Malolactic Fermentation

©Copyright Ben Rotter 2002-2008
www.brsquared.org/wine

1 Introduction

Malolactic fermentation (MLF, or "malo") is an important winemaking process conducted on most red grape wines and some white grape wines. It is also used with some fruit wines. The following article gives information concerning the conditions necessary for MLF, its affects, prevention, progress, suitable wine type candidates for it and yeast compatibility.

2 Malolactic fermentation bacteria

MLF is conducted by lactic acid bacteria (LAB), of the genera Lactobacillus, Oenococcus, Pediococcus, and Leuconostoc.
Not all LAB are desirable for MLF. Oenococcus oeni (formerly Leuconostoc oenos) is the most beneficial, and probably the most frequently occurring species of LAB in wine. Species associated with wine spoilage are generally members of Lactobacillus and Pediococcus genera. The Lactobacillus genus, for example, can cause acescence (excessive acetic acid) by metabolising sugar or tartaric acid [Radler and Yannissis, 1972]. Many LAB metabolise pentoses, tartaric acid and glycerol. The term "malolactic fermentation bacteria" (MLB) is commonly used to refer to those LAB strains which are more desirable for MLF. They are more resistant to low pHs such as those in wine (and in which other LAB find it more difficult to live) and they prefer to metabolise malic acid over sugars and citric acid (and they do not metabolise tartaric acid or glycerol). Oenococcus oeni, a desirable strain, may also metabolise glucose to produce carbon dioxide, lactic acid, acetic acid and ethanol (it follows the heterolactic pathway (6-PG/PK pathway)) [Garvie, 1986; Axelsson, 1993; Henick-Kling, 1993] but will degrade malic acid before degrading any glucose present under non-growing conditions.

The differentiation between LAB types is based the basis of sugar metabolism, cell shape and physiological features. Homofermentary strains are defined as those which transform sugar exclusively to lactic acid, whereas heterofermentary strains transform sugars into lactic acid, ethanol, acetic acid, glycerol, mannitol and other polyalcohols, and carbon dioxide.
Heterofermentative MLB can metabolise citric acid to predominantly acetic acid, lactic acid and carbon dioxide [Dittrich, 1977; Subramanian and SivaRaman, 1984; Martineau and Henick-Kling, 1995]. The co-fermentation of citric acid and glucose by O. oeni has been seen to increase its growth rate and biomass production [Salou et al., 1994; Ramos and Santos, 1996; Liu, 2002].

Lactobacillus contains both homo- and heterofermentative strains, whilst Pediococcus species are all homofermentative, and Leuconostoc species are all heterofermentary. Pediococcus and Leuconostoc usually cease growing below pH 3.5.
The following table describes various species:

Table of some LAB Species
HomofementantPediococcus cerevisiae
Pediococcus damnosus mainly present in musts and at high pH after malolactic fermentation
Pediococcis pentosaceus sometimes present lower populations
Pediococcis parvulus
Lactobacillus casei sometimes present lower populations
Lactobacillus plantarum mainly present in musts
Lactobacillus casei
Streptobacterium
Heterofermentant Leuconostoc gracile
Oenococcus oeni most resistant to low pH
Leuconostoc mesenteroides sometimes present lower populations
Lactobacillus hilgardii
Lactobacillus fructivorans
Lactobacillus desidiosus
Lactobacillus brevis
Lactobacillus hilgardi sometimes present lower populations
Lactobacillus brevis sometimes present lower populations

(Cell shape and grouping may depend on the medium in which the bacteria grow.)

3 Effects of MLF

3.1 General

The reaction of interest during MLF is the conversion of L(-) malic acid to monocarboxilic (L-) or D(-) lactic acid and carbon dioxide. (Whether L- or D- lactic acid is produced depends on the LAB strain and substrate attacked.) All LAB follow this pathway for MLF. It is worth noting that some commercial malic acid is racemate, a mixture of L(-) and D(+) malic acid. Only L(-) malic acid will be converted by MLB.

Malic acid ---> Lactic acid + Carbon dioxide
COOH-H2OC-H2C-COOH ---> CH3-CHOH-COOH + CO2
For every 1 gram of malic acid metabolised, 0.67 grams of lactic acid and 0.33 grams (or 165 ml) of CO2 are produced.

