|
A Technical Guide for Wine Producers
|
|
RECENT ARTICLES | WYNBOER HOME
CURRENT VINEYARD AND CELLAR EVENTS
The choice between spontaneous and inoculated malolactic fermentation (MLF)
The role of yeasts in the alcoholic fermentation of wines has long been known to and understood by most winemakers and their vinification practices are applied accordingly, but knowledge about the importance of the malolactic fermentation (MLF) of wines is still scant.The impact it has on the quality and microbiological stability of wines is often not recognised. It often happens therefore that winemakers decide, from a cost savings point of view, to leave the MLF to the natural flora. This saving may end up costing them a great deal more than they bargained for.
The bacteria that cause MLF in wines were only isolated and described in 1960 and classified as Leuconostoc oenos. In 1995 they were reclassified as Oenococcus oeni.
The three basic goals of MLF in vinification are the deacidification of the wine, the improvement of the bacteriological stability, as well as the changes in the aroma profile of the wine. The acid reduction may result in an increase in pH of 0,1 - 0,2 units.
The microflora of wine is very complex, but only lactic acid bacteria are able to convert malic acid into lactic acid. The generaLactobacillus, Leuconostoc, Oenococcus and Pediococcus are able to do so, but Oenococcus oeni is the preferred species for the MLF of all wines, as it is well adapted to grow in a wine with a high alcohol content, and low pH with limited nutrients.
Malolactic fermentation can obviously take place without inoculation or winemakers may decide to inoculate with MLF pure yeast cultures instead. The onset of MLF is extremely dependent on the initial bacterial count, however. The MLF will only become noticeable if at least 1 million MLF bacteria/ml are present. The growth and multiplication of the MLF bacteria are also heavily influenced by the pH, alcohol content, sulphur dioxide content and available nutrients in the wine. The wine temperature and yeast strain used for alcoholic fermentation will also play a role in this regard.
The spontaneous MLF in wine is therefore highly unpredictable as there are so many unknown factors that come into play. For example, it may only start months after completion of the alcoholic fermentation. Improved cellar hygiene will also limit the population of all flora, which makes spontaneous MLF even more unpredictable. In the case of spontaneous MLF there is very little control over either the timing or the bacteria that induce the MLF. Spontaneous completion may occur if the bacterial count is high enough, but this is obviously uncertain. The undesirable Lactobacillus and Pediococcus species usually dominate in wines with a high pH and may cause undesirable flavours such as mousiness, bitterness and volatile acid. There is also an increased risk that biogenic amines such as histamines may be formed, which may be extremely harmful to allergic persons. Improved control over the onset stage and the MLF tempo, with a reduced potential of spoilage, can therefore be obtained by inoculating wines with selected bacterial cultures. The inoculation with pure yeast cultures can take place at various stages. It is usually done towards the end or after completion of alcoholic fermentation.
The development of commercial pure cultures of predominantly Oenococcus oeni has increased over the years. The development of molecular biology has also made it possible to distinguish between strains of the same species. By so doing it is possible to follow the dynamics of the bacterial population during MLF. It is, for example, possible to follow the occurrence of natural and inoculated bacteria during MLF.
The orderly management of MLF has become increasingly important in view of the trend to produce wines with a high alcohol content. Although Oenococcus oeni is well adjusted to the drastic wine environment, wines with an alcohol content of 14 - 15% A/V will remain a challenge. Although the natural Oenococcus oeni will always occur, inoculation with pure yeasts that ensure a high population will at least increase the possibility that desirable bacteria conduct the MLF.
Reference:
Bartowsky, E J, McCarthy, J M & Henschke, P A, 2005. The Australian & New Zealand Grapegrower & Winemaker. Annual Technical Issue 2005: 49 - 52.
|
|
Wynboer is incorporated in WineLand, magazine of the SA wine producers.
Visit our sister sites:
|
| UP |
COPYRIGHT (C) 2000 WineLand |