Chemical evaluation and sensory relevance of thiols in South African Chenin Blanc wines

Wilson, Christine Leigh (2017-03)

Thesis (MScAgric)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: South African Chenin Blanc is gaining recognition for its high quality both domestically and abroad. As the most widely-planted cultivar in the country, there is interest in research which can provide additional knowledge to producers and further increase Chenin Blanc wine quality. One of the sensory modalities contributing to wine quality is wine aroma, which is studied through sensory analysis and the chemical quantification of volatile compounds. Commercially-available South African Chenin Blanc wines had been characterized previously for a variety of chemical compounds, but not for thiols. Thiols, including 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA), are volatile sulphur compounds which are important to the ‘tropical’ and ‘green’ aromas of many wines, especially Sauvignon Blanc. The main aims of this research were to chemically characterize 3MH and 3MHA levels in a variety of commercially-available dry South African Chenin Blanc wines and explore the sensory contribution of these compounds to Chenin Blanc wine aroma. Chapter 3 reported the chemical analysis results of 3MH and 3MHA in South African Chenin Blanc Wines and explored trends within the chemical results. Chapters 4, 5, and 6 addressed the sensory relevance of thiols to South African Chenin Blanc wines. In Chapter 3, both 3MH and 3MHA were quantified in South African Chenin Blanc wines at levels above their odour thresholds. The average levels found were 893 ng/L for 3MH and 23 ng/L for 3MHA, with ranges of 380-2929 ng/L for 3MH and 0-305 ng/L for 3MHA. Significant differences were found for 3MHA levels by wine age, vine age, wood contact, price, and lees contact were found, while 3MH only differed significantly for wine origin. In Chapters 4 and 5, the sensory contribution of thiols was analysed through interaction studies. In Chapter 4, interactions of a thiol (3MH), an ester (ethyl hexanoate), and a terpene (linalool) in partially-dearomatized Chenin Blanc wine were analysed by descriptive analysis. Interaction effects were identified, such as the antagonism between the ‘tropical’ attributes of 3MH and the ‘floral’ character of linalool. The second interaction experiment, reported in Chapter 5, analysed combinations of 3MH and 3MHA in different matrices by projective mapping (PM) with intensity. This study showed that the perception of thiols was affected by the volatile and non-volatile wine matrix. The addition of an intensity measure to the ultra flash profiling step of the method provided more detailed data, which made the rapid sensory method better suited to interaction studies. In all sensory studies, wines with high thiols, especially high 3MHA, were described with ‘tropical’ and ‘green’ terms In Chapter 6, polarized projective mapping (PPM) was used to characterize commercial South African Chenin Blanc wine aroma, and sensory results were compared with extensive volatile chemical analyses. Results showed a sensorial and chemical opposition between wooded and unwooded wines. The levels of 3MHA in the wines correlated with the unwooded wines and thiol-related descriptors. PPM was applied for the first time to wine, validating a method which increases the maximum sample size of wines in rapid sensory analysis. The results of this research made contributions to the sensorial and chemical characterization of South African Chenin Blanc wines, as well as the validation of PPM and PM with intensity in wine. The knowledge that thiols are present in Chenin Blanc wines, together with existing research on practices affecting thiols can help inform viticultural and oenological decisions in the future of Chenin Blanc winemaking.

