Multiple headspace solid-phase microextraction for quantifying volatile free fatty acids in cheeses

Abstract

Multiple headspace solid-phase microextraction (MHS-SPME) has been utilized for the quantitative determination of 9 volatile free fatty acids (FFAs) in cheeses, in combination with gas-chromatography and flame-ionization detection (GC-FID). Variables affecting HS-SPME and MHS-SPME were optimized to attain adequate sensitivity while allowing correct application of the MHS method. Thus, the MHS-SPME method was successfully performed when using 0.3g of cheese and 1mL of NaCl (sat. solution), which is subjected to four consecutive extractions using the carboxen–polydimethylsyloxane (CAR–PDMS) as the commercial SPME coating, 40min of HS extraction time at 45°C, and 6min of desorption time in the GC injector at 290°C. The MHS-SPME permitted the calculation of β values, which range from 0.72±0.01–0.95±0.02, depending on the cheese studied. Later, this β parameter is used to perform quantitation for the 9 volatile FFAs after just a single HS-SPME extraction, using an external solvent calibration curve. The validity of the utilization of an external solvent calibration was tested with aqueous standards of volatile FFAs, getting average recoveries higher than 81.2%. Quantitation by MHS-SPME was free of matrix interferences despite measuring a complex cheese sample. The optimized method was validated, presenting inter-day reproducibility values (as RSD in %) lower than 13%, and limits of detection down to 7µgkg−1. The method was also compared with a conventional extraction method such as solid-phase extraction for the studied cheeses elaborated with goat milk, generating comparable results. To our knowledge, this is the first time that MHS-SPME has been applied to volatiles in cheeses.