Malic acid: Preparation for malolactic fermentation

Malolactic fermentation (MLF) has been practiced by winemakers for many a year but the benefits have not always been widely acclaimed. But, as Jackson (Wine Science: Principles and Applications, 3rd ed., Elsevier, 2008) notes, the controversy seems to be over and it is now generally accepted that the process can impact wines as follows (Lerm et al., Malolactic Fermentation: The ABCs of MLF, S. Afr. J. Enol. Vitic., 31 (2), 2010):

• Deacidification, with a resultant increase in pH
• Contribute to the microbial stability through removal of malic acid as a possible carbon substrate; and
• Modification of the wine's aroma profile.

In its simplest terms, MLF is the conversion of the "hard" malic acids into the softer lactic acid by lactic acid bacteria (LAB). But what is the origin of the malic acid so unceremoniously neutered? And what was the path that it trod to get here? I explore these questions in this post.

As shown in the figure below, malate makes its first appearance in the grape berry in Stage I of its development. Malic acid is produced in the berry by one of two processes: (i) Fixation of carbon dioxide by PEP carboxylase (an enzyme that catalyzes the addition of bicarbonate to PEP to form oxaloacetate and inorganic phosphate) and (ii) synthesis from sugars via glycolysis and the Tricarboxylic acid cycle (TCA). The oxaloacetic acid produced in the PEPC process is reduced to malate by the enzyme malate dehydrogenase (MDH)

Grape berry development (Source:www.extension.org)

It is estimated that tartaric and malic acids constitute between 70% and 90% of total berry acid content, with malic acid's contribution on the order of 23% - 40%. Malic acid concentration increases constantly in the first stage of development, increases sharply in Stage II, and then declines rapidly in Stage III. Prior to véraison, malic  acid levels are the highest of any of the organic acids in the berry, reaching levels of up to 25 g/L, with a resultant pH of 2.5. Pre- and post-véraison occurrences, however, contribute to a significant reduction in malic acid levels (Straus et al., Malic Acid in Wine, S. Afr. J. Enol. Vitic., 27 (2), 2006):

• Malic acid in the berry vacuoles are diluted by water influx in Stage II
• There is a significant decrease in L-malic acid biosynthesis post-véraison
• The slowing of glycolytic carbon flow during véraison results in increased glucose and fructose in the berry vacuole and a decrease in malic acid synthesis via pyruvic acid in the TCA cycle
• The biosynthesis of malic acid via the PEPC cycle is reduced due to véraison-induced lack of PEPC transcription 
• During véraison, chlorophyll is degraded so the berry shifts its metabolism from sucrose respiration to malic acid respiration.

As a result of the pre- and post-véraison retrenchments, malic acid concentration will decline to between 4 and 6.5 g/l; in some cases, getting down to 1 g/l.

A point of note is the role that malic acid plays in the development of flavor and color compounds in the berry. While the berry turns its attention to its reproductive phase -- and directs the attention of its sugars to plumping and sweetening the berry --  it fails to utilize those energy sources for the development of the compounds that are so important for wine quality. But, as I have mentioned previously, the berry is not interested in producing great wines; it is interested in genetic survival. Malic acid, then, is respired to meet the berry's energy needs for development of the flavor and color compounds.

Of the three reasons listed for malolactic fermentation, two would seem to be intrinsically related to malic acid: de-acidification and substrate for microbial instability. And de-acidification would only seem to be appropriate in the cooler-climate regions. In the warmer regions, where low acidity and high pH are already an issue, the downsides associated with the pursuit of malolactic fermentation has to be significantly outweighed by the aroma-enrichment benefits.

In his study of solutes in grape berries, Coombe (Distribution of Solutes within the Developing Grape Berry in Relation to its Morphology, AJEV, 38 (2), 1987) showed the evolution of malate levels during grape development by measuring its presence at four stages identified thusly:

1. Close to véraison, berries hard and free; 6.2ºBrix
2. Berries ripening; 10.2ºBrix
3. Early ripe stage; 17.4ºBrix
4. Overripe; 26.4ºBrix

Some of his key findings, re malate, were as follows:

• Malate was the most abundant solute in the flesh of unripe berries but declined during development by proportions comparable with the degree of increase that occurred in glucose and fructose concentrations
• The decline in malate was most notable between 10ºBrix and 17ºBrix
• The levels of malate in the skin and brush showed smaller changes as the berries developed (they were smaller to begin with and declined less over the stages of berry development)
• The malate decline in the skin occurred earlier in the regions closest to the pedicel
• Within the flesh, the lowest malate concentrations were found where vascular bundles occur.

Jackson noted that the latter point may result from "malate metabolism initiating around the axial vascular bundles and progressing outwards." Regardless, at maturity, the malic acid levels in the skin may be higher than the levels in the flesh.

The overall regional climate will determine the final berry malate levels. In colder climates, the rate of berry respiration is low and this results in immature grapes at harvest with high total acidity (TA) and low pH (Jackson, Straus et al.). In this situation, malic acid can be as much as 50% of the TA in the grape berry, resulting in a sour tasting wine. In warmer climates, conversely, the rate of malic acid respiration is higher with a resultant low (insufficient) TA and high pH at harvest. Wines made from these grapes can have a flat taste and may be susceptible to microbial spoilage (Jackson). Further, these wines may not age well. Malic acid content is thus a key factor in the determination of optimal harvest date.

According to Straus et al., mature grapes have between 2 and 6.5 g/l at maturity with levels above that only present in grapes harvested after cold summers in the cool-climate viticultural regions of the world. In those cases, the levels could rise to as much as 15 - 16 g/l. And the levels at harvest are, for the most part, the levels in the wine when the decision regarding malolactic fermentation is made. The type of crushing equipment used may have some impact on malic acid levels but that is minimal (Jackson).  Also, according to Jackson, during alcoholic fermentation, the yeasts may increase the wine pH by converting some of the malic acid to lactic acid but this is highly variable by yeast strain and has not been established to any reliable degree.

So, after its many contributions along the way; its sacrifices to keep the berry alive during its most vulnerable time, this is malic acid's reward. A neutering; a sex change operation by some egotistic, self-serving winemaker who wants to make his/her wine softer. Where are your cojones man? Well that is not a question that I can answer. When I next visit this topic I will look at the other player in this drama: the lactic acid bacteria.


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