Characterising sensory interactions between volatile phenols and other taint-causing compounds in South African red wines

McKay, Marianne (2019-04)

Thesis (PhDAgric)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: South African red wine competes for limited market space, and must maintain consistent, excellent quality. One of the important modalities in assessing the quality and typicality of wine is odour perception (Hopfer et al., 2015). Most investigations quantify aroma compounds by chemical/analytical means and compare levels to odour detection thresholds (ODTs) provided by scientific literature. If malodourous compounds are present at concentrations above their ODTs, they may be considered a threat to wine quality as they exhibit odour activity values >1 (Louw et al., 2010; Prida & Chatonnet, 2010). A review of the literature reveals that studies in wine can use inappropriate ODTs for work carried out in a new wine matrix, and matrix effects on the activity and perception of a compound are often ignored. To properly scientifically evaluate the effect of any compound to wine aroma, formal sensory evaluation in the study matrix is essential (Villamor & Ross, 2013; Perry & Hayes, 2016). Certain volatile phenols (VPs) can contribute to a continuum of smoke-taint related off-flavours including ‘burnt’, ‘bretty’, ‘smoky’, and ‘ashy’ attributes in wine (Jiranek, 2011; Kennison et al., 2011) at higher levels but are generally accepted as being benign to wine aroma at subthreshold levels (Boidron et al., 1988; Prida & Chatonnet, 2010). Useful sensory studies on VPs in wine have been carried out (Simpson et al., 1986; Boidron et al., 1988; Chatonnet et al., 1992; Kennison et al., 2008; Petrozziello et al., 2014), but at supra-threshold levels, and the effect of combinations of subthreshold levels of VPs is not taken into account. The main aims of this research were to characterise the sensory contribution of specific VPs at low levels to red wine odour, and to assess their effect on the perception of attributes through interactions with themselves and other compounds known to be involved in specific wine offflavours. Chapter 2 addresses key knowledge and gaps within the literature around origin, perception and odour detection threshold of VPs associated with specific off-flavours (including ‘smoke’ and ‘ashiness’), and previous studies concerning interaction of aroma compounds which may have relevance to the current study. Chapter 3 investigated trends within the chemical (GC-MS) and sensory (Descriptive Analysis) results for twelve commercial samples of smoke-affected wine. Associations between negative attributes and bushfire events prior to harvest were found. Results also showed that certain sensory effects could have resulted from combinations of subthreshold levels of VPs. The need arose to formally test perceptual interactions at subthreshold levels to see if various effects could be explained. It was thus decided to investigate effects of three VPs: guaiacol, ortho-cresol, 4-ethylphenol, and two compounds associated with certain off-flavours in wine, viz. 3- isobutyl-2- methoxypyrazine (IBMP) and 2,4,6-trichloroanisole (TCA). The work was carried out in partially de-aromatised Shiraz, and it was necessary to establish if ODTs provided in the literature were appropriate for this matrix. Formal sensory work is known to be complex, time-consuming and expensive, particularly in establishing ODTs, and thus a pragmatic sensory approach to the work is outlined in Chapter 4. Chapters 5 and 6 address the perceptual effects of combinations of two, three, four and five offflavour compounds on red wine aroma, which has not been conducted on this scale before. The results of this DA sensory study showed olfactory opposition between clean controls, wines spiked with single compounds (generally fruity and sweet-associated), and wines spiked with complex combinations of VPs and IBMP (linked to negative attributes). Chapter 7 demonstrates another pragmatic approach, using projective mapping (PM) with a large sample size (n=18). Comparable results to the DA interaction study for four compounds in red wine were shown. Chapter 8 investigates effects of combinations of two VPs on four cultivars in order to establish whether there were perceptual olfactory effects that were common to all cultivars, or whether the matrices responded differently from an aroma perception perspective, and shows that samples spiked with combinations of VPs and IBMP show consistently negative olfactory attributes that are independent of cultivar. This research contributes to the sensorial and chemical characterization of selected VPs in red wines, and shows that subthreshold levels of VPs in combination with very low levels of IBMP and TCA can lead to olfactory interactions that cause various olfactory effects, some of them negative. This may help inform winemaking decisions, particularly when dealing with smokeaffected grapes, and/or cultivars that naturally have higher levels of methoxypyrazines, like Merlot and Cabernet Sauvignon. This study also emphasises the importance of understanding effects of VPs on wine aroma, and escalating awareness and sensitivity to these issues in the wine industry.

