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.

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.

A method for the determination of deoxynivalenol (DON) and its derivatives 3-acetyl-deoxynivalenol (3-ADON) and 15-acetyl-deoxynivalenol (15-ADON) in dairy products, cereal flour and dairy cow feed was developed using high performance liquid chromatography (HPLC). Samples were extracted with ultrapure water, and the extract was cleaned up by immunoaffinity column chromatography and detected by HPLC with ultraviolet detection (HPLC-UVD). The limit of quantitation of the method was 100 μg/kg for all three analytes in dairy products, cereal flour and feed. Recoveries from these three sample matrices spiked at concentration levels ranging from 100 to 500 μg/kg were higher than 90% for DON and higher than 80% for 3-ADON and 15-ADON. This method proved to be simple, fast, sensitive, highly selective, and suitable for the simultaneous analysis of DON, 3-ADON and 15-ADON in dairy products, cereal flour and feed.

An high performance liquid chromatographic (HPLC) method for the determination of four sweeteners and one preservative in fermented milk and jam was developed in this paper. Water was used as the extraction solvent. The chromatographic separation was performed on a reversed phase C18 column using a mobile phase composed of a mixture of 20 mmol/L ammonium acetate buffer solution (pH 4.6) and methanol by isocratic elution. The ratio between mobile phase A and B was 85:15 (V/V) for the detection of benzoic acid and sorbic acid and 95:5 (V/V) for the detection of saccharin sodium and acesulfame K. The analytes were detected using an ultraviolet detector at 254 nm and quantified by an external standard method. The recoveries for benzoic acid, sorbic acid, saccharin sodium and acesulfame K in fermented milk at spiked levels of 1–20 mg/kg were 92.0%–102.4%, and the recoveries in jam at spiked levels of 5–20 mg/kg were 92.3%–105.8%. The limit of quantification (LOQ) for all analytes was 1 and 5 mg/kg in fermented milk and jam, respectively.

The probiotic Lactobacillus plantarum LIP-1 was cultured in modified MRS medium at high cell density to investigate the effect of changes in medium components (the types and amounts of nitrogen source, carbon source and buffer salt, total amount of carbon and nitrogen, and carbon/nitrogen ratio) on the viable cell count and survival rate after freezedrying of this strain. The optimal medium was found to be composed of carbon source (sucrose) 33.35 g/L, nitrogen source (yeast powder, soy peptone and casein peptone, 3:3:5, m/m) 10.80 g/L, and buffer salt (sodium acetate, potassium dihydrogen phosphate and sodium citrate, 2.5:1:1, m/m) 14.85 g/L (namely 0.13 mol/L). The viable cell count of strain LIP-1 cultured in the optimized medium after freeze-drying was 9.767 (lg(CFU/mL)), which was distinctly higher than that cultured in MRS medium. The modified medium allowed high-cell-density fermentation of Lactobacillus plantarum LIP-1 while maintaining its viability during freeze-drying.

A method for the determination of eight colorants, new red, amaranth, allura red, ponceau, tartrazime, sunset yellow, indigotine and brilliant blue in fermented milk by high performance liquid chromatography was developed. The analytes were extracted from samples with weakly alkaline water at 7.7–7.9 and cleaned up on a polyamide solid phase extraction cartridge. The analysis was performed on a C18 column with gradient elution using 0.02 mol/L sodium acetate solution-methanol as mobile phase. The results showed that the recoveries of seven colorants other than indigotine at spiked concentration levels of 0.5–5.0 mg/1 000 g in fermented milk were higher than 90%, while the recovery of indigotine was around 80%. The limit of detection of the method was 0.5 mg/kg. The repeatability relative standard deviation was less than 5%, indicating good precision. With the advantages of accurate results, simplicity and good repeatability, this method is suitable for the analysis of colorants in fermented milk.

A method for the determination of nucleotides in milk powder by high performance liquid chromatography (HPLC) was developed. Nucleotides were extracted with water, deproteinated with 0.5% acetic acid and cleaned up by an SAX solid phase extraction (SPE) column. The analysis was performed on C18 column using 0.1 mol/L phosphate as the mobile phase. The recoveries of spiked samples at three levels ranging from 0.3 to 50 mg/100 g were higher than 90%. The limit of quantification was 0.3 mg/100 g. This method was accurate, simple, reproducible, and suitable for the analysis of nucleotides in milk powder.

A method for determining lutein in dairy products by HPLC is described. Samples were extracted with acetone and separated with a mobile phase made up of methanol and methyltert-butyl ether. The analysis was performed by UV detection at 445 nm, and the external standard method was used for quantification. This method has good accuracy and precision. The spiked recoveries in milk and milk powder ranged from 86.9% to 101.2% with a relative standard deviation of 2.56% to 4.05%. The detection limit was 0.02 mg/kg, and a linear range of 0.02 to 4 mg/kg was obtained.