外文翻译CHANGES IN ODOUR OF BARTLETT PEAR BRANDY INFLUENCED BY SUNLIGHT IRRADIATION.docx

CHANGES IN ODOUR OF BARTLETT PEAR BRANDY INFLUENCED BY SUNLIGHT IRRADIATIONI. Kralj Cigi6, L. Zupani-KraljUniversity of Ljubljana, Faculty of Chemistry and Chemical TechnologyAkereva 5, SI- 1000 Ljublj ana, SloveniaABSTRACTSensory evaluation of Bartlett pear brandy revealed differences in taste and smell between the brandy in colourless and green bottles. To determine the differences in chemical composition, headspace sampling with solid-phase microextraction combined with gas chromatography-mass spectrometry was applied. The major aroma components in pear brandy stored in green bottles were ethyl trans-2-cis-4-decadienoate (68 ppm), ethyl trans-2-trans-4-decadienoate (21 ppm) and methyl trans-2-cis-4-decadienoate (27 ppm). In pear brandy kept in colourless bottles, the concentration of trans-2-cis-4-isomeres was lower and the concentrations of other possible isomeres were higher. Results show a pronounced difference in the concentration ratios among isomeres of ethyl 2,4-decadienoate and methyl 2,4-decadienoate in both samples. Sunlight irradiation was found to be the reason for these changes. It was confirmed that trans-2-cis-4-isomere partly converts to the other three isomeres during UV irradiation.INTRODUCTIONAroma is one of the most important qualities of distilled beverages, e.g. brandy. It is determined by the presence and the level of volatile compounds (complex mixture of carbonyl compounds, alcohols,carboxylic acids and their esters) and all production stages can noticeably contribute and influence the final aroma composition of brandies.Earlier studies on Bartlett pear aroma have shown that the character impact compounds are methyl and ethyl esters of trans-2-cis-4-decadienoic acid 1 . The pleasant pear-like odour has been found to change during fermentation as a result of the trans-2-cis-4-decadienoates being partly isomerized to trans-2-trans-4-decadienoates, which have a non-typical smell 2. The odour characteristics of some volatile compounds are sometimes affected by their geometric isomerism 3. Headspace (HS) sampling with solid-phase microextraction (SPME) in combination with gas chromatography-mass spectrometry (GC-MS) is the main method for analysing aroma compounds 1 .The aim of our work was to determine the differences in chemical composition of the aroma of Bartlett pear brandy stored in green and colourless bottles and to determine why differences occur.EXPERIMENTALStandard solutions of ethyl trans-2-cis-4-decadienoate (90+%, Aldrich) were prepared in ethanol (absolute p.a., Merck) and water (40:60, v/v). The ratio of ethanol and water was chosen according to the ethanol content in Bartlett pear brandy.UV irradiation UV irradiation was performed with 50 W Hg high-pressure lamp (Osram, Ultra-vitalux). The samples wereplaced in vials 1 cm from the UV lamp.Headspace GC-MS analysisFor headspace analysis solid-phase microextraction was used. A manual holder for solid-phase microextraction wa:; ,)btained from Supelco. SPME stationary phase was polydimethylsiloxane with film thickness 100 gm from Supelco. 5 mL aliquot of each sample were thermostated at 40 C and the extractions were performed for 15 rain.The analytes were desorbed from the SPME fibre in the GC injector at 220 C and separated on VOCOL fused silica capillary column (60 m, 0.25 mm 1D, 1.50 !am film thickness, Supelco) and analysed using massspectrometer with electron impact ionization (HP 5890 II gas chromatograph, 5989 A mass spectrometer,!i wlett-Packard). Temperatures of the ion source, quadrupole mass analyzer and GC-MS interface were 200, 100 and 250 C respectively. The temperature program was: 70 C for 4 rain, and then heated up to 210 C at 15 C/min.RESULTS AND DISCUSSIONChromatograms obtained by GC-MS after headspace sampling with SPME of the two pear brandy samples are presented in Figure 1 (a, b). Eight chromatographic peaks were identified as: methyl cis-2-trans-4-decadienoate (1), methyl trans-2-cis-4-decadienoate (2), methyl