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The impact of malolactic fermentation on South African rebate wine
1Institute for Wine Biotechnology, Stellenbosch University
Key words: Malolactic fermentation, lactic acid bacteria, rebate wine, distillate, brandy
Introduction
Vinification includes two important fermentation processes, namely alcoholic fermentation, which is conducted by yeast, and malolactic fermentation (MLF), which is performed by lactic acid bacteria (LAB). During MLF l-malic acid is converted to l-lactic acid and CO2 (Lonvaud-Funel 1995). MLF in wine is desirable for three reasons: (1) to decrease the acidity, (2) production of flavour compounds, and (3) to increase microbiological stability. LAB occur naturally on grapes and in juice (Wibowo et al. 1985). The LAB associated with the vinification process include species of the genera Lactobacillus, Leuconostoc, Oenococcus and Pediococcus (Stiles & Holzapfel 1997; Lonvaud-Funel 1999). The growth of LAB is influenced by factors such as pH, temperature, SO2, ethanol concentration and the presence of nutrients (Wibowo et al. 1985; Britz & Tracey 1990).
Rebate wine is the base wine that is used in the production of brandy. During the production of brandy very little, or hardly any, SO2 is used (Leaute 1990). Sulphur dioxide is the winemaker's most important antimicrobial aid and it is also an effective anti-oxidant. Rebate wines also have a lower alcohol content (10-11%) and are fermented at 18°C. Rebate wines are consequently more susceptible to microbial spoilage than table wines. Many rebate wines might therefore be susceptible to MLF, due to the favourable growth conditions for the natural LAB that are present after the alcoholic fermentation.
The aims of these studies were to determine the extent of MLF in South African rebate wine, as well as the influence on the quality of the rebate wine and the resulting distillate, and to identify the LAB species responsible for the occurrence of spontaneous MLF (Du Plessis et al. 2002, 2004).
Experimental procedure
The complete layout of the experimental procedures can be found in Du Plessis et al. 2002 and 2004. Aroma profiles of the brandy distillates were compiled using the brandy aroma wheel (Jolly & Hattingh 2001). LAB were isolated from the juice and wine, and identified.
Results and discussion
Presence of lactic acid bacteria
LAB numbers in grape juice from the 1998, 1999 and 2000 vintages varied from 7 x 104 to 8 x 105 cells/ml, 2 x 102 to 4 x 103 cells/ml and 2 to 9 x 104 cells/ml, respectively. There were fewer LAB in the juice and wine of 1999 than in 1998 and 2000 (Fig. 1). A difference in climate could possibly explain the lower cell count. The lower LAB counts caused MLF to take longer in 1999 than in 1998 and 2000. In rebate wines having undergone MLF, the LAB count was higher than 1 x 106 cells/ml, while wines that did not undergo MLF, had a lower count. This trend was observed in experimental and commercial rebate wine samples.
Lactobacillus plantarum was the dominant species in the grape juice, but the viability of this species was drastically reduced in the course of the alcoholic fermentation. Oenococcus oeni was the dominant species in rebate wines having undergone spontaneous MLF. In three commercial rebate wines spontaneous MLF was induced by Lactobacillus hilgardii and Lactobacillus paracasei. In certain rebate wines Lactobacillus brevis, Lactobacillus vermiforme and Lb. paracasei developed once MLF was completed. The Lactobacillus species were identified during all the stages of rebate wine production, but the counts were usually low. There is a direct correlation between the presence of lactobacilli and a reduction in the rebate wine and distillate quality. The evaluation panel showed a preference for rebate wine samples in which only O. oeni occurred. No Pediococcus species were isolated or identified in any rebate wine.
Monitoring of commercial rebate wines
In 1998 14% of the commercial rebate wines monitored underwent complete MLF before distillation. A further 50% of these commercial wines underwent partial MLF. In 1999 3% of the commercial wines underwent complete MLF before distillation and 39% partial MLF. In 2000 10% of the commercial wines underwent complete MLF before distillation and 45% partial MLF. Results clearly indicate that spontaneous MLF does indeed occur, but that the extent may differ from year to year.
