Wine Fermentation

The second type of problem that can arise during  fermentation is the development of an unpleasant aroma. What is deemed "objectionable" may depend upon the circumstances and aromatic profile of the wine. Some compounds, such as hydrogen sulfide (H2S), are universally perceived as negative while others, like the floral esters, may be desired in some wines or at some concentrations. Yeast strains vary in their abilities to produce aromatic substances and the production of these compounds is influenced by the composition of the grape juice.

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 display a dependence upon juice composition and strain variability. Both are easier to prevent than to treat. However, complete avoidance of these problems requires a sophisticated chemical analysis of juice composition that is generally beyond the scope of the typical winery. In many cases, fermentation progression appears completely normal immediately prior to the appearance of a problem.

1. Know your juice: The key to completion of the yeast fermentation is the availability of sufficient nutrient resources to allow cell growth and sustained metabolic rates in the presence of increasing ethanol concentrations. It is important therefore to have some sense of the starting nutritional content of the must or juice and to supplement up to the needs of the yeast. Over-supplementation while not leading to arrest of the fermentation will impact the spectrum of aromatic compounds produced generally leading to less complexity.

The goal of fermentation management is to develop a robust population of microorganisms that will complete the fermentation and resist any biotic or abiotic stress that may occur along the way in addition to the production of positive aromatic characters contributing to complexity. Grape juice is rich in nutrients, particularly in sugars from which energy will be derived.

The crushing of grapes brings the microbes of the berry surface in contact with the nutrients of the fruit. Ample carbon, nitrogen, sulfur and phosphate sources exist in the typical berry to promote the growth of a variety of bacteria and yeasts. Micronutrients are also present and only the most nutritionally fastidious organisms will be unable to proliferate. How grapes are processed can create conditions that select for the growth of a subset of the organisms originally present. This can be used to the advantage of the winemaker to enrich for desired populations.

The fruit harvesting conditions can also impact the flora present in the juice or must. Once grape surface organisms are brought into contact with the nutrients of the grape, they can accelerate growth and metabolism.

Fermentation management begins in the vineyard. This is especially true for native flora fermentations but even inoculated fermentations can be influenced by the nature of the organisms present on the grapes. Grapes, leaves, bark, even the trellis system and irrigation systems support microbial flora in vineyards. The type of microbes best adapted to the localized environment of the plant will dominate that surface, but rarely is a single species of organism able to completely dominate an environmental niche.

Fermentation is a microbial process. The type of fermentation that occurs is dependent upon several key factors: the presence and types of nutrients, the absence of inhibitory conditions, and permissive chemical and environmental conditions. Successful fermentation management practices provide sufficient, but not excessive, nutrients, utilize fermentation conditions that favor the growth of, and domination by, large populations of Saccharomyces, and minimize stress to those populations.

Grape juice contains a rich source of substrates derived from plant photosynthesis. The major sugar substrates in grape are the two hexoses, glucose and fructose, but other hexoses and pentoses may be formed as well. Fermentation of grape sugar is largely conducted by the yeast Saccharomyces. Through the process of glycolysis, the 6-carbon hexoses are broken down into 2 molecules of the 3 carbon pyruvate.

Fermentation refers to a metabolic process of energy generation that uses an organic compound as a terminal electron acceptor.  Organisms obtain energy necessary to fuel biological work from the rearrangement of chemical bond energy, the movement of protons or the movement of electrons. In the case of the fermentation of sugars, the energy of light captured by the plant during photosynthesis and used to create sugar is recaptured in the catabolic process that breaks down sugars to create the high energy molecule adenosine triphosphate (ATP).