Malolactic fermentation in wine
Numerous species of lactic acid bacteria (LAB) have been isolated from wine, which can ferment various wine ingredients to lactic acid.
Lactic acid bacteria use glucose, fructose, pentose, malic, tartaric and other acids for their metabolism. Depending on which ingredient they use in their metabolism, they can act in two ways:
- Biological decomposition of malic acid into lactic acid – contributes to the quality of wine, making it less acidic, harmonious and softer, gentler and finally fully mature. With the decomposition of malic acid, wine becomes more stable with regard to lactic acid bacteria – the causes of spoilage
- Bacterial transformation of other ingredients – sugars, alcohol, acids … – leads to wine spoilage
During the wine production process, the proportion of acids decreases, due to the excretion of tartaric acid (potassium acid tartrate) and due to the decomposition of strong malic acid.
Malic acid has a bitter taste, and during malolactic fermentation it turns into weaker and milder lactic acid. Wines in which malolactic fermentation has been carried out have a softer, milder and more rounded taste, and the aroma is much more complex than in wines in which malolactic fermentation has not occurred.
The content of malic acid in grapes is 3-5 g/L, and sometimes up to 10 g/L. One smaller part is consumed during alcoholic fermentation, and the other, larger part is converted into wine and transformed in the process of lactic fermentation. The reduction in total acids can be 1-3 g / L, and sometimes more.
Lactic fermentation bacteria are resistant to: low pH, high alcohol content, low storage temperature and the addition of SO2.
The main products of malolactic fermentation are lactic acid and CO2. Secondary products can be formed during fermentation and they affect the taste and smell of wine in a certain way. These are, for example, acetaldehyde, acetic acid, diacetyl, ethyl lactate, acrolein …
Factors affecting the work of bacteria of malolactic fermentation:
- Alcohol – some bacteria are sensitive at 5-6% vol, while some are active at 14-15% vol. alcohol. The higher the alcohol content, the slower the breakdown.
- Temperature – to reduce malic acid in wine is most favorable 20-25 ° C. Degradation does not occur below 15 ° C or above 30 ° C. Therefore, in wines that contain little acid and do not want malolactic fermentation, the temperature should be taken into consideration.
- pH – Lactic acid bacteria decompose malic acid only above pH 3.2, and according to some data they are most active in the range of 4.3-4.8. At the acidity most commonly encountered in wine (pH 3-4), fermentation of malic acid will begin before the pH is closer to 4.
- SO2 – sulfurization delays the onset of malolactic fermentation. 50-70 mg / L of free or bound SO2 prevents malolactic fermentation.
- Polyphenols – inhibitory properties of barrel or tannin polyphenols
In which wines is malolactic fermentation desirable, and in which should it be prevented?
If the grapes or must contain less acid, after alcoholic fermentation the wine must be sulphurised, the first flow must be made earlier and the cellar temperature maintained up to 15 ° C. When wines are more acidic, with a higher proportion of malic acid, the wines must later be swollen from the lees, in order to release from the yeast cells the ingredients necessary for the growth of bacteria of malolactic fermentation, and should be less sulfurized.
It is desirable to carry out malolactic fermentation in:
- white wines naturally containing more acids (above 8 g / L) – poor grape ripening, in general northern vineyards
- red wines, which achieve optimum quality by longer maturation (aging wines)
- champagne wines
- white wines, which require a certain personality of the age they achieve by longer maturation
Malolactic fermentation is undesirable in:
- wines that naturally contain less acid
- white dry wines, with a fruity character and to which the amount of acids 6.8-7.5 g / L with 11-12 vol% alcohol ensures the harmony of taste
- wines obtained from varieties that have special organoleptic properties, because the typical taste and aroma changes in malolactic fermentation can cover the personality of the variety (for example, Rhine Riesling)
- rosé wines, which are characterized by freshness of taste
Reasons for conducting malolactic fermentation:
- Deacidification – the acidity of the wine is reduced by 1-3 g / L
– increase in pH by 0.1-0.3 units
– significant for strongly acidic wines
– may be undesirable for low acid wines
- Changing the aroma of wine – depends on the type of bacteria, the composition of the juice and the level of aeration
– due to the formation of numerous by-products, the aroma of wine becomes more complex
- Bacteriological stability of wine – consume nutrients used by other organisms
Measuring malic and lactic acid in wine
For measuring malic and lactic acid in wine, enzymatic methods are very simple and accurate to use. Enzymatic analysis imply a series of chemical reactions to covert a compound of interest into one that can be measured on a spectrophotometer via light absorbance. In this kind of reaction NAD+ reacts with malic acid to produce an equivalent amount of NADH, which can be measured at 340 nm (simillar with lactic acid). Kits for this assay are commercially available.
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Author: Tajana Frančić, mag.nutr.
