Yeasts: the microscopic martyrs

You don’t have to be a genius to taste the difference between the flavour and alcoholic kick of grape juice and wine, so it’s worth taking a few minutes to get to know those responsible.  It’s hard to get excited about microscopic unicellular fungi, and these little guys don’t get much press coverage, but according to scientists yeasts are responsible for around 400 out of the total 1,000 (40%) flavour compounds found in wine. 

To make wine, yeasts gobble the sugar in grape juice and turn it into alcohol, carbon dioxide gas and lots of complex flavours.  Depending on your preference, the flavours can be ‘good’, like the nutty, spice and savoury notes, or they can be ‘bad’, such as the horse manure flavour found in some big red wines.  In Champagne, yeasts are responsible for both the bubbles (carbon dioxide) and the lovely toasty flavours. 

A winemaker, aiming to make a particular style of wine, needs to work with the yeast to achieve their goal.  Yeast varieties occur naturally in the air, and the first critical decision a winemaker makes is whether to allow these wild yeasts to ferment the grape juice or to use a cultured strain of yeast instead.  Wild yeasts impart some extra flavour complexity into the wine, and in doing so subtly stamp a wine with individuality.  However, wild yeasts are unpredictable with risk of ‘bad’ flavours and spoilage.  A cultured yeast can significantly reduce those risks, but at the expense of individuality.  Nowadays cultured yeasts are being developed that can deliver all sorts of quality benefits from specific ‘good’ flavours, to reduced alcohol and even targeted health benefits. 

Whichever the choice of yeast, the winemaker must then work with the yeast to control fermentation, mainly through steady temperature control, to ensure the final flavour and alcohol are just right.  After the yeasts have died (in the very alcohol they produce) the wine may be left with the dead yeast cells (‘lees’) to extract further toasty, smooth flavours.  Finally the yeast will be filtered out of the wine before bottling.  So yeasts are really the unsung heroes of wine: martyrdom for the good of the party.

At Summertown Wine Café we have detailed wine making notes on all the wines, so if you want to taste the work of the wild or cultured yeasts for yourself, come along and do a tasting anytime.

Rob Malcolm