Bacteria also use only 0.3-2 g/l of sugars yielding about 100 mg/l of D-lactic acid [Krieger et al., 2000].

3.2 Acidity

3.2.1 TA, pH, acid species

MLF affects TA, pH and acid species through the following factors:
  • a chemical deacidification usually reducing titratable acidity by about 1-4.6 g/l (as tartaric)
  • a pH increase of between 0.1 and 0.45 units (more typically 0.1-0.25)
  • the fact that the newly formed lactic acid (like that in milk) appears softer on the palate than the converted malic acid (like that in apples).
    •
    These factors can give a wine a rounder, softer, mellower mouth-feel.

    MLB also produce substances which give these sensations more directly, and/or integrate with bitter and astringent substances in wine. This is predominantly seen in red wines and appears to be dependant on MLB strain.

    3.2.2 Volatile acidity (acetic acid)

    Most LAB cause a slight increase in volatile acidity. Acetic acid increases in dry wine due to citric acid metabolism and chetonic compounds. This increase is by 0.05-0.2 g/l according to Krieger et al. [2000], though Riesen [1999] quotes up to 0.3 g/l. This increase is not usually perceptible, though the formation of acetic acid esters does have an impact. [Riesen, 1999]. The increase is partly due to LAB attacking trace sugar (usually 0.3-2 g/l of sugar [Krieger et al., 2000]) and citric, pyruvic, and chetoglutaric acids, in addition to acetaldehyde. Acetic acid production is higher during MLF at higher pHs [see Wibowo et al., 1985].
    Acetic acid is produced during bacterial growth in must and wine (from sugar and organic acid metabolisation), not during malic acid degradation. If the MLB grow fast, more acetic acid is produced. Thus, when a high population MLB culture is inoculated into a wine there is less growth, and therefore, a considerably lower production of acetic acid.

    3.3 Microbial stability

    Under certain conditions, an MLF can increase the microbial stability of a wine.
    Some spoilage bacteria attack malic acid. Therefore, by reducing the malic acid content of a wine using the favourable method of controlled MLF, a more microbially stable wine can potentially result.
    MLB also consume nutrients (amino acids, nitrogen bases, vitamins) and this reduction in nutrient availability has been thought to increase microbial stability by limiting the potential growth of spoilage orgaisms. However, it has been shown that wines which have completed MLF can still support Oenococcus oeni, lactobacilli or pediococci [Costello et al., 1983]. Indeed bacterial growth may continue after MLF, particularly if SO2 levels remain low [Davis et al., 1986].
    Additionally, it should be kept in mind that MLFs which raise the pH above 3.5 can cause a potential increase in microbial instability due to providing a more favourable environment for spoilage organisms.

    3.4 Aromatic/Taste Modification

    Many people claim the alteration MLF makes in flavour alone (without regard for acidity) is not perceptible. Some studies have confirmed this [see Davis et al., 1985] and many might be used to support this argument [Asmundson and Kelly, 1990; Axelsson, 1993; Buckenhüskes, 1993; Henick-Kling, 1995; Henick-Kling et al., 1989; Sandine and Heatherbell, 1985; Sharpe, 1981; Wibowo et al., 1985; van Vuuren and Dicks, 1993]. However, at least one other study showed that similar wines which have undergone MLF can be distinguished from those which have not [McDaniel et al., 1987]. Certainly, MLB can produce a whole host of products during MLF (succinate, acetate, acetoin, lactate, diacetyl, mannitol, 2-butanol, 1,3-propanedoil, and many more) depending on the available substrates (acids, sugars, polyols, etc). These compounds are claimed by many to potentially modify vegetal characters, and potentially impart nutty, lactic, and/or earthy aromas.
    Undesirable odours brought about by MLF are usually associated with pediococci or lactobacilli, or MLF occurring above pH 3.5; whereas MLF by Oenococcus oeni below pH 3.5 is less likely to produce off-odours [Jackson, 1994].

