Degradation of Furfuryl Mercaptan in Fenton-Type Model Systems

Chapter 24

Degradation of Furfuryl Mercaptan in Fenton-Type Model Systems 1

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I. Blank , E. C. Pascual , L. B. Fay , R. H . Stadler , B . A. Goodman , and C . Yeretzian 1

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Nestec Ltd., Nestlé Research Center, Vers-ehez-les-BIanc, P.O. Box 44, CH-1000 Lausanne 26, Switzerland Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, United Kingdom

Downloaded by FUDAN UNIV on February 22, 2017 | http://pubs.acs.org Publication Date: January 15, 2000 | doi: 10.1021/bk-2000-0754.ch024

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The stability o f furfuryl mercaptan (Fur-SH) was studied in aqueous solutions in the presence of reagents for the Fenton reaction. The impact of hydrogen peroxide, iron, ascorbic acid, and E D T A was studied by incubating them in various combinations with Fur-SH for 1 h at 37°C. About 80% of Fur-SH was lost in the presence o f hydrogen peroxide ( H O ) and the ferrous iron generating system Fe(III) and ascorbic acid. Volatile components formed were mainly dimers of Fur-SH, i.e. Fur-S -Fur with n = 1-3, difurfuryl disulfide being the major compound. In the model systems containing Fe(II) and H O , the volatiles generated account for about 1/3 o f the total Fur-SH lost. Electron paramagnetic resonance spectroscopy in the presence o f the spin traps P O B N , D M P O , and D E P M P O indicated the formation o f hydroxyl and carbon-centered radicals arising from H O and FurSH, respectively. 2

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Furfuryl mercaptan (Fur-SH) has been suggested to be a key odorant o f coffee (/). Its sensory relevance, evidenced by various groups (2, J), is due to the roasty, coffee-like aroma note and low odor threshold o f 0.01 ng/L air (4). The concentration of Fur-SH in roast and ground coffee was determined to be 1-2 mg/kg (5). However, only 20-40 Mg/kg o f Fur-SH were detected in coffee brews (50 g/L), which correspond to about 1/3 of the total amount (6). This might be explained by the low extractability o f Fur-SH during preparation o f the coffee beverage or high sensitivity of Fur-SH to oxidative processes. Formation of hydrogen peroxide ( H 0 ) in liquid coffee has been reported (7). In the presence of transition metals, H 0 produces hydroxyl radicals ( Ό Η ) via the Fenton reaction (8) as shown below: 2

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H 0 + Fe(ll) 2

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O H + OH" + Fe(lll) €> 2000 American Chemical Society

Parliment et al.; Caffeinated Beverages ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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Caffeine has been shown to be an effective radical scavenger, trapping O H radicals and forming 8-oxocaffeine (P). Yet,«OH radicals may also attack aromaactive thiols, which may then lead to a distortion of the coffee aroma. The objective of this work was to study the stability of Fur-SH in Fenton-type model systems. We were aiming at (i) quantifying the losses of Fur-SH caused by Fenton reagents, (ii) identifying the corresponding volatile degradation products, and (iii) characterizing short-lived radical species.

Experimental Procedures


Materials Furfuryl mercaptan (Fur-SH), benzyl mercaptan (Ben-SH), difurfuryl monosulfide (DFMS), difurfuryl disulfide (DFDS) and ascorbic acid were from Aldrich (Buchs, Switzerland). a-(4-Pyridyl-l-oxide)-iV -/-butylnitrone (POBN), 5,5dimethyl-l-pyrroline-JV-oxide (DMPO) were from Sigma (Dorset, England, U K ) . Ethylenediaminetetraacetic acid ( E D T A , disodium salt), hydrogen peroxide ( H 0 ) , ferric chloride (FeCl * 6 H 0 ) , and sodium dodecyl sulfate (SDS) were from Merck (Darmstadt, Germany) and 5-(diethoxyphosphoryl),5-methyl-l-pyrroline-iV-oxide (DEPMPO) was from Calbiochem-Novabiochem (Beeston, Nottinghamshire, U K ) . T

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Sample Preparation The aqueous solutions were prepared fresh before use, i.e. ascorbic acid (20 mM), E D T A (25 mM), H 0 (1.5 %), F e C l · 6 H 0 (10 mM), K H P 0 (20 m M , p H 5.5), and Fur-SH (3.3 m M , dissolved in aqueous SDS, 3.3 mg/mL). The reaction was initiated by adding Fur-SH to mixtures as indicated in T a b l e J . Samples were incubated (1 h, 37°C) and adding ethanol (0.1 mL) terminated the reaction. E P R measurements were made on solutions to which a spin trap was added immediately prior to the Fur-SH. Chemical analyses were performed after adjusting the p H to 3.5 and extracting neutral compounds with diethyl ether (1 mL). The organic phase was centrifuged (5 min, 3500 rpm) and analyzed by G C . 2

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Table I. Experimental Design to Study the Effect of Fenton Reagents on the Loss of Furfuryl Mercaptan (Fur-SH) in Aqueous Model Systems 8

Reagents

#?

