Biodiversity of Saccharomyces yeast strains from grape berries of wine-producing areas using starter commercial yeasts

Authors


  • Present address: Eva Valero, Departamento de Agroalimentación, IMIDRA. Apto. 127. 28800-Madrid, Spain.

  • Editor: Isak Pretorius

Correspondence: Eva Valero, Finca El Encin. Apto. 127, Madrid, Spain. Tel.: +34 91 887 9488; fax: +34 91 887 9492; e-mail: milvable@uco.es

Abstract

The use of commercial wine yeast strains as starters has grown extensively over the past two decades. In this study, a large-scale sampling plan was devised over a period of 3 years in three different vineyards in the south of France, to evaluate autochthonous wine yeast biodiversity in vineyards around wineries where active dry yeasts have been used as fermentation starters for more than 5 years. Seventy-two spontaneous fermentations were completed from a total of 106 grape samples, and 2160 colonies were isolated. Among these, 608 Saccharomyces strains were identified and 104 different chromosomal patterns found. The large majority of these (91) were found as unique patterns, indicating great biodiversity. There were differences in biodiversity according to the vineyard and year, showing that the biodiversity of Saccharomyces strains is influenced by climatic conditions and specific factors associated with the vineyards, such as age and size. Strains that were terroir yeast candidates were not found. The biodiversity of S. cerevisiae strains after harvest was similar to that in the early campaign; moreover, a temporal succession of S. cerevisiae strains is shown. This fact, together with the differences in biodiversity levels verifies that other factors were more important than commercial yeast utilization in the biodiversity of the vineyard.

Introduction

Traditional wine fermentation is a complex heterogeneous microbiological process involving the sequential development of various yeasts and other microorganisms present in musts, such as moulds and lactic and acetic acid bacteria. However, it is accepted that strains of Saccharomyces cerevisiae, known as ‘wine yeast’, are especially well adapted to this process and play a major role in the fermentation of grape musts (Rankine, 1968; Martini & Vaughan-Martini, 1990; de Barros Lopes et al., 1998). The origin of S. cerevisiae in spontaneous fermentation is rather controversial (Pretorius, 2000). Some authors consider that S. cerevisiae comes from the microbial community resident in the wineries. In the vineyard, yeasts may be transported from the soil to the grapes by various insects or by the wind. Surprisingly, fermentative species of Saccharomyces occur in very low numbers in grapes, the predominant microorganisms being apiculate yeasts and other oxidative species (Fleet & Heard, 1993). On the other hand, Mortimer & Polsinelli (1999) observed that damaged grape berries are rich depositories of S. cerevisiae, showing that the vineyard can be a natural store of S. cerevisiae. The importance of each yeast source – vineyard or winery – may vary greatly, depending on a large variety of factors, such as climatic conditions, including temperature and rainfall, the geographical location of the vineyard, the amount of SO2, antifungal applications, the harvest technique, the grape variety, the age of the vineyard, and the soil type (Pretorius, 2000).

Since the beginning of the 1980s, the use of active dried S. cerevisiae yeast starters has become increasingly common. Today, the majority of wine production is based on the use of commercial strains, which have been isolated from vineyards or wineries and selected for their superior properties for winemaking. This ensures rapid and reliable fermentations and reduces the risk of sluggish or stuck fermentations and of microbial contamination. The use of selected S. cerevisiae strains has greatly improved the reliability of the fermentation process and the quality of wines.

On the other hand, there is increasing interest in both indigenous strains of S. cerevisiae and wild yeast species that may contribute to the overall sensorial quality of wine, even in guided fermentations using selected S. cerevisiae starter cultures, and in the use of indigenous S. cerevisiae strains in mixed starter cultures tailored to reflect the biodiversity of a given region. Extensive ecological surveys using molecular methods of identification have been carried out with the aim of selecting new yeasts better adapted to local fermentation conditions (Pretorius et al., 1999; Khan et al., 2000; van der Westhuizen et al., 2000a). These and other publications (Versavaud et al., 1995; Lopes et al., 2002) report a great diversity of genetic patterns among the enological fermentative microbial communities. Saccharomyces cerevisiae strains seem to be widely distributed in a given viticultural region, and they can be found in consecutive years (Vézinhet et al., 1992; Torija et al., 2001); there are also strains predominant in fermenting microbial communities (Frezier & Dubourdieu, 1992; Sabate et al., 1998), suggesting the occurrence of specific native strains that can be associated with a terroir.

