|This is the final article in a series of three which covers the fungal and bacterial origins of wine aromas. These articles detail esters, aldehydes, volatile fatty acids, volatile phenols, sulphurous compounds and higher alcohols. The old adage “one man’s trash is another man’s treasure”, holds true with most of these compounds. The article herein will deal primarily with volatile acidity and hydrogen sulphide, which are nearly unanimously classified as wine faults by vintners.
Some sulphurous compounds can be pleasant, as is the case with grape derived thiols that are intrinsic to the “passion fruit”, “box wood”, and “grapefruit” aromas of Sauvignon blanc. However, the sulphurous compounds which are purely derived from yeast are not considered valuable to the vintner.
The volatile fatty acids are likely only considered to be contributors of positive aromas by those who produce vinegar. In wine, the volatile fatty acids are responsible for a major fault when they accumulate beyond their sensory threshold.
Volatile fatty acids
The volatile fatty acids found in wine consist primarily of short-chain fatty acids (tails of less than 6 carbons) and medium-chain fatty acids (tails with 6 – 12 carbons). The short and medium-chain fatty acids are the most studied fatty acids in wine and are responsible for what is known as volatile acidity (VA).
VA is a measure of all the steam distillable volatile acids present in wine. These can include acetic, lactic, formic, butyric and propionic acid (Zoecklein et al., 1999). Other organic acids, excepting acetic acid, are of little consequence to wine flavour and aroma. Around 90% of all the volatile acidity in wine comes from acetic acid, which, in conjunction with ethyl acetate, possesses a vinegar-like aroma (Pretorius & Lambrechts, 2000). Yeast produce acetic acid during fermentation within the range of 100 mg/ℓ – 200 mg/ℓ, depending on the yeast strain and vigour of fermentation (e.g. temperature and juice nutrient status) (Boulton et al., 1996). This usually occurs during the beginning lag phase of fermentation (Whiting, 1976). Excessive acetic acid production is usually an indicator of microbial spoilage by Acetobacter and Gluconobacter (Boulton et al., 1996).
Acetic acid from microbial sources is derived through various pathways. One mode is through the degradation of sugars by lactic acid bacteria via the phosphoketolase process (the way in which bacteria can break down residual sugar). Alternatively, acetic acid can simply be produced as part of the citric acid cycle. Acetobacter and Gluconobacter can oxidise ethanol to acetic acid enzymatically with alcohol dehydrogenase (first oxidised to acetaldehyde then to acetate with aldehyde dehydrogenase) (Swiegers et al., 2005).
Acetic acid (e.g. high VA) is a common issue when creating ice wines. Under ice wine conditions, the yeast are under high osmotic stress conditions. In order to adapt to this condition, yeast cells will exude glycerol, which prevents the movement of water from the yeast cell into the must. Glycerol is formed through a NADH dependent enzymatic reaction. The subsequent conversion of NADH to NAD+ changes the redox balance, which is corrected for by the production of acetic acid, shifting the redox potential back to equilibrium (Erasmus et al., 2004). Thus, this results in excessive levels of acetic acid.
Giudici and Zambonelli (1992) suggest that perhaps the reason for acetic acid production by yeast in normal table wines is due to acetic acid’s role as an intermediate in the formation of acetyl Coenzyme A (CoA) from acetaldehyde. However, Boulton et al. (1996) state that a consensus has not been reached concerning the mechanism of the enzymatic formation of acetic acid.
Pretorius and Lambrechts (2000) suggest that a typical VA of an unspoiled wine is around 200 – 400 ppm. There is no given “threshold of detection” for VA. The perception of these compounds can differ between wines, since high levels of sugar and ethanol mask them (Corison et al., 1979; Zoecklein et al., 1999). Further, winemakers expect an increase in VA of about 60 – 120 mg/ℓ in barrel-aged wine after one year. This is not necessarily due to microbial spoilage, but rather the degradation of the hemicellulose of the oak barrel itself. Also, phenolic compounds can oxidise over time to form peroxide, which oxidises to acetaldehyde and, after, to acetic acid (Zoecklein et al., 1999).
