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Influence of the wine yeast NT 202 on the progress of malolactic fermentation
Heinrich du Plessis & Neil Jolly, ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Pinotage, Merlot, yeast, lactic acid bacteria.
Introduction
In winemaking there are two important fermentation processes, firstly the alcoholic fermentation, conducted using yeast and secondly the malolactic fermentation (MLF), conducted using lactic acid bacteria (LAB). During malolactic fermentation, L-malic acid is converted to L-lactic acid and CO2 (Lonvaud-Funel, 1995). Malolactic fermentation can occur spontaneously as a result of LAB that occur naturally, or it can be induced with commercial starter cultures. The most important reasons why MLF is required in South African wines, is the contribution it makes to the flavour complexity and microbiological stability of the wine. There are many factors that influence the growth of LAB in wine and therefore the progress of MLF; one of those factors being the wine yeasts used to perform the alcoholic fermentation, which may have an inhibiting or stimulating effect on MLF. Yeasts produce compounds i.e. ethanol, SO2 and medium chain fatty acids that inhibit the growth of LAB. Both yeasts and LAB compete with each other for nutrients. Other yeasts produce compounds which may promote the growth of LAB and nutrients are also released during yeast autolysis (Wibowo et al., 1985; Ribéreau-Gayon et al., 2000).
The wine yeast, NT 202, is a product of the yeast development programme at ARC Infruitec-Nietvoorbij. It is not genetically modified and was initially released for the production of Pinotage wines. Since its recent commercialisation it has also been used for red wine production from other grape cultivars. The effect of NT 202 on MLF is not generally known, so the aim of this study was to determine what effect NT 202 has on the progress of MLF in red wine.
Experimental procedure
Wine was made from Pinotage and/or Merlot grapes from the Nietvoorbij experimental farm for three consecutive vintages. Nietvoorbij’s standard, small-scale vinification techniques were used (Experimental Wine Judging Committee, 2003, 2004, 2005). NT 202 was compared with two other commercial red wine yeasts that are commonly used by South African winemakers (yeasts coded as A and B). The analyses of the wines after alcoholic fermentation are given in Table 1. These wines were used for the MLF trials. A variety of commercial LAB cultures, i.e. Viniflora oenos, Viniflora CH 35, Viniflora CH 16 (Lake International Products), Lalvin 41 (Lallemand) and a treatment where MLF occurred spontaneously, were investigated. The progress of MLF was monitored using paper chromatography and chemical analyses confirmed that it was completed.
Results and discussion
In 2003 the progress of MLF was tested in Pinotage wines only. From Table 2 it is clear that the wines fermented with NT 202 completed MLF in a much shorter period than wines fermented with Yeasts A and B, respectively. Even the NT 202 wines that underwent spontaneous MLF completed MLF in a shorter period than the Yeast A and B wines that had been inoculated with commercial MLF cultures. The alcohol content of these wines was very high (Table 1) and may be the reason for the slow progress of MLF in the wines that had been fermented with Yeast A or B. Surprisingly, MLF was completed more quickly in the NT 202 wines which had a higher alcohol concentration than the Yeast A and B wines. Yeasts A and B are both established and often used in red wine production. Yeast A is known to inhibit MLF and Yeast B is considered to be a "neutral yeast", i.e. it neither inhibits nor stimulates MLF.
In 2004 the progress of MLF in both Pinotage and Merlot was investigated (Table 2). Yeast B was left out of the investigation because despite its "neutral" status, it produced results similar to Yeast A in 2003. As in 2003, MLF was completed in a much shorter period in both Pinotage and Merlot wines fermented with NT 202, regardless of the MLF treatment. The only exception was the Viniflora oenos treatment which completed MLF in 21 days in Pinotage wines produced with NT 202 and Yeast A, respectively.
In 2005 the focus was on Merlot exclusively, which is considered a difficult cultivar due to worldwide problems with successful induction and progress of MLF. Once again MLF was completed much more rapidly in NT 202 wines than in Yeast A wines. The spontaneous MLF treatment of the wines produced using NT 202 completed MLF at the same time as the wines that had been inoculated with commercial MLF cultures.
It is still unclear if NT 202 produced compounds that stimulated MLF or was just less inhibiting than the other yeasts. This matter requires further investigation. Winemakers should also note that the use of NT 202 may encourage the occurrence of spontaneous MLF, which could have a negative effect on wine quality.
Recommendations
If MLF is required in a red wine, a yeast strain such as NT 202, which is not inhibiting, should be used. NT 202 is especially recommended for alcoholic fermentation where the induction of MLF may be problematic, e.g. Merlot. For more information about the influence of other yeasts on MLF, the relevant yeast suppliers should be contacted.
Acknowledgements
Our gratitude to MLF culture suppliers (Lallemand and Lake International Technologies), the technicians at Wine Microbiology at ARC Infruitec-Nietvoorbij, as well as ARC and Winetech for financial support.
For further queries contact:
References
Experimental Wine Judging Committee. 2003. ARC Infruitec-Nietvoorbij Harvest programme 2003, Stellenbosch.
Summary
In winemaking there are two important fermentation processes, firstly the alcoholic fermentation, conducted by yeast and secondly the 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 can occur naturally or can be induced with imported commercial starter cultures. Malolactic fermentation contributes to the flavour complexity and microbiological stability of the wine. There are many factors that influence the growth of LAB in wine, one of those being the wine yeasts used to perform the alcoholic fermentation. Yeast produce compounds i.e. ethanol, SO2 and medium chain fatty acids that inhibit the growth of LAB. Other yeasts can stimulate the growth of LAB by releasing nutrients during yeast autolysis. The wine yeast, NT 202 was originally selected for the production of Pinotage wines, but since its recent commercialisation it has also been used for red wine production from other grape cultivars. The effect of NT 202 on MLF is not generally known. The aim of this study was to determine what effect NT 202 had on the progress of MLF in red wine.
Results show that wines fermented with NT 202 completed MLF in a much shorter period than wines fermented with reference commercial red wine yeasts. This result was confirmed in both Pinotage and Merlot. The use of NT 202 also resulted in faster completion of spontaneous MLF. It is unclear if NT 202 produced compounds that stimulated MLF or was just less inhibiting than the other yeasts. This matter requires further investigation. Winemakers should also note that the use of NT 202 may encourage the occurrence of spontaneous MLF, which could have a negative effect on wine quality.
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