Preserving biodiversity is also important in order to ensure the conservation of gene pools of technological importance. With regard to this, several studies have been performed with the aim of assessing the impact of winemaking practices – including the extensive use of active dried yeast – on the natural microbial community. Monoculture practice was described as having a negative effect on the biodiversity of non-Saccharomyces wine yeast in a wine-producing region of Chile (Ganga & Martínez, 2004). Our results from a large-scale study in two different wine-producing areas, the Vinho Verde region in the north of Portugal and the Languedoc region in the south of France, show that dissemination of commercial yeast in the vineyard is restricted to short distances and limited periods of time, and that they do not become implanted systematically in the ecosystem (Valero et al., 2005).

Against this background, the present study was performed with two aims: first, to examine S. cerevisiae biodiversity and its natural population dynamics over a 3-year period in the vineyards surrounding wineries where active dry yeasts were used as fermentation starters; and second, to establish a strain collection contributing to the preservation of S. cerevisiae genetic resources. The results from the Portuguese winemaking area were published recently by Schuller et al. (2005), and the present article gives the results from the French winemaking region.

Materials and methods

Sampling plan and fermentation

Grapes were harvested in three vineyards (A, B and C) around a winery, located in the Languedoc region, around the Mediterranean city of Montpellier. The vineyards were situated at distances of 30 and 80 km apart. In each vineyard, six sampling points were defined according to the predominating wind direction at a distance of between 100 and 1000 m from the winery, as shown in Fig. 1.

Figure 1.

 Geographical localization of the vineyards (A, B and C) in the Languedoc wine region of France, with an indication of the wineries and the sampling sites AI-AVI, BI-BVI and CI-CVI.

In order to evaluate the diversity among fermentative yeast communities during the last stage of grape maturation and harvest, two sampling campaigns were performed, before (early campaign) and after (later campaign) harvest. The gap between the two campaigns was about 10 days. This study was carried out over a period of 3 consecutive years (2001–2003); samples were always collected from the same area at a maximum radius of 5 m. With the present experimental design, 36 grape samples were collected each year. The grape variety was always Carignan, with the exception of the sample point situated closest to the north of the winery, where it was Mourvèdre in vineyard A, Cabernet in vineyard B and Merlot in vineyard C.

Approximately 2 kg of grapes, including the stems, were harvested in aseptic conditions from each sampling point and placed directly into sterile plastic bags, which were transported to the laboratory in cool bags. At the laboratory, grapes were crushed by hand in the plastic bags; these were then opened, and 180 mL of juice was poured into 250-mL sterile fermenters. The fermenters were placed in a temperature-controlled room at 20°C with mechanical agitation. Fermentation progress was monitored daily by weight determinations.

Yeast isolation

The yeast community present in the fermentation was evaluated when the must weight was reduced by 70 g L−1, corresponding to the consumption of about two-thirds of the sugar content. Must samples were diluted and spread on plates with YEPD medium (yeast extract 1% w/v, peptone 1% w/v, glucose 2% w/v, agar 2% w/v), and incubated for 48 h, after which 30 colonies selected at random were collected from each spontaneous fermentation.

