The case for spontaneous fermentation of wine grapes

Saccharomyces cerevisiae (SC) is the yeast species which completes the alcoholic fermentation process in both inoculated and spontaneous fermentations but the role of non-Saccharomyces (non-S) yeasts in the process should not be discounted. Spontaneous fermented wines carry a higher risk of spoilage but bring along the benefits of increased complexity, improved mouthfeel, and a higher degree of flavor integration (Jolly, et al.). This post, drawing heavily on the work of Jolly, et al., takes a closer look into the functioning of non-S yeasts in alcoholic fermentation.

Absent an inoculation, all yeasts found in grape must and wine will originate from one or more of the following sources: vineyard, grapes, or winery processing equipment (E.J. Bartowsky). Wine-associated yeasts are identified in the table below.

The general consensus was that all non-S yeasts died shortly after the beginning of alcoholic fermentation but, according to Jolly, et al., that is not borne out by more recent reasearch. Rather, the progression, they say, is as follows:

• H. uvarum is usually present in the highest numbers initially, followed by various Candida spp.
• The majority of the non-S yeasts disappear during the early stages of a vigorous fermentation
• May be a result of :
• Slow growth
• Inhibition of the combined effects of SO2, low pH, high ethanol, oxygen deficiency, nutrient limitation, and size or dominance of SC inoculants
• Non-S yeasts that do survive and are present till the end of the fermentation (Z. bailii, Pichia spp.)may have a higher tolerance to ethanol.

Jolly, et al., have conducted a literature survey which has identified a number of benefits that accrue to the practitioners of spontaneous fermentation:

• Lower ethanol yields -- as the authors point out, these yields "are sometimes the result of wines with higher residual sugar"
• A range of metabolic products to include terpenoids, esters, higher alcohols, glycerol, acetaldehyde, acetic acid, and succinic acid
• Hydrolization of glycosolated flavorless precursors by the enzyme ß-glucosidase to form free volatiles that can improve the flavor and aroma of wine 
• Several flavor and aroma compounds are present in the grapes as glycosolated flavorless precursors
• Enzyme ß-glucosidase not encoded by the SC genome
• Several of the non-S yeasts possess varying degrees of the enzyme
• Contribution to flavor production -- Non-S yeasts can be divided into neutral and flavor-producing yeasts. P. anomala, K. apiculata, and Candida pulcherima are flavor-producing non-S species, with the latter being known as a high producer of esters.
• Some non-S yeasts can consistently produce high glycerol concentrations during fermentation
• Glycerol important for regulating cell redox potential during fermentation
• Glycerol also contributes to smoothness, sweetness, and complexity of wine
• Glycerol production can also be associated with increased acetic acid production.

In summary, there are risks associated with spontaneous fermentation but those risks seem to be more than offset by the benefits that accrue to the user. And the number of great wines in France and Italy that utilize this approach attest to the fact that most of the leading producers have arrived at this conclusion.

Bibliography
E.J. Bartowsky, Bacterial spoilage of wine and approaches to minimize it, Letters in Applied Microbiology.
Neil P. Jolly, et al., Not your ordinary yeast; non-Saccharomyces yeasts in wine production uncovered, FEMS Yeast Research, 14 (2).
Loureiro and Malfeito-Ferreira (Spoilage yeasts in the wine industry, International Journal of Food Microbiology 86, 2003).

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