    3.5 Loss/Gain of Fruitiness and Vegetative/Grassy Notes

    Non-specific MLB strains can reduce or mask wine esters, resulting in a reduction of varietal characteristics and fruit aromas.
    However, some MLB strains (Leuconostoc oeni in particular) have been shown to increase these compounds (blueberry, raspberry, cherry, pineapple). Current theory is that a direct bacterial action releases varietal compounds from odourless precursors in wine. [Krieger et al., 2000; Riesen, 1999].
    A reduction in excessive vegetative and grassy notes caused by MLB has been seen in wines made from underripe grapes, though the mechanism for this is unknown. [Krieger et al., 2000; Riesen, 1999].

    3.6 Loss of Colour

    A reduction in colour intensity can be caused by MLB directly (not because of pH increase). This is due to the metabolic activity of bacteria on phenolic compounds [Lonvaud-Funel, 1995]. MLF has been shown to change colour hue (lower 520 nm) [Riesen, 1999].

    3.7 Astringency

    MLF can decrease astringency. Under the influence of MLF alone, total phenols may decrease (gelatin index, measured at 280 nm). [Riesen, 1999]

    3.8 Tannins and Anthocyanins

    MLF can increase the polymerisation of tannins and anthocyanins. [Riesen, 1999]

    3.9 Ethyl-lactate

    Often exceeds the taste threshold of 60-110 mg/l giving a wider and fuller palate.

    3.10 Diacetyl

    3.10.1 What is diacetyl?

    Diacetyl is a substance produced by many LAB which smells like warm/hot butter. It is commonly used to flavour "movie" popcorn and gives the characteristic buttery aroma to MLFed, lees-stirred Chardonnays that are so popular today.

    3.10.2 Levels of diacetyl

    Threshold levels vary for different wines. Martineau et al [1995] claimed diacetyl thresholds of 0.2 mg/l in Chardonnay, 0.9 in Pinot Noir, and 2.8 in Cabernet Sauvignon. Krieger et al. [2000] claim a level of 2-7 mg/l can give a "butter or cheese" note that makes the aroma heavy and unpleasant and Jackson [1994] writes that 1-4 mg/l adds "a desirable complexity to the fragrance" but at concentrations over 5-7 mg/l the buttery aromas becomes overt and undesirable. However, Dharmadhikari [2002] reported that 2-3 mg/l and 4-5 mg/l of diacetyl enhanced wine aroma in whites and reds respectively. It is clear from this information that acceptable levels vary depending on wine style and, most likely, personal preference for diacetyl aswell.

    On testing wines from 20 different regions, 28 different producers and 8 vintages, Bartowsky et al. [2002] found that Chardonnay (of 24 wines) showed levels of diacetyl from 0.3 to 0.6 mg/l (mean value 0.4 mg/l) whilst reds (Cabernet Sauvignon, Merlot and Shiraz) showed 0.3 to 2.5 mg/L (mean value 1.1 mg/l) for 43 wines.

    3.10.3 The production of diacetyl

    Diacetyl appears to be a metabolic by-product of citric acid metabolisation by Oenococcus oeni [Nielsen and Prahl, (1995) and Shimazu et al., (1985)]. The amount produced depends on:
  • the LAB strain
  • cell multiplication (the lower the cell multiplication, the lower the production of acetate and diacetyl; under stressed slow cell growth conditions more diacetyl is produced ad less acetic acid produced)
  • the environmental conditions affecting the LAB
  • the amount of citric acid available
  • sulphur dioxide (SO2) content
  • the redox potential (oxygen content) of the wine.
    •

    Research suggests that
  • citric acid metabolisation begins at the same time that malic acid degradation occurs, but that the degradation of the citric acid is slower
  • the higher the initial concentration of citric acid, the more diacetyl will be produced from MLF
  • diacetyl concentration increases as citric acid is metabolised by MLB and decreases again when most of the citric acid has been consumed
  • maximum diacetyl concentration tends to coincide with the exhaustion of malic acid during MLF [Nielsen, 1995]
  • semi-aerobic (2-4 mg/l oxygen) MLF yielded higher concentrations of diacetyl, whereas anaerobic (<0.2 mg/l oxygen) conditions yielded much lower concentrations [Nielsen, 1999]
  • the higher the pH, the lower the diacetyl production [see Wibowo et al., 1985]
    •

    The diagram shows the main metabolic pathway for citric acid metabolisation by Leuconostoc oeni.