Selection of Saccharomyces and molecular identification

To rapidly discriminate between Saccharomyces and non-Saccharomyces, every isolate was evaluated according to its ability to grow in a medium containing l-lysine as the sole nitrogen source (Barnett et al., 1990). The Saccharomyces strains not able to grow on l-lysine medium were further identified by pulsed-field gel electrophoresis (PFGE). To establish chromosomal profiles, yeast chromosomal DNA was prepared in plugs and analysed using the TAFE (transverse alternating field electrophoresis) system (Geneline, Beckman), as previously described (Blondin & Vézinhet, 1988). The gels were run for 6 h at 250 V with a 35 s pulse time, and then for 20 h at 275 V with a 55 s pulse time, at a constant temperature (14°C). Designations for observed distinct patterns were A1–A5, B1–B25 and C1–C77, corresponding to isolates from vineyards A, B and C, respectively. Identification of commercial yeasts was carried out by comparison of chromosomal patterns of 23 commercial yeasts used in the wineries and the different Saccharomyces strain isolates (Valero et al., 2005). Some examples of chromosomal patterns of the Saccharomyces strains isolated are shown in Fig. 2.

Figure 2.

 Examples of chromosomal profiles of commercial yeast and natural isolates of spontaneous fermentations. Profile B01 was identical to ICV-D254.

Differentiation between the indigenous Saccharomyces sensu stricto strains isolated was performed by PCR restriction fragment length polymorphism (PCR-RFLP) analyses of the internal transcribed spacer (ITS1) region of the 18S rRNA gene. The ITS1 region was amplified with the NS1/ITS2 primer pair, and the PCR products were digested with HaeIII and MspI restriction endonucleases and separated by electrophoresis as described by Redzepovic et al. (2002).

Results

Three vineyards (A, B and C) in the Languedoc region (south of France) were selected to study the evolution of Saccharomyces strain populations over a period of three harvest seasons (2001–2003). Two sampling campaigns were performed, one before and the other after the harvest, to evaluate in greater detail the fermenting yeast temporal distribution. In total, 106 grape samples were collected, of which 72 completed spontaneous fermentations. From these fermentations, 2160 colonies were isolated.

A large proportion of non-Saccharomyces strains was found in the isolates after fermentation, representing 72% of the total yeasts isolated over the 3 years. Analysis of 79 non-Saccharomyces isolates from the four fastest fermentations, by PCR-RFLP of the rRNA gene ITS region (Granchi et al., 1999), showed that these strains mainly belonged to the genus Kloeckera (data not shown). It is noteworthy that 2002 was an atypical year, owing to heavy rainfall (50% above normal) before and during the harvest, resulting in a greater application of antifungal sprays, which may explain the reduced number of Saccharomyces isolates (12%).

Based on the l-lysine method (Barnett et al., 1990), 608 Saccharomyces strains were selected from the 2160 isolates collected during the 3 years. These strains are not distributed in the same way, in terms of either space or time; 323 Saccharomyces strains were isolated in vineyard C, 194 in vineyard B, and only 91 in vineyard A. The same phenomenon occurred with the different harvests; the largest proportions (50%) of Saccharomyces strains were found in 2001 and 2003 (50% and 46%, respectively), and the number was 10 times smaller in 2002 (5.4%).

Molecular identification of the Saccharomyces strains by PFGE revealed a total of 104 different chromosomal profiles (Table 1). A large majority of chromosomal profiles (91) were found as unique patterns, and only 13 karyotypes were found in more than one fermentation.

Table 1.   Chromosomal patterns of 608 yeast isolates from spontaneous fermentations of collected grape samples from vineyards A, B and C, during the harvest of 2001–2003
SiteNumber of
isolates
Number of
Saccharomyces
strains
Number of
distinct
patterns
Number of
total patterns
Common
patterns
Vineyard A
2001
 E
  AINF
  AII300
  AIII AIV AV
AVI
NF
 L
  AI302612
  AII AIIINF 
  AIV30301 
  AV300 
  AVINF 
2002
 E
  AI300
  AIINF
  AIII AIV AV
AVI
1200
 L
  AI AII AIII AIV12001
  AVNF 
  AVI3011 
2003
 E
  AINF1
  AII30301 
  AIII AIV AVNF 
  AVI300 
 L
  AI3001
  AII AIIINF 
  AIV3041 
  AV300 
  AVINF 
Vineyard B
2001
 E
  BI BIINF 1
  BIII30101 B01
  BIV301 
  BV3051 B01
  BVINF 
 L
  BI3028217
  BII30291 B04C37
  BIII3011 B04C37
  BIV302914 
  BV BVI600 
2002
 E
  BI BII BIII BIV BV1500
  BVINF
 L
  BI3002
  BII30141 
  BIII BIV600 
  BV30221 
  BVI300 
2003
 E
  BI BIINF
  BIII300
  BIV BV BVINF
 L
  BINF5 
  BII300  
  BIII30272  
  BIV30263 B23C09
  BV
BVI
NF  
SiteNumber of
isolates
Number of
Saccharomyces
strains
Number of
distinct
patterns
Number of
unique
patterns
Common
patterns
  1. E, early campaign; L, later campaign; NF, not finished; NC, not collected.

