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THE INTERACTION BETWEEN MALOLACTIC BACTERIA, WINE GRAPE CULTIVARS AND SOUTH AFRICAN WINE YEASTS INTRODUCTION Although malolactic fermentation (MLF) may occur spontaneously, it is still necessary in some cases to inoculate wines with a specific commercial culture so as to bring about the process. However, the success of such an induced MLF is not guaranteed in all cases. The purpose of this study was to obtain more insight into the process of MLF and investigate the nature and extent of the interactions between bacteria, wine grape cultivars and South African wine yeasts in order to pass on possible procedures to winemakers which may ensure a successful MLF. EXPERIMENTAL PROCEDURE Tests were conducted on Chardonnay and Sauvignon blanc which had the following analyses before fermentation. Primary fermentations were conducted with yeast strains Vin 7, NT 7 (KAROOVIN), Vin 13, NT 45 (HEMELVIN), N 96 and NT 116 (STELLENVIN). Both Viniflora oenos and EQ 54 were used to initiate MLF. The course of the MLF was monitored on a weekly basis.
RESULTS AND DISCUSSION 1. The interaction between bacteria, yeasts and wine grape cultivars The interaction between the bacteria, yeasts and Chardonnay and Sauvignon blanc of Nietvoorbij is indicated in Figures 1 and 2. The yeasts Vin 7, N 96 and Vin 13 were used to conduct the primary fermentation (in this part of the experiment).
FIGURE 1. The course of malolactic fermentation (MLF) in Chardonnay (A) and Sauvignon blanc (B) wines, respectively fermented with yeast strains N 96, Vin 7 and Vin 13 to demonstrate the yeast/bacteria interaction. The MLF was induced with Viniflora oenos. The course of MLF (inoculated with Viniflora oenos) in a Chardonnay wine which was fermented primarily with Vin 7, N 96 and Vin 13, can be seen in Fig. 1A. The wine fermented with Vin 7 completed the MLF after 35 days (Fig. 1A). It is clear that very little of the initial amount of malic acid in the wine fermented with N 96 was decarboxilated. The wine fermented with Vin 13 did not have a complete MLF either. The course of MLF in Sauvignon blanc wines may be seen in Fig. 1B. The course of MLF in this wine fermented with Vin 7 was successful and occurred rapidly and completely. The MLF of the wine fermented primarily with N 96, however, was completed after 34 days, while the wine fermented with Vin 13 still had a residual malic concentration of 1,34 g/l after 34 days. Similar results were obtained in Chardonnay and Sauvignon blanc wines where the same yeasts were used, but with the addition of EQ 54 to initiate the MLF (Fig. 2A and 2B). The reason why the Sauvignon blanc wines which had been fermented with N 96 still had not completed the MLF after 34 days (Fig. 2B), as opposed to the previous Sauvignon blanc (Fig. 1B), is not quite clear. The fact that another bacterial strain had been used to induce the MLF could possibly be a factor. Further investigations in this regard are required.