Sensorial evaluation of rebate wines and distillates
A triangular test was done to determine whether the evaluation panel could distinguish between rebate wines and distillates having undergone MLF and those that did not undergo MLF. The panel could indeed distinguish between these two treatments, which indicates that the two treatments differed significantly from each other. In most instances the panel preferred those that had not undergone MLF, mainly because many rebate wines that had undergone MLF displayed a lack of aroma and freshness.
There were more groups among the distillates, however, where it was not possible to distinguish between the two treatments. The aroma profiles of distillates that had undergone MLF and those that did not undergo MLF, are indicated in Fig. 2. Smooth associated, herbaceous and fruity aromas were more prominent in distillates that had not undergone MLF, while sweet associated aromas, such as chocolate and caramel, were more clearly noticeable in the samples that had undergone MLF. Negative aromas, such as solvent or chemical, were also more prominent in distillates that had undergone MLF. Malolactic fermentation did not influence all aroma components in rebate wine, but did have a significant influence on the fruity and negative aroma components.
Conclusion
This study indicated that LAB occur in high numbers in juice and rebate wine and are able to conduct MLF. In the majority of instances O. oeni was the species responsible for the occurrence of spontaneous MLF, but Lb. brevis, Lb. hilgardii, Lb. paracasei and Lb. vermiforme were also found in rebate wines. There is a direct correlation between the presence of Lactobacillus species and the reduction in quality of the rebate wine and distillate. It was also found that spontaneous MLF occurs in South African rebate wine and that this definitely has an influence on the sensorial quality of the rebate wine and the brandy distillate.
Recommendations
Winemakers should inhibit the growth of the natural LAB population by storing rebate wine at low temperatures (10°C) as soon as the alcoholic fermentation is completed or by adding lysozyme at the beginning or after the alcoholic fermentation.
Acknowledgements
We would like to express our gratitude to the following persons and organisations: Distell for conducting trials on a commercial scale, the analyses, as well as their participation in the sensorial evaluations; ARC Infruitec-Nietvoorbij for the use of their distillation apparatus and for determining the aroma profiles of the distillates; the winemakers and the various cellars for their collaboration; the National Research Foundation and Winetech for financial support.
Contact person: Heinrich du Plessis
References
Britz, T.J. & Tracey, R.P. (1990) The combination effect of pH, SO2, ethanol and temperature on the growth of Leuconostoc oenos. Journal of Applied Bacteriology 68, 23-31.
Du Plessis, H.W., Steger, C.L.C., du Toit, M. & Lambrechts, M.G. (2002) The occurrence of malolactic fermentation in brandy base wine and its influence on brandy quality. Journal of Applied Microbiology 92, 1005-1013.
Du Plessi, H.W., Dicks, L.M.T., Lambrechts, M.G., Pretorius, I.S. and du Toit, M. (2004) Identification of lactic acid bacteria isolated from South African brandy base wines. International Journal of Food Microbiology 91, 19-29.
Jolly, N.P. & Hattingh, S. (2001) A brandy aroma wheel for South African brandy. South African Journal of Enology and Viticulture 22, 16-21.
Leaute, R. (1990) Distillation in alambic. American Journal of Enology and Viticulture 41, 90-103.
Lonvaud-Funel, A. (1995) Microbiology of the malolactic fermentation: Molecular aspects. FEMS Microbiology Letters 126, 209-214.
Lonvaud-Funel, A. (1999) Lactic acid bacteria in the quality improvement and depreciation of wine. Antonie van Leeuwenhoek 76, 317-331.
Stiles, M.E. & Holzapfel, W.H. (1997) Lactic acid bacteria of foods and their current taxonomy. International Journal of Food Microbiology 36, 1-29.
Wibowo, D., Eschenbruch, R., Davis, C.R., Fleet, G.H. & Lee. T.H. (1985) Occurrence and growth of lactic acid bacteria in wine. A Review. American Journal of Enology and Viticulture 36, 302-313.
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