AFRIKAANSE OPSOMMING: Suid-Afrikaanse Chenin Blanc begin toenemende erkenning geniet as hoë gehalte wyne plaaslik sowel as in die buiteland. As die mees aangeplante kultivar in Suid-Afrika is daar ‘n behoefte aan navorsing wat addisionele kennis aan verbouers kan verskaf om die kwaliteit van Chenin blanc wyn te bevorder. Een van die sensoriese modaliteite wat bydrae tot wynkwaliteit is wynaroma. Wynaroma kan bestudeer word met behulp van sensoriese analise en chemiese kwantifisering van vlugtige verbindings. Kommersieel beskikbare Suid-Afrikaanse Chenin blanc wyne is voorheen gekarakteriseer in terme van ʼn verskeidenheid chemiese verbindings. Hierdie analises het egter nie tiole ingesluit nie. Tiole, insluitende 3-merkaptoheksan-1-ol (3MH) en 3-merkaptoheksielasetaat (3MHA) is vlugtige swaelverbindings wat ‘n belangrike rol speel in terme van ‘tropiese’ en ‘groen’ aromas van verskeie wyne, veral Sauvignon Blanc. Die hoofdoelwitte van hierdie navorsing was om die vlakke van 3MH en 3MHA chemies te bepaal vir ‘n verskeidenheid kommersieël-beskikbare droë Suid-Afrikaanse Chenin Blanc wyne asook die verkenning van die sensoriese bydrae wat hierdie verbindings tot Chenin blanc aroma maak. Hoofstuk 3 rapporteer die chemiese analise resultate van 3MH en 3MHA in Suid-Afrikaanse Chenin blanc wyne en verken die tendense daarvan. Hoofstukke 4, 5 en 6 bespreek die sensoriese relevansie van tiole in Suid-Afrikaanse Chenin blanc wyn. In Hoofstuk 3 word resultate gewys waar beide 3MH en 3MHA gekwantifiseer is bo hul aroma opsporingsdrumpels. Die vlakke wat gevind is, was 380-2929 ng/L, met ‘n gemiddeld van 893 ng/L, vir 3MH en 0-305 ng/L, met ‘n gemiddeld van 23 ng/L, vir 3MHA. Beduidende verskille is gevind vir 3MHA vlakke met betrekking tot die ouderdom van die wyn, houtbehandeling, prys, en gismoerkontak terwyl 3MH vlakke slegs beduidend verskil het met betrekking tot die oorsprong van die wyn (‘wine of origin’). In Hoofstukke 4 en 5 is die sensoriese impak van tiole ondersoek met behulp van interaksie studies. In Hoofstuk 4 is die interaksie van ‘n tiol (3MH), ‘n ester (etielheksanoaat) en ‘n terpeen (linaloöl) in Chenin Blanc wyn wat gedeeltelik ontgeur is met behulp van beskrywende sensoriese analise geanaliseer. Interaksie effekte is geïdentifiseer soos antagonisme tussen ‘tropiese’ eienskappe van 3MH en die ‘blomagtige’ karakter van linaloöl. Die tweede interaksie eksperiment, bespreek in Hoofstuk 5, is uitgevoer om kombinasies van 3MH en 3MHA in verskillende matrikse met behulp van projeksiekartering met intensiteit te analiseer. Hierdie studie het gewys dat die persepsie van tiole geaffekteer word deur die vlugtige en nie-vlugtige wynmatriks komponente. Die toevoeging van ‘n intensiteitsmeting tot die beskrywende stap van projeksiekartering het aanleiding gegee tot meer detail in die datastel, wat die vinnige sensoriese evalueringsmetode beter aangepas het vir interaksiestudies. Tydens al die sensoriese eksperimente is wyne met hoër tiole, veral hoë 3MHA, beskryf as ‘tropiese’ en ‘groen’. In Hoofstuk 6 is gepolariseerde projeksiekartering gebruik om kommersiële Suid-Afrikaanse Chenin Blanc wyne se aroma te karakteriseer. Sensoriese resultate is vergelyk met uitgebreide chemiese analise van ‘n wye verskeidenheid van vlugtige komponente in wyn. Resultate het ‘n sensoriese en chemiese opposisie tussen gehoute en ongehoute wyne uitgewys. Die vlakke van 3MHA in die wyne het met ongehoute wyne en tiool-verwante beskrywende sensoriese terme gekorreleerd. Gepolariseerde projeksiekartering is vir die eerste keer gebruik om die sensoriese eienskappe van wyne te beskryf, dus is ‘n metode gevalideer waar ‘n groter aantal wyne tydens ‘n vinnige sensoriese evalueringsmetode as te vore geëvalueer kan word. Die resultate van hierdie studie het bydraes gelewer tot die sensoriese en chemiese karakterisering van Suid-Afrikaanse Chenin Blanc wyne, sowel as die validasie van gepolariseerde projeksiekartering en projeksiekartering met intensiteit vir die sensoriese evaluering van wyn. Die kennis tiole teenwoordig is in Chenin Blanc wyne te same met die bestaande navorsing oor praktyke wat die vlakke van tiole in wyne beïnvloed, kan help om wingerd- sowel as wynkundige besluite in toekomstige Chenin blanc wynbereiding te rig.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101250
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