One of the most important parameters for the measurement of quality is the aroma/flavour profile of a wine (1). Up till now, more than 1000 compounds have been identified in grapes and wine. To add to the complexity of the wine matrix, the individual concentrations of these compounds may vary considerably (2). The aroma profile will also be influenced by production processes, be it in the vineyard or cellar and with an infinite number of variations possible in the production process, the final aroma profile of a wine is a complex matter to say the least (3).

There are various influencing factors that play a role in determining wine aroma composition. These include, amongst many more, climatic conditions (e.g. altitude above sea level), viticultural practices (e.g. canopy management) and enological practices, e.g. fermentation conditions, on which this article will focus (1).

Even though some aroma impact compounds exist for some varieties, seldom can the sensory perception of wine aroma be attributed to a single compound (1). The aroma attributes of a specific compound depends not only on its concentration or the specific odour threshold value (lowest concentration at which it can be detected), but also its interaction with other aroma compounds, be it the enhancement (even compounds present below their odour threshold) or suppression of another compound (1). Because of the complexity of the wine matrix it is almost impossible to predict the interaction between aroma compounds, but certain actions, like the selection of a specific yeast strain, could aid in driving the aroma profile to a certain extent (2). This is an important tool as it has been shown that a difference in flavour profile solely as a result of the choice of yeast strain, can be detected not only by trained panels and wine professionals, but more importantly, wine consumers (6). This implies that besides choice of viticultural practices and grape selection, selecting a specific yeast strain (usually Saccharomyces cerevisiae) for fermentation, as well as a bacteria strain for MLF, can greatly modify the aroma, flavour, mouthfeel, colour and chemical complexity of a wine, making this a tool to create a specific flavour profile according to market specifications (3).

The compounds that play a role in wine appearance, aroma, flavour and mouthfeel can be derived from three sources: the grapes, microbial modification during fermentation and then maturation, be it bottle ageing or wood maturation (3).

Grape-derived compounds do not only provide the basic wine structure, but also results in distinct varietal characteristics (3). The main grape-derived aroma compounds belong to the groups of monoterpenes, norisoprenoids and methoxypyrazines. Some examples of these include rose-like geraniol in Chardonnay, spicy eugenol and guaiacol in Gewürztraminer and floral, fruity and berry-like β-damascenone and violet-like β-ionone in Cabernet Sauvignon, Shiraz and Pinotage (6). While monoterpenes and norisoprenoids are very important in Muscat and aroma-rich varieties, fermentation-derived aroma compounds play a larger role in ‘neutral’ cultivars. The following section will focus on yeast-derived fermentation aroma compounds, although MLF also makes a significant contribution towards the final wine aroma profile.

While the main purpose of yeast is to metabolise sugar in order to produce ethanol and carbon dioxide, this microbial culture also produces a myriad other metabolites that, despite being present in small amounts, significantly alters the wine aroma profile and have a significant sensorial impact (3). Yeast strains are able to modify the wine aroma via three mechanisms (3):

1) via the extraction of compounds from solids in the grape must;
2) modification of grape-derived aroma compounds and
3) producing flavour-active metabolites.

The biosynthetic pathways responsible for aroma production via these mechanisms are influenced by various factors, to name a few (3):

  1. a) viticultural factors;
  2. b) composition and pH of grape must;
  3. c) nature and prevailing temperature of grape must and
  4. d) technological aspects and vinification methods.

As previously mentioned, the yeast can modify grape-derived aroma compounds for e.g. esters, higher alcohols and lactones in Chenin blanc contributes to varietal aroma; mercapto components formed during fermentation in Sauvignon blanc adds to passion fruit, guava and other tropical aromas and iso-amyl acetate adds to banana aromas in Pinotage (6). The table below also lists some of the most important yeast-derived aroma compounds important in determining the final wine aroma profile that serves as an important quality parameter (5).

Major aroma impact compounds produced and modified by yeast during fermentation

Volatile Acids
  • produce 0.2-0.7 g/L acetic acid during fermentation
Alcohols
  • ethanol: influence volatility of other aroma compounds
  • higher alcohols: positive or negative effect on wine aroma
  • involves degradation of amino acids
Carbonyl Compounds
  • acetaldehyde: 10-75 mg/L produced (bruised apple; oxidation)
  • diacetyl: small amount (0.2-0.3 mg/L) produced by yeast (butter )
Volatile Phenols
  • off-odours: medicinal, barnyard
  • vinyl-phenols: stabilise colour in red wine
  • Brettanomyces: ethyl-phenol (negative sensory impact)
Esters
  • influence fruity and floral aromas
  • dependant on: yeast strain, fermentation temp., precursors
  • acetate esters: ethyl acetate (fruity); iso-amyl acetate (banana, pear); 2-phenylethyl acetate (honey, rose, flower)
  • ethyl esters: ethyl hexanoate and ethyl octanoate (apple
Volatile Sulphur Compounds
  • low sensory threshold (generally negative to wine quality)
  • positive: thiols (grape-derived compounds modified by yeast)
  • guava, passion fruit, grapefruit, gooseberry (Sauvignon blanc)
  • release and modification is yeast strain dependant
Monoterpenes
  • grape-derived: aromatic (free) and non-aromatic glucose-bound
  • free form: fruity and floral
  • yeast release bound form via β-glucosidase activity; add to aroma

 

It has also been shown that chemical changes that occur as a result of ageing, either bottle or wood, may also alter the wine composition and quality (1). During the ageing period, compounds are extracted from wood (oak lactones) and these add to aroma complexity. Certain compounds are also transformed and/or liberated from bound forms, which mean they can then play a role in the aroma perception of the wine.

Due to the fierce competition in the wine industry, wine producers are being forced to investigate and understand consumer preferences and expectations and produce wine accordingly. This has become a market-driven industry whereby winemakers are challenged with responding to consumer sentiments and preferences (3). One of the tools in a winemaker’s arsenal that is available to address this challenge is the selection of the microbial populations that will be responsible for fermentation. Therefore the yeast and bacteria strain(s) can be seen as a flavour-impact tool to produce a certain style of wine. This will only be possible with an understanding of the impact aroma compounds and the role the selection of the correct yeast and bacteria can play in the production and or modification of these compounds. This is the reason for the extensive and careful research that goes into the development of all Anchor yeast and bacteria cultures. This way we ensure not only optimal fermentation, but also optimal contributions to the final aroma profile.

So take a big whiff…

References:
1. Wine aroma-important aspect of wine quality. www.newworldwinemaker.com
2. Sensory perception. www.newworldwinemaker.com
3. Swiegers J.H., Bartowsky E.J., Henschke P.A. & Pretorius I.S., 2005. Yeast and bacterial modulation of wine aroma and flavour. The Australian Journal of Grape and Wine Research, 11, 139-173.
4. The complete A-G understanding to waking up your wine. www.newworldwinemaker.com
5. The impact of yeast on the sensory quality of wine. www.newworldwinemaker.com
6: Cordente A.G., Curtin C.D., Varela C. & Pretorius I.S., 2012. Flavour-active wine yeasts. Applied Microbiology and Biotechnology. DOI 10.1007/s00253-012-4370-z