Media samples were collected by centrifugation at 5000 rpm for 8 min at -20°C. The supernatant was removed and 1.5 mL of ice-cold (-20°C) 60% methanol was added to the pellets to terminate metabolic activity. The mixtures were transferred into 2 mL pre-weighted Eppendorf tubes, and each tube frozen and thawed five times in liquid nitrogen and subsequently centrifuged at 15000 rpm and -20°C for 15 min. The supernatants were stored at -80°C. The remaining materials were suspended in 1 mL of ice-cold (-20°C) 60% methanol in water and centrifuged same as above. Subsequently, the resulting supernatant was mixed with the first supernatant and carefully transferred into a 1.5 mL Eppendorf tube, after which 10 μL of a 0.2 mg/mL ribitol (Sigma-Aldrich, USA) solution was added as internal standard and the samples dried under nitrogen gas. The dried samples were mixed with 50 μL methoxylamine hydrochloride /pyridine (20 mg/mL) (Sigma-Aldrich, USA) and incubated in a water bath at 37°C for 80 min. The compounds were blended with 80 μL MSTFA (N-Methyl-N-(trimethylsilyl) trifluoroacetamide) (Sigma-Aldrich, USA) and incubated in a water bath at 37°C for 80 min, after which the samples were centrifuged at 10000 g for 5 min and a 100 μL aliquot of the supernatant transferred to a fresh Eppendorf tube and used directly for GC-TOF/MS detection on a gas chromatograph-mass spectrometer (Trace GC2000 DSQ, Agilent, USA).