Some aroma compounds of importance to the quality of Ferdinand de Lesseps and Kerner wines

Rogers, Ilva Margaret (2000-03)

Thesis (MScAgric)--University of Stellenbosch, 2000.

Thesis

ENGLISH ABSTRACT: Ferdinand de Lesseps grapes have a distinctive fruity varietal character and were often used judiciously by wine-makers to enhance the fruity bouquet of some white table wines. Ferdinand de Lesseps grape juice was investigated to identify the main contributing compounds responsible for its unique and intense aroma. The juice was recovered from grapes under anaerobic conditions and extracted using Freon 11. The concentrated extract was analysed using a combination of capillary gas chromatography-mass spectrometry and gas chromatography-sniffing techniques. Thirty-two compounds were reported. With the aid of GC-sniffing, it was concluded that the hybrid note of the Ferdinand de Lesseps grape was most likely attributed to the presence of 2,5-dimethyl-3(2H)-furanone and 0- aminoacetophenone. Esters, which featured prominently in the juice, consisted mainly of ethyl butanoate, ethyl- and methyl- 3-hydroxy butanoate, and to a lesser extent, ethyl 3- hydroxy hexanoate and ethyl-3-hydroxy propanoate. These esters are most likely responsible for the sweetish pineapple aroma. According to EEC regulations, the use of non- Vinifera grapes in the production of commercial wines is prohibited. The presence of 2,5-dimethyl- 3(2H)-furanone in wine could therefore be interpreted as a labrusca indicator should it be suspected that Ferdinand de Lesseps grapes were used in the blend. The quality of some South African Kerner table wines often is rated inferior owing to an unwanted odour described as "Elastoplast" or "medicinal". This odour is encountered occasionally in wines from other cultivars such as Gewurztraminer, Weisser Riesling, Muscat de Frontignan and Chenin blanc. The identification of the compounds responsible for the offodour was investigated, as well as possible relationships between its occurrence and viniviticultural practices. The compound responsible for the "medicinal" off- odour was identified as p-vinyl guaiacol. It is formed via decarboxylation of ferulic acid by yeast during alcoholic fermentation. lts concentration is directly related to the concentration of ferulic acid and the yeast strain. Other factors affecting p-vinyl guaiacol formation in Kerner wines are region, microclimate, exposure of grapes to sunlight and grape maturity. Wines made from grapes harvested from the warmer climatic regions and exposed to direct sunlight and increased maturity contained higher levels of p-vinyl guaiacol. Viticultural practices that can be applied to limit the formation of the p-vinyl guaiacol precursor in grapes are the use of a canopy manipulation to protect the grapes from direct sunlight and an earlier harvesting date. Various winemaking techniques can be used to lower p-vinyl guaiacol levels in wine. Careful consideration must be given to the choice of yeast, as different yeast strains differ considerably with their ability to form p-vinyl guaiacol. Fining oxidised grape juice with phenol adsorbing agents such as activated charcoal, polyvinylpolypirrollidone, casein, gelatin (particularly in combination with bentonite and "kieselsol") led to decreases in the p-vinyl guaiacol levels. Although fining with activated charcoal was the most effective must treatment for reducing the "medicinal" aroma of Kerner wines, it stripped the wines of colour and character. Excessive oxidation of must followed by fining involves extra costs and time for the winemaker and would not be used in making the reductive style of wines.

AFRIKAANSE OPSOMMING: Ferdinand de Lesseps druiwe en wyne vertoon unieke vrugtige varieteitskarakter en is dikwels deur wynmakers in versnitte gebruik om die geur van sekere wit tafelwyne te verbeter. In hierdie studie is Ferdinand de Lesseps druiwe ondersoek om die belangrikste komponente wat verantwoordelik is vir die prominente, unieke aroma daarvan te identifiseer. Die sap is onder anaerobe toestande herwin en met Freon 11 ekstraheer. Die gekonsentreerde ekstrakte is met behulp van 'n kombinasie van kapillêre gaschromatografie-massaspektrometrie en gaschromatografie-snuif tegnieke analiseer. Twee-en-dertig komponente is gerapporteer. Met die behulp van GC-snuiftegnieke is bevestig dat die hibried karakter in Ferdinand de Lesseps druiwe hoofsaaklik aan die teenwoordigheid van o-amino-asetofenoon en 2,5-dimetiel-3(2H)- furanoon toegeskyf kan word. Die vernaamste esters in die sap was etielbutanoaat, etiel- en metiel-3-hidroksibutanoaat en in 'n mindere mate etiel-3-hidroksiheksanoaat en etiel-3- hidroksipropanoaat. Dié esters is waarskynlik verantwoordelik vir die soet pynappel aroma. Volgens EEG regulasies word die gebruik van nie- Vinifera druiwe vir die produksie van komersiéle wyne verbied. Die teenwoordigheid van 2,5-dimetiel-3(2H)-furanoon in wyn kan dus interpreteer word as 'n labrusca indikator in gevalle waar gebruik van Ferdinand de Lesseps druiwe vermoed word. Die kwaliteit van sommige Suid-Afrikaanse Kerner wyne word dikwels verlaag vanweë die teenwoordigheid van 'n geur wat beskryf word as "Elastoplast" of "medisinaal". Dié reuk word soms waargeneem in wyne van ander kutivars soos Gewurztraminer, Weisser Riesling, Muskaat de Frontignan en Chenin blanc. Die identiteit van die komponent wat vir dié ongewenste reuk verantwoordelik is, asook moontlike verwantskappe met wingerd- en wynkundige praktyke is ondersoek. Die komponent wat vir die medisinale karakter verantwoordelik is, is geïdentifiseer as para-vinielguajakol. Dit word deur dekarboksilasie van feruliensuur deur gisselle tydens alkoholiese gisting gevorm. Die konsentrasie van paravinielguajacol in wyn is direk verwant aan die konsentrasie feruliensuur en die gisras. Ander faktore wat para-vinielguajacol-vorming in Kerner wyn affekteer is streek, mikroklimaat, blootstelling van druiwe aan direkte sonlig en die rypheidsgraad van die druiwe. Wyne wat berei is van ryper druiwe uit warmer streke en wat direk aan sonlig blootgestel was, het hoër konsentrasies para-vinielguajacol bevat. Lowerbestuurspraktyke met behulp waarvan druiwe teen direkte sonligblootstelling beskerm word, is van die vernaamste wingerdkundige praktyke wat aangewend kan word om die vorming van die voorlopers van paravinielguajacol in druiwe te verlaag. Verskeie wynbereidingstegnieke kan gebruik word om die para-vinielguajacol vlakke in wyn te verlaag. Gisraskeuse is van besondere belang omdat gisrasse grootliks verskil in hul vermoë om para-vinielguajacol te vorm. Breibehandelings van geoksideerde sap met middels soos geaktiveerde koolstof, PVPP, kasseïen, gelatien (veral in kombinasie met bentoniet en "kieselsol") het die vlakke van para-vinielguajacol in wyne verlaag. Hoewel geaktiveerde koolstof die doeltreffendste breimiddel vir die verlaging van para-vinielguajacol was, het dit te veel kleur en geur uit die wyn verwyder. Oormatige oksidasie van mos gevolg deur breibehandelings sal meer geld en tyd verg en word nie aanbeveel vir wyne wat in 'n reduktiewe styl berei word nie.

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