AFRIKAANSE OPSOMMING: Suid-Afrikaanse rooiwyn kompeteer vir beperkte markruimte en moet uitstekende gehalte op ’n konsekwente basis onderhou. Een van die belangrikste modaliteite wanneer die gehalte en tipiese aard van wyn geassesseer word, is reukwaarneming (Hopfer et al., 2015). Die meeste ondersoeke kwantifiseer reukverbindings chemies/ analities en vergelyk vlakke met reukwaarnemingsdrempels (odour detection thresholds (ODT’s)) wat in die wetenskaplike literatuur verskaf word. Indien onwelriekende verbindings in konsentrasies hoër as hulle ODT’s teenwoordig is, kan hulle as ’n bedreiging vir wyngehalte beskou word as hulle reukaktiwiteitswaardes (odour activity values) van > 1 vertoon (Louw et al., 2010; Prida & Chatonnet, 2010). ’n Literatuuroorsig toon dat studies van wyn ongepaste ODT’s kan gebruik vir werk wat in ’n nuwe wynmatriks uitgevoer word, en in baie gevalle word matriks-effekte op die aktiwiteit en waarneming van ’n verbinding geïgnoreer. Om die effek van enige verbinding op wynaroma behoorlik wetenskaplik te evalueer, is formele sensoriese evaluering in die studiematriks noodsaaklik (Villamor & Ross, 2013; Perry & Hayes, 2016). Sekere vlugtige fenole (VF’e) kan bydra tot ’n kontinuum van rooksmaak-verwante wangeure, insluitend ‘gebrande’, ‘brett-agtige’, ‘rokerige’ en ‘asagtige’ kenmerke, in wyn (Jiranek, 2011; Kennison et al., 2011) teen hoër vlakke, maar wat oor die algemeen aanvaar word as onskadelik vir wynaroma teen vlakke onder die drempel (Boidron et al., 1988; Prida & Chatonnet, 2010). Nuttige sensoriese studies oor VF’e in wyn is onderneem (Simpson et al., 1986; Boidron et al., 1988; Chatonnet et al., 1992; Kennison et al., 2008; Petrozziello et al., 2014), maar teen vlakke effe hoër as die drempel, en die effek van kombinasies van hierdie VF’e onder die drempel is nie in ag geneem nie. Die vernaamste doelwitte van hierdie navorsing was om die sensoriese bydrae van spesifieke VF’e teen lae vlakke tot die reuk van rooiwyn te karakteriseer, en om hulle effek op die persepsie van eienskappe afkomstig van interaksies met mekaar en met ander verbindings wat in spesifieke wyn-wangeure betrokke is, te assesseer. Hoofstuk 2 ondersoek belangrike kennis en gapings in die literatuur oor oorsprong, waarneming en die reukwaarnemingsdrempels van VF’e wat gekoppel word aan spesifieke wangeure (insluitend ‘rook’ en asserigheid’), en vorige studies oor die interaksie van aromaverbindings wat moontlik relevant is vir die huidige studie. Hoofstuk 3 ondersoek tendense in die chemiese (GC-MS) en sensoriese (beskrywende analise, BA) resultate vir twaalf rookgeaffekteerde kommersiële wynmonsters. Verbintenisse is gevind tussen negatiewe kenmerke en bosbrande voor oes. Die resultate toon ook dat sekere sensoriese effekte kon ontstaan het uit kombinasies van sub-drempel vlakke van VF’e. Die behoefte het ontstaan om perseptuele interaksies teen sub-drempel vlakke formeel te toets om te sien of die verskillende effekte verklaar kon word. Daar is dus besluit om die effekte van drie VF’e te ondersoek: guajakol, orto-kresol, 4-etielfenol, en twee verbindings gekoppel aan sekere wangeure in wyn, nl. 3-isobutiel-2-metoksipirasien (IBMP) en 2,4,6-trichloro-anisool (TCA). Die werk is uitgevoer op Shiraz waarvan die aroma gedeeltelik verwyder is, en daar moes eers bepaal word of die ODT’s wat in die literatuur verskaf word, toepaslik was vir hierdie matriks. Formele sensoriese werk is kompleks, tydrowend en duur, veral wanneer ODT’s bepaal word, en ’n pragmatiese sensoriese benadering tot die werk word dus in Hoofstuk 4 uitgestippel. Hoofstukke 5 en 6 kyk na die perseptuele effekte van kombinasies van twee, drie, vier en vyf wangeurige verbindings op rooiwyn-aroma, wat nie tevore op so ’n groot skaal uitgevoer is nie. Die resultate van hierdie BA- sensoriese studie toon reukverwante opposisie tussen skoon kontroles, wyn waarby een verbinding gevoeg is (gewoonlik vrugtig en soet assosiasies), en wyn waarby komplekse kombinasies van VF’e en IBMP (wat verband hou met negatiewe kenmerke) gevoeg is. Hoofstuk 7 demonstreer nóg ’n pragmatiese benadering, waarvoor projective mapping (PM) met ’n groot monster (n = 18) gebruik is. Resultate wat vergelykbaar is met die BA-interaksie studie is vir vier verbindings in rooiwyn getoon. Hoofstuk 8 ondersoek die effekte van kombinasies van twee VF’e op vier kultivars om te bepaal of daar perseptuele reukeffekte was wat algemeen is aan alle kultivars en of die matrikse verskillend gereageer het vanuit ’n aromawaarnnemingsperspektief. Daar is gevind dat monsters waarby kombinasies van VF’e en IBMP gevoeg is, konsekwent negatiewe reukkenmerke getoon het wat onafhanklik was van kultivar. Hierdie navorsing dra by tot die sensoriese en chemiese karakterisering van die gekose VF’e in rooiwyn en wys dat VF-vlakke onder die drempel, in kombinasie met baie lae vlakke van IBMP en TCA, kan lei tot reuk-interaksies wat ’n verskeidenheid reuk-effekte kan veroorsaak, waaronder negatiewe effekte. Dit kan help om wynbereidingsbesluite in te lig, veral wanneer daar gewerk word met druiwe wat deur rook geaffekteer is en/of met kultivars wat natuurlik hoër vlakke van metoksipirasiene het, soos Merlot en Cabernet Sauvignon. Hierdie studie beklemtoon ook die belangrikheid van ’n begrip van die effekte van VF’e op wynaroma, en van ’n verhoging van bewussyn van en sensitiwiteit vir hierdie kwessies in die wynbedryf.

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