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Odour-Port Gas-Chromatography Dilution Assay for the Determination of the
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Aroma Impact Components of Ciders
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Andrew GH Lea and Graeme D Ford
Reading Scientific Services Ltd., Whiteknights, The University, Reading RG6 6LA,
UK
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Abstract
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Headspace extracts of cider volatiles were prepared and analysed by GC-MS to
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establish the identity of the major components. Extracts were run by odour-port GC
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and the aroma descriptors recorded by three assessors. Progressive serial dilutions of
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the extracts were re-assessed by odour-port GC until no more odour could be
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detected. From this exercise, a relative ranking of the odour impact of individual
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cider volatiles could be established. The most odour-active component was shown to
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be 2-methyl 4-pentyl 1,3-dioxane, the condensation product of acetaldehyde with 1,3
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octanediol, a compound which is relatively restricted to apples and pears.
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Keywords
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Cider; aroma compounds; odour threshold values; gas chromatography; dioxane
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Introduction
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The major volatiles of all alcoholic beverages are similar and comprise a range of
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esters and alcohols derived from yeast fermentation. Individual beverages are
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typified and distinguished by specific components which tend to be derived from the
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raw material used. Because of their widely differing concentrations and odour
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thresholds, it is impossible to be certain which components make the greatest
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sensory impact by simple chemical analysis. Use of odour-port GC analysis allows
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the sensory character of each component to be assessed. Serial dilution of the
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extracts, followed by repeated chromatography until no more odours can be
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detected, allows the sensory importance of each volatile in the extract to be ranked.
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This technique was used here.
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Extraction of Cider Headspace Volatiles
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A commercial UK cider (50 ml) was equilibrated at 35ºC and the headspace was
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sampled for 30 mins in a closed loop stripping apparatus using a 1.5 mg (Grob)
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charcoal trap. The extract was desorbed with 4 x 15 μl of carbon disulphide and
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stored deep-frozen in a sealed vial until required for use. Hedonically, this extract
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on a paper ‘sniffing strip’ was considered to be valid and characteristic of the cider
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under study.
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GC-MS identification of cider volatiles
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Separation of the extract (1 μl) was carried out on a fused silica capillary coated with
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the polar phase OV 351 (50m x 0.32mm), temperature programmed from 55º - 200º
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at 2º/min. Peaks were identified using a VG 7070 MS in the EI mode, using standard
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spectral libraries where appropriate (Figure 1)
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Odour Port Analysis
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Separation of the extract (2 μl ) was carried out on a polar CP-Wax 52CP column
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(25m x 0.53 mm) temperature programmed from 30º - 200º at 4º/min. The effluent
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was split to an FID detector / chart recorder and to a home-made odour-port
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assembly. Odour descriptors for each peak were recorded independently by three
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separate assessors, who wrote their assessment of each peak on the FID chart as it
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eluted. The FID peaks were correlated with the GC-MS identifications obtained on
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the analytical column.
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Odour Port Dilution Analysis
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The extract was serially diluted 1:3 in carbon disulphide in six separate steps (final
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dilution 1:729). At each step an odour-port analysis was carried out by the human
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assessors, who recorded the dilution at which the odour for each individual peak was
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no longer detectable. From this data the relative odour-impact of each component
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was calculated, at its natural concentration in the headspace of the cider under study.
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Results and conclusions
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The composite data for the odour descriptors and their relative impact is given in the
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Table. Some unidentified components were also detected by odour-port GC at
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intermediate dilutions. It was of interest to find that the most odour-active
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component in the headspace was 2-methyl 4-pentyl 1,3-dioxane (Figure 2). At the
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time that this work was carried out (1990) the structure of this compound was
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unknown. However, it had already been recognised by a number of other workers as
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a component of probable mass 172 (major ion m/z 69) eluting just before ethyl
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octanoate (Hubert et al 1990), while Williams and Tucknott (1978) had already
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identified its ‘acid drop’ aroma by GC odour port analysis on a polar Carbowax 20M
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column. Lea (1995) suggested its probable structure, which was eventually
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confirmed by Dietrich et al (1997).
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2-methyl 4-pentyl 1,3-dioxane is the condensation product between acetaldehyde
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generated from fermentation and 1,3 octanediol, an unusual alcohol which occurs
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naturally (and also in glycosidic form) in apples and pears (Beurle et al 1996, 1997).
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Because of its origin, this dioxane is likely to be characteristic of fermented cider
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and perry and may be virtually unique to these products. Dietrich et al (1997)
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reported its presence in French cider at up to 24 mg/l in two isomeric forms. Ours is
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the first report of its likely importance to the characteristic odour of fermented
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ciders.
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References
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Beuerle, T., Schreier, P., Brunerie, P., Bicchi, C., Schwab, W. (1996) Absolute
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configuration of octane-1,3-diol derivatives in apple fruits. Phytochemistry, 43, 145-
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149.
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Beurle, T and Schwab W. (1997) Octane 1,3 diol and its derivatives from pear
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fruits. Lebensm Unters Forsch A 205 215-217
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Dietrich C., Beurle T., Withopf B., Schreier P., Brunerie P., Bicchi C. and Schwab
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W. (1997) . Absolute configuration and conformation of 1,3 dioxanes from cider. J
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Agric. Fd Chem. 45 3178 – 3182
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Hubert C., Brunerie P., Le Quere JM and Drilleau J-F (1990) Les composes volatils
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du cidre – extraction rapide et dosage. Science des Aliments 10 603-618
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Lea A G H (1995) ‘Cidermaking’ in Fermented Beverage Production (1st edition)
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pp 66-96. Lea AGH and Piggott JR (eds). Blackie, London (2nd edition publ Kluwer
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Academic, NY, 2003)
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Williams AA, and Tucknott OG (1978) The volatile aroma components of
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fermented ciders J Sci Fd Agric 29 381-397
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TABLE
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GC-Odour Port Descriptors and Odour Values for Cider Extract
Component
Descriptors
Relative Odour Value
(smallest numbers are those
of greatest impact)
2-methyl 4-pentyl 1,3-dioxane
Green, acidic, fruity, cidery
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Ethyl 2-methyl butyrate
Banana, fruity, estery, fresh
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2-phenyl ethanol
Roses, floral
3
3 unknowns
Fatty, greasy
3
Octanal
Fatty, green, metallic
9
Nonanal
Fatty, green, metallic, waxy
9
Ethyl hexanoate
Banana, estery, fruity
9
1 unknown
Winelike
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Iso-amyl acetate
Pear drops, estery
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2 and 3-methyl butanol
Fusel, green, grassy
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Eugenol + 4-ethyl phenol (co-elute)
Cloves, spicy
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4 unknowns
Cidery, winelike
81
2-phenyl ethyl acetate
Perfumed, floral, ‘winey’
81
7 unknowns
Rubbery, yeasty, soupy
243
Ethyl decanoate
Weakly fatty, leafy
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Ethyl octanoate, dodecanoate
Hexanal, heptanal
Hexanol, octanol
Decanal, decenal
Methyl butyl octanoate
Diethyl succinate
Heptanoic, octanoic, nonanoic acids
4-ethyl guaiacol
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No detectable aroma on GC
odour-port assessment
> 243
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Figure 1 – GC Trace of Cider Headspace extract with major peaks identified
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C5H11
O
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O
CH3
Figure 2 - The structure of 2-methyl 4-pentyl 1,3-dioxane