Sulphur compounds can be pleasant or disagreeable and generally have a low threshold of detection. Volatile sulphur compounds, such as thiols, are responsible for the ripe/fruity aromas of Sauvignon blanc and are formed during fermentation (Tominaga et al., 1998). They can also contribute a “box-tree like aroma” in the same variety (Tominaga et al., 1996). Further, sulphurous compounds at low concentrations may cause a perceived “minerality” in some wines (Goode, 2005). Additionally, Lactobacillus can metabolise methionine, which forms volatile sulphur compounds, such as methanethiol, dimethyl disulphide and propionic acid. Oenococcus oeni also metabolises methionine; current research suggests that the most significant by-product is propionic acid, which contributes a chocolate aroma and may be partially responsible for the pleasing and complex aroma profile of malolactic fermentation (Pripis-Nicolau et al., 2004).
The molecule H2S is the most studied sulphur compound. Normally considered aversive, H2S can have a pleasing aromatic impact by providing a “yeasty” flavour to wine at low levels. Higher concentrations of H2S have a “rotten egg” aroma and a very low sensory threshold of 10 – 100 µg/ℓ Pretorius & Lambrechts, 2000). In order to synthesise sulphur-containing amino acids, yeast can reduce sulphite to sulphide, which is then enzymatically combined with a nitrogenous compound to form cysteine or methionine. If those nitrogenous compounds are not present, the result is the release of hydrogen sulphide, which freely bypasses the cell wall (Kaiser, 2010). Deficiencies in vitamin B5 have also been found to be limiting in juices that produce H2S. This vitamin is important for the formation of Coenzyme A, which is necessary for the formation of methionine and cysteine. Without this enzyme, these amino acids cannot form, and the sulphur produces hydrogen sulphide (Wang et al., 2003). However, a vitamin deficiency is extremely rare and difficult to test for in a lab (Boulton et al., 1996).
H2S can also result from:
- Reduction of elemental sulphur from spray residues (relatively uncommon).
- Presence of other sulphur containing compounds (glutathione).
- High or very low juice turbidity (recommended ~0.5% turbid).
- Low redox potential of must (e.g. reductively held or tall/skinny tanks).
- Release of bound sulphurous compounds in yeast lees during lees ageing.
The impact of lees on H2S production isn’t always negative though. The mannoproteins of yeast can form disulphide bridges with sulphur compounds and lessen their aromatic impact.
Excessive SO2 use leads to the formation of H2S by inhibiting acetaldehyde reduction to ethanol. If a deficiency is also present in O-acetylserine and O-acetylhomoserine (precursory compounds necessary for the formation of sulphur containing amino acids), H2S is produced from the enzymatic reduction of sulphite (from SO2). This gives the yeast a sulphur source to produce these amino acids (Margalit, 2004). A high metal ion (e.g. residual copper from Bordeaux mixture) concentration within the must suppresses cellular respiration, which lowers redox potential and ultimately, elevates H2S levels (Boulton et al., 1996). If the redox potential of a must is not increased during H2S formation (e.g. aerated must), the H2S can react with other compounds such as ethanol and sulphur containing amino acids to form mercaptans (most notably, methyl mercaptan). The details of the mechanism of the formation of mercaptans are currently unknown. These molecules create a pungent, rotten cabbage aroma. Mercaptans can be easily oxidised to form a less aromatic disulphide. This misleads winemakers into believing the problem has dissipated, although mercaptans can re-form under reductive bottle conditions. Yeast strains differ widely in their propensity to form H2S and are chosen based on this characteristic.
Wine is commonly referred to as a “complex matrix”. By breaking wine down into its fundamental components, we can begin to understand how to better manage our vineyards and wineries to attain the wine styles that our markets desire. Volatile fatty acids and sulphurous compounds which are purely microbially derived should be avoided. It is crucial to understand how these compounds arise and how winemakers and viticulturists can manage them effectively and efficiently.
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