Milk & Dairy Testing

Garehbaghi, Sanam, et al. Microchemical Journal 204 (2024): 111138.

The concentration of lactose in cow's milk and human milk ranges from 115 to 144 mM and approximately 230 mM, respectively. A lactose concentration below 4.7% in milk can serve as a biomarker for mastitis infection. Therefore, it is significant to measure lactose in various matrices, particularly in milk, dairy products, and other dairy-derived items.

An enzyme-based electrochemical biosensor was developed for the sensitive determination of lactose. The enzyme coupling system employed consists of β-galactosidase (β-Gal) and glucose oxidase (GOx). Ruthenium (IV) oxide (RuO₂) within multi-walled carbon nanotube-RuO₂ (MWCNT-RuO₂) nanocomposites, immobilized on a glassy carbon electrode, acts as the electrochemical mediator, similar to second-generation enzyme biosensors.

The functional mechanism of the biosensor was discussed, explaining how the final product of the enzymatic reaction, hydrogen peroxide (H₂O₂), is chemically oxidized by RuO₂, leading to the reoxidation of the generated Ru back to RuO₂ on the electrode surface. This allows the oxidation of H₂O₂ to occur at a lower potential of +0.40 V, enhancing the selectivity of the biosensor. The analytical parameters of the developed lactose biosensor were validated through repeated measurements. The accuracy of the lactose biosensor was ensured, demonstrating good reproducibility (RSD % = 2.68) and repeatability (RSD % = 4.12).

The selectivity of the biosensor was investigated with respect to various sugars and ionic species that may be present in milk samples, with no significant interference effects detected. Moreover, the accuracy of the lactose biosensor was tested by analyzing spiked samples and semi-skimmed milk (SS-milk) samples with certified lactose values. The limits of detection (LOD) and quantification (LOQ) were calculated to be 0.036 mM and 0.121 mM, respectively.

Shishov, Andrey, Egor Nizov, and Andrey Bulatov. Journal of Food Composition and Analysis 116 (2023): 105083.

According to the World Health Organization, the tolerable daily intake of melamine is 2.5 mg kg^-1 body weight per day. Dairy products and foods made from milk adulterated with additives may contain melamine. Therefore, despite the ban on this additive, strict control of melamine content in milk and dairy products is still necessary.

This study demonstrates that deep eutectic solvents can effectively perform liquid-phase microextraction of melamine from dairy products. In the developed procedure, milk samples are mixed with a salt (sodium sulfate) to precipitate proteins, and melamine is extracted into a hydrophobic eutectic solvent phase based on thymol and nonanoic acid. After phase separation, the analyte is detected in the eutectic solvent phase. In the developed analytical procedure, medium-chain fatty acids act as precursors of the extraction solvent, facilitating melamine mass transfer due to strong interactions between carboxyl and amino groups. Terpenoids, as precursors of the extraction solvent, also show a synergistic effect in melamine extraction, possibly due to π-π interactions. This observed phenomenon is used to effectively separate highly hydrophilic analytes from the aqueous phase, followed by determination using high-performance liquid chromatography.

Under optimal conditions, the concentration range for detection is 0.1 to 30 mg L^-1, with a detection limit of 0.03 mg L^-1. The procedure is fast (7 minutes), miniaturized (100 μL of extraction solvent), and reproducible (RSD % < 5 %). The developed procedure was optimized and successfully applied to determine melamine in real samples of milk powder and whole milk powder.

Zhang, Wenhua, et al. Journal of Food Composition and Analysis 132 (2024): 106362.

β-Hydroxy-β-methylbutyrate (HMB) is a five-carbon organic acid and a derivative of α-ketoisocaproic acid, a metabolic product of leucine in the human body. Due to the instability of HMB, it is commonly converted to its calcium salt, CaHMB, for storage and use. As a globally recognized innovative nutrient, HMB has been widely applied in various food products, including dairy, chocolate products, beverages, and energy bars.

An analytical method based on solid-phase extraction (SPE) purification and high-performance liquid chromatography (HPLC) has been developed for determining the content of calcium β-hydroxy-β-methylbutyrate (CaHMB) in milk and dairy products. The quantification is conducted using an external standard method with a diode-array detector (DAD) HPLC. The limit of quantification (LOQ) is 0.10 g/100 g (calculated based on 10 times the standard deviation). The average recovery rates measured at three concentration levels range from 92.40% to 103.00%, with relative standard deviations (RSD) between 1.25% and 3.77%. Analysis of actual samples showed that CaHMB was detected in infant formula, formula milk powder, and milkshake samples, with content ranging from 0.60 to 8.66 g/100 g. This method demonstrates good purification efficiency, excellent separation specificity, and strong reproducibility, accuracy, and reliability, making it suitable for the determination of CaHMB content in milk and dairy products.

Peng, Hao, et al. Food Chemistry 458 (2024): 140246.

Many international organizations and countries have established strict maximum residue limits (MRLs) for pesticides in milk. Specific regulations have been developed for foods intended for children and infants to limit pesticide levels in infant formula as much as possible. There is an urgent need for effective analytical methods to detect multiple pesticides in milk and its products, with high sensitivity, selectivity, and accuracy.

This study developed a simple, sensitive, and rapid method for the simultaneous determination of 99 pesticides in fat-containing milk samples. This novel emulsification-demulsification purification method, combined with an automatic demulsification-dehydration box, allows for quick single-step purification operations and high throughput. It also enables effective and selective lipid removal. Analysis was conducted using low-pressure gas chromatography-tandem mass spectrometry (LPGC-MS/MS). Under optimal conditions, the target pesticides showed good linearity in the range of 5 to 250 μg/kg, with recovery rates of 70-120% at spiking levels of 5, 10, and 20 μg/kg in cow's milk, goat milk, and almond milk, respectively. The limit of quantification (LOQ) for most pesticides was 5 μg/kg, with relative standard deviations (RSD) of less than 20%.