Alcoholic Fermentation for Winemaking

Thursday, October 1, 2020

 By Neslihan Ivit, Perennia Winemaking & Innovation Specialist

Since grape harvest has already begun in Nova Scotia, the wine cellars are getting full of grapes to be processed, and fermentations are starting. As all winemakers know, there is nothing more important than a complete fermentation during this season. The fermentation process is crucial not only for ethanol production but also for the organoleptic parameters and quality of the final product. Therefore, a complete analysis of the chemical composition of the must (grape juice) is essential for the fermentation process. Some of the vital parameters are as follows:

  • Brix and reducing sugars – These two parameters indicate the potential alcohol, although the final ethanol concentration may vary depending on the fermentation conditions.
  • pH – pH is determined by the sum of acids and cations in the must. It is important to check this parameter before fermentation since the activity of the yeasts and bacteria will depend on the pH. Moderate pH of the must is 3 to 3.5. The lower pH values may hinder the completion of fermentation. In extreme cases, it is recommended to make acidity adjustments before fermentation. On the other hand, higher pH values render the microbiological stability of the wine difficult to achieve further in the winemaking process and after bottling. 
  • Acidity parameters –  The total or titratable acidity of must considers all types of acids present, including organic and inorganic. The main organic acids in must are tartaric and malic acid. Their levels in the must, and consequently in wine, affect the organoleptic characteristics, as well as cold and microbiological stability.
  • Yeast Assimilable Nitrogen – The nitrogen which can be used by the yeast for the fermentation process. Checking this value before the beginning of fermentation leads to better fermentation management. It is necessary to keep in mind that the nitrogen requirement of each yeast strain is different and also depends on various fermentation parameters, including the amount of sugar that is expected to be consumed by the yeast.

Figure 1. White wine fermentation (left) and red wine fermentation (right).

These parameters complement the factors to be considered while deciding the style of the wine to be produced. Some of the styles of alcoholic fermentation of grape juice are listed as follows:

  • Inoculation commercial active dry yeasts (ADY) – It is possible to choose among different ADYs depending on the variety of the grape and style of the wine. It allows winemakers to decrease the risk of having a stuck fermentation and to achieve the predetermined wine style with desired organoleptic characteristics. On the other hand, some winemakers and researchers believe that commercial ADYs create “uniformity” in wine characteristics.
  • Spontaneous fermentation – In this method, the yeasts which are already present on the grape skin as well as in the winery environment are used for fermentation. This allows winemakers to create unique wine characteristics. However, these fermentations continue to be a “risk” to winemakers since the completion of fermentation and regularity in wine quality are not guaranteed. Some winemakers may choose to re-inoculate with ADY after the native yeasts reach a certain alcohol level.
  • Pied de cuve – This is the traditional method of preparing a winery-made fermentation starter from grapes. The method involves collecting some grapes close to harvest date, crushing them, and fermenting the fresh juice. This small volume of preparation, which is rich in different yeast strains coming from the vineyard, is used to inoculate subsequent batches of harvested grapes. It is a good idea to prepare different batches and choose the pied de cuve that has the best fermentation activity and aromatic profile. This might be time-consuming during the busy harvest season; however, it decreases the lag time of the fermentation take-off and the risk of spontaneous fermentation.

Co-inoculation of yeast & bacteria – If malolactic fermentation is desired to be performed after alcoholic fermentation, it is possible to inoculate with commercial lactic acid bacteria (LAB). Although generally done sequentially after alcoholic fermentation is completed, it can be done as a co-inoculation with yeast during alcoholic fermentation. If this method is used, it is necessary to choose a LAB strain suitable for co-inoculation with yeast.