Atypical aging (ATA) of white wines

Atypical aging is an aroma defect that may occur in rather young white wines and is not necessarily related to aging.

The practical significance of ATA

• ATA is a white wine flavor defect widely spread in all wine growing countries.

• It is distinct from other kinds of white wine aging (oxidative aging, petrol flavor, post-bottling reduction flavor….) in both sensory and chemical terms.

• The attention paid to ATA by producers and consumers is variable and does not necessarily reflect the frequency of its real occurrence in a given wine-growing area.

• In some countries, it is not assumed to be a distinctive wine fault.

• By some wine critics, it is even considered as an intrinsic expression of terroir.

• By many enologists, it is considered the most intractable problem in white wine making in modern times.

How does ATA show up in sensory terms?

By smell:

Descriptors, group I: Descriptors, group II:

- mothballs - damp towel

- naphthalene - wet wool

- floor polish - dirty dish rag

- acacia blossom - washing machine

- lemon blossom - urine deposits

- soap

- detergent powder

- jasmine

- dry linen

On the palate:

- meager, thin, no volume

- metallic bitterness

- some astringency

- light color

In all wines affected by ATA, the fruity, floral, or mineral varietal aroma has disappeared to a large extent.

sensory transition to reduction flavor

(sensory confusion)

Viticultural causes of ATA

drought UV-radiation

soil nutrient deficiencies

over-cropping early harvest green cover

poor physiological ripeness(not necessarily correlated to Brix)

lack of positive aroma compounds

lack of O2 -radical scavengers

lack of yeast-assimilable N

variable contents of grape-derived indole-3-acetic acid

increased production of yeast-derived indole-3-acetic acid (from tryptophane)

alc. fermentation

Chemical pathway of ATA formation

2-aminoacetophenone (AAP) is the main impact compound of atypical aging:

Indole-3-acetic acid (IAA) is the precursor of AAP:

indole-3-acetic acid (IAA)

3-methyl-indole (skatole)

2-aminoacetophenone (AAP) SO2 + O2

O2-

O2 - -

HO2-

oxygen radicals

(odorless)

(mothballs, soap, detergent powder, acacia blossom, dry linen)

1. The conversion of IAA into AAP is initiated by oxygen radicals generated when SO2 is added the first time post fermentation. 2. It requires only minute amounts of dissolved oxygen (< 0.5 mg/L) and is not related to oxidative aging.

3. Red wines do not produce AAP because their total phenols (~ 2000 mg/L) scavenge oxygen radicals in contrast to white wines ….displaying only ~ 200 mg/L total phenols.

Viticultural means to prevent ATA(Central European cool-climate conditions)

Starting point:

- ATA is a viticultural problem resulting from a physiological stress in the vines.

- White wines produced from stressed fruit display higher concentrations of the odorless precursor (IAA) … and lower concentrations of oxygen radical scavengers.

Counter-measures in the vineyard:• Make sure water supply / irrigation around véraison (July/August)

• Reduce green cover

• Make sure enough organic matter in the soil (> 2.5 % humus)

• Adjust nitrogen supply to approximately 60 kg/ha N

• Reduce yield, avoid overcropping

• Allow for full physiological ripeness (transparent skins easily to separate from the pulp)

Enological means to prevent ATA

Means Purpose Efficiency

Skin contact (6-12 hours) Extraction of more grape phenols (radical scavengers). Extraction of more primary aroma compounds masking AAP.

moderate

Severe juice clarification (to less than 100 NTU residual turbidity)

Removal of juice solids binding a portion of the precursor IAA.

moderate

SO2 addition to grapes and juice Preventing juice oxidation none

Tannin additions (cannot achieve phenol level of red wines)

Addition of radical scavengers none

Addition of complex yeast nutrients Addition of radical scavengers none to very moderate

Cool wine storage Delay of all chemical reactions none to very moderate

Protection against oxygen uptake Preventing formation of oxygen radicals none

Ascorbic acid addition (15 g/hl) to wine Addition of a radical scavenger effective

1. Timely addition of ascorbic acid is the only reliable way to prevent ATA in wines prone to develop it.

2. Since the first addition of SO2 after fermentation initiates the reaction leading to ATA, ascorbic acid (15 g/hl) should be added simultaneously or a few days after. It stalls ATA formation when added at a later point of time.

3. The reaction leading to ATA is irreversible. Ascorbic acid nor any other treatment cannot remove ATA already developed.

Impact of aroma intensity of 38 white wines (Germany 2011 + 2012) upon the olfactory perception of AAP

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00.0

0.5

1.0

1.5

2.0

2.5

R² = 0.424541708862104

R² = 0.440701679400192

addition of AAP (µg/l) until detection thresholdLinear (addition of AAP (µg/l) until detection threshold)addition of AAP (µg/l) until rejection as quality wine

intensity fruit aromatics (0-5)

ad

diti

on

of A

AP

(µ

g/l

)

AAP detection threshold is around 0.5 µg/L, AAP rejection threshold is 1.0 µg/L approximately, depending on the wine matrix. The stronger the wine aroma, the less perceptible is AAP due to masking effects.

Testing a wine for ATA potential(accelerated aging test)

Take a wine sample (200 mL min.)

Add SO2 if necessary (~ 70 mg/L)

Clarify by centrifugation or filtration

Store both flasks at 50-60° C (incubator) during 3-4 days.

Cool down to ambient temperature.

Identify ATA by olfactory comparison of both flasks.

If the sample without ascorbic acid displays ATA (or if the sample with ascorbic acid smells better), the wine is prone to produce ATA and should be treated immediately.

Sample I:100 ml-flask with screwcap,with 15 g/hl ascorbic acid

Sample II:100 ml-flask with screwcap,without ascorbic acid

Special requirements for wines supplemented with ascorbic acid

Problems caused by the presence of ascorbic acid Solution

Ascorbic acid interferes with SO2 measurements by iodometric titration, enhancing SO2 readings.

Use more specific methods for measuring free and total SO2.

Ascorbic acid is spontaneously oxidized by dissolved oxygen.Avoid uncontrolled oxygen uptake: - Carefully top storage containers and keep them without

liquid surface in contact with air; - do not use wooden barrels for storage; - do not use pumping over for removal of reduction flavor;- do not add ascorbic acid prior to fermentation

Ascorbic acid increases the tendency to produce reduction flavor if a wine is prone to it, especially after bottling with screw caps.

Use copper sulfate to remove reduction flavor.Add copper sulfate (0.1 g/hl) before bottling to prevent the appearance of post-bottling reduction flavor.

Ascorbic acid decreases copper solubility / stability. Do not bottle with more than 0.3 mg/L Cu+ (0.1 g/hl CuSO4), check copper content before bottling (AAS).

Conclusion:

It is not a shame to have a wine affected by ATA, but it‘s disgraceful to have it year after year without taking counter-measures.

For further informations:Volker Schneider

Email: Schneider.Oenologie@gmail.comwww.Schneider-Oenologie.com