## Materials and Methods
### Qualitative and Quantitative Methods  
6.1 The Determination of Diacetyl and 2,3-Pentanedione in Packaged Beer Using Headspace Gas Chromatography  
Internal standard stock solution  
Preparation: Pour about 90 mL cold (0 \({}^{\circ}\)C) ethanol in a 100 mL volumetric flask. Place the flask in a tray with melting ice. Add 150 uL 2,3-hexanedione using a 250 uL syringe. Make up to 100 mL at 0 \({}^{\circ}\)C with cold (0 \({}^{\circ}\)C) ethanol. Stability: In refrigerator 1 week. Internal standard working solution  
2.0 mL stock solution (A) in 200 mL 5% (\(v/v\)) ethanol  
Preparation: Pour about 50 mL cold (0 \({}^{\circ}\)C) 5% (\(v/v\)) ethanol/water in a 200 mL volumetric flask. Pipette 2.0 mL of the cold (0 \({}^{\circ}\)C) stock solution (A) into the flask. Make up to 200 mL with cold (0 \({}^{\circ}\)C) 5% (\(v/v\)) ethanol/water and mix. Transfer the solution to the dispenser when used. Stability: In refrigerator 1 week. 2.6.2. The Determination of Acetaldehyde, Dimethyl Sulfide, Esters, and Higher Alcohols by Gas Chromatography  
X-Internal Standard Stock Solution  
Preparation: Pour 50 mL ethanol in a 100 mL volumetric flask. Pipette 3.0 mL 4-heptanon and 30.0 mL n-butanol into the flask. Make up to 100 mL with distilled-water and mix. Stability: In refrigerator (0-10 \({}^{\circ}\)C) 1 month. Y-Internal Standard Working Solution  
Preparation: Pour 50.0 mL ethanol and about 400 mL distilled water in a 1000 mL volumetric flask. Pipette 20.0 mL of the stock solution (A) into the flask. Make up to 1000 mL with distilled water and mix. Transfer the solution to the bottle with dispenser when used. Stability: In refrigerator (0-10 \({}^{\circ}\)C) 1 month. The normal procedure for sample preparation is as follows: Slowly fill a 250 mL graduated measuring cylinder with the cold (0-10 \({}^{\circ}\)C) beer sample. Remove the excess of beer and/or foam by using a waterjet suction pump to obtain 250 mL. Pipette 2.0 mL (volumetric pipette or dispenser) of the internal standard working solution B and D into an infusion bottle. Immediately add the beer sample to the infusion bottle. Close with a screwcap and rubber septum and carefully mix. In this way the samples are prepared one by one. After a series of samples has been prepared, each infusion bottle should be kept closed for at least 5 min to prevent foaming over. Prepared samples can be stored in the refrigerator (0-10 \({}^{\circ}\)C) for 2 days. Preparation: Pour about 50 mL of the 250 mL prepared sample (with internal standard) into another clean infusion bottle, close and place in a water bath of 40 \({}^{\circ}\)C, place a (vinyl-covered) lead weight around the neck of the bottle to prevent floating, (Lead donut 50 mm, Tamson 486K32 or order via GSC), pierce a disposable needle through the septum to prevent overpressure, after 30 min of conditioning inject about 0.5 mL of the headspace. 2.6.3. Spectroscopic Analysis of Polyphenols  
The spectrophotometric analyses were performed using a UV-1601 spectrophotometer from Shimadzu (Duisburg, Germany) and were replicated three times for each extract or calibration point (\(n=3\)). 2.6.4. Determination of Total Phenolic Content (TPC)  
The total phenolic content was determined based on a spectrophotometric method using the Folin-Ciocalteu reagent according to the method of [PERSON] et al. (1999) [31]. This reagent leads to oxidizes the phenolic compounds, which turns the solution to blue. The TPC was quantified from a calibration curve prepared with gallic acid standard and expressed as mg of gallic acid equivalents (GAE) per 100 mL of sample (mg GAE 100 mL\({}^{-1}\)). The calibration curve was drawn, and the equation of linear regression was obtained:  
\[\text{y}=1.552\text{x}+0.208,\,\text{R}^{2}=0.962\]  
2.6.5. Determination of Flavonoid Content (TFC)  
Total flavonoid content was performed using the aluminum chloride (AICI\({}_{3}\)) based on the protocol described by [PERSON] et al. (2003) [32].