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New Wine Yeasts For South African Winemakers
Rodney Hart & Neil Jolly
Key words: wine, yeast selection, yeast breeding, yeast evaluation
INTRODUCTION
This project was started during a period when the local wine industry experienced a variety of serious fermentation problems. Problems with the locally selected yeasts could not be solved by turning to imported yeasts as many of these were not well adapted to local conditions. This Winetech/ARC funded project continues in spite of the fact that an extensive range of active dried wine yeasts (ADY) yeasts (local and imported) are currently available in South Africa. The ever-changing requirements of the dynamic wine industry cause many of the yeasts to fall short of the specific requirements of the winemaker thereby creating a need for new and better adapted yeasts. In the initial phases a comprehensive grape sampling and Saccharomyces cerevisiae yeast isolation strategy from different viticultural areas of the Western Cape was undertaken (Khan et al., 2000; Van der Westhuizen et al., 2000). Concurrent with this, the natural isolates, together with commercial strains, were used in a yeast breeding program resulting in 460 hybrid yeasts (Farmer, 2001). All the yeasts are currently in storage in the ARC Infruitec-Nietvoorbij yeast culture collection and genebank.
Ongoing consultations with the wine industry identified specific criteria for yeast development and seven yeasts (NT range) were developed and commercialised under licence agreement to Anchor Bio-Technologies. The yeast development program is still ongoing to satisfy the ever changing needs of winemakers who are continually striving to improve wine quality in an era of increasing competition. Changing market trends and new production technologies also dictate different wine styles and yeast development must keep pace with this.
Some of the areas currently receiving attention include yeasts suitable for the production of 1) aromatic white wines; 2) lower alcohol wines from grapes with high sugar content; 3) Stellenbosch region Chardonnay; and 4) brandy base wine. Current strategy is to select yeasts from the culture collection, evaluate them on laboratory-scale followed by evaluation during small-scale experimental wine production. This article highlights some of the progress made during the 2006 and 2007 vintages.
MATERIALS AND METHODS The yeast strains used in this study are shown in Table 1. The commercial yeasts served as reference yeasts in the respective sections of the study.
Laboratory-scale evaluations
The fermentation potential of the experimental yeasts strains were investigated in laboratory-scale (500 mL) fermentation trials. A previously frozen grape must was mixed thoroughly and aliquoted into fermentation vessels (750 mL glass bottles). After sterilisation (121°C for 20 minutes) 24 hour yeast cultures in YPD broth (Biolab, Merck) were inoculated (1% inoculum). Commercial yeast strains were included in the trials as references. Fermentations were conducted at an ambient temperature of ca. 15°C, and the CO2 weight loss was measured and used to monitor the fermentations. The residual glucose was also periodically checked, by using ClinistixÒ (Bayer Corporation, USA), until the fermentation went to dryness (total residual sugar < 5 mg/L). All fermentations were done in duplicate and strains meeting the specific selection criteria were used in subsequent small-scale (20 L) winemaking trials.
Small-scale winemaking
The grapes used for vinification originated from the ARC Infruitec-Nietvoorbij Farm vineyards and the chemical analyses of the base must are shown in Table 2. The wines were made in duplicate according to the standard Nietvoorbij cellar method (Eksperimentele Wynbeoordelingskomitee, 2006, 2007) with the exception that the yeasts were inoculated as wet cultures (24 h cultures in YPD medium at 2%) after the respective yeast suspensions were allowed to acclimatise. The active dried experimental yeasts (ADEY) were rehydrated in the way specified for VIN 13 commercially. Fermentations were conducted at an ambient temperature of 14°C and 24°C for the white and red wines, respectively.
Chemical and sensory evaluation
The wines were subjected to sensory and chemical analyses upon completion of fermentation trials. Chemical analyses included total SO2 (Ripper method, Cape Winelab [Pty] Ltd., Stellenbosch; Integral Bio-Tech Laboratory [Pty] Ltd., Paarl); volatile acidity, residual sugar and alcohol (Winescan method, The Institute for Wine Biotechnology, University of Stellenbosch). Descriptive sensory evaluation was done by different panels consisting of seven trained wine tasters five months after production. A ten centimetre unstructured line scale was used and the judges were asked to rate “fruity” and “vegetative”; “berry” and “plum”; “citrus/limey” aroma intensity (undetectable to prominent) for Sauvignon blanc, Pinotage and Chardonnay, respectively. All wines were also rated on general quality (unacceptable to excellent). The average values of the duplicate wines are reported.
