What pH increase is generally observed during a MLF?
Depending on the initial L-malic acid concentration, the relative pH increase consecutive to MLF is generally between 5 and 10% of the initial value. That is, a wine with a pH of 3.5 could see its pH increase up to 3.85 after MLF. However the increase in pH cannot be genuinely correlated with the quantity of L-malic acid consumed because other organic acids in the grape are equally important: tartaric acid, citric acid, succinic acid and pyruvic acid. Besides L-malic acid, MLF bacteria also degrade small quantities of citric and pyruvic acid for example.
It should be noted here that for L-malic acid only the L isomer is transformed by the MLF bacteria.
What is the total decrease in acidity generally observed during a MLF?
In general it is considered that the decrease in total acidity is approximately 0.4 g/L of H2SO4 per gram of L-malic acid consumed. This appears to be the most important impact the MLF bacteria have on the organoleptic qualities of the wine.
Why does malolactic fermentation modify wine acidity?
During MLF, multiple biological transformations occur, the most noteworthy being a drop in total acidity (approximately 0.4 g of H2SO4 per gram of consumed L-malic acid) and an increase in pH (a 5 to 10% rise in pH can be observed during MLF). This is due to the transformation of a di-acid, L-malic acid, into a mono-acid, L-lactic acid, and also due to other organic acids: citric acid, pyruvic acid… that are used by the lactic bacteria during MLF. For these reasons, besides microbiological stability, there is an increasing demand for MLF on dry white wines or extremely acidic rosé wines. It should be noted that if a must is acidified with malic acid as authorized by European regulations since 2009 (see effective regulation conditions); the preparation used contains a racemic combination of both isomeric forms of malic acid (L and D). In this case, lactic bacteria development will lead to degradation of L form of the isomer, leaving only d-malic acid to remain in the wine.
What role does citric acid play for lactic bacteria?
L-malic acid is not the only substrate for lactic bacteria in wine. While present at a much lower concentration than malic acid, citric acid is also an important substrate for wine bacteria. Citric acid is systematically consumed during MLF but at a much slower speed than malic acid, to such an extent that when MLF is completed, wines still contain up to 100 - 200 mg/L, i.e. 30 to 60% of the initial citric acid concentration. Nevertheless citric acid is an important substrate for lactic bacteria due to the fact that at the start of their growth phase, its degradation is essential for synthesizing membrane lipids. While L-malic acid degradation is favorable for wine quality, that of citric acid is questionable. Citric acid degradation leads either to the production of volatile acidity, acetoinic compounds (diacetyl, acetoin, butanediol) or lipids. In favorable growth conditions, bacteria develop rapidly and they have high lipid requirements. Consequently the degradation pathway of citric acid is mainly used to provide lipids. Conversely, in limiting growth conditions, the bacteria will use the citric acid mainly to produce acetoinic compounds. The most well-known one, diacetyl, is responsible for buttery odors.
The more sluggish the MLF, the more diacetyl is formed. After malolactic fermentation diacetyl concentration is quite variable; from 2 to 10 mg/L and sometimes higher. It is preferable that the diacetyl content should not exceed 5 to 6 mg/L in wine. Within this range we can consider that the diacetyl contributes to the wine’s bouquet; above this it is detrimental.
The use of a malolactic bacteria starter is the most efficient tool for limiting diacetyl production: on the one hand the selected strains intrinsically produce little diacetyl, on the other, carrying out a straightforward and regular MLF limits its production.
Several types of lactic bacteria; homo or heterofermentative?
Which lactic bacteria are found in wine? ?
Certain strains of Œnococcus œni have remarkable oenological aptitudes, the best being utilized for producing malolactic bacteria starters. Others strains can be detrimental to wine quality, such as indigenous strains that produce biogenic amines. The other most frequently encountered species in oenology are Lactobacillus plantarum in the must and Pediococcus damnosus in certain wines at the end of maturation.
What is a bacterium?
What are the principal differences between lactic bacteria and acetic acid bacteria in wine?
Lactic bacteria belong to the group of Gram positive bacteria. Acetic acid bacteria are Gram-negative bacteria. Gram staining is a method for distinguishing bacteria according to their cell wall structure. Gram staining today remains a benchmark method of analysis in bacteriology. Gram positive bacteria retain the stain used for the analysis, thus responding positively to the staining.
Apart from this difference, it is important to underline that acetic acid bacteria are aerobic bacteria, that is, they require oxygen for development, while lactic bacteria are facultative anaerobic (aerobic tolerant). They develop best in a medium without oxygen but tolerate its presence.
Finally, from an oenological point of view, the major difference is the fact that acetic acid bacteria lead to the production of acetic acid while lactic bacteria produce principally lactic acid.