Determination of 4-vinylgaiacol and 4-vinylphenol in top-fermented wheat beers by isocratic high performance liquid chromatography with ultraviolet detector

ENGINEERING, TECHNOLOGY AND TECHNIQUES

Determination of 4-vinylgaiacol and 4-vinylphenol in top-fermented wheat beers by isocratic high performance liquid chromatography with ultraviolet detector

Mingguang Zhu^I; Yunqian CuiII,^*

^ICollege of Bioengineering; Tianjin University of Science and Technology; P.R. - China

^IIChina-Germany Brewing Technical Center; Shandong Provincial Key Laboratory of Microbial Engineering; Shandong Polytechnic University; Jinan, Shandong, P.R. - China

ABSTRACT

This work aimed at determining 4-vinylguaiacol and 4-vinylphenol in the top-fermented wheat beers using different wavelength and the mobile phase for HPLC. Best results for isocratic elution were obtained at 260 nm and the mobile phase comprising methanol/ultrapure water/phosphoric acid (400/590/10, V/V). Under these conditions, the retention time of 4-vinylguaiacol and 4-vinylphenol was 25 and 27min, respectively.

Key words: 4-vinylguaiacol, 4-vinylphenol, Top-fermented wheat beers, HPLC, phenolic off-flavor

INTRODUCTION

Despite being considered undesirable and cataloged as a "phenolic off-flavor" (POF) in bottom-fermented beers (Thurston et al. 1981; Thurston et al. 1986), 4-vinylguaiacol (4VG) and 4-vinylphenol (4VP) are well-known aroma compounds found in top-fermented beers, including Belgian white beers (brewed with unmalted wheat), German Weizen beers and Rauch beers (brewed with malted wheat) (Coghe et al. 2004; Vanbeneden et al. 2006; Vanbeneden et al. 2008). Both the 4VG and 4VP have a lower sensory threshold value, which are reported to be 0.3 and 0.2 mg/L, respectively (Thurston et al. 1981; BDAS Online. 2006; Vanbeneden et al. 2006). Even at trace levels, they can also produce a significant medicinal, phenolic, clove-like, smoky, or BBQ flavor in beers (Thurston et al. 1981; Thurston et al. 1986; McMurrough et al. 1996; Coghe et al. 2004; Vanbeneden et al. 2008). Hence, they are of vital importance for the overall flavor perception in these top-fermented beers.

The 4VG and 4VP have the similar structure (Fig. 1), (Vanbeneden et al. 2006; Vanbeneden et al. 2008). Their molecular weights are 150.18 and 120.15, respectively. The universal method to determine the 4VG and 4VP is by High Performance Liquid Chromatography (HPLC) with gradient pump (McMurrough et al. 1996, Shinohara et al. 2000; Vanbeneden et al. 2008). Nevertheless, there are also different analysis methods for different detectors. For example, for fluorescence monitor, mobile phase is H₂O/CH₃OH/H₃PO₄=540:450:10 and excitation and emission wavelength is 259 nm and 341 nm, respectively (McMurrough et al. 1996); for U.V. detector, mobile phase is H₂O/CH₃CN/H₃PO₄=599:400:1 and wavelength is 230 nm (Shinohara et al. 2000); for electrochemical detector, mobile phase is H₂O/CH₃OH/H₃PO₄=745:245:10, and the working electrode is Ag/AgCl, +1200 mV, 100 nA (Vanbeneden et al. 2006; Vanbeneden et al. 2008), etc.

This work aimed to study the determination of 4VG and 4VP in top-fermented wheat beers by isocratic HPLC with ultraviolet detector.

EXPERIMENTAL

Reagents and materials

HPLC grade 4VG (>98%, 2-methoxy-4- vinylphenol) and 4VP (10% in propylene glycol) were purchased from SAFC (UK) and Alfa Aesar (Tianjin, China), respectively. HPLC grade methanol and acetonitrile were obtained from Tianjin Siyou Chemical Reagent Co, Ltd (Tianjin, China) and Jinan Institute of Chemical Engineering (Jinan, Shandong, China), respectively. Phosphoric acid (85%) was analytical grade. The ultrapure water used in this study was deionized and filtered by a Milli-Q Plus water purifying system (Bedford, MA, USA). Scarlett barley malt was procured from Zhongliang Malt Division (Dalian, China). Wheat malt was obtained from Weyermann (Bamberg, Germany). Hops were supplied from Barth-Haas Group (Beijing, China), which contained 4.9% α-acids (Analytica-EBC) (Cui et al. 2010).

