## 3 Results and Discussion
### Odour Thresholds in Sparkling Wines  
An 'absolute' or odour detection threshold is the concentration range below which the odour or taste of a substance is not detectable and above which individuals with a normal sense of smell or taste can readily detect the presence of the substance [22; 30]. For populations, the detection threshold is defined as the concentration at which 50% of the group can detect the odorant [30]. The concentrations of ethyl isobutyrate, and ethyl isovalerate and ethyl hexanoate at 5, 10 and 15 weeks post-disgorging were above their respective odour thresholds (Tables 1 and 5). By 15 weeks, concentrations of 1-hexanol were below its odour threshold for all wines and ethyl-2-methylbutyrate only exceeded its odour threshold in PN. Ethyl octanoate was below its odour threshold in PN, OC and IW. By 15 weeks the 2-phenylethanol concentrations were above its sensory threshold except for the B and PN wines. Ethyl butanoate was above its odour threshold according to [PERSON] [11], which was calculated in 10% water/ethanol. Conversely, [PERSON] et al. [13] used a 10% water/ethanol solution with a pH of 3.2 which resulted in its odour threshold increasing from 20 ug/L to 125 ug/L. Therefore, as wine is an acidic medium, we found that ethyl butanoate was below its odour threshold (125 ug/L) at all stages of analysis.  
Another matrix effect within wine is referred to as an aroma buffer, and is caused by the numerous different VOCs found in wine. This aroma buffer effect describes how certain fuel alcohols, acids, esters and volatile phenols are not perceived separately within a wine [31]. This phenomenon is generally attributed to the presence of ethanol and a variety of fermentation-derived aromatic compounds. Since several compounds in alcoholic beverages are generally found coexisting as a result of fermentation, this causes the odour in question to be perceived as a mixture. Many aromatic compounds have similar odours and therefore, if one is lacking in concentration, the difference in the wine's profile would be negligible or undetectable by the average taster [31].  
There are many factors that affect the concentrations of VOCs in sparkling wines including production techniques, grape variety, fining agents, yeast, ambient secondary fermentation temperature, type of sugar used during production, on-lees collar storage temperature, _liquer d'explotition_ ingredients, oxygen ingress i.e., at disgorging, as well as closure type, SO\({}_{2}\) levels, carbon dioxide levels (CO\({}_{2}\)) and ethanol levels. The impact of CO\({}_{2}\) on the perception of sugar and acidity in sparkling wine was confirmed by [PERSON] [32]. Furthermore, increased CO\({}_{2}\) concentrations in sparkling wines resulted in increased intensity of mouthfeel attributes, with the attributes burn, bite, carbonation/bubble pain, and foamy showing the greatest differences in wines and after-numbing and tingly showing the least [33]. However, the impact of CO\({}_{2}\) and oxygen at disgorging on the concentrations of the VOCs studied is not known, yet both are likely to affect VOC concentrations as rising and collapsing bubbles act as continuous \"portals\" for volatile aromas in Champagne wine [34]. Additionally, carbonation has been reported to increase extremely volatile compounds i.e., ethyl butanoate, but not impact less volatile compounds, e.g., isoamyl acetate, due to a specific effect that is dependent on the physiochemical characteristics of each compound [35].