    3.10.4 The breakdown or binding of diacetyl

    Saccharomyces cerevisiae irreversibly reduces diacetyl, and sulphur dioxide (SO2) binds reversibly with diacetyl. The addition of SO2 will initially decrease the concentration of diacetyl, and increase the quantity of bound SO2. When the diacetyl-SO2 complex dissociates, however, the diacetyl and free SO2 levels will increase.

    Diacetyl is also reduced by LAB irreversibly to acetoin and 2,3-butanediol (a sweet-tasting polyol (a polyalcohol is an alcohol with more than one hydroxyl group per molecule)). These have no influence on wine aroma in the normal concentrations they are found in. [De Revel et al., 1989]

    3.10.5 Minimising diacetyl

    For a low diacetyl concentration, the best approach is to inoculate with MLB at end of fermentation (when the yeast population is high) and keep the wine on its lees (to allow the yeast and LAB to convert the diacetyl to acetoin and 2,3-butanediol).

    3.10.6 Maximising diacetyl

    For a high diacetyl concentration, rack/clarify the wine in order to reduce yeast population before inoculating MLB and stabilise the wine as soon as the malic acid is catabolised. Malolactic nutrients are often required in this case, since the necessary nutrients for MLF have been removed due to the racking/clarifying procedure. (0.13-0.15 g/l (0.5 g/U.S. Gal) is recommended.)

    3.10.7 Diacetyl and red wines

    Since diacetyl is not considered desirable in red wines, MLF is usually conducted on yeast lees (or the wine is left in contact with MLB lees). This results in the diacetyl being catabolised by the yeast (or MLB).

    4 Living conditions

    The limiting factors on MLF include total and free sulphur dioxide (SO2) concentration, alcohol content, pH, and temperature. Each LAB has it's own limits (and culture manufacturers usually provide these). The following limitations provide general guidelines (these are based on Oenococcus oeni unless where otherwise stated).

    4.1 Total and free SO2 concentration

    Most LAB are sensitive to free SO2 concentrations above of 10-20 mg/l or above. Total SO2 is usually kept below a maximum value of 70 mg/l (for reds) and 40 mg/l (for whites). Molecular levels of 0.6 mg/l are inhibitory, whereas levels of 1.2-1.8 mg/l are strongly inhibitory. Keeping molecular levels below 0.2 mg/l is desirable. It should be noted that O. oeni can degrade acetaldehyde. Thus, if SO2 were previously added to the wine/must and subsequently became bound with acetaldehyde then the action of MLB upon this bound complex will results in an increase in free SO2.

    4.2 Alcohol content

    MLB are sensitive to ethanol and usually struggle above an abv of 13.5% exhibiting very slow (or non-existent) growth. Leuconostoc oeni appears to adapt to high alcohol environments over time but loses this adaptation when returned to lower abv environments. Lactobacillus is the most resistant to ethanol and Oenococcus oeni (formerly named Leuconostoc oeni) appears to be the most sensitive [Jackson, 1994].

    4.3 pH

    MLB favour higher pH's. The optimum pH will depend on the strain and the culturing environment. For most strains, minimal growth occurs at pH 3.0. Generally, pH's above 3.2 are advised. Optimal growth for O. oeni occurs at pH 4.2-4.8. Optimal activity occurs between pH 3.0 and 4.0 and decreases as pH rises - inhibition is experienced at pH 4.5.

    4.4 Temperature

    MLF will proceed faster at higher temperatures. When no SO2 is present in the wine the optimum temperature range for MLF is 23-25°C (73-77°F). (Maximum malic acid degradation will occur at 20-25°C (68-77°F) [Ribéreau-Gayon et al., 1975].) However, this decreases with increasing concentrations of SO2 causing 20°C (68°F) to be more suitable. The following table indicates MLF speed at given temperatures.

    Delay Temperature
    slowed by months / essentially no malic acid decarboxylation 10°C (50°F)
    slowed by weeks12-13°C (54-55°F)
    begins to slow15°C (59°F)
    optimum20-25°C (68-77°F)
    MLB death>30°C (> 86°F)

    Most strains of Oenococcus oeni either cease to grow or grow very slowly below 15°C (59°F). However, cells remain viable at low temperatures. [Jackson, 1994].

    4.4 Population

    MLB require a certain population level to be reached before they can begin MLF. A population of around 105 cells/ml is a good starting level. Due to the stress inoculated MLB undergo in adapting to the wine environment, inoculations of MLB are often increased to population loads of at least 108 cells/ml.