HPLC system

LC-10AT HPLC (Shimadzu) and Rheodyne 7725i manual sampler were used. Absorption spectra were recorded with a UV spectrophotometer, model SPD-10Avp. Isocratic separation was performed on a reversed-phase C18 column (4.6 mm×150 mm, 5 µm particle size, Diamonsil). The measurements were carried out at room temperature. The flow rate was at 1 mL/min. The injection volume was 20 µL.

Preparation of 4VG and 4VP standard solutions

According to the density of 4VG and 4VP as 1.10 and 1.04 mg/L, respectively, the standard solution of 4VG and 4VP were prepared as mentioned in Table 1.

Preparation of samples

The top-fermented wheat beers were fermented in the pilot-scale brewery (20 hL) in Shandong Polytechnic University. Brewing process was carried out as follows. barley malt (102 kg), wheat malt (68 kg), brewing water (5.95 hL), mashing procedure: 44ºC (20 min), 52ºC (40 min), 65ºC (70 min), 72ºC (15 min), 78ºC (10 min), wort lautering by lauter tun, boiling for 90 min in wort kettle, wort clarification in whirlpool, original gravity is 11ºP, fermentation temperature 20ºC (5 days), lagering period 7 days at 0ºC (Cui et al. 2010).

Mobile phases and samples were filtered through 0.45 µm pore PVDF membrane filters and then decarbonated by sonication. All the samples were protected from light during operation to minimize the photo-isomerisation reaction.

The determination of the optimized absorption wavelength

Wavelength ranges were set between 190-300 nm on the ultraviolet spectrophotometer to confirm the optimal absorption wavelength.

The determination of the optimal mobile phase

The 4VG and 4VP standard solutions were mixed at the rate of 1:1. The mobile water/phosphoric acid as shown in Table 2.

The determination of 4VG and 4VP concentrations in top-fermented wheat beers

The top-fermented wheat beers were injected into HPLC under the conditions of optimized absorption wavelength and mobile phase in this study so as to determine the levels of 4VG and 4VP.

RESULTS AND DISCUSSION

The determination of the optimum absorption wavelength

The wavelength (190~300 nm) scanning maps of 4VG and 4VP are shown in Figure 2, respectively. There was a relatively better absorption peak at 260 nm for both the 4VG and 4VP. Furthermore, according to the proper property of acetonitrile and methanol, the absorption values of them were less than 0.005 and 0.01 at 260 nm, respectively. Hence, it was an appropriate choice to set the wavelength at 260 nm to separate the 4VG and 4VP entirely.

The determination of the optimal mobile phase

The 4VG and 4VP standard solutions were mixed at the rate of 1:1 and their standard solutions with different mobile phases shown in Table 2 were injected into HPLC system. The corresponding spectra are shown in Figure 3.

Figure 3C

The drawing of 4VG and 4VP standard curves

The five concentration gradients of 4VG or 4VP were prepared and injected into HPLC according to standard operations. Peak areas were obtained as shown in Table 3. Consequently, standard curves of 4VG and 4VP were drawn (Fig. 4).

Determination of 4VG and 4VP concentrations in top-fermented wheat beers

The top-fermented wheat beers were injected into HPLC under the conditions of optimized absorption wavelength and mobile phase. The, spectrum of top-fermented wheat beers and close- up spectrum of 4VG and 4VP are shown in Figures 5 and 6, respectively.

The 4VG and 4VP peaks in Figure 6 were substituted in the standard curve equations of 4VG and 4VP in Figure 4. After calculation, the contents of 4VG and 4VP in the top-fermented wheat beers found were 2.6133 and 1.0095 mg/L, respectively, which was similar to the findings of Wackerbauer et al. (1982) and Schwarz et al. (2012).

CONCLUSION

In summary, under the existing conditions (LC-10AT, 7725i manual sampler, SPD-10Avp UV spectrophotometer, reversed-phase C18 column), the optimum HPLC conditions were wavelength at 260 nm, the mobile phase methanol/ultrapure water/phosphoric acid (400/590/10, V/V), flow rate 1.0 mL/min and injection volume 20 µL. This resulted the analysis time of the 4VP and 4VG as 25 and 27 min, respectively.

ACKNOWLEDGMENTS

We thank Weiyue Zhu (Shandong Polytechnic University) for constant support and guidance in HPLC operation.

Received: March 08, 2012;

Accepted: August 01, 2013.

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Publication Dates

• Publication in this collection
  06 Jan 2014
• Date of issue
  Dec 2013

History

• Received
  08 Mar 2012
• Accepted
  01 Aug 2013