Sensory
In the literature, various descriptors have been applied to smoke tainted grapes and wine. These include smoky, burnt, earthy, burnt toast, smoked salmon, tobacco, beetroot, drying, ash, cigar box, truffle, charcoal, charred, cold ash, fungal, tar, ashtray, bacon, roast meat, leather, salami, coffee, chocolate and gamey. In general, they are regarded as negative characteristics but some characteristics come through in low concentrations from toasted barrels and are regarded as providing some complexity to the finished wine.
Grapes
The smoke taint characters are less easily detected through sensory analysis of grapes. The high concentration of sugars may mask many of the compounds and the concentrations in the berry are generally lower than in the resultant wine. This indicates that taint compounds may be present in conjugated forms within the grape.
Wine
The sensory components that have been ascribed to smoke tainted wines (see above) are primarily aroma and retronasal sensed characters. The taste characters are described as bitter, astringent and drying.
Thresholds
Guaiacol
The threshold concentrations of compounds at which the aroma or taste can be perceived is of great interest. If a compound has a low aroma/taste threshold then it only requires a small amount of that compound to be present for it to be detected organolepticly.
The aroma and taste threshold for guaiacol has previously been evaluated in wines. Simpson et al. (1986) determined the aroma threshold in a neutral dry white wine to be 20 µg/L. Other estimates are 21 µg/L in water (Wasserman 1966) and 30 µg/L in water:alcohol (Riboulet 1982 – cited in Simpson et al. 1986). Boidron et al. (1988) obtained aroma thresholds of 5.5 µg/L in water, 20 µg/L in a model wine, 95 µg/L in a Ugni blanc wine and 75 µg/L in a Merlot red wine. These odour thresholds are higher than that determined by the AWRI in white wine but some are similar to that determined in red wine (6 µg/L in white juice and between 15 and 25 µg/L in red wine).
Wine matured in oak typically contains between 10 and 100 µg/L of guaiacol. However, guaiacol above 80 µg/L in wine has been found to have a negative effect on wine aroma (cited in Pollnitz et al. 2004). Guth (1997) used a water+alcohol (90+10 %v/v) mix to assess the threshold of guaiacol where samples were taken into the mouth and odour assessed retronasally. The threshold was determined as 10 µg/L.
The taste threshold for guaiacol in water has been determined as 13 µg/L (Wasserman 1966). However, 90% of the panel reported a loss of taste when their noses were pinched closed and the author concluded that there was a significant retronasal component of ‘taste’. Guaiacol is a recognised taint of apple and other juices, being produced by bacterial contamination (e.g. Alicyclobacillus acidoterrestris). Sensory studies have determined the aroma and taste thresholds of guaiacol in water and apple juice (Table 1). Pre-trained panels can detect a lower threshold than untrained panels and juice results appear much lower than for those observed in wine previously.
The detection threshold for guaiacol varies markedly between panellists, up to 80-fold with aroma and 470-fold with taste. A similar wide range of detection thresholds for smoke taint aroma of up to 245-fold were demonstrated in the WA project wine tastings (Kennison pers. comm. 2007).
Such broad ranges of detection thresholds need to be considered when tasting panels are developed.
Table 1. Thresholds for guaiacol determined in apple juice, and water.
1 Panel pre-trained to recognise guaiacol 2 Cited in Eisele and Semon (2005) 3 Coefficient of Variation
4-methylguaiacol
The aroma profile of 4-methylguaiacol has been described as burning wood, smoky, leather, spicy and ash. Boidron et al. (1988) reported a threshold of 10 µg/L in water, 30 µg/L in a model wine and 65 µg/L in both Ugni blanc and Merlot wines. In wines matured in oak, the concentration of 4- methylguaiacol ranged from 1 to 20 µg/L (Pollnitz et al. 2004).
Wasserman (1966) also determined aroma and taste thresholds of 4- methylguaiacol in water being 90 µg/L and 65 µg/L respectively. Wittkowski et al. (1992) showed that the perceived aroma of 4-methylguaiacol in water changed with concentration, exhibiting musty aromas at 10 µg/L and smoky and caramel like aromas at 100 µg/L.
Other compounds
Other components from smoke affected grapes that may be transferred into wine have also been studied. These include:
• 4-ethylguaiacol - elicits odours described as smoky, clove, spice and bacon. The aroma threshold of this compound has been determined as 25 µg/L in water, 47 µg/L in a model wine, 70 µg/L in a white wine and 150 µg/L in red wine (Boidron et al. 1988). A threshold of 33 µg/L in a model wine is reported in Francis and Newton (2005).
• 4-ethylphenol – shows medicinal, phenolic, horse stable, horse urine, ink, band-aid and pungent descriptors, and a threshold of 130 µg/L in water, 440 µg/L in a model wine, 1,100 µg/L in white wine and 1,200 µg/L in red wine (Boidron et al. 1988).
• 2,6-dimethoxyphenol - in water it has an aroma threshold of 1,850 µg/L and a taste threshold of 1,650 µg/L (Wasserman 1966).
• Eugenol - has an aroma likeness with cloves. Thresholds of 7 µg/L in water, 15 µg/L in a model wine, 100 µg/L in white wine and 500 µg/L in red wine have been determined (Boidron et al. 1988). Thresholds of 5 and 6 µg/L in a model wine were reported in Francis and Newton (2005).
• Furfural - has been described as an almond aroma. Thresholds of 8,000 µg/L in water, 15,000 µg/L in a model wine , 65,000 µg/L in white wine and 20,000 µg/L in red wine have been determined (Boidron et al. 1988). The compounds 4-ethylguaiacol and 4-ethylphenol are commonly associated with the ‘band-aid’, ‘medicinal’, ‘wet dog’ type aromas produced by Brettanomyces/Dekkera spp. yeasts.