#2

#3

#4

#5

#6

#7

#8

Fur-SH FeCI

100 10 10 10 50 800 20

100

100

-

10 50 800 30

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100 10 10 10

100 10

10 10 50 800 30

100 10 10

100

-

100 10

-

50 800 30

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800 70

50 800 50

50 800 40

3

H 0 2

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EDTA Ascorbic acid Phosphate buffer Water

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800 100

The values correspond to volumes expressed in μL· The total volume of each sample was 1 mL. In all samples, the initial concentration of Fur-SH was 3.3 Mmol. 8


232 Capillary Gas Chromatography A Hewlett Packard gas chromatograph (HP-5890) equipped with a HP-7673A autosampler and cold on-column injection was used. Samples were analyzed on an OV-1701 fused silica capillary column (30 m χ 0.25 mm, film thickness 0,25 μπι). The column pressure was 80 kPa using helium as carrier gas. The effluent was split 1:1 to a flame ionization detector (FID) and a flame photometric detector (FPD). The G C oven was temperature programmed (4).


Quantification Benzyl mercaptan (Ben-SH) was used as internal standard (0.5 m L of 49.4 mg/100 mL diethyl ether) added to the samples after quenching with ethanol. The p H was adjusted to 3.5 and the clean-up was performed as described above. FID detection was used for quantification. Response factors were determined using mixtures of known amounts of Ben-SH and the compounds to be quantified (e.g. 1.75 for Fur-SH). Gas Chromatography-Mass Spectrometry ( G C - M S ) Electron impact (EI) and positive chemical ionization (PCI) mass spectra were obtained on a Finnigan M A T 8430 mass spectrometer at 70 eV and 150 eV, respectively. Ammonia was used as reagent gas for PCI. Volatile compounds were sampled via a cold on-column injector (HP-5890 G C ) using the conditions described above. Relative abundances of the ions are given in %. Electron Paramagnetic Resonance ( E P R ) Spectroscopy This was made at room temperature at X-band frequencies (-9.5 GHz) using a Bruker ESP300E computer-controlled spectrometer incorporating an ER4103TM cylindrical microwave cavity. A l l spectra were collected in 1024 data points using a modulation frequency of 100 K H z . Most spectra were recorded as 1 derivatives o f the microwave absorption and displayed as functions of absorption versus magnetic field at a constant microwave frequency (except for one spectrum with P O B N for which a 2 derivative spectrum is shown). st

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Results and Discussion Loss of F u r - S H in Fenton-type Model Systems The degradation of Fur-SH and concomitant formation of reaction products was investigated on a series of eight Fenton-typs model systems at p H 5.5 (Table II). Sample #1 (full Fenton model) contained all o f the reagents for a Fenton reaction. E D T A was used to prevent precipitation of Fe(III) and ascorbic acid reduced Fe(III) to Fe(II), which initiated decomposition of H 0 to the hydroxyl radical ( Ό Η ) . The reference (sample #8) was a 3.3 μπιο! Fur-SH solution. 2

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233 The concentration of Fur-SH found after reaction was strongly dependent on the composition of the model system (Table II). The most significant loss of Fur-SH, i.e. 80%, was observed in sample #1 containing the typical Fenton reagents H 0 and Fe(II). The presence of E D T A was not essential under the experimental condition chosen, as about 70% of Fur-SH were lost in sample #4, which contained no E D T A . Similarly, the absence of ascorbic acid did not prevent degradation of Fur-SH, where a 65% loss was observed in sample #5. This might be explained by the ability of Fur-SH or its degradation products to reduce Fe(III) to Fe(II). 2

Table II. Effect of Fenton Reagents on the Loss of Furfuryl Mercaptan (FurSH) in Aqueous Model Systems 8


Reagents # 1 : Full Fenton system

#2: #3: #4: #5: #8:

NoFe(lll) NoH 0 No EDTA No ascorbic acid Reference sample 2

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Concentration of Fur-SH (jumol)

Loss of Fur-SH

0.7 1.8 2.3 1.0 1.1 3.3

80 45 30 70 65

Degradation of Furfuryl Mercaptan in Fenton-Type Model Systems

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