## 2 Materials and Methods
### Headspace SPME GC/MS Analysis of 3-MBT Content  
In order to analyze the 3-MBT content, volatiles from the headspace of 5 mL of the sample (in 20 mL glass SPME vials with 10 \(\upmu\)L ethyl butyrate solution) were adsorbed on a divinylbenzene/carboux/polydimethylsiloxane (DVB/CAR/PDMS) solid-phase micro-extraction (SPME) fiber (50/30 \(\upmu\)m, 1 cm, Sigma Aldrich, Oakville, ON, Canada), according to the protocols of [PERSON] et al. [26]. The prepared samples were homogenized by vortexing for 10 s, followed by preincubation without the fiber in the oven of an SPME autosampler (GC Sampler 80, Agilent Technologies, Santa Clara, CA, USA) connected to a 7890A gas chromatograph (GC) coupled to a 5975C single quadrupole mass spectrometer (MS) detector (Agilent Technologies, USA) for 5 min at 45 \({}^{\circ}\)C while agitated at 300 rpm. The fiber was exposed to the sample headspace for 30 min at 45 \({}^{\circ}\)C while agitated at the same speed. Fibers were conditioned at 250 \({}^{\circ}\)C for 20 min after each analysis to prevent the cross-contamination of non-targeted semi-volatile compounds. Volatiles were thermally desorbed from the SPME fiber in the GC injection port at 250 \({}^{\circ}\)C for 3 min, and then separated on a polar DB-Wax column (100% polyethylene glycol, 30 m \(\times\) 0.25 mm ID \(\times\) 0.25 \(\upmu\)m, J&W Scientific, Folsom, CA, USA) with helium as carrier gas at a flow rate of 0.9 mL/min. Chromatograph conditions were optimized for the analysis of 3-MBT but allowed the other non-targeted compounds to elute. The GC oven temperature was programmed at 35 \({}^{\circ}\)C for 4.36 min, followed by an increase of 15 \({}^{\circ}\)C/min until 230 \({}^{\circ}\)C, which was maintained for 5 min. The total GC run time was 22 min. The interface, ion source, and quadrupole temperatures were 250 \({}^{\circ}\)C, 230 \({}^{\circ}\)C, and 150 \({}^{\circ}\)C, respectively. Eluted compounds were simultaneously detected in SCAN (mass range of 35-500 \(m/z\)) and Selected Ion Monitoring (SIM) modes, operating in electron impact ionization mode at 70 eV. The SIM-targeted detection of 3-MBT was performed by tracking the ions 69 (target ion), 41 (qualifier ion 1), and 102 (qualifier ion 2). 3-MBT was identified on the chromatograph by comparing its retention times and accurate mass spectra with standards (Aroxa, Surrey, UK) and using the Mass Spectra Library (NIST MS Search 2.2 and MS Interpreter) on Chemstation software (version E.02.02.1431, Agilent Technologies, Santa Clara, CA, USA). Treatments were analyzed in triplicate.  
Standards were obtained in the form of capsules containing 100 ng 3-MBT per capsule. The standard was dissolved; serially diluted in 5% ethanol (to mimic the matrix of beer) to concentration levels of 2000, 1000, 500, 250, and 125 ng/L; and similarly analyzed through SPME-GC/MS to create the calibration curve. This calibration curve was employed for the quantification of 3-MBT in control and treated beer samples, and the concentrations were corrected for the matrix effect %, the latter being calculated following Equation (4).  
\[\%\ Matrix\ effect=\left(\frac{\text{area spiked sample}-\text{area non}- \text{spiked sample}}{\text{area standard in solution}}-1\right)\ \times\ 100 \tag{4}\]  
To ensure that the identified 3-MBT in the different beers were above their limits of detection (LOD) and the quantified 3-MBT were above their limits of quantification (LOQ), the sensitivity was assessed in terms of LODs and LOQs as three and ten times the signal-to-noise ratio (S/N), respectively. For linearity, the coefficient of determination (R\({}^{2}\)) was subsequently calculated, and the linear range was also established.