Vineyard C
2001
 E
  CI3028127 
  CIINF  
  CIII302420 C05 B23C09 C18 C19
  CIV30205 C23 C26
  CV30286  
  CVI300  
 L
  CI30024 
  CII30301 C19 C33 C36 B04C37
  CIII30124  
  CIV301710 C33
  CV30282 B04C37
  CVI301412 C05 C19 C36
2002
 E
  CI CII CIII1800
  CIV CV CVI     
 L
  CI30211
  CII CIII CIV     
  CV CVI1500 
2003
 E
  CI CIINF1
  CIII30271 
  CIVNC 
  CV CVINF 
 L
  CI3030120C62
  CII30288 C67
  CIII3095 C18 C71
  CIVNC  
  CV30277 B04C37 C62 C67 C71
  CVI300  

Concerning the geographical distribution of repeated patterns, only two (B04C37 and B23C09) were found in different vineyards (B and C) and 11 in different sites at the same vineyard. Repeated patterns in a single vineyard were always found in vineyard C, with the exception of pattern B01, corresponding to the chromosomal profile of the commercial yeast ICV-D254, found in two different sites in vineyard B in the early campaign in 2001. In vineyard C, patterns C23 and C26 were found in different sites, in the early campaign only, and C33 and C36 in the later campaign, both in 2001. C05 and C19 were found in the early and later campaign in the same years, but in different sampling sites. In 2003, three repeated patterns were found only in the later campaign (C62, C67 and C71) and one other (C18) was found in two fermentations, one in the early campaign in 2001 and the other in the later campaign in 2003. Pattern B23C09 was found in two vineyards, in the early campaigns of 2001 in vineyard C, and in the later campaign of 2003 in vineyard B. The chromosomal profile most widely distributed was B04C37; this was found in five fermentations from two different sites in vineyards B and C in 2001 and in one site in vineyard C in 2003, always in the later campaign.

As mentioned previously, the first sampling campaign was performed some days before the harvest, and the second a few days after the harvest, in a time frame of about 10 days. This study revealed a succession of Saccharomyces strains, given that the patterns of autochthonous strains from the early campaign never appeared in the later campaign. Nevertheless, it is possible that some differences can be attributed to the fact that different grape bunches were collected. Although these were situated close together, their microbial communities may have varied. In contrast to the results obtained in the Vinho Verde region of Portugal (Schuller et al., 2005), where spontaneous fermentation was verified rarely from grapes collected some days before the harvest, in our study, 54% of grape samples collected in the early campaign were able to ferment spontaneously compared to 83% of postharvest samples. The numbers of Saccharomyces strains collected were 173 and 436, in the early stage and late stage, respectively, a result that, according to Schuller et al. (2005), shows that the last stage of grape maturation appears to favour fermentative yeast proliferation on the grape surface.

The fermentation profiles of 72 grape samples that completed spontaneous fermentation are shown in Fig. 3. Whereas the Portuguese results (Schuller et al., 2005) show that only Saccharomyces strains were isolated after fermentation, in French wineries many non-Saccharomyces strains were involved in the autochthonous fermentations. Fifty-eight percent of fermentations were exclusively carried out by non-Saccharomyces strains; the large majority of these fermentations were produced from grape samples collected in 2002.

Figure 3.

Figure 3.

