Fermentation Science – Good Stuff!

We all enjoy the benefits of fermentation science, but do we really know very much about the subject?  Fermentation is the key to creating alcoholic beverages, and many other consumables.  For wine (beer, liquors, etc.), fermentation is the biological process by which sugar molecules are converted to ethyl alcohol plus carbon dioxide and heat.  There are other by-products created during fermentation that add subtle nuances of flavor and texture to wines.  Since this is so important to creating our favorite beverage, it seems an appropriate subject for Carl’s Corner. 

Photosynthesis promoted by sunlight on green grape leaves creates carbohydrate molecules that nourish the grapevine and generate sugars in grapes.  These sugars are primarily fructose and glucose, molecules closely related to sucrose, common table or baking sugar.  Fermentation involves yeasts consuming these grape sugars to produce ethyl alcohol and give off carbon dioxide, the gas that makes carbonated beverages and sparkling wines effervesce.  So long as sugar and yeast cultures are present in the grape must, fermentation will most likely continue.  Complete fermentation creates a dry wine with no residual sugar (RS) content.  There are techniques available to interrupt and stop fermentation to leave RS in a wine.  Or, winemakers can simply add sugar back to sweeten a wine.  Leaving or adding RS to a wine is really a separate subject – a future Carl’s Corner.

Carbon dioxide is mostly released from the fermenting grape must, and, in most cases, will dissipate and not be noticeable in a bottled “still” wine.  However, if one caps or seals the wine container to trap and hold carbon dioxide, a sparkling or frizzy wine, like Champagne, can be produced (another Carl’s Corner topic).  This carbon dioxide gas is important early on as CO2 gas is more dense than air and will blanket the vapor space above the fermenting grape must & skin cap.  This protects the fermenting juice from over exposure to oxygen in air that can promote too much oxidation and reduce overall wine quality. 

Fermentation is an exothermic reaction.  Exothermic is a term that means heat is generated and released during the reaction, and can build up to increase temperature.  Most wine fermentations are done at temperatures between 55 and 85 degrees Fahrenheit.  White wines are best fermented at cooler temperatures which help capture and maintain fresh fruit aromas and flavors.  Higher temperatures for red wine fermentations are common as more extraction of color, flavor and tannin takes place.  However, temperatures above 85 degrees can introduce cooked or baked aromas and flavors which debit overall wine quality.  Temperatures over 100 degrees can actually kill yeast cultures and stop the fermentation process.  Thus, temperature control is very important, and numerous technologies are employed by winemakers to manage fermentation temperature.  Further, research efforts continue to develop and improve methodology available to winemakers for temperature control of fermentation. 

A key tool that winemakers use in fermentation is yeast strain selection.  Some yeasts work best at low temperatures and others at higher temperatures.  The same yeast may ferment slower or faster depending on temperature, and can generate additional flavor compounds related to the fermentation temperature.  Cooler fermentations, common with white wines, can produce more volatile ester compounds that offer fruity and floral aromas, like banana, apple, pineapple, rose, honeysuckle, and many more.  These cooler fermentations take longer to finish, often requiring 25-35 days for sugars to be converted to alcohol.  Warmer fermentations, common with red wines, tend to produce less volatile esters, compounds that offer different fruit aromas and flavors, like black cherry, blackberry, toasted coconut, mocha chocolate, coffee, toffee and lavender.  These warmer fermentations are completed much more quickly, typically within 7-10 days. 

Another impact of temperature during fermentation is the level of extraction of color, flavor and tannins from the grape skins and seeds.  At a warmer temperature, there is a greater level of extraction.  And, polysaccharides, complex carbohydrate molecules related to sugars, will autolyze faster at higher temperatures to create a softer and smoother mouthfeel in the finished wine.  Learning how to use different yeast strains and manipulating fermentation conditions, especially temperature, are very important to a winemaker as he/she develops better understanding and a personal style of winemaking. 

There is, of course, much more to this concept of fermentation science.  Hopefully this post has provided some basic information that will help you appreciate the complexity of wine production.  The next time you taste a particularly wonderful wine from your favorite winery, tip your Stetson to the winemaker for his/her efforts to understand and manipulate fermentation science to help produce that special wine. 

A recent article in WineMaker magazine, Dec 2017-Jan 2018 Volume 20 No. 6, entitled Fermentation Dynamics, How Temperature Affects Style by Alex Russan, provided the inspiration and a lot of good background information for this Carl’s Corner post.