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A Technical Guide for Wine Producers
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Malolactic fermentation - a priority at Nietvoorbij
Heinrich du Plessis, ARC Infruitec-Nietvoorbij, Stellenbosch
Introduction
Wine production includes two important fermentation processes, i.e. the alcoholic fermentation conducted by yeast and malolactic fermentation (MLF), conducted by lactic acid bacteria (LAB). Malolactic fermentation entails the conversion of L-malic acid to L-lactic acid and CO2 (Lonvaud-Funel, 1995). This secondary fermentation process may occur spontaneously as a result of naturally occurring LAB, or it can be induced by imported commercial cultures. The most important reasons why MLF is required, are the reduction of total acid, the production of flavour compounds and the contribution to microbiological stability of the wine. The advantages of inoculation with LAB are better control over the induction and progress of MLF, as well as certainty about the specific LAB. Despite research into factors that influence the growth of LAB in wine (Wibowo et al, 1985; Britz & Tracey, 1990), the success of induced MLF cannot be guaranteed (Krieger, 1993).
Industry funded research into factors influencing MLF, as well as problems with the successful induction thereof, has been conducted at Nietvoorbij for the past 10 years. The factors that influence MLF have already been thoroughly studied and over the past three years the focus has been moved to characterisation of locally isolated LAB.
Performance of commercial LAB cultures
Each year existing and new commercial LAB cultures are evaluated to determine their MLF performance. Cultures that have already been evaluated include Viniflora oenos, Viniflora CH 35, Viniflora CH 16, Bitec D, Bitec Vino, EQ 54, Lalvin 31, Lalvin 41, Enoferm Alpha, Provino and Enodoc ML. Although the above-mentioned cultures can conduct a successful MLF, cultures differ with regard to the tempo at which the process takes place (Loubser, 1997; Du Plessis, unpublished data). Certain cultures are therefore better adapted to conditions occurring in South African wines, while others are more sensitive and easily hampered. Possible reasons for the failure of commercial LAB cultures include: incorrect inoculation practices, insufficient inoculation dosage, wine parameters that fall outside the desired boundaries (pH < 3.2, SO2 > 50 mg/L and alcohol > 14%), inhibition by yeasts, use of poor or sensitive cultures, as well as low viability of cultures.
Interaction between LAB and yeasts
Yeasts compete for nutrients and also produce compounds such as ethanol and SO2 that impair the growth of LAB. The SO2 production differs from yeast to yeast, however, not all yeasts inhibit the progress of MLF. Certain yeasts, e.g. NT 202, support the process or may even stimulate it. The inhibiting effect of the yeast is emphasised in cultivars where MLF is difficult to induce, while the use of a non-inhibiting yeast benefits the onset of MLF. Certain yeasts such as N 96, NT 112, NT 50 and VIN 13 may hamper the onset of MLF (Du Plessis, unpublished data). The end result is that MLF is more difficult to induce and therefore takes longer to be completed.
'Chardonnay' and 'Merlot' factor
In general white wines do not undergo MLF, but in the case of Chardonnay this is sometimes required. Although the reason(s) for the problems with MLF remain unclear, inhibiting phenolic compounds and/or the lack of certain nutrients have been suggested as reasons why MLF is more difficult to induce in Chardonnay. Results with regard to the course of MLF indicate that Chardonnay differs not only from region to region, but even within the same region. Certain cellars struggle annually with MLF. Individual cellar practices might shed light on the problem. As already mentioned, certain yeast strains are more inhibiting, and this is aggravated in Chardonnay (Loubser, 1999a; Du Plessis, unpublished data). If one or more wine parameters fall outside the boundaries, the chances of successful MLF decreases. The use of a sensitive LAB culture may also cause slow and incomplete MLF.
Merlot is the red wine version of Chardonnay and often experiences problems with the progress of MLF. As with Chardonnay, there are more questions than answers, but the same principles apply.
Other factors influencing MLF
The influence of temperature on the growth of LAB and the occurrence of MLF have been thoroughly researched (Van der Westhuizen & Loos, 1981; Wibowo et al, 1985). However, our results confirm that MLF occurs much more rapidly at temperatures of 20§C and above, than at 15°C and below (Loubser, 1999b). As in the case of temperature, the role of SO2 has also been thoroughly studied. High SO2 concentrations (40 - 50 mg/L) may delay or completely inhibit the onset of MLF. The inhibitory effect of SO2 is also aggravated in cultivars such as Chardonnay and Merlot. Some of the commercial LAB cultures are also more sensitive to SO2 than others.
The nutrient supplements provided with the commercial LAB cultures were also investigated. Nutrient supplements may improve the progress of MLF, but the improvement is influenced by the parameters of the wine. The addition of nutrient supplements drastically reduced the progress of MLF or had no effect. The use of nutrient supplements is of no use when most of the wine parameters fall outside the boundaries.
Malolactic fermentation occurs more rapidly in oak barrels than in steel tanks. The use of wooden chips and staves would therefore beneficial to the progress of MLF in steel tanks.