    All these factors act in synergy and the tolerance limit for one factor can change if the other factors have an inhibiting affect on MLF.
    Inhibition of MLB does not necessarily mean the bacteria have died. A bacterial population may simply be dormant and become active when conditions are more favourable. This explains the often seen delay of MLF in bottled wines, as well as the increase in MLF activity in cellared wines over the spring as cellar temperatures rise.

    5 LAB development

    MLB require a certain population level to be reached before they can begin MLF. This means that natural/indigenous MLFs (where LAB have not been inoculated into the must or wine) can have a lag phase of weeks to several months before they begin MLF.
    This lag phase is prolonged the lower the pH. Additionally, the pH affects which species of LAB will be dominant in the must or wine [Bousbouras and Kunkee, 1971]. At low pHs Oenococcus oeni is the primary MLB genus, different strains of which will dominate throughout MLF. At higher pHs, however, Lactobacillus and Pediococcus dominate over Leuconostoc [Costello et al., 1983].
    During MLF, the population often reaches 1 million cells/ml. (Leuconostoc oeni in particular requires a population of more than 106-107 CFU/ml to begin MLF.) Their population is usually 103-104 CFU/ml following fermentation, and may rise to 106-108 CFU/ml once growth initiates substantially.
    After exponential growth, cell populations deline rapidly depending on the conditions (e.g. elevated tempertures or SO2 will increase the rate). It is possible that the population of strains of Lactobacillus and Pediococcus may increase at this point.

    6 Why inoculate

    The MLF growth (lag) phase associated with spontaneous MLF (wild/uncultured strains) presents a time of increased risk from spoilage organisms due to the non-SO2 environment and the potential production of volatile acidity. Inoculating with a prepared MLB culture avoids the problems associated with the MLB growth lag phase by immediately providing the population necessary to conduct MLF. It is important that the MLB have enough nutrients to develop. Yeast (Saccharomyces cerevisiae var. "bayanus" in particular) can reduce the nutrients available to MLB considerably. Winemakers often add a MLB nutrient when inoculating with MLB to assist their development.

    Additionally, spontaneous MLB can cause unpleasant aromas and tastes, which can include sweat, mould, sulphate, phenols, sauerkraut, or a bitter and oily aftertaste. The source and chemical composition of these compounds is not known, but it has been noted that they tend to occur in high pH and low SO2 environments, indicating Lactobacillus and Pediococcus or wild strains of Leuconostoc. Inoculation of cultured MLB avoids these unpleasant flavours.

    Cultured MLB strains are consistent. When relying on wild MLB, there can be no certainty of consistent results in consecutive wines. Some winemakers feel, however, that wild MLB tend to lend more complexity to their wines. Additionally, some areas in the world which have been making wine in the same cellar (and region) for centuries may have resident MLB which are relatively stable and favourable for MLF, thus providing reliable wild MLB strains for MLF.

    7 Mixed cultures

    Some believe there are advantages in conducting MLF with mixed MLB cultures.

    8 Timing of inoculation of cultured MLB

    8.1 Sugars

    Inoculation of MLB is usually conducted after all sugars (<2 g/l of reducing sugars is a suitable level) have been fermented by yeast. This avoids the possibility of MLB metabolising the sugar and producing unwanted products such as acetic acid. Some research suggests that pre-fermentation inoculation results in higher VA concentrations [Semon et al., 2001]. When considering the timing of inoculation, the compatibility of yeast and LAB compatibility should also be considered. (Competition for nutrients can occur with Saccharomyces cerevisiae var. "bayanus" strains, for example.) Some winemakers, however, inoculate MLB a few days into alcoholic fermentation in an attempt to minimise contamination through the subsequent ability to add SO2 to the wine sooner.

    8.2 Nutrients, alcohol and SO2

    Interactions between co-existing yeast (Saccharomyces cerevisiae) and Leuconostoc oeni can cause problems with MLF. Saccharomyces cerevisiae releases ethanol, sulphur dioxide (SO2) and medium chain fatty acids which inhibit O. oeni [Lonvaud-Funel et al., 1988].
    Additionally, the yeast's utilisation of complex nutrients such as amino acids during the early stages of fermentation can complicate ensuing MLB growth.