 Fermentation profiles of must samples collected in the early (grey) and late (black) sampling campaigns. Chromosomal patterns of strains isolated from the spontaneous fermentations are indicated. The predominating strains are underlined. Repeated patterns are highlighted in grey. Initial and residual sugar in spontaneous fermentations with Saccharomyces strains are indicated by bars.

Figure 3.

Figure 3.

 Fermentation profiles of must samples collected in the early (grey) and late (black) sampling campaigns. Chromosomal patterns of strains isolated from the spontaneous fermentations are indicated. The predominating strains are underlined. Repeated patterns are highlighted in grey. Initial and residual sugar in spontaneous fermentations with Saccharomyces strains are indicated by bars.

Figure 3.

Figure 3.

 Fermentation profiles of must samples collected in the early (grey) and late (black) sampling campaigns. Chromosomal patterns of strains isolated from the spontaneous fermentations are indicated. The predominating strains are underlined. Repeated patterns are highlighted in grey. Initial and residual sugar in spontaneous fermentations with Saccharomyces strains are indicated by bars.

Fermentations in which Saccharomyces strains participated were generally accomplished by a mix of Saccharomyces and non-Saccharomyces strains in different proportions, varying between 3% and 100% of Saccharomyces strains. These strains dominated in 20 fermentations, but only five of these were carried out exclusively by Saccharomyces strains. Spontaneous fermentations, mixed or not, were generally carried out by one to 20 Saccharomyces strains, with a predominance of one or more strains accompanied by a few or many minority strains, or by a very heterogeneous yeast community with no prevalent strain(s). Studies describing both situations have also been published (Khan et al., 2000; van der Westhuizen et al., 2000a, b).

As can be seen in Fig. 3, the greatest number of strains were involved in fermentations of must from grapes collected in 2001 from vineyard C. Grape samples from vineyard A produced a lower number of spontaneous fermentations, only five in the 3 years studied, all accomplished by only one S. cerevisiae strain, which was always different. Two of these represented 100% of the yeasts isolated. Grape samples from vineyard B produced 10 spontaneous fermentations, of which six were carried out by only one Saccharomyces strain and four by two to 14 strains. Of 15 spontaneous fermentations produced from grapes collected in vineyard C, only four were carried out by a single strain and 11 by more than one strain, varying between two and 20.

If we consider the number of Saccharomyces strains involved in spontaneous fermentation by years, we find that 78 different Saccharomyces strains were involved in 18 fermentations of grape samples collected in 2001, four different strains in four fermentations performed in 2002, and 22 strains in seven fermentations from grapes collected in 2003. In addition, the later campaign resulted in a greater number of spontaneous fermentations involving a greater number of Saccharomyces strains.

It is important to point out that the distribution of strains is not associated with the capacity to predominate in fermentation. The most widely distributed strain (B04C37) was involved in five fermentations and dominated in only two of these (BII-2001 and CV-2001), being a minority strain in the others (BIII-2001, CII-2001 and CV-2003). In the latter case, this strain accounted for only 3–20% (one to six strains) and was accompanied by one to seven other strains. Commercial yeasts were only found in three fermentations. Chromosomal pattern B01 was identical to that of commercial yeast ICV D254, initially isolated in this region, and was found in two fermentations (BIII-2001 and BV-2001) in the early campaign. Pattern C72 was identical to that of K1M-ICV INRA, found in fermentation CIII-2003 in the later campaign. These three fermentations were mixed Saccharomyces and non-Saccharomyces and did not dominate the fermentations in any case. Whereas ICV D254 was the only Saccharomyces strain found in these fermentations, K1M-ICV INRA was accompanied by another four Saccharomyces strains, the majority strain being pattern C71 (Fig. 3).

