Yeast & ML Bacteria in Winemaking at Stephen Ross
During the 2024 vintage at Stephen Ross, all fermentations were carried out using naturally occurring yeast and malolactic (ML) bacteria. Over the past two years, many winemakers I have met in both California and New Zealand have emphasized the benefits of native or indigenous fermentations—highlighting slower, cleaner, and cooler fermentations that result in wines with a greater expression of terroir. In recent years, our wines were fermented using a combination of cultured yeast and ML bacteria, alongside indigenous yeast and ML bacteria. However, in 2024, we fully converted to relying solely on naturally occurring yeast and bacteria.
In early August 2024, we harvested grapes from the 29 Pinot Noir vines growing directly in front of the winery to create a pied de cuve (PdC). Essentially, we initiated a small-scale fermentation about ten days prior to the onset of harvest. This PdC was treated like all red grapes received during last year’s harvest—it was blanketed with carbon dioxide to prevent the growth of acetic acid bacteria. We left it in the cellar, performed daily punch-downs, and after about eight days, fermentation began. To support yeast growth, we continued daily punch-downs and introduced food-grade oxygen at the beginning of fermentation. After about a week, fermentation was complete, yielding a relatively large yeast population. This culture was then used to initiate fermentation for the first several batches of grapes in 2024.
The first grapes received during the season were Pinot Noir for sparkling wine, followed by Sauvignon Blanc. These lots, or cuvées, were given the opportunity to undergo spontaneous fermentation using wild yeast present on the grape skins. However, when no significant reduction in sugar levels occurred after about a week, we introduced a small amount of the PdC to initiate alcoholic fermentation. In essence, the PdC served as an insurance policy to ensure fermentation proceeded when spontaneous fermentation did not start on its own.
Early in the harvest season, we also began cross-inoculating must or juice by transferring a small amount of actively fermenting juice to new lots of grapes. However, as the yeast population in the cellar naturally built up, fermentations began spontaneously in both red must and white juice without any intervention.
We experienced fermentations that were free from off-odors, remained at stable temperatures, and progressed at a steady, controlled pace—yielding outstanding results. Malolactic fermentation in both Chardonnay and all red wines occurred naturally during the fall and winter, despite our practice of steam-cleaning barrels.
So, should this process be called Native fermentation or Indigenous fermentation? I prefer the term "Naturally Occurring Fermentation," as the microorganisms originate from either the grapes or the cellar. These microbes exist all around us, and as winemakers, our role is to create a sanitary, temperature-controlled setting in which they can thrive and contribute to the winemaking process.