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SULPHIDES

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by

Roland Harrison

on 4 August 2014

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Transcript of SULPHIDES

SULPHIDES
Description in wines
Sources
Factors influencing presence/extent
The presence and extent of sulphides in wine is influenced by a number of factors. Some of these relate to the production of sulphides and others to their form.
So-called sulphides are characterised by low olfactory thresholds and have both unpleasant and pleasant aromas
Divided into 3 main chemical groups
It is possible to classify these chemicals in different ways, e.g. low/high boiling points, pleasant/unpleasant aromas, but to understanding their activity in wines is best done from a chemical point of view.
1. H2S and other sulphides
2. Mecaptans (also known as thiols)
3. Disulphides
H2S is the major compound responsible for sulphide faults
H2S smells of rotten eggs
Dimethyl sulphide smells of cooked vegetables
Diethyl sulphide smells of rubber
Unpleasant aromas
This is a large group of compounds, of which the most important are:
Methyl mecaptan (methanethiol) smells of rotten cabbage, burnt rubber
Ethyl mecaptan (ethanethiol) smells of burnt match
Pleasant aromas
3MH smells of grapefruit, passion fruit
3MHA smells of box tree, passion fruit
4MMP smells of box tree, broom
Benzenemethanethiol smells of gunflint, smoky
Wine style
There is a debate about the role of sulphides in wine style
Varietal thiols
Cork vs. screw-cap
So-called complexity
A number of thiols (e.g. 3MH, 3MHA and 4MMP) are associated with the aroma of particular parietal varies (i.e. New Zealand Sauvignon blanc)
Some writers (i.e. Alan Limmer) argue that wine styles have evolved as a result of historical norms for redox potential associated with cork closures
A number of so-called sulphides (e.g. ethyl mercaptan) at low concentrations are not unpleasant and can add interest to the aroma of some wines
Yeasts
Sensory thresholds
Redox potential
Carbon disulphide smells sweet, ethereal, slightly green
Dimethyl disulphide smells of cabbage
Diethyl disulphide smells of garlic, burnt rubber
Solution composition
The redox potential is influenced by the composition of the solution, especially by dissolved oxygen (DO) but also but the presence of antioxidants
Tannins
SO2
Yeast lees
Scavenges O2 to maintain a reduced environment
In red wine, intereact with O2 to buffer the solution against oxidation
Reacts only slowly with O2 so not relevant to redox in this context. Binds products of oxidation
Closures
Yeast assimable nitrogen (YAN)
Yeasts are able to make use of the amino acid cysteine as a nitrogen source
Cysteine also contains sulphur which is also released and recombined in other compounds (e.g. sulphides)
Composition of the must
Jamie Goode quotes Sam Harrop MW saying that 2.2% of all screw-capped wines as faulty due to vegetal/rubber characteristics
Mecaptans have lower threshold concentrations compared to equivalent disulphides: e.g. methyl mercaptan = 0.2-2 ppb whereas dimethyl disulphide = 12-50 ppb
Disulphides are reduced to mercaptans at low redox potential
Also methyl and ethyl thioacetates (MeCOSMe, MeCOSEt) and trisulphides
Acid hydrolysis can produce mecaptans
Kinetics studies indicate that reduction occurs on an observable time-scale in wine-like solution
Measurements of oxygen transmission and free SO2 remaining in wine support the view that corks transmit air at a slightly higher rate than screw-caps
Limmer suggests, based on clustering of free SO2 data that the variation in the permeability of graded cork is similar to that of screw-caps.
He also points out that differences in estimated permeability may result from the method used (diffusion, pressure) but that sensory results suggest that corks are less permeable than synthetics.
Both cork length and grade are important for permeability.
Although Limmer says, "[Low levels of mercaptans] impact from an organoleptic perspective towards the end of the palate imparting a 'mineral' or bitter/hard/astringent aspect. This has the appearance of shortening or closing up the palate, so the wine does not display a fine fresh long finish, but ends abruptly, and somewhat harshly."
Production of H2S during AF controlled by enzymes responsible for reducing sulphate and biosynthesis of S-containing amino acids (cysteine, methionine)
Elemental S
H2S by chemical reduction of S
Yeast available S
If must content of cysteine and methionine are sufficiently high, their formation by yeast is reduced and H2S production suppressed
Tall-form fermentors lead to more rapid drop in redox potential.
Important for elemental S and yeast metabolism?
Yeast strain
Concentrations > 1-5 mg/L can result in excessive H2S
Timing of last application
Follow regional guidelines
Form of S
Quantity of H2S inversely proportional to particl size
Differ with respect to application rates and fuming temperature
Small particle size easier to apply effectively and fume at lower temperatures, both contribute to lower application rates and less residue
The redox potential determines the proportion of reduced and oxidised species in a chemical system. [Analogously, the pH determines the proportion of dissociated and undissociated species of weak acid systems]. Because O2 is such a strong electron acceptor, its presence tends to shift the redox equilibria in the oxidised direction. But other species can also accept electons and therefore act as oxidants.
Probably the most important influence on the types and amounts of sulphides in wine
Must clarification
Moderate clarification likey to reduce H2S formation by removing elemental S
Excessive clarification may lower YAN
Bentonite use during white wine fermentation
Lees can absorb H2S and mercaptans that may be released subsequently
Full transcript