After the surprising observation that S. paradoxus, normally associated with oak species (Quercus robur or Quercus mongolica) in Europe, the Far East and North America (Naumov et al., 1992, 1998), appeared to occur in far greater numbers than S. cerevisiae in the indigenous population of Saccharomyces sensu stricto in Croatian vineyards, we wanted to investigate whether this was a particular case or could occur in the Languedoc region of the south of France. The analysis of the 104 indigenous strains with different karyotypes isolated in this region by PCR-RFLP analyses of the ITS1 region of the 18S rRNA gene (Redzepovic et al., 2002) indicated that only one (pattern A04) of the 104 strains with different chromosomal profiles was S. paradoxus (Fig. 4). This strain was found in the later campaign of 2003, in vineyard A, and completely dominated the fermentation, as can be seen in Fig. 3 (AII-2003). This strain exhibited a bad fermentation performance compared with the majority of S. cerevisiae strains, taking more than 45 days to complete the fermentation.

Figure 4.

 Examples of PCR-RFLP patterns of the ITS1 region of some Saccharomyces strains isolated from spontaneous fermentations. Pattern A04 was identified as Saccharomyces paradoxus and the others as Saccharomyces cerevisiae.

Discussion

It is well known that grape yeast communities vary from area to area and from vintage to vintage (Frezier & Dubourdieu, 1992; Vézinhet et al., 1992; Schütz & Gafner, 1994). Several of these studies have been carried out in wineries of different regions of France, from spontaneous fermentations. Although these studies gave interesting conclusions, a larger-scale study of grape-associated yeast in the vineyards was necessary, in order both to evaluate the biodiversity and natural dynamics of autochthonous populations of Saccharomyces and to evaluate the impact of the use of commercial selected yeasts on biodiversity.

In the present study, 104 different chromosomal patterns were found among 608 Saccharomyces strains selected from 2160 isolates obtained from three different vineyards in the Languedoc region, over a 3-year period. This same study was carried out in the Vinho Verde region in the north of Portugal, published recently by Schuller et al. (2005). Important differences were observed between the two studies with regard to the proportion of Saccharomyces strains found in the isolates after fermentation. In Portugal, mtDNA RFLP (HinfI) patterns of all the isolates after fermentation showed a Saccharomyces-type profile, whereas in France a large proportion of non-Saccharomyces strains were found (Fig. 2). The non-Saccharomyces strains represented 66% of the total yeasts isolated over the 3 years. These data confirm previous reports indicating that S. cerevisiae is not present in large numbers in vineyards (Pretorius, 2000). The majority of non-Saccharomyces strains were isolated in 2002, probably due to heavier than usual rainfall. As previously described (Longo et al., 1991; Angulo et al. 1993; Ganga & Martínez, 2004), these conditions both produced musts with lower sugar content and slower fermentations and made it necessary to increase the antifungal treatment of the vines, which may at least in part explain the decrease in S. cerevisiae strains during the fermentation. Nevertheless, some fermentations carried out exclusively by non-Saccharomyces strains were also able to complete the fermentation (e.g. AI-2002 or AIV-2002 in postharvest campaigns), producing 10–11% (v/v) of ethanol. This fact was previously observed by Torija et al. (2001), who showed the presence of non-Saccharomyces strains in fermentation stages with a high ethanol content. However, in practice the impact of non-Saccharomyces strains would be less, as a result of the addition of SO2 to industrial fermentations in wineries.

The methodology used, based on analysis of the yeast community after spontaneous fermentation, permitted the selective isolation of Saccharomyces wine yeasts, which do not appear on the grapes in great abundance. As a contribution to the still vigorous debate about the origin of wine yeast (Vaughan-Martini & Martini, 1995; Martini et al., 1996; Mortimer & Polsinelli, 1999; Martini, 2003), our results indicate the presence of a sufficient number of S. cerevisiae strains in the vineyard to carry out a spontaneous fermentation if the sample size permits, as proposed by van der Westhuizen et al. (2000b). It should be noted that among the 30 colonies analysed per fermentation, an average of about four different Saccharomyces biotypes per sample was observed, varying between one and 21 different biotypes. This indicates that the number of colonies analysed per sample was high enough to reflect the initial biodiversity. Nevertheless, our data refer only to yeast strains capable of surviving the conditions imposed by fermentation, and therefore give a distorted picture (underestimation) of the kinds of strain that really occur in vine. However, we regard our approach as an acceptable compromise that allows good estimation of population composition, but no precise description in terms of relative strain abundance in nature is possible.

