Fermentation of Wine: Yeasts Produce a Bouquet of Aromas

Since lately I feel like I've been spending most of my time endlessly writing lab reports,  I figured it would be appropriate to share a snippet of an experiment from my Wine Production & Analysis class.

The fermentation process of wine can be a bit tricky, as there are quite a few things to consider while the freshly pressed grape juice transforms and ferments into a wine. In order for the juice to become alcoholic, we need two things: yeast and sugar. Yeast is essentially needed for a juice to undergo fermentation, but can greatly influence the quality and aroma attributes of a wine. In class, we performed an experiment to see the effects of 15 different yeast strains in juice during the course of fermentation, using the 2011 Chardonnay harvested in the UC Davis campus vineyards. 

 {Glasses of Chardonnay--each glass was introduced to a different strain of yeast}

{My aroma notes for each of the yeasty wine samples--it was amazing how much the aroma attributes varied. (Oh, and please excuse the typos!)}

Cheers,
B

[Read below if interested as to why some of the wines varied in aroma--in scientific explanation]:



It has been observed and commonly agreed by many studies that there is a correlation between yeast strain added to a wine/juice and its influence on the aroma of wine during fermentation.  When introduced to a wine sample, yeasts begin to produce esters, which are characteristic of how some aroma attributes are present in wines, specifically volatile acetate esters and fatty acid ethyl esters.  Such aroma attributes include the floral, fruity, solvent-like, sweaty, or soapy characteristics--and the aroma of the wine depends on the type of ester structure that is present, as well as the carbon chain length of the ester.  Sensory analysis provides a sufficient and convenient method for the determination of significant amounts of volatile esters in wines. Scientific approaches can be performed to determine esters present in wine samples, one of them including HPLC, (or high performance liquid chromatography) but was not implemented in this particular experiment since the focus was to observe changes in the overall smell (or bouquet) of 15 yeast-treated UCD 2011 Chardonnay samples during the course of two weeks.

Results among different yeast strains varied throughout the course of the experiment. While some samples maintained pleasant aromas, others suffered awful chemical-like characters that were simply off in smell.  Certain strains of yeasts are known to exude typical smells in a wine but the yeast is only a fraction of the reason why some of the wines smelled the way that they did. Off-characteristics are in most cases due to microbiological and environmental issues. (This includes things such as including DAP, co- and mono-inoculum use, rate of growth of the yeasts, vitamin and mineral & nutrients deficiency, excess H2S formation, ammonium and amino acid count, phosphate levels, or even long chain fatty acids and sterols--to name a few).

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