    Later inoculations result in more efficient MLB growth when Saccharomyces cerevisiae var. "bayanus" strains are used. If a bayanus strain is used, it is recommended that MLB inoculation be conducted after complete alcoholic fermentation (avoiding competition nutrients). Additionally, MLF nutrients should be added to the wine since bayanus yeasts tend to monopolise and consume most nutrients.

    Bacterial inhibition decreases towards the end of fermentation. This is probably connected with the death phase of yeast as SO2 produced during fermentation reduces and binds with other compounds and dying yeast cells release nutrients useful to the MLB.

    However, some winemakers inoculate before the end of alcoholic fermentation, taking advantage of the low alcohol, higher nutrient environment. Assuming MLF completes before or soon after alcoholic fermentation, this allows SO2 (which provides security against bacterial and oxidative attack) to be added to the wine sooner rather than later.

    However, other work does not show higher VA concentrations, stuck fermentations, or yeast-MLB incompatability [Beelman and Kunkee, 1985].

    9 Measuring MLF progress

    Most home winemakers do not have access to advanced lab equipment to measure microbial populations or the concentration of substrates and products. Therefore, the progress of MLF is usually measured by paper chromatography (PC). In this test, drops of the tested wines are placed on the chromatographic filter paper and dried, with reference standards of tartaric, citric and malic acids. The spotted paper is then (usually) rolled up like a cylinder and left vertically to absorb chromatographic solvent via capillary action. The solvent travels vertically up the paper, separating out the organic compounds present. When the solvent has almost penetrated to the top of the paper (usually after 8 hours), the paper is removed and left to dry.

    The chart can then be viewed and the presence of each individual acid assessed. The background appears blue, with yellowish spots appearing up the paper. Each acid can only travel a certain distance up the paper. Tartaric, citric, malic, lactic, and succinic acids are represented in order at heights progressively up the paper.

    Below is a theoretical diagram of a chromatographic paper. Tartaric, citric, malic and lactic acids are indicated by the letters T, C, M, and L, respectively. Whilst each acid travels the full certain distance up the paper, it can be seen that the interpretation of acids occupying regions can be helpful.


    Images 1 to 3 show wines which have progressively undergone MLF to a greater extent. In wine 1, there is a small presence of lactic acid and a high level of malic acid present. In wine 2, less malic is shown to exist and more lactic has been produced as a result of MLF. Wine 3 shows very little malic acid and a high level of lactic, suggesting that this wine has nearly completed MLF. Paper chromatography indicates malic acid concentrations above about 100 mg/l (wines may continue to undergo MLF over 30 mg/l without showing any malic spot on the paper). Therefore, wine 3 may have a little way to go until MLF is entirely complete, but it is almost there.

    The tartaric spot on 2 is weak. In wine's where the predominant acids are tartaric and malic (such as from grapes), there is a possibility that a high proportion of the acid in the wine is malic. If this is the case, a complete MLF might leave the wine with an unacceptably low TA and an excessively high pH. Therefore, these parameters should be monitored.

    If, alternatively, a wine has a weak malic acid concentration, there is little point in conducting a MLF at all as it would have little affect on the wine.

    The diagram of wine 4 is included to indicate what the paper looks like when citric acid is present.

    10 MLF and Oak

    LAB enzymes appear to react with soluble substances in oak barrels, creating a wider range of flavours in a wine than would be produced in an inert vessel.

    LAB become resident in the wood or tartrate layer of cooperage. A wine that has not been through, nor is intended to go through, MLF (MLF- wine) cannot, therefore, safely be made in a (MLF+) barrel which has previously stored MLFed wine.

    11 MLF and Sorbate

    Sorbate should not be added to a wine which is to (or may) undergo MLF. MLB transform sorbate into ethoxyesadiene which has a strong smell of grass or geranium. When this aroma is perceptible in wine, the wine is generally considered ruined.

    12 MLF and Lees

    Lees provide nutrients for MLB. By stirring the yeast lees, nutrients are re-exposed to the bacteria assisting their development. Lees in the presence of MLF also tends to lead to lower diacetyl concentrations, since the yeast metabolise the diacetyl to acetoin and 2,3-butanediol.
    Lees stirring is usually conducted on fine lees (mainly yