Simultaneous determination of sorbic and benzoic acids in milk products using an optimised microextraction technique followed by gas chromatography

A rapid and reliable method for direct determination of sorbic and benzoic acids in milk products was developed by dispersive liquid-liquid microextraction (DLLME) and gas chromatography with flame ionisation detector (GC-FID). A response surface methodology (RSM) based on a central composite design (CCD) was applied for optimisation of the main variables, such as volume of extraction and dispersive solvents, pH and salt effect. The primary extraction of sorbic and benzoic acids were performed in 8 mL NaOH (0.1 M) in a closed-vessel system. Carrez solutions (potassium hexaferrocyanide and zinc acetate) were used for protein sedimentation. The best simultaneous extraction efficiency was identified using acetone and 1-octanal as dispersive and extraction solvents, respectively. For DLLME, central composite design resulted in the optimised values of microextraction parameters as follows: 475 µL of dispersive and 60 µL of extraction solvents, 2 g NaCl at pH 2.5. Under optimum conditions, the calibration curve was linear over the range 0.1-50 μg mL(-1) and the square of correlation coefficient (R(2)) was 0.9992 for sorbic acid and 0.9994 for benzoic acid. Relative standard deviation (RSD %) was 6.1% and 3.1% (n = 5) for sorbic and benzoic acids, respectively. Limits of detection were 150 ng g(-1) for sorbic acid and 140 ng g(-1) for benzoic acid and recoveries were 88% and 103.7% respectively. Good reproducibility (RSD %), short extraction time and no matrix interference were advantages of the proposed method which was successfully applied to the determination of sorbic and benzoic acids in milk products.