Diagnostic test for ‘reductive’ wine characters (Cu/Cd test)

The following procedure (commonly referred to as the copper/cadmium [Cu/Cd] test) outlines a quick simple aroma only diagnostic test that can be used to determine what type of volatile sulfur compounds are causing a wine to have a ‘reductive’ wine fault or character. ‘Reductive’ wine characters can be caused by hydrogen sulfide, sulfhydryls (formerly known as mercaptans), disulfides or dimethyl sulfide, each of which have distinct aromas and different winemaking treatment options to remove them.

Hydrogen sulfide (H2S)
H2S has the aroma of ‘rotten egg gas’. During alcoholic fermentation, yeast will excrete H2S into the fermenting juice when placed under stress; for example, when the yeast starts to run out of nitrogen. Australian grape juices can be low in nitrogen and winemakers often supplement the juice with a soluble nitrogen source, such as diammonium phosphate (DAP) at the first sign of H2S development during fermentation. To calculate an appropriate DAP addition, access the AWRIs winemaking calculator. The AWRI strongly recommends that yeast assimilable nitrogen (YAN) be analysed before making decisions on DAP additions.

Unwanted sulfhydryls
Sulfhydryls are compounds that contains a -SH group. These compounds are also known as ‘thiols’ or ‘mercaptans’, with aromas variously described as ‘cabbage’, ‘garlic’, ‘onion’ and ‘rubber’. Both methanethiol (MeSH) and ethanethiol (EtSH) are formed directly as a result of yeast metabolism, and can also be produced in wine post-bottling by the breakdown of methyl thioacetate and ethyl thioacetate, respectively. The aroma of methanethiol (MeSH) is described as ‘rotten eggs’ and ‘cabbage’, and it has a sensory threshold of 0.02 – 2.0 µg/L. The aroma of ethanethiol (EtSH) is described as ‘onion-like’ and ‘rubber-like’. The sensory threshold for ethanethiol is 1.1 µg/L.

Disulfides
Sulfhydryls such as EtSH and MeSH can be rapidly oxidised to produce symmetrical or asymmetrical disulfides. Examples of symmetrical disulfides are diethyldisulfide (DEDS) and dimethyldisulfide (DMDS), respectively. The aroma of DMDS is described as ‘onions’ and ‘cooked cabbage’ and its sensory threshold is 29 µg/L. The aroma of DEDS is described as ‘burnt rubber’ and ‘garlic’ and its sensory threshold is 4.3 µg/L.

Dimethyl sulfide
Dimethyl sulfide (DMS) is one of the major compounds found in aged wines and is formed during maturation. It is produced from the breakdown of a sulfur-containing amino acid called S-methyl methionine. At low concentrations it might contribute to the body of aged white wines and has a ‘vegetable’, ‘truffle’, or ‘blackcurrant’ character. At higher concentrations, the aroma of DMS is perceived as a fault and is described as ‘asparagus’, ‘cooked corn’, ‘cooked tomato’ or ‘molasses’. The sensory threshold for DMS is between 30 and 60 µg/L.

Principle of Cu/Cd test
The principle of this test is that different reductive volatiles react with different fining agents, including copper and cadmium salts (Note that cadmium sulfate is not an allowable fining agent and is only to be used in this diagnostic test). A fining trial can be conducted using these agents to determination of the type of reductive fault present in a wine to determine the appropriate course of action to remove the fault.

Reagents
1) 1% w/v Copper (II) sulfate solution (1 g of CuSO4.5H2O in 100 mL 10% Ethanol)
2) 1% w/v Cadmium (II) sulfate solution (1 g of CdSO4.8 H2O in 100 mL 10% Ethanol)

Caution:
Cadmium is TOXIC.
Do not taste samples to which cadmium has been added. Assess by aroma only.
Cadmium sulfate is not an allowable wine additive or processing aid and is only to be used in this diagnostic test.

