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1、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
2、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 e
3、thyl 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. Result
4、s 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
5、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 n
6、oticeably 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
7、 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 microextrac
8、tion (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 w
9、hy 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 irradia
10、tion 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 stat
11、ionary 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
12、 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 2
13、50 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 identifi
14、ed 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
15、-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 ch
16、romatograms 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-
