Quick, who discovered the process of fermentation?  I’ll share the answer at the end of this article.  Suffice it to say, the concept has been known for just over two hundred and fifty years.

Let’s go back to first year college or perhaps even high school chemistry as fermentation is, in essence, just a chemical reaction.  Fermentation is the process of converting sugar (from grapes) to ethanol (ethyl alcohol) through the metabolism of yeast.  The process is anaerobic (oxygen free) and generates heat in the form of carbon dioxide (CO2).  In fact, sugar that has been infused with yeast, looks like it’s boiling as it releases carbon dioxide.
The true chemical equation is:

C6H12O6 yields 2C2H5OH + 2CO2

If a generous amount of oxygen is available, more cell building energy is produced through the production of carbon dioxide and water.  However, for the production of wine (alcohol), it is highly advantageous to limit the supply of oxygen as this will, in turn, limit the production of water.  After all, we are looking to produce alcohol and not water.

During fermentation, several “intermediate” compounds are produced that may be detected as secondary aromas.  These are compounds such as Acetaldehyde, Ethyl Acetate, and ester derivatives.  But far more crucial in this process is the production of energy that is released as heat.  For a wine cellar manager, it is vital to remove at least a portion of this waste heat.  If the heat generated is not allowed to dissipate, the temperature of the fermenting liquid will rise to levels that may damage or kill the yeast cells and thus stop the fermentation.  When this occurs, it can be very difficult to restart the process.  In order to efficiently control this risk, larger fermentation vessels often require some form of surrounding refrigeration layer to control the temperature of the fermenting juice.

Measuring the progress of fermentation is most easily documented through the measurement of the density of a sample of the juice.  While the density can be determined through floating of a calibrated hydrometer, the device is susceptible to misreading due to the alcohol content of the solution (the hydrometer is calibrated to read sugar in a water solution).  Hand held density meters are now often utilized with the data downloaded into a computer for computation.  Some larger producers have tanks with in line densitometers or pressure sensors that feed data into a computer. 

The time required for fermentation is mostly a function of temperature.  In general, red wine fermentation is completed with 4-7 days.  Whites, on the other hand, may require several weeks as the juice is fermented as much lower temperatures.  Fermentation may be halted purposely by adding alcohol to the juice, thus leaving residual unfermented sugars.  This is common in the production of certain wines such as Port.

There are many alternative methods of fermentation: in the bottle, in a vessel (most commonly in a barrel), carbonic maceration, malolactic (ML) and secondary. 

Fermentation in the bottle plays an important role in the production Sparkling Wine.  Here, a little sugar is added to the bottle at the end of the primary fermentation and then sealed.  The secondary fermentation produces dissolved carbon dioxide in solution that is released when the bottle is opened.  Fermentation in a vessel may occur in either an oak barrel or a stainless steel or cement vat.  Carbonic maceration is the fermentation process that applies to whole, unbroken clusters of grapes in the presence of added carbon dioxide.  Under these conditions, intracellular fermentation occurs within the grapes producing a small quantity of alcohol and traces of many aromatic compounds.  This process usually takes between one and three weeks.  Producing a wine from only carbonic maceration is nearly impossible as some of the grapes at the bottom of the vessel will inevitably break due to weight of the grapes above.  Malolactic fermentation refers to the conversion of the more acidic and stronger Malic acid to the softer and more full lactic acid plus carbon dioxide.  Actually, the word fermentation here is a misnomer, even though carbon dioxide is produced as an end product.  Malolactic “fermentation” is accomplished by Lactic Acid Bacteria which always occurs after primary alcoholic fermentation and thus is often referred to as “secondary fermentation.”  Usually, ML fermentation is desirable in wines that have excessive acidity, softening the wine and adding flavor and complexity.

By the way, it was Louis Pasteur who discovered the process of fermentation; this in addition to his in depth studies of viral diseases and vaccines, rabies, beer, Silkworm diseases and molecular disymmetry. 

If you enjoyed this article, you would enjoy reading about Tannins and wine Acidity.

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Tags: carbonic maceration, fermentation, malolactic fermentation