3) 10% w/v Ascorbic acid (10 g ascorbic acid in 100 ml of 10% Ethanol)

Procedure

  1. Place 50 mL of wine into four separate glasses.
  2. Label the four glasses: (1) control, (2) copper, (3) cadmium, and (4) ascorbic acid +copper
  3. Add 1 mL of reagent 1, the copper sulfate solution to glass 2 (‘copper’ glass),
  4. Add 1 mL of reagent 2, the cadmium sulfate solution to glass 3 (‘cadmium’ glass),
  5. Add 0.5 mL of reagent 3, the ascorbic acid solution to glass 4 (‘ascorbic + Cu’ glass), wait 2 minutes, then add 1 mL of reagent 1, the copper sulfate solution to the same glass 4.

Wines glasses

  1. Assess and compare the aroma of the control (glass 1) to the aroma of the other three glasses, noting down any differences observed. Note that this is an AROMA ONLY diagnostic test. DO NOT TASTE THE SAMPLES. An assessment sheet template can be found here.

Interpretation

Glass 1
(control)
Glass 2
(copper)
Glass 3
(cadmium)
Glass 4
(Ascorbic +
Cu)
Type of
reductive
compounds
Presence of
offensive odour
Odour gone Odour gone Odour gone H2S
Odour gone No change Odour gone Sulfhydryls
Odour gone Slight
improvement
Odour gone H2S and
sulfhydryls
No change No change Odour gone Disulfides
No change No change No change Dimethyl sulfide

Note: Have several people perform the assessment. A triangle test can be used to determine if real differences exist.

Reference: Adapted from Zoecklein, B.W.; Fugelsang, K.C.; Gump, B.H.; Nury, F.S. Wine analysis and production. New York: Chapman and Hall; 1995; pp428-429.

Treatment for removal

H2S:

  • During fermentation
    Excess H2S can be removed from red wines by aerating at the first racking, thus volatilising the H2S. Supplement the juice or must with a soluble nitrogen source, such as diammonium phosphate at the first sign of development during fermentation. Addition of DAP after the first half of the fermentation (the yeast growth phase) is less effective at removing reductive characters and can result in wines with high residual nitrogen content.Minimise the formation of excess H2S in white wines by either settling, centrifuging or filtering the must before fermentation, which removes high-density solids which might contain elemental sulfur. Note that excessive clarification at the juice stage however can also lead to stuck fermentation issues.
  • After fermentation
    Copper sulfate (CuSO4) fining can be used to remove sulfhydryls such as H2S, MeSH and EtSH from red and white wines. Copper sulfate reacts with these compounds to form copper salts, which were previously thought to be insoluble and easy to remove via filtration. However, recent studies have shown that copper sulfide does not precipitate out after reacting as once was thought, but may act as a dissolved species, a large proportion of which may remain in the wine post-filtration.
  • Careful laboratory trials should precede any CuSO4 additions to bulk wine, as an instability can result if the copper concentration in the wine exceeds approximately 0.5 mg/L (even lower in some wines). While there is no specified maximum limit for copper in wine in Australia, there are limits in many of Australia’s export markets (e.g. USA 0.5 mg/L), which should be taken into account when making copper additions. More information can be found on the Removing unwanted ‘reductive’ volatile sulfur compounds page.

Unwanted sulfhydryls:
Removal of sulfhydryls in red and white wines can be achieved by fining with copper sulfate (CuSO4). Copper sulfate reacts with sulfhydryls to form highly insoluble salts i.e.:

CH3CH2SH + Cu2+ → Cu(CH3CH2S)2

It is important to note however, that the addition of copper can also cause by-product formation of an unreactive disulfide complex via the below reaction:

3CH3CH2SH + Cu2+ → Cu-S-CH2CH3 ↓ + CH3CH2S-SCH2CH3

Disulfides:
Removal of disulfides requires the creation of reducing conditions, with the addition of ascorbic acid and SO2, in order to reduce these compounds back to the reactive sulfhydryl species which may then be removed by treatment with copper. Note that copper is not as effective at removing these mercaptans as it is at removing H2S, as discussed above.

Dimethyl sulfide:
As dimethyl sulfide does not bind to copper it can be difficult to remove the off-aroma; however, removal might be possible by sparging with nitrogen or by using reverse osmosis.