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Native Fermentation by David Ramey
The focus of the presentation was a comparison of "native" or "natural" yeasts against "commercial" or "industrially-prepared" yeasts for primary fermentation of wine. Many of the conclusions were drawn from years of practical experimentation by Californian wineries, and in particular through Mr. Ramey's own experiences at Chalk Hill Winery.

The Chalk Hill Winery is established in the Sonoma Valley in California. Its vineyard area is 1100 acres, and produces 75,000 cases of wine annually, 45,000 of which is chardonnay. Mr. Ramey, who has a Master of Science degree from Davis University, was taught to use commercial yeasts through his University training, and it was not until he completed his qualifications and worked at various wineries did he run experiments on a regular basis dealing with native fermentation.

The experimentation focussed on the "French varieties" of chardonnay, cabernet sauvignon, and sauvignon blanc. An outcome of comparative studies lead the winemakers involved to conclude that the "French approach" to winemaking often produced "tastier" wines than those produced under the "New World" framework - the winemaking practices used to establish the comparisons involved extensive lees contact, full barrel fermentations, full malolactic fermentation, and the absence of skin contact.

Mr. Ramey indicated that his French winemaking experience in 1979 in Pomerol, where he returned in 1989, had encouraged him to reconsider the opportunities to harness native fermentation in California. Simultaneously, other experimentation occurred in California during the 1989 vintage on the same subject by other wineries. Results indicated that wines fermented with natural yeasts produced notable differences in the texture or mouth-feel of the wines, being; richer, rounder, "fatter" yet also possessing more delicate characteristics as well - a phenomenon difficult to explain given that "delicacy" and "fatness" are usually paradoxical terms. Subsequent comparative experiences in the next vintages confirmed the initial impressions.

Chalk Hill Wines pursued the "Burgundian Model";

  • No Skin Contact
  • Cluster Pressed
  • Full Barrel Ferment in French Oak
  • Full Malolactic Fermentation
  • Full Natural Yeast Fermentation
  • Fined with Whole Milk and Isinglass
  • Bottled with Minimal Filtration.
Presentation of comparative data of "natural" versus "industrial" yeast fermented wines, stemming from five years' of Chardonnay data and four years' Sauvignon Blanc data, indicated that wines fermented with natural yeasts;
Demonstrated better oak integration, in that the oak influence tended to be less obvious.

Reduced "butter" character as a result of lower concentrations of diacetyl. All yeasts (which are still active towards the conclusion of fermentation) contain an enzyme which reduced the prevalence of diacetyl (which lends to the "buttery character"). Since natural yeast ferments last longer, the yeast stay in suspention longer, putting them in contact with the diacetyl as it is produced by bacteria. It was suggested that this presented a more integrated overall style.

Bisulfite compounds, which are created microbiologically, occurred to a greater extent in (some) commercial-yeast fermentations than in natural-yeast fermentations.

Residual sugar levels tended to be higher in naturally fermented wines (1.5 grams per litre, compared to 0.9 grams per litre in wines fermented from commercial yeasts). It was suggested that the variation was minimal - both levels characterising dry and microbiologically stable wines. It was also suggested that the differences in residual sugar could explain the textural differences in the wines.

Volatile acidity was marginally higher in naturally fermented wines, but the variation being at a statistically insignificant level (this indeed can lead to greater complexity at low levels).

Flavournoid tannins, which is characterised in colour depth "yellowness" in young Chardonnay, was measured at a lower level in naturally fermented wines (8.73 - industrial vs. 8.29 - natural @ 280 nanometres). It was thought that because industrial yeasts take shorter time to ferment, the yeast spent comparatively less time in suspension than did natural yeast before falling to the bottom of the tank or barrel. Consequently, there was better opportunity for the active natural yeasts to reduce the tannin content as the suspended natural yeast had longer contact with the wine than the industrial yeast. It was recognised that French winemakers often stirred the wine following fermentation in order to achieve this increased exposure of yeast to the wine.

It was Mr. Ramey's view that there was one principal difficulty encountered with natural yeast fermentations, being the prevalence of "stuck" fermentations. Following one vintage (1992) of frequent stuck fermentations, an investigation of the various fermentation batches identified that those barrels with high sugar concentration which were stuck had completed their malolactic fermentations, whereas those with low sugar concentrations had not completed the malolactic fermentation. It was noted that one French observation was that primary (yeast) fermentation and secondary (malolactic) fermentations cannot occur simultaneously. However this rule had never seemed to be applicable in the USA where it was common that the fermentations could coexist. It was determined that the reason in this case had been the common use of industrial yeasts in the United States, and therefore natural-yeast fermentations would need to be treated differently.

An examination of the differences between industrial and native yeasts ensued. Californian winemakers tended to use dry industrial yeasts, as wet yeasts could lead to fermentation problems if the timing of inoculation was incorrect. It was concluded that dry yeasts were very vigorous and robust, and could be "nutrient scavengers". Comparatively, native yeasts tended to be more delicate or fragile. It was the relationship between the bacteria in the wine and the yeast that proved to be the cause of the stuck ferments.

It was seen that bacteria can promulgate before the comparatively more delicate natural yeasts had completed the primary fermentation, and as such the bacteria could interfere with the fermentation's completion through;

  • Nutrient competition, or
  • Toxin formation
In order to combat the bacteria from interfering with the yeast fermentation, two adjustments were initiated;
  • The winemakers ceased inoculating for malolactic fermentation, using only natural yeast with natural bacteria - however this only partly addressed the problem.
  • Sulfur dioxide was added (25mg/l) on the day after harvest to keep bacteria in check until complete ferment - the first day, with no added sulfur dioxide, allowed tannins to oxidise and precipitate out on the yeast cells, enhancing the character of the wine without compromising the yeast's capacity to complete the primary fermentation.
Characterisation of yeasts;
  • It was advised that it was easy to recognise differences in yeasts "under the microscope" in that natural yeasts varied significantly in shape and character whereas industrial yeast was homogenous in character.
  • Further sampling also identified that different strains varied from vineyard to vineyard, although this variation appeared not to make notable differences to the resulting wines.
A tasting of four Chalk Hill Wines followed the presentation which involved;
  • 1994 Chardonnay - Native Yeast
  • 1994 Chardonnay - Industrial Yeast
  • 1994 Chardonnay - Final Blend (which included a small component from the first two wines presented) with 95% Native Yeast composition
  • 1993 Chardonnay - Native Yeast demonstrating some aged characters.