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A Technical Guide for Wine Producers
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OPTIMUM TEMPERATURE - A DEFINITE PREREQUISITE FOR SUCCESSFUL MALO-LACTIC FERMENTATION INTRODUCTION Malolactic fermentation (MLF) entails the bacterial conversion of L-malic acid to L(+)-lactic acid and CO2, while bringing about a reduction in the total acid (Wibowo et al., 1985). This process can also change the sensory composition of wine through the production of aroma compounds which include inter alia acetoin, di-acetyl and ethylacetate (Davis et al., 1985). This secondary fermentation process is conducted by Leuconostoc oenos, as well as other malolactic bacteria. Various factors including pH, SO2 levels, alcohol percentage and temperature, can act synergistically and influence the metabolic activities of these bacteria negatively (Van der Westhuizen & Loos, 1981). The purpose of the study was therefore to focus on temperature as a limiting factor and use the findings to explain why MLFs during the 1997 season were slower than usual. At the same time the importance of temperature and the influence thereof on the course of MLF are clearly demonstrated. Winemakers are once again sensitised to the negative influence which temperature may have. EXPERIMENTAL PROCEDURE Chardonnay and Sauvignon blanc wines were used in this study and experiments were conducted at 15°C, 20°C and 22-23°C. Viniflora oenos was used to initiate the secondary fermentation. The course of MLF was monitored weekly. RESULTS AND DISCUSSION
Fig. 1. The course of malolactic fermentation at various temperatures in a Sauvignon blanc wine (using frozen must from the 1997 season) fermented with NT 45. Fig. 2. The course of malolactic fermentation at various temperatures in a Sauvignon blanc wine (using frozen must from the 1997 season) fermented with Vin 13. The MLF of Chardonnay and Sauvignon blanc wines prepared on a laboratory scale (using frozen must from the 1997 season), was positively influenced by temperatures of 20°C and 22-23°C, while 15°C obviously inhibited the course thereof (Fig. 1-4). Experiments conducted in Nietvoorbij's experimental cellar during the 1998 season confirm the results of the above-mentioned laboratory experiments. The course of MLF at 15°C, 20°C and 22-23°C in Chardonnay and Sauvignon blanc can be seen respectively in Fig. 5A and 5B. Temperatures of 20°C definitely made a positive contribution to the successful course of MLF, while the process occurred more slowly at 15°C. From this research it is thus clear that the optimum temperature for the successful/complete course of MLF is 20°C and higher.
Fig. 3. The course of malolactic fermentation at various temperatures in a Chardonnay wine (using frozen must from the 1997 season) fermented with NT 45. Fig. 4. The course of malolactic fermentation at various temperatures in a Chardonnay wine (using frozen must from the 1997 season) fermented with Vin 13. The course of MLF in the industry during the 1997 season was notably slower than usual. To a large extent this phenomenon can be explained by the influence of temperature. Data obtained from the Department of Agricultural Meteorology at Nietvoorbij clearly indicate that the average temperatures, the average maximum temperatures, as well as the average minimum temperatures for the Stellenbosch region were considerably lower during the 1997 pressing season than the temperatures for the corresponding periods of 1996, 1998 and the long term averages of the past 32 years (P D Haasbroek, personal communication). Furthermore the average maximum, as well as the average minimum temperatures for April 1997 (the month in which malolactic fermentations had to begin, once alcoholic fermentation had taken its course) were considerably lower compared to the corresponding periods of 1996 and 1998. The average maximum temperature for April 1997 was 3°C lower compared to April 1996, and 2,3°C lower compared to April 1998. The average minimums for the same periods were respectively 1,7°C and 1,4°C lower. Lower temperatures during the 1997 pressing season were also observed in other wine producing regions.
Fig. 5. The course of malolactic fermentation (MLF) at 15°C, 20°C and 22-23°C in a Chardonnay (A) and Sauvignon blanc (B) wine. The wines were primarily fermented with Vin 7, and Viniflora oenos was used to induce MLF. Therefore the very late pressing season of 1997 obviously meant that MLFs were initiated much later than usual. At that specific stage environmental temperatures were already lower than usual (as indicated above) and this necessarily had a detrimental effect on the course of MLF. SUMMARY Results of Van der Westhuizen & Loos (1981) support findings of the 1997 and 1998 research which clearly indicated 20°C to be more favourable than 15°C for the more rapid/ thorough course of MLF. Consequently these results show without any doubt that optimum temperature plays an extremely critical role in the successful course of MLF and that temperature per se can act as a limiting factor. RECOMMENDATIONS
ACKNOWLEDGEMENTS Ewarda Swart and Helena Kritzinger are thanked for the efficient way in which they conducted the malic acid analyses. The Vine and Wine Industry is thanked for financial support of project WW10/06. LITERATURE DAVIS, C., WIBOWO, D., ESCHENBRUCH, R., LEE, T.H. & FLEET, G.H. 1985. Practical implications of malolactic fermentation: A review. Am. J. Enol. Vitic. 36, 290-301. VAN DER WESTHUIZEN, L.M., & LOOS, M.A. 1981. Effect of pH, temperature and SO2 concentration on the malo-lactic fermentation abilities of selected bacteria and on wine colour. S. Afr. J. Enol. Vitic. 2, 61-65. 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. Am. J. Enol. Vitic. 36, 302-313. |
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