Differentiation between the four species of the Saccharomyces sensu stricto group (S. cerevisiae, S. bayanus, S. pasteurianus and S. paradoxus) of the Saccharomyces strains isolated in the Languedoc region in France indicated that all belonged to the species S. cerevisiae, except for one strain of S. paradoxus. The previously reported large distribution of S. paradoxus in Croatian vineyards (Redzepovic et al., 2002) is a particular case, not generalizable to other winemaking areas such as that analysed in this study, supporting the idea suggested by the authors of the presence of specific indigenous yeasts that are better adapted to a specific grape-growing area.

The large majority of the 104 chromosomal patterns of S. cerevisiae strains identified were unique, demonstrating an enormous biodiversity of indigenous S. cerevisiae strains in this region of France. Considering the ratio between the number of Saccharomyces isolates and the number of patterns as an approximate biodiversity estimation, our overall results (about six strains per pattern) showed similar values to those found in Portugal by Schuller et al. (2005) and in previously published studies on the genetic diversity of indigenous S. cerevisiae strains in other viticultural regions of France (Vézinhet et al., 1992; Versavaud et al., 1995). In our study, this general estimation includes different situations, in contrast to the Portuguese results, where no apparent correlation between the number of strains involved in a fermentation and sampling site, year or vineyard was found (Schuller et al., 2005). If we make an estimation per vineyard, we find that the biodiversity was significantly greater in vineyard C, where only four strains per chromosomal pattern were found, and estimated biodiversity was much less in vineyard A, where 18 isolates of Saccharomyces per karyotype were found. The value for vineyard B was eight. We could not consider the influence of the grape variety in this study, given that Carignan was principally used in the three vineyards, with the exception of one sample per vineyard, corresponding to fermentations AI, BI and CI from the grape varieties Mourvedre, Cabernet and Merlot, respectively, for vineyards A, B and C. As the three vineyards are geographically close, they are included in the same climatic zone, meaning that intra-annual differences in terms of greater or lesser biodiversity of autochthonous S. cerevisiae strains per vineyard must be attributed to specific factors associated with the vineyard, such as the age and size (Pretorius et al., 1999), which would have a positive effect on the biodiversity of S. cerevisiae strains. As a reference, winery C, where the greatest biodiversity was found, was established in 1937 and is the largest winemaking area in the region, as well as one of the largest in Europe, with 2250 ha of vineyard, whereas winery A, where biodiversity was less, was established in 1951 and has c. 700 ha.

We also observed important differences when estimating biodiversity per year; whereas in 2001 the number of strains per karyotype was five, in 2002 it had doubled, and in 2003 it was in between (seven). The strong decrease in the biodiversity of S. cerevisiae strains in 2002, in accordance with the observations of other authors (Longo et al., 1991; Angulo et al., 1993), was probably due, as we mentioned previously, to the particular climatic conditions of this year. An increase in the biodiversity of S. cerevisiae strains was observed when weather conditions returned to normal in 2003. Further studies could be designed in order to explore each of these factors in greater depth.

The yeast community of each year was characterized by the appearance of many new patterns, indicating the fact that the behaviour of the large majority of the strains was not perennial. This may be attributable to the fact that only 12 × 2 kg of grapes per vineyard and year were sampled, and this may have been insufficient to detect the entire biodiversity of the given area. The last stage of grape maturation appears to favour fermentative yeast proliferation on the grape. This is due to damage to the grape skin, and leakage of must from the berries, attracting insects, which are the probable source of yeast on these grapes. A first sampling campaign was performed some days before the harvest, and a second a few days after the end of the harvest, in a time frame of 10 days, in order to assess the temporal distribution of fermenting yeast populations during the harvest. According to Rosini et al. (1982), only 5% of the grapes collected before vintage contain yeast, this number being much higher (60%) during vintage. Our results show that before vintage, 40% of samples were able to ferment spontaneously, although only 11% contained S. cerevisiae strains, compared to 60%, of which 30% contained S. cerevisiae strains, in postharvest samples. The estimated biodiversity of associated strains in the early and later campaign was five and six strains per chromosomal pattern, respectively; therefore, the biodiversity of grape-associated yeast in the later campaign did not seem to increase significantly in our studied area, in contrast to the results from the Vinho Verde region of Portugal (Schuller et al., 2005). Furthermore, as occurred in the Portuguese study, autochthonous strain patterns from the early campaign did not appear in the later sampling campaign, showing a temporal succession of S. cerevisiae strains.