Drying of experimental yeasts
Promising experimental yeasts were dried by Anchor Bio-Technologies, Cape Town in their pilot plant. These yeasts were subsequently evaluated in laboratory-scale fermentation trials and small-scale winemaking as ADEY.
RESULTS AND DISCUSSION
The process of developing yeasts by natural selection and breeding is time consuming. It starts with identification of industry needs and culminates with production of an experimental dried yeast that is then subjected to further evaluation (small-scale and/or commercial-scale). Feedback is received from various industry sources, including industry committees, yeast suppliers and representatives, as well as one on one visits with winemakers.
Screening of yeasts for the production of aromatic white wines
Yeast strains suitable for production of aromatic white wines from grapes of non-aromatic cultivars were identified as an industry priority (Karien Lourens, personal communication, 2004). As VIN 7 was previously recommended for use in neutral cultivars (Marais, 2005) and was also shown to have thiol releasing abilities (Swiegers et al., 2006), the potential of hybrid yeasts descended from VIN 7 in this regard was investigated. Fermentation trials were conducted in Sauvignon blanc as this cultivar has been shown to produce grapes with aroma-inactive, non-volatile, bound thiols (imparts guava aromas) that can only be released by wine yeast during fermentation (Swiegers et al., 2007).
The eight selected VIN 7 descendants i.e. NT 2, NT 40, NT 46, NT 51, NT 52, NT 53, NT 54 and NT 55 represent the most promising based on standard chemical analyses following laboratory-scale screenings in 2005 (data not shown). The small-scale wine production trials done in 2006 were with Sauvignon blanc grapes from a vineyard that did not historically deliver grapes with strong cultivar characteristics. These grapes were chosen so that the impact of the yeast on the wine aroma could be more clearly noted. Indications were that only NT 54 and NT 55 were able to produce wine equal or better than that of the references based on chemical analyses (Table 3). Sensory evaluation indicated that, although these wines were judged to be of a similar general quality to the reference wines, the NT 54 wine was more vegetative, but less fruity than the VIN 13 and NT 116 reference wines, whilst the NT 55 wine was more fruity than the reference wines. The wines produced by the remaining experimental yeasts were regarded as being of lower quality (data not shown).
The trials were repeated in the 2007 vintage with the same eight yeasts and an additional two wine yeasts i.e. KC/7 and KC/9. The Sauvignon blanc grapes and reference yeasts used were as for the 2006 trials. The new yeasts (KC/7, KC/9) were previously isolated from the Constantia region known for good quality Sauvignon blanc wine.
The experimental yeasts NT 40 and NT 55 were able to produce wine equal to that of the references based on chemical analyses (Table 4). These results were complemented by the sensory evaluation results that showed that the NT 40 wine was more vegetative, but less fruity than the VIN 13, VIN 7 and NT 116 reference wines. In support of the 2006 results, the NT 55 wine was again fruitier than the reference wines. The wines produced by the remaining experimental yeasts were regarded as lower in quality (data not shown). As NT 55 showed potential for two consecutive vintages this yeast is a good a candidate for further evaluations.
Screening of wet culture yeasts suitable for lower alcohol wine (< 14%) production
Climate change, together with issues of optimum ripeness and consumer preferences for lower alcohol wines presents challenges to the winemaker regarding wines, produced from grapes harvested at optimal maturity. These wines, usually produced from grapes with higher sugar content, have high alcohol levels (14% and above). Therefore, yeast strains suitable for the production of lower alcohol level wine from grapes with higher sugar content were identified as an industry priority. Low alcohol producing yeasts are also an international priority as was emphasised by a keynote speaker (Dr Vladimir Jiranek) at the recent 8th International Symposium on Innovations in Enology, Stuttgart (Gardner et al., 2007).
Genetic modification has the potential to create yeasts with lower alcohol production (Malherbe & van Rensburg, 2004). However, the South African wine industry will not easily use genetically modified (GM) yeasts due to the negative public opinion and the highly sensitive European export market that is largely against GM food products (Cape Winemakers Guild, 2006). The natural selection and breeding of yeast with lower sugar to alcohol conversion rates is therefore the only viable alternative. Simultaneously it will also enhance the natural, green image of wine. However, this path of yeast development will be difficult, since alcohol reduction might only be achieved in very small increments.
