Guidelines to reduce smoke taint in grapes and wine

By: Karien O’Kennedy - 13 Apr 2016

Reviewed by Marianne McKay

Harvesting grapes from vines that have been exposed to bushfire smoke can result in wines containing smoke taint, commonly described as having “smoky, burnt and ash” aromas and “cigarette, ash tray, acrid and metallic” flavours. It is believed that smoke particles and associated aroma compounds enter the vine by absorbing onto the protective waxy cuticle layer or moving into the stomata on leaves and thereafter finding their way through the phloem into grapes.

The main source of the smoke taint can be attributed to the various volatile phenols, with guaiacol and 4-methylguaiacol (4MG) being the most predominant compounds. These volatile phenols can be found in grape juice and wine in free and glycosylated (linked to sugars) forms. Only the free form is odorous. Guaiacol and 4MG are also extracted into wine during oak maturation (from oak toasting) at levels that contribute positively to the aroma of the wine. Guaiacol can also naturally be found in non-smoke exposed, non-oaked wines through hydrolysis of fruit-derived precursors. Research conducted over the past few years (mostly in Australia) has led to the development of guidelines on how to handle smoke tainted fruit. Here is an adapted version of the proposed guidelines:

  1. A high pressure cold water wash in the vineyard can be effective to remove ash but only after canopy leaf plucking. Without leaf plucking it can make matters worse. Rinsing of the grapes once harvested might be more practical. It will not affect the volatiles already in the grapes but it will help to remove excess ash and smoke particles that contribute to the “ashtray taste”. The grapes need to be allowed to drain and dry off afterwards.
  2. Hand harvest the grapes – minimise the breaking / rupturing of skins as long as possible. It has been found that the volatile phenols responsible for the smoke taint are more concentrated in grape skins.
  3. Remove leaves and other MOG – it has been shown that leaves contain higher volatile phenol concentrations than grapes.
  4. Process grapes cool – it has been shown that grapes processed at 10°C had less extraction than grapes processed at 25°C.
  5. Limit skin contact – in the case of white grapes do whole bunch press and keep press fractions separate from free run juice. Only add enzymes after pressing to free run juice to improve settling. Use “purified from glycosidases” enzymes. A large proportion of the volatile phenols occur in a non-odorous glycosylated form that can be hydrolysed by glycosidases naturally occurring in non-purified white wine processing enzymes.
  6. In the case of red wine fermentation, limit juice and skin maceration time. Do not do cold soaking and do not do extended skin contact after fermentation. Do not use enzymes to improve extraction. Extraction enzymes, apart from extracting more volatile phenols from skins, can also contain glycosidase side-activities, if un-purified.
  7. Pre-fermentation fining with activated carbon can effectively remove a large percentage of the volatile phenols. This is however only a solution for white juice since most activated carbons remove colour as well. Combine with bentonite to assist with flocculation and removal of the carbon from the juice. Specially developed activated carbon for the removal of smoke taint and not colour was developed in Australia, but at this stage cannot be imported into South Africa as it is seen as a “hazardous chemical” by shipping authorities.
  8. Use aroma enhancing (for its masking effect), fast fermenting yeast strains to limit skin contact in the case of red wine maceration. Avoid using yeasts with glycosidase activity and avoid using colour absorbing yeasts. Maintain high yeast viability with proper nutrition and temperature management – dead yeasts absorb colour.
  9. Use a malolactic starter culture that does not have glycosidase activities as this too can release glycosidically bound smoke-derived volatiles in wines. Do not do spontaneous malolactic-fermentation (MLF) as most naturally occurring bacteria do contain glycosidase activities.
  10. Consider the addition of oak alternatives or fermentation/finishing tannins. The addition of oak and tannin actually increases smoky volatile phenols, but the contribution of oak lactones, vanillin and hydroxyl-methyl-furfural increases complexity and thereby mask the smoky taints.
  11. Reverse osmosis and solid phase adsorption of wine – this technique has been proven to be effective in removing a large percentage of smoke-derived volatile phenols. However it also removes oak-derived volatile phenols and diminishes fruity aromas.
  12. Do not blend smoke tainted wines with non-smoke tainted wines, since fruity aromas associated with young wines diminish over time and thus their masking effect of negative aromas. Non-odorous glycosylated volatile phenols can also be hydrolysed over time via chemical hydrolysis, releasing the odorous volatile phenols. This means a blend that seemed fine initially could become problematic overtime.
  13. Market the wine for early consumption for the same reasons mentioned above. Wines that were acceptable for consumers at bottling could become non-acceptable six or 12 months down the line.

References:

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