FIGURE 2. The course of malolactic fermentation (MLF) in Chardonnay (A) and Sauvignon blanc (B) wines, respectively fermented with yeast strains N 96, Vin 7 and Vin 13 to demonstrate the yeast/bacteria interaction. The MLF was induced with EQ 54. These results, together with the fact that Chardonnay in general renders the course of MLF more difficult, emphasise the statement that cultivar, as well as yeast strain responsible for primary fermentation, definitely play a role in the successful course of MLF. The combined effect of yeast strain (N 96) and cultivar (Nietvoorbij Chardonnay) definitely had a detrimental effect on the successful/complete course of MLF. Comparable results from 1996 and 1997 (data not shown) indicated that Vin 7 in Chardonnay and Sauvignon blanc was less "limiting" with regard to MLF than N 96. In this regard the results from the 1996 and 1997 seasons support the 1998 results. The fact that Vin 13 did not support the course of MLF to the same extent as Vin 7 was also noticed in the 1996 and 1997 seasons. However, the extent to which it limited the course of MLF was not as drastic as in the case of N 96. This is an aspect which requires further investigation, considering the fact that Vin 13 is the yeast strain most commonly used in the industry. During the 1997 season Vin 13 comprised more than 30% of the total sales of active dry wine yeast. The possibility of seasonal differences in this regard cannot be excluded either. With N 96 being one of the parents of Vin 13, the negative bacteria/yeast interaction may possibly be considered as a genetically transferable characteristic. Another factor which may impact negatively on the functioning of malolactic bacteria is SO2, since very low levels thereof (free SO2 = 10 mg/l) may have an extremely limiting effect on malolactic bacteria. During the 1996/97 vintage the possibility was raised that the "negative" functioning of N 96 might possibly be ascribed to the fact that during primary fermentation the yeast strain produces SO2 which has a negative influence on malolactic bacteria during the secondary fermentation. Gafner et al. (1997) indicated that the production of high levels of SO2 by sparkling wine yeasts also used for primary alcoholic fermentations may result in an increase in the time required to initiate or complete MLF. It is thus clear that SO2 production by yeasts responsible for primary fermentation could indeed play a role in the optimal functioning of malolactic bacteria during the secondary fermentation. The issues of SO2 production by locally produced yeasts during the primary fermentation, and the subsequent influence thereof on MLF, are currently being investigated. 2. The interaction between bacteria and newly released Nietvoorbij yeasts (for white wine production) The interaction between bacteria and newly released yeasts by Nietvoorbij Microbiology was investigated in Chardonnay and Sauvignon blanc. New yeast strains NT 7, NT 45 and NT 116, as well as two existing yeasts, namely Vin 7 and N 96, were used in this part of the study. The course of MLF in the above-mentioned wines may be seen in Fig. 3A and 3B.
FIGURE 3. The course of malolactic fermentation (MLF) in Chardonnay (A) and Sauvignon blanc (B) wines, respectively fermented with yeast strains Vin 7, NT 7, NT 45, N 96 and NT 115 to demonstrate the yeast/bacteria interaction with Nietvoorbij's new releases of yeast strains. The MLF was induced with Viniflora oenos. It is quite clear that yeast strain Vin 7, as well as NT 7 and NT 45, "supported" the course of MLF in both cultivars. The course of MLF in both cultivars fermented with N 96 and NT 116, however, was not yet completed after 50 and 34 days respectively. The reason why NT 116 did not "support" the course of MLF to the same extent as the above-mentioned yeasts in this instance is not clear. Further investigations in this regard must certainly be undertaken. If the negative bacteria/yeast interaction (as mentioned above) is possibly transmitted genetically, it may well be that the "negative conduct" of NT 116 can also be explained by the fact that N 96 was used as one of the parent strains in the breeding process. Another aspect, concurring with the latter presumption, is that the inclination of N 96 (as one of the parents) to SO2 production may also have been transferred to NT 116 (daughter strain) and that the possible production of SO2 (by NT 116) may have had a negative influence on malolactic bacteria. Further investigations are required to determine the extent of the negative interaction between the bacteria and the new yeasts. The role of possible SO2 production by yeasts during primary fermentation and the subsequent influence on the course of MLF also require further investigation. From the past three years' results (data not shown), as well as the latest results, it is clear throughout that yeast strain N 96 in combination with Chardonnay definitely has a negative influence on the successful course of MLF. The extent of the interaction of other yeasts such as Vin 13 and NT 116 requires further investigation. RECOMMENDATIONS
ACKNOWLEDGEMENTS Ewarda Swart and Helena Kritzinger are sincerely thanked for the efficient way in which the malic acid analyses were conducted. The ARC and the Vine and Wine Industry are thanked for financial support of this very important project. LITERATURE GAFNER, J., ALBISSER, S., HOFFMANN, P., & PULVER, D. 1977. Ecology of yeasts and bacteria in vineyards and during winemaking - Molecular and physiological studies with practical conclusions. 18th International Specialized Symposium on Yeasts. Beld, Slovenia. LOUBSER, P.A., 1997. Die werkverrigting van appel-melksuurkulture in Suid-Afrikaanse wyne. Wynboer Tegnies. Mei 1997. T10-T11. |
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