Solid phase microextraction (SPME) combined with gas-chromatography and olfactometry-mass spectrometry for characterization of cheese aroma compounds

The applicability of solid phase microextraction (SPME) as a technique for the concentration of cheese aroma for analysis by gas chromatography-mass spectrometry (GC-MS) and gas chromatography-olfactometry (GC-O) was assessed in this preliminary study. Three each of the following cheese varieties were examined: cheddar, hard grating and mold-ripened blue. Volatile components were concentrated by Carboxen-PDMS SPME fibres for 16 h (overnight) and analysed by GC-MS and GC-O. Odor compounds, which could be perceived at the olfactory port (OP), were matched with electron impact (EI) and methanol chemical ionization (CI) mass spectra. The volatile compounds identified were compared to previously reported cheese aroma compounds. Of the components identified via olfactometry, methanethiol, methional, dimethyl trisulfide and butanoic acid were present in all of the cheeses implying their essential role in the formation of basic cheese aroma. A number of alkyl-pyrazines were also found to impart roasted nutty, raw potato and savoury broth-like notes in some of the cheeses. In all cases, the aroma active compounds identified via olfactometry were in agreement with those reported in the literature. In a separate study, it was demonstrated on a number of cheeses that the adsorption of most important aroma volatiles increased consistently up to 16 h; i.e. sulfur compounds, lactones, pyrazines, phenolic compounds and benzene derivatives. For the strong-smelling pecorino and blue cheeses, however, some analyte displacement effects were observed. Data indicated that a sampling time between 9 and 16 h was appropriate and displacement/competition reactions tended to occur in strong cheeses, i.e. those which have undergone extensive lipolysis e.g. pecorino, very high concentration of butanoic and hexanoic acid, and/or β -oxidation e.g. blue cheese, with high concentrations of 2-heptanone and 2-nonanone.