquid chromatography (HPLC). The sample was saponified with potassium hydroxide solution to release zeaxanthin, extracted with a mixed solvent of ethyl ether, n-hexane and cyclohexane and frozen for degreasing. The chromatographic separation was performed on a C30 column with gradient elution. The analyte was detected with an ultraviolet detector and quantified by an external standard method. The limit of quantification was 100 μg/100 g and the limit of detection was 30 μg/100 g for zeaxanthin in milk powder. The method recoveries at spiked concentration levels of 90–811 μg/100 g were 85.8%–102.2%, with relative standard deviations ranging from 1.43% to 2.76%. This method was accurate, repeatable, precise and sensitive.

A reversed-phase high performance liquid chromatography (RP-HPLC) method for the simultaneous quantitative determination of vanillin, methyl vanillin, ethyl vanillin and coumarin in milk and dairy products has been established. Vanillin compounds from samples were extracted with acetonitrile, blown to dryness under nitrogen, and then cleaned up with n-hexane. The chromatographic separation was achieved using a mixture of 20 mmoL/L ammonium acetate (pH 5.6), acetonitrile and methanol as the mobile phase with gradient elution on a C18 column. The analysis was carried out using a UV detector at a wavelength of 267 nm and the analytes were quantified by an external standard method. Good linear relationships in the range of 0.05–5.00 μg/mL were observed for all analytes. The limit of quantification was 0.2 mg/kg and the limit of detection was 0.06 mg/kg for the four analytes in liquid milk, milk powder, yogurt, lactic acid fermented beverage, ice cream, and cheese. The recoveries for vanillin, methyl vanillin, ethyl vanillin and coumarin in these samples at spiked concentration levels of 0.2–2.0 mg/kg were 80.6%–110.0%, 80.1%–109.6%, 80.4%–109.5% and 80.1%–105.9%, with relative standard deviations of 1.87%–7.70%, 1.45%–9.80%, 1.66%–9.52% and 1.16%–9.52%, respectively. This method was rapid, accurate, repeatable and simple and could be applied to determine vanillin compounds in milk and dairy products.

A method for the determination of lactose in low-lactose milk and lactose-free milk by using ion chromatography was developed. Lactose from samples was extracted with 3% acetic acid as a protein precipitator. The method was performed on a CarboPacTM PA20 column by gradient elution. The analyte was detected with an electrochemical detector and quantified by an external standard method. The limit of quantification was 100 mg/kg and the limit of detection was 50 mg/kg for lactose in milk. The recoveries at spiked concentration levels of 100–500 mg/kg were 92.3%–103.4%, with relative standard deviations between 1.44% and 4.43%. This method proved to be rapid, accurate, repeatable, sensitive and simple.

An ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method for the quantitative determination of salicylic acid in milk and dairy products was developed. The sample was extracted with 1% acidified acetonitrile, and then the extract was purified by solid-phase extraction (SPE) with an HLB cartridge. The separation was performed on an Acquity UPLC BEH C18 chromatographic column (50 mm × 2.1 mm, 1.7 μm）with gradient elution using 0.1% aqueous formic acid-acetonitrile as the mobile phase. The target compound was monitored under the negative ion mode with an electrospray ionization (ESI) source and quantified by external standard method. The results demonstrated that the linear range of the proposed method was from 0.5 to 50.0 ng/mL with a correlation coefficient (R2) > 0.999. The average recovery of salicylic acid in different matrixes spiked at low, medium and high levels varied from 68.3% to 111.7%, and the relative standard deviations (RSDs) of precision were between 3.3% and 15.2%. The limits of detection (LODs) were 5.0 and 10.0 μg/kg in milk and milk powder, respectively. This method can meet the requirements for the determination of different salicylic acid levels in milk and dairy products.

