Chapter 6

Wine Fermentation

Problem Wine Fermentations

Problem fermentations can be divided into two broad categories: issues with fermentation rate progression and off-character formation. Both types of problems are sporadic and chronic, and are easier to prevent than to treat. In many cases, fermentation progression appears completely normal immediately prior to the appearance of a problem. Fermentation behavior is inherently difficult to predict due to the number of potential variables. Careful analysis of fermentation conditions (e.g., alcohol level, quantity of residual fermentable sugars, volatile acidity, and malic acid level) can provide clues to the reason for poor fermentation performance.

Sluggish or Stuck Fermentations

Sometimes alcoholic fermentation becomes sluggish during middle and later phases of alcoholic fermentation, whereas in other cases, cessation of fermentative activity may be abrupt. Premature cessation of yeast growth and fermentation results in a wine with unfermented sugars and an ethanol concentration lower than expected. From a commercial standpoint, sluggish or stuck fermentations are a problem due to their sweet taste, inferior sensory quality, and can cause significant economic losses for a winery. It is often difficult to determine the difference between a fermentation that is merely slow, but will complete, and one that is or soon will be stuck.

Causes of Sluggish or Stuck Fermentations

There are many factors that can impede yeast metabolism and result in a sluggish or stuck fermentation. When apparently normal fermentations slow down or stop abruptly, factors such as nutrient depletion and temperature shock, especially high temperatures with high ethanol concentrations, or unsatisfactory starter culture may be involved. However, seldom is any single factor the cause of fermentation problems. More frequently, synergistic interaction of several limiting or inhibitory conditions listed below negatively impacts fermentation. The following are causes of sluggish or stuck fermentations that can prevent alcoholic fermentation from developing correctly, some of which are unavoidable and others of which are the result of inappropriate fermentation management decisions.

Strategies for Overcoming Sluggish or Stuck Fermentations

Successful restarting of a stuck fermentation depends upon two critical factors: proper pre-conditioning of the yeast to be used as the inoculum and knowledge of the cause of the fermentation arrest. The latter will directly impact the former, as the tolerances of the strain used in the re-inoculation must compensate for the specific stresses of the arrested fermentation. If fermentation is inhibited due to a nutrient deficiency, addition of a yeast nutrient is often enough to restart fermentation. In some cases, simply bringing the yeast culture back up into suspension, slowly warming the tank and/or re-inoculating with a fresh yeast culture may jump-start the sluggish fermentation.

Production of Off-Aromas and Off-Flavors

The second class of fermentation problems, besides sluggish and stuck fermentations, concerns the production of off-characters, off-flavors, or aroma compounds detracting from overall wine quality. Off-character formation may occur prior to the establishment of Saccharomyces, during the early stages of fermentation, during the active phase of fermentation, at the end of fermentation, or during aging on the yeast lees. Pre-fermentation aromas often come from the activity of non-Saccharomyces yeasts and bacteria. The types of off-characters that can form during fermentation fall generally into one of two main categories: sulfur-containing volatiles and esters

Sulfur-Containing Volatiles

An important class of spoilage compounds are the sulfur-containing volatiles. Sulfur-containing volatile compounds are perhaps the most challenging of the off-aromas that can form during fermentation (Section 16.3). This group includes compounds which are very volatile and have unpleasant odors generally described in terms of rotten eggs, skunk aroma, garlic, or onion.

Esters

Esters can contribute positively to the aroma of a wine. In low concentrations, these compounds are perceived as generically fruity or floral and can boost the awareness of the innate fruit and floral characteristics of the grape varietal.

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