Browsing by Author "McKay, Marianne"

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    Characterising sensory interactions between volatile phenols and other taint-causing compounds in South African red wines
    (Stellenbosch : Stellenbosch University, 2019-04) McKay, Marianne; Buica, Astrid; Bauer, Florian; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
    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.
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    Investigation of olfactory interactions of low levels of five off-flavour causing compounds in a red wine matrix
    (Elsevier, 2019) McKay, Marianne; Bauer, Florian F.; Panzeri, Valeria; Buica, Astrid
    The qualitative sensory perception of individual and of complex mixtures of five compounds, guaiacol (‘burnt note’), o-cresol (‘phenolic/tar’), 4-ethylphenol (4-EP, ‘leather/barnyard’), 2-iso-butyl-3-methoxypyrazine (IBMP, ‘green pepper/herbaceous’), and 2,4,6-trichloroanisole (TCA, ‘cork taint/ mouldy’) were tested in a partially de-aromatised red wine matrix using descriptive analysis by a trained panel of eleven judges. Compounds were characterised at peri- and sub-threshold concentrations using a partial D-optimal statistical design and response surface methodology. Results indicated that complex mixtures in red wine elicit an olfactory response that could not be predicted from the attributes or descriptors of single compounds. Positive sweet/fruity attributes were more intense in solutions containing fewer off-flavour compounds. Novel findings of this study include that IBMP at sub- and peri-threshold levels shows perceptual interaction with volatile phenols at the same levels, and samples containing combinations of these compounds manifested herbaceous and burnt characteristics. Olfactory interactions of this many off-flavour compounds have not been investigated previously in one study. The findings have direct implications for wines made from cultivars that are known to contain these compounds, and add to the understanding of the behaviour and impact of very low levels (peri- and sub-threshold) of volatile phenols, IBMP, and TCA derived from various sources during winemaking.
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    Testing the sensitivity of potential panelists for wine taint compounds using a simplified sensory strategy
    (MDPI, 2018-10-24) McKay, Marianne; Bauer, Florian; Panzeri, Valeria; Buica, Astrid
    The odor detection threshold (ODT) of a compound is the lowest concentration at which individuals can reliably perceive a difference between a sample and its corresponding control, with 50% performance above chance. Wine is a complex matrix, and ODTs used in studies on wine can be based on inappropriate matrices and informal sensory methodologies. Formal studies confirming ODTs in wine are relatively scarce in the literature, and are complex and expensive to carry out. In this study, the sensitivity of panelists to previously published ODTs for five compounds: Guaiacol, o-cresol and 4-ethyl phenol, 3-isobutyl-2-methoxypyrazine (IBMP), and 2,4,6-trichloroanisole (TCA) associated with off-flavor/taint issues in wine, was investigated. The study was carried out in partially de-aromatized young Shirazwine (unwooded) using a simplified version of the formal sensory approach. A triangle test in triplicate was carried out with 34 panelists, at the ODT for each compound, in one day. The study explored whether previous training affected panelists’ sensitivity for threshold differences. Results showed that samples spiked with volatile phenols were significantly different (p = 0.01) to controls. The spiked TCA and IBMP samples were not significantly different from the control in either case. Judges were better able to detect compounds if they had prior experience or training in wine evaluation. Despite some limitations, this pragmatic approach may be useful when carrying out sensory studieswith fairly limited resources andwithin tight timelines, as it provides helpful information on panel members and detection thresholds for a specific matrix.