Handbook of Enology: Volume 1. Pascal Ribéreau-Gayon

FIGURE 1.37 Breakdown of S. cerevisiae karyotypes during alcoholic fermentation in red grape tanks in the Fz vineyard (Pessac‐Léognan, France) in 1989 (Frezier, 1992). b, c, and d designate the start, middle, and end of alcoholic fermentation, respectively. Tanks I and II (Merlot) and III and IV (Cabernet Sauvignon) were filled on the 1st, 3rd, 17th, and 23rd day of the harvest, respectively.

The tank filling order and the grape variety have little effect on the clonal composition of the populations of S. cerevisiae spontaneously found in the winery. The daily practice of pumping over the red grape must, with pumping equipment used for all of the tanks, and the contact of must with the atmosphere in the cellar enriched with S. cerevisiae populations probably ensure the dissemination of the same strains in the winery. In white winemaking, the wine press facility plays the same role as an inoculator.

      The same major strain is frequently encountered for several consecutive vintages in the same vineyard in spontaneously fermenting red must tanks. In 1990, one of the major strains was the same as the previous year in the red must tanks of the Fz vineyard. Other strains appeared, however, which had not been isolated in 1989.

      When sterile grape samples are taken, pressed sterilely, sulfited at winemaking levels, and fermented in the laboratory in sterile containers, one or several dominant strains responsible for spontaneous fermentations in the winery exist in some samples. These strains are therefore present in the vineyard. In practice, they probably begin to multiply as soon as the grapes arrive at the winery. A few days into the harvest, they infest the winery equipment, which in turn ensures a systematic inoculation of the fresh grape crop.

The presence each year of the same dominant strain in the vineyard is not systematic (Table 1.8). In the Fz vineyard, the Fzlb2‐89 strain could not be isolated in 1991, although it was present in certain vineyard samples in 1990, 1992, and 1994. In 1993, another strain proved to be dominant in spontaneous fermentations of sterile grape samples.

The spontaneous microflora of S. cerevisiae seems to fluctuate. At present, the factors involved in this fluctuation have not been identified. In a given vineyard, spontaneous fermentation is not systematically carried out by the same strains each year, and there is thus no specific strain that is one of the vineyard's characteristics. Ecological observations do not confirm the notion of a vineyard‐specific yeast. Furthermore, some native strains, dominant in a given vineyard, have been found in other nearby or distant vineyards. For example, the Fzlb2‐89 strain, isolated for the first time in a vineyard in Pessac‐Léognan, was later identified not only in the spontaneous fermentation of dry white and red wines of other vineyards in the same appellation but also in relatively distant wineries as far away as the Médoc. This strain has since been selected and commercialized under the name Zymaflore F10.

TABLE 1.8 Rate of Occurrence of the Dominant Fzlb2‐89 Karyotype in Microvinifications Carried Out on Sterile Grape Samples (I, II, and III) in the Fz Vineyard (The Number of Clones Analyzed is Indicated in Parentheses)

In some cases (Figure 1.38), S. cerevisiae populations with a large clonal diversity carry out spontaneous must fermentation. Many strains coexist. Their proportions differ from the start to the end of fermentation and from one tank to another. Is it more favorable for the sensory characteristics of wine to have a fermentation with low or large clonal diversity? Few scientific works have addressed this question until now. Howell et al. (2006) showed that analytical profiles of wines obtained from mixed fermentations are different from those obtained with yeast monocultures. These cannot be reproduced by blending different monoculture wines. Evidently, metabolic interactions explain the differences observed between mixed fermentation and blending of monocultures. The contribution of a yeast clone to the aroma profile of a wine resulting from polyclonal fermentation cannot simply be predicted by this clone's population. Thus, the direct relationship between the diversity of yeast strains and wine complexity has not received until now any scientific demonstration and must be considered with care. In the Bordeaux region, the coexistence of a large number of yeast strains is often associated with slow fermentations, and sometimes even stuck fermentations before full depletion of sugars. No single strain seems to be capable of asserting itself. On the other hand, the presence of a small number of dominant strains generally characterizes complete and rapid spontaneous fermentations. These dominant strains are found from the start to the end of fermentation.

FIGURE 1.38 Breakdown of S. cerevisiae karyotypes in tank I of red grapes from the LG vineyard (Pomerol, France) in 1989 (Frezier, 1992). b, c, and d designate the start, middle, and end of alcoholic fermentation, respectively.