With respect to the impact of the utilization of commercial yeast as a fermentation starter in the wineries, our study appears to show that the biodiversity of autochthonous species of S. cerevisiae remains very close to that reported in other studies, including fermentations in wineries where no commercial wine yeast strains have been used (Frezier & Dubourdieu, 1992; Vézinhet et al., 1992; Versavaud et al., 1995; Sabate et al., 1998; Torija et al., 2001). Furthermore, the fact that we found very different levels of biodiversity in the three vineyards studied (A, B and C) around the wineries that had utilized commercial yeast in large quantities for a long time verifies that other factors were more important than commercial yeast utilization for the biodiversity of the vineyard. This is because dissemination of commercial yeast in the vineyard surrounding the winery was almost completely absent (Valero et al., 2005). Only two chromosomal patterns identical to that of commercial yeasts were found: B01, which corresponds to the profile of S. cerevisiae strain ICV D254, found in vineyard B; and C72, which corresponds to the profile of S. cerevisiae strain K1M ICV-INRA, in vineyard C. This fact could be an indication of previous dissemination, but this cannot be confirmed, as strain ICV D254 was initially isolated from the same region of the south of France where the study was carried out. No commercial yeasts were found from winery A, and one colony, isolated in 2003 in winery C, had the same profile as K1M ICV-INRA, used in all three French wineries for the last 5–15 years. Furthermore, no implantation in the fermentation was produced, as the presence of indigenous strains was not affected, and only one isolate corresponding to this profile was found, accompanied by non-Saccharomyces and another four S. cerevisiae strains.

Spontaneous fermentations, mixed or not, were generally carried out by one to 20 Saccharomyces strains. This is in agreement with other studies reporting the presence of one or two predominating strains, and a varying number of ‘secondary’ strains (Querol et al., 1992a, b; Schütz & Gafner, 1993; Versavaud et al., 1995; Constanti et al., 1997; Lopes et al., 2002), or the presence of many different strains with no prevalence (Sabate et al., 1998; Pramateftaki et al., 2000). The occurrence of both situations has also been described (Khan et al., 2000; van der Westhuizen et al., 2000a, b). The most widely distributed strain in this study (B04C37) did not show a perennial appearance or wider geographical distribution, as it was involved in only five fermentations, four of which were in 2001. For this reason, we cannot conclude that any one strain can be considered as a terroir yeast.

The present work, together with that carried out in Portugal (Schuller et al., 2005), is a large-scale survey of vineyard-associated strains performed in order to obtain a better understanding of the ecology of S. cerevisiae strains. We consider that these studies give interesting conclusions, allowing improved determination of factors influencing the biodiversity of indigenous populations of wine yeast. Studies of this nature are indispensable for the preservation of biodiversity and genetic resources, and as a basis for further biotechnological applications.

Acknowledgements

This study was supported by grant no. 657 C2 from the cooperation agreement between the Portuguese Institute for International Scientific and Technological Cooperation (ICCTI) and the French Embassy in Lisbon and the Marie Curie Fellowship of the European Community programme of Quality of Life under Contract QLK4-CT-2001-51873. The authors wish to thank C. Camarasa for their support during grape collection and sample processing. We would like to express our sincere gratitude to D. Delteil from ICV-Montpellier for his help in the selection of sample sites in France. We also appreciate the kind assistance of the enologists (B. Agay, E. Feneuil and E. Bru) and Directors (J.L. Refle, J. Combette and R. Bruno) of the wineries, facilitating sampling campaigns in the vineyards.

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