Selection and evaluation of local LAB
Induced MLF is recommended, but expensive commercial cultures are not always successful under South African wine conditions. The local LAB are better adapted and spontaneous MLF is therefore a regular occurrence. The dangers of spontaneous MLF include a negative contribution to flavour, production of volatile acids, increased ethyl carbamate production and production of biogenic amines. It is important, however, to distinguish potentially good LAB.
During the past three years local LAB strains have been isolated from juice and wine and their MLF ability investigated. Approximately 200 strains have been isolated, four of which are being tested in experimental wine production trials. The most promising isolates will be further characterised and used in commercial trials. Thereafter, negotiations will take place with suppliers of LAB cultures about the possibility of drying and commercialisation. The eventual goal is local LAB cultures that are able to conduct rapid and complete MLF under South African conditions.
Recommendations
Winemakers should decide before primary fermentation whether a specific wine has to undergo MLF. The wine should then be made in such a way that all the limiting factors such as pH, temperature, alcohol and SO2 levels fall within the required parameters. The selection of a suitable LAB culture is very important, as well as the use of a nutrient supplement as recommended. A yeast strain that does not inhibit MLF should preferably be used. The yeast supplier can be contacted for relevant information. These recommendations are especially important with regard to Chardonnay, Merlot and other problematic cultivars.
Should any MLF problems be experienced / regular spontaneous MLF occur in wine / or to become part of MLF research, contact Heinrich at tel (021) 809-3063, fax (021) 809-3002, email dplessishe@arc.agric.za.
Acknowledgements
Piet Loubser (Lallemand Suid-Afrika) for his contribution to the results. Winemakers from the respective cellars; suppliers of LAB culture (Lallemand and Lake International Technologies) and Winetech for financial support.
References
Britz, T J and 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.
Krieger, S A 1993. The use of active dry malolactic starter cultures. Wine Industry Journal, February, 56 - 62.
Lonvaud-Funel, A 1995. Microbiology of the malolactic fermentation: Molecular aspects. FEMS Microbiology Letters 126, 209 - 214.
Loubser, P A 1997. Die werkverrigting van appelmelksuurkulture in Suid-Afrikaanse wyne. Wynboer Tegnies, Mei, T10 - T11.
Loubser, P A 1999a. Die interaksie tussen appelmelksuurbakterieë, wyndruifkultivars en Suid-Afrikaanse wyngiste. Wynboer Tegnies, Mei, T48 - T50.
Loubser, P A 1999b. Optimum temperatuur - 'n besliste voorvereiste vir suksesvolle appelmelksuurgisting. Wynboer Tegnies, Mei, T56 - T57.
Van der Westhuizen, L M and Loos, M A 1981. Effect of pH, temperature and SO2 concentration on the malolactic fermentation abilities of selected bacteria and on wine colour. South African Journal of Enology and Viticulture 2, 61 - 65.
Wibowo, D, Eschenbruch, R, Davis, C R, Fleet, G H and Lee, T H 1985. Occurrence and growth of lactic acid bacteria in wine. A Review. American Journal of Enology and Viticulture 36, 302 - 313.
Summary
Malolactic fermentation - a priority at Nietvoorbij
In winemaking there are two important fermentation processes, e g the alcoholic fermentation, conducted by yeast and malolactic fermentation (MLF), conducted by lactic acid bacteria (LAB). During malolactic fermentation, L-malic acid is converted to L-lactic acid and CO2. Malolactic fermentation reduces the acidity of the wine, contributes to the flavour complexity and microbiological stability of the wine. Malolactic fermentation can occur naturally or can be induced with imported commercial starter cultures. Induced MLF provides better control over time of onset and rate of completion, as well as the specific LAB strain that carries out the process. The occurrence of MLF and factors affecting MLF has been the focus of our research for the last decade.
The performance of commercially available LAB starter cultures was investigated and it was found that some starter cultures performed better than others under the conditions found in South African wines. The interaction between the wine yeast and LAB had a big impact on the induction and progress of MLF. Certain yeast, e g NT 202 may stimulate MLF, while others i e N 96, NT 112, NT 50 and VIN 13 may delay the onset and completion of MLF. The combination of wine yeast and grape cultivar also has a big effect on the progress of MLF. Some grape cultivars, e g Chardonnay and Merlot do not support MLF as well as other cultivars. The inhibiting effect of the yeast is accentuated in these cultivars. It is clear, however, that the wine should be prepared so that the wine parameters are conducive to MLF, especially in cultivars known to be problematic. Temperatures higher than 20°C are more beneficial to the progression of MLF, than temperatures of 15°C and lower. Addition of nutrient supplements can be very beneficial to the progress of MLF. However, the addition of these supplements will not help if the wine parameters are not conducive to MLF. Wine kept in oak barrels completes MLF faster than the same wine that was stored in stainless steel tanks. Therefore, wood chips and staves can be of benefit to MLF taking place in steel tanks.
In the last three years we have already isolated 200 local LAB strains from must and wine and their ability to conduct MLF is being investigated. Four isolates look promising and have been used in experimental winemaking trials. These isolates will be further characterised and used in commercial trials. Our eventual aim is to have local LAB cultures adapted to South African conditions that are able to rapidly induce and complete MLF.
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