Quantification of Lactulose and Epilactose in the Presence of Lactose in Milk using a dual HPLC analysis

Instant quantification of sugars in milk tablets using near-infrared spectroscopy and chemometric tools

Milk tablets are a popular dairy product in many Asian countries. This research aimed to develop an instant and rapid method for determining sucrose and lactose contents in milk tablets using near-infrared (NIR) spectroscopy. For the quantitative analysis, a training set composed of laboratory-scale milk samples was generated based on a central composite design (CCD) and used to establish partial least squares (PLS) regression for the predictions of sucrose and lactose contents resulting in R² values of 0.9749 and 0.9987 with the corresponding root mean square error of calibration (RMSEC) values of 1.69 and 0.35. However, the physical difference between the laboratory-scale powder and the final product milk tablet samples resulted in spectral deviations that dramatically affected the predictive performance of the PLS models. Therefore, calibration transfer methods called direct standardization (DS) and piecewise direct standardization (PDS) were used to adjust the NIR spectra from the real milk tablet samples before the quantitative prediction. Using high-performance liquid chromatography (HPLC) as a reference method, the developed NIR-chemometric model could be used to instantly predict the sugar contents in real milk tablets by producing root mean square error of prediction (RMSEP) values for sucrose and lactose of 5.04 and 4.22 with Q² values of 0.7973 and 0.9411, respectively, after the PDS transformation.

An indirect ELISA system for the detection of heat-stable Pseudomonas endopeptidases (AprX) in milk

Heat-stable endopeptidases in raw milk, especially the alkaline metallopeptidase AprX secreted by Pseudomonas spp., are a well-known challenge for the dairy industry. They can withstand UHT treatment and may cause quality defects over the shelf life of milk products. Therefore, we established an indirect ELISA for the detection of Pseudomonas AprX in milk. We developed a 2-step sample treatment for milk contaminated with AprX to avoid the interference of milk proteins with the detection system. First, casein micelles were destabilized by the detraction of Ca²⁺ using trisodium citrate; then, AprX was concentrated 10-fold using hydrophobic interaction chromatography. The recovery of AprX in spiked milk samples after the 2-step treatment was 43 ± 0.1%. Specific antibodies for purified AprX from Pseudomonas lactis were produced to establish the ELISA. Western blot experiments showed that the binding affinity of these antibodies depended on the sequence homology of the AprX from P. lactis and several other Pseudomonas spp. The indirect ELISA, which was completed in 6 to 7 h, had a limit of detection of 21.0 ng mL^-1 and a limit of quantification of 25.7 ng mL^-1. Milk proteins or milk endogenous peptidases were not detected by the antibodies. The ELISA had high precision, with a CV between 0.2 and 0.8% measured on the same day (intraday) and 5.6 and 6.8% measured on 5 separate days (interday). Milk samples were spiked with different AprX activity levels [7.5-150 nkat Na-caseinate/o-phthalaldehyde (OPA) mL^-1] and evaluated by ELISA. The recovery of the ELISA was 92.3 ± 1.6 to 105 ± 4.7%. The lowest AprX activity quantifiable in the spiked milk samples was 500 pkat Na-caseinate/OPA mL^-1. The proof of concept to detect heat-stable Pseudomonas AprX in milk by ELISA was established.

Rapid HPLC method for monitoring of lactulose production with a high yield

An HPLC method suitable for rapid monitoring of lactulose production by isomerization from lactose was developed. The separation of lactose and lactulose under hydrophilic interaction liquid chromatography (HILIC) mode was achieved with resolution 1.5 within 5 min. Since isocratic elution was used, there is no extra time necessary for the column equilibration. Application of the method was illustrated on monitoring lactulose isomerization with catalysis of sodium hydroxide in the presence of sodium tetraborate at 70 °C (pH = 11). The conversion yield obtained for lactulose was 86%, and corresponding purity 76%. For the first time, a polyhydroxy stationary phase for separation of lactose and lactulose is reported.

Hydrophilic interaction chromatography in food matrices analysis: An updated review

This review focuses on the most recent papers (from 2011 to submission date in 2017) dealing with the analysis of different organic components in foods (i.e. nucleobases, nucleosides, nucleotides, uric acid, and creatinine, amino acids and related compounds, choline-related compounds and phospholipids, carbohydrates, artificial sweeteners and polyphenolic compounds), using hydrophilic interaction liquid chromatography (HILIC) combined with different detection techniques. For each compound class, the investigated food matrices are grouped per: foods of animal origin, vegetables, fruits and related products, baby food, and other matrices such as drinks and mushrooms/fungi. Furthermore, the main advantages of HILIC chromatography respect to the other commonly used techniques are discussed.