Fermentation Management: Issues, Options and Variables

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.

In addition to energy in the form of ATP generated via fermentation, yeast need appropriate nutrients to build new cells and to maintain metabolic activity.· Macronutrients are those elements needed in relatively large quantities as they form the building blocks of cellular material: carbon for cell lipids, sterols, polysaccharides, nucleic acids, proteins, co-factors and metabolites; nitrogen for nucleic acids, proteins, co-factors and metabolites;· phosphate for nucleic acids, proteins, co-factors and metabolites; and sulfur for proteins, co-factors and metabolites. A shortage of any one of these nutrient components will lead to a reduced ability to create new cells or biomass. The biomass that is formed may be metabolically healthy but fewer cells means an overall slower fermentation rate.

Cells also need micronutrients, components needed to facilitate enzyme-catalyzed reactions. Minerals and vitamins are micronutrients. A limitation of one or more of these components may prevent cellular reactions from occurring and, if severe enough, will likewise limit growth. If the component is not required for growth but is needed for fermentation, maximal biomass may be formed with reduced rates of sugar catabolism per cell. For example, zinc is a cofactor of alcohol dehydrogenase, the enzyme required to transfer electrons generated during sugar catabolism to acetaldehyde, forming ethanol. If insufficient zinc is present, electrons may accumulate that will then inhibit further fermentation. The cell is a dynamic entity, needing to balance metabolic activities with twin biological priorities: creation of new cells via the process of cell division, and survival.· Both serve to preserve the genotype of that organism.

In addition to proper nutrition, stress can also limit cell division, cell metabolism or both. Stressors can be biotic (such as a nutrient restriction or accumulation of toxic intermediate or end products) or abiotic (low pH, temperature extreme, osmotic extreme). In the case of wine, production stress can arise from the metabolic activities of other microorganisms present prior to or during the fermentation. Also, as ethanol increases, tolerances to other stressors, such as low pH and temperature, decreases. Similarly, the presence of another stressor, such as a temperature shock, may decrease the cells ability to tolerate ethanol.

In the typical case, and particularly for native flora fermentations, the flora of the grape itself as well as of the winery can have a dramatic impact on the fermentation and its progression. Grape flora may be normal residents: that is, members of the biofilm coating the grape.· Or they may be transient, and on the grape because of deposition of soil, dust, or insect vectors. Whatever their origin, these microbes may be able to compete with Saccharomyces for nutrients, deplete the juice of nutrients, or produce end products that may be inhibitory.

Management of a fermentation is, therefore, a complex process of making sure adequate nutrients are present, minimizing the inhibitory potential of non-Saccharomyces organisms, maintaining a proper temperature to assure continued ethanol tolerance, astute use of antimicrobials, such as sulfite, and choosing the correct inoculation practices for the wine. It is also important to understand how factors prior to initiation of the fermentation may impact the progression of that fermentation.