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 andGluconobacter (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) …