An ultra performance liquid chromatography-tandem mass spectrometry method for the simultaneous quantitative determination of four tetracycline residues in dairy products, including milk, yogurt and milk powder, has been established. In this method, the pretreatment and instrument operating conditions were optimized. Tetracycline residues were extracted into EDTA-Mcllvaine buffer and then cleaned up by an HLB solid-phase extraction cartridge. After the residues were eluted with a mixture of methanol and acetonitrile (1:9, V/V), the eluate was blown to dryness under nitrogen. The resulting residue was then redissolved. The analysis was carried out using positive ion electrospray ionization under the multiple reaction monitoring mode. The limits of quantification were 10 μg/kg for tetracycline, oxytetracycline, chlortetracycline and doxycycline in milk powder; the limits of quantification were 2 μg/kg in milk and yogurt. The recoveries of the tetracyclines fortified at levels from 2 to 100 μg/kg ranged from 63.1% to 100.6% with relative standard deviation of 1.8%–10.7%, and the matrix effects were 100.5%–122.1%. In conclusion, this method is accurate, rapid and highly sensitive and exhibits good repeatability and wide linearity. Thus it is suitable for the determination of tetracycline antibiotic residues in milk, yogurt and milk powder.

An ultra performance liquid chromatography-tandem mass spectrometry method for the simultaneous quantitative determination of four cephalosporin residues in dairy products, including milk, yogurt and milk powder, has been established. The samples were extracted with 5% formic acid acetonitrile. After being purified by solid-phase microextraction using an Oasis PRIME HLB cartridge, the extract was blown to dryness under a stream of nitrogen gas and then re-dissolved. The analysis was carried out using a positive electrospray ion source in the multiple reaction monitoring mode. The limit of quantification was 32 μg/kg for cefalexin, cefapirin, cefalonium and cefquinome?in milk powder; the limit of quantification was 4 μg/kg in milk and yogurt. The recoveries at spiked concentration levels of 4-150 μg/kg were 63.3%-112.1%, with relative standard deviations of 1.6%-10.5%. The matrix effects were 108.77%-379.91%. This method was characterized by short analysis time and was suitable for the determination of cephalosporin residues in different dairy products.

An ultra performance liquid chromatography-tandem mass spectrometry method for simultaneous quantitative determination of four quaternary ammonium compounds in dairy products, including milk, yogurt and milk powder, has been established. Quaternary ammonium compounds from samples were extracted with methanol and cleaned up by solid phase extraction using a weak cationic exchange reversed-phase adsorbent (WCX). The chromatographic separation was achieved using a mixture of formic acid and methanol (2:98, V/V) as the elution solvent. The eluate was evaporated to dryness under a stream of nitrogen gas and the residue was re-dissolved prior to analysis using a positive electrospray ion source in the multiple reaction monitoring mode. The limit of quantification was 10 μg/kg for dodecyl trimethyl ammonium bromide, benzylcetyldimethyl ammonium chloride, tetradecyl dimethyl benzyl ammonium chloride and didecyl dimethyl ammonium chloride in all three dairy products. The recoveries at spiked concentration levels of 10-200 μg/kg were 81.4%-105.6%, with relative standard deviations ranging from 0.97% to 6.04%. The matrix effect was in the range of 90.89%-234.94%. This method was rapid, accurate, repeatable and was suitable for the determination of quaternary ammonium compounds in various dairy products.

An inductively coupled plasma-optical emission spectrometry (ICP-OES) method for the determination of aluminum in baby formula was developed. The sample was digested by microwave digestion and analyzed by ICP-OES. Quantitation was performed using an external standard method. The detection limit was 0.5 mg/kg for 0.5 g of the sample digested and diluted to 25 mL. Linear relationship between emission intensity and aluminum concentration in the range of 0.01 to 50.0 μg/mL was found with a correlation coefficient (r) of 0.999 5. Average recoveries of spiked samples were between 92.2% and 107.0% with relative standard deviations (RSDs) varying from 1.43% to 2.34%. The proposed method permits efficient and accurate determination of aluminum in baby formula with wide linear range, low detection limit, good precision and high recovery and good repeatability.

In January 2013, it was reported that dicyandiamide (DCD) residues were detected in New Zealand milk and milk products, which has received worldwide concern. Therefore, the development of methods to detect dicyandiamide in milk and dairy products has become a hot topic. In this paper, we review methods used to detect dicyandiamide in milk and dairy products such as HPLC-MS/MS, UPLC-MS/MS and UFLC-MS/MS, including sample extraction and purification, chromatographic column selection, mobile phase composition optimization, mass spectrometry monitoring mode and matrix effect of samples.