cis-2-cis-4-decadienoate (3), methyl trans-2-trans-4-decadienoate (4), ethyl cis-2-trans-4-decadienoate (5), ethyl trans-2-cis-4-decadienoate (6), ethyl cis-2-cis-4-decadienoate (7), and ethyl trans-2-trans-4-decadienoate (8).A comparison of the two chromatograms revealed that the amount of trans-2-cis-4-decadienoates (2 and 6) decrease and the amount of other isomeres of 2,4-decadienoates (peaks 1, 3, 4, 5, 7 and 8) increase when brandy is stored in colourless bottles (Figure 1 a, b).Figure 1: GC-MS chromatograms after SPME sampling showing the composition of Bartlett pear brandy stored in green bottle (a) and in colourless bottle (b). Peak identities are as follows: methyl cis-2-trans-4-decadienoate (1), methyl trans-2-cis-4-decadienoate (2), methyl cis-2-cis-4-decadienoate (3), methyl trans-2-trans-4-decadienoate (4), ethyl cis-2-trans-4-decadienoate (5), ethyl trans-2-cis-4-decadienoate (6), ethyl cis-2-cis-4-decadienoate (7), and ethyl trans-2-trans-4decadienoate (8).The peaks in the chromatograms can be divided in two groups: peaks 1 to 4 and peaks 5 to 8 (Figure 1).Compounds in each group have very similar mass spectra and it was determined that the group with the shortest retention times belong to four geometrical isomeres of methyl-2,4-decadienoate. The second group of compounds are isomeres of ethyl-2,4-decadienoate. Since the mass spectra of geometrical isomers are similar, it is impossible to distinguish among different isomeres. However ethyl trans-2-cis-4-decadienoate was identified by comparing retention times of peak 6 with an authentic standard. Ethyl cis-2-trans-4-decadienoate and ethyl trans-2-trans-4-decadienoate were determined by their volatility. Cis-2-trans-4 isomeres are the most volatile and trans-2-trans-4 isomeres the least volatile among four possible isomers and we can conclude that peak 5 is the ethyl cis-2-trans-4-decadienoate and peak 8 is ethyl trans-2-trans-4-decadienoate. The fourth peak in the second group can only he the ethyl cis-2-cis-4-decadienoate. A similar elution order was proposed for isomeres of methyl-2,4-decadienoate.We speculated that changes in composition of isomeres were caused by sunlight irradiation. To confirm this,a solution of ethyl trans-2-cis-4-decadienoate in ethanol-water (40:60, v/v, concentration 100 ppm) was exposed to UV light. The results are shown in Figure 2. During irradiation the concentration of ethyl trans-2-cis-4-decadienoate decreases and the concentrations of ethyl cis-2-trans-4-decadienoate, ethyl cis-2-cis-4-decadienoate and ethyl trans-2-trans-4-decadienoate increase. This experiment confirms that sunlight irradiation is responsible for the changes in the composition of the isomeres of 2,4-decadienoate and consequently influence on pear brandy aroma.Figure 2: Changes in peak area for isomeres of ethyl 2,4-decadienoate during UV irradiation of a standard solution of ethyl trans-2-cis-4-decadienoate in colourless vial.The aroma of Bartlett pear brandy did not alter while being stored in green bottles at the standard roomtemperature for 3 months. Furthermore, the concentration ratios among isomeres of ethyl 2,4-decadienoate in green bottles remained constant during exposure to UV irradiation.The esters in pear brandy were quantified by comparing peak area of a standard solution of ethyl trans-2-cis-4-decadienoate. Due to a lack of standards the concentrations of all esters were expressed relatively to the same standard. Quantitative data for esters of 2,4-decadienoic acid are presented in Table 1.The sum of isomeres of ethyl 2,4-decadienoate and isomeres of methyl 2,4-decadienoate is equal in both samples, but the concentration ratios among isomeres are different for brandy stored in green and colourless bottles.Table 1: Quantitative data for esters of 2,4-decadienoic acid in Bartlett pear brandyCONCLUSIONSThe main compound in the aroma of Bartlett pear brandy is ethyl trans-2-cis-4-decadienoate (68 ppm) and it is responsible for the pleasant odour and taste. When exposed to UV light ethyl trans-2-cis-4-decadienoate is transfor