During a 2005 investigation into slow fermenters during small-scale Colombar wine production it was observed that three yeasts i.e. NT 2, NT 51 and NT 53 all produced wine with marginally less alcohol than the reference yeasts (D 47; CY 3097 [Lalvin, Lallemand] and VIN 13). In another investigation in 2006 a further four naturally isolated yeasts were also shown to produce wines with marginally lower alcohol than the reference yeasts. To investigate whether this observed lower alcohol could be repeated or was perhaps due to experimental error the yeasts were re-evaluated in 2007. Pinotage was again chosen for the trial and the resultant wines from 26°B grapes all had alcohol content below 15%. Unfortunately, only two yeasts showed promise i.e. KC/9 and NT 53 (Table 5).
As the experimental yeasts NT 53 and KC/9 both produced wines that were slightly lower in alcohol than the reference yeasts on two occasions, further investigation should be done. Although the reduction may be marginal, additional hybrid breeding and selective screening will now be conducted in an attempt to decrease the sugar to alcohol conversion rate in subsequent generations.
Stellenbosch region Chardonnay Three yeasts i.e. JV 23, MCB C6, and TV 21 all natural isolates from Chardonnay grapes, were previously evaluated as wet cultures and shown to have potential for non-wooded Chardonnay production. They were subsequently dried by Anchor Bio-Technologies in their pilot plant. These yeasts together with three other ADEY i.e. NT 239, NW 3 and HR5 were used for small-scale winemaking during 2006. The chemical analyses of the wine making trials showed that only NT 239 and JV 23 produced wine equal to that of the reference yeasts (data not shown).
Sensory evaluation of the wines indicated that only HR5 of above-mentioned three ADEY produced a wine with a better Chardonnay (limey and/or citrus) character and more body than that of wine fermented by the reference yeast VIN 7 (data not shown). The ADEY i.e MCB C6, NT 239, JV 23 and HR 5 were re-evaluated in small-scale wine making trials during the 2007 vintage. NT 239, JV 23 and HR 5 produced wine equal to that of the VIN 7 reference (Table 6). The wine produced by MCB C6 was regarded as of lower quality (data not shown). As JV 23 has consistently been producing good quality wine in the active dried form over two vintages (2006 and 2007), the wine making trials were scaled up to pilot-scale (100 L) in 2008.
Brandy base wine yeast
Starting in 1996 twenty-two yeast strains developed by ARC Infruitec-Nietvoorbij were evaluated as potential brandy yeasts. Subsequent small-scale and large-scale fermentations, followed by small-scale, pilot-scale and large-scale distillations with accompanying chemical analyses identified one yeast with potential for the preparation of a good quality brandy. This yeast strain i.e. NT 117 was dried by Anchor Bio-Technologies and evaluated on commercial scale in fermentations and distillations carried out in 1999 and 2000 by a commercial brandy producer.
The distillates were matured in barrels and evaluated annually for three years. The outcome lead to a recommendation that this yeast receive further attention as an alternative to the existing brandy base wine fermenting yeast strains (Du Toit, 2004). The yeast was subsequently dried again and evaluated in the 2005 season. A decision against commercialisation was taken, since NT 117 was found to be no better than VIN 13 and WE 372, respectively (G. Reid, personal communication, 2008).
GENERAL COMMENTS AND CONCLUSIONS
As discussed above, the different sections of the yeast development program are at different stages in reaching their specific targets. Some are far advanced e.g. brandy yeast and yeast for the production of aromatic wine, while others are only at the beginning stages e.g. low alcohol yeasts. Since 2004, seven experimental yeasts (JV 23, MCB C6, NT 239, NW 3, T 19, W 2 and NT 117) have been dried by Anchor Bio-Technologies. It is envisioned that NT 55 will be the next so that it can be evaluated on a larger scale in the near future. The evaluation of the yeast in the dried form is integral to the overall yeast development project. Promising yeasts in the wet form do not always dry successfully, or may not perform the same way once dried. Evaluations over vintages are also important to eliminate vintage differences in grape must composition.
Once an active dried yeast has shown its merit in the dried form during small-scale wine production, it needs to be evaluated by industry on a commercial scale. Here the willingness of the South African winemakers to test and evaluate new strains must be commended. Without this valuable input, no relevant decision on further development can be made. A decision on commercialisation will only be taken once feedback is obtained from industry. In the meanwhile, ongoing consultations with the wine industry will keep identifying new areas for yeast development so that the South African wine industry remains at the forefront regarding the most suitable yeast per application.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge:
For further information please contact Rodney Hart at hartr@arc.agric.za.
LITERATURE CITED
Cape Winemakers Guild, 2006. South African wine industry. [WWW document]. URL. http://wosa.co.za/sa/sustainable_gmyeast_statement.php [accessed on 19 June 2008].
Du Toit, M., 2004. Final report for Winetech: The selection and breeding of brandy yeasts with enhanced ester formation, Institute for Wine Biotechnology, Department of Viticulture and Oenology Stellenbosch University 7600 Stellenbosch.
Eksperimentele Wynbeoordelingskomitee, 2006. Parsprogram 2006 Landbounavorsingsraad (LNR) Infruitec-Nietvoorbij, Stellenbosch.
Eksperimentele Wynbeoordelingskomitee, 2007. 2007 Parsprogram van die Landbounavorsingsraad (LNR) Infruitec-Nietvoorbij, Stellenbosch.
Farmer, J., 2001. Progress report for Winetech: The selection, breeding, evaluation and characterisation of new wine yeasts, ARC Infruitec-Nietvoorbij, Private Bag X5026, Stellenbosch, 7599.
Gardner, J., McBryde, C., Astorga, M., Walker, M. & Jiranek, V., 2007. Tackling the impacts of climate change on wine making through novel and improved wine yeasts. 8th International Symposium Innovations in Enology. Messe-Kongresszentrum, Stuttgart-Killesberg, Germany 20. – 23 April 2007.
Khan, W., Augustyn, O.P.H., Van der Westhuizen, T.J., Lambrechts, M.G. & Pretorius, I.S., 2000. Geographic distribution and evaluation of Saccharomyces cerevisiae strains isolated from vineyards in the warmer, inland regions of the Western Cape in South Africa. S. Afr. J. Enol. Vitic. 21, 17-31.
Malherbe, D. & Van Rensburg, P., 2004. Let the yeast do the work for you: Less alcohol per bottle. Wineland. [WWW document]. URL http://www.wynboer.co.za/recentarticles/0406yeast.php3 [accessed 19 June 2008].
Marais, J., 2005. Factors affecting Sauvignon blanc wine quality: A South African perspective. [WWW document]. URL http://www.wynboer.co.za/recentarticles/200502sb.php3 [accessed on 19 June 2008].
Swiegers, H., Francis, I.L., Herderich, M.J. & Pretorius, I.S., 2006. The choice of yeast for fermentation offers great potential to adjust the aroma of Sauvignon Blanc wine. [WWW document]. URL http://www.newworldwinemaker.com/article_content.asp?id=203 [accessed on 08 Apr 2008].
Swiegers, H., King, E., Travis, B., Francis, L. & Pretorius, I.S., 2007. Enhancement of Sauvignon blanc wine aroma through yeast combinations. [WWW document]. URL http://www.wynboer.co.za/recentarticles/200712sauvignon.php3 [accessed on 19 June 2008].
Van der Westhuizen, T.J., Augustyn, O.P.H. & Pretorius, I.S., 2000. Geographical distribution of indigenous Saccharomyces cerevisiae strains isolated from vineyards in the coastal regions of the Western Cape in South Africa. S. Afr. J. Enol. Vitic. 21, 3-9.
SUMMARY This project was started during a period when the local wine industry experienced a variety of serious fermentation problems. Many of the local and imported active dried wine yeasts that were available to South African winemakers at the time fell short of the specific requirements. Consequently, a wine yeast development program was initiated at ARC Infruitec-Nietvoorbij that resulted in seven yeasts (NT range) being developed and commercialised between 1997 and 2004. This program is ongoing and currently various avenues of yeast development, as determined by the wine industry, are receiving attention. These include yeasts suitable for the production of aromatic white wines, lower alcohol wines from grapes harvested at high levels of ripeness, regional Chardonnay and a brandy base wine. Yeasts for each of these aspects are at different developmental stages and this article conveys some of the progress that was made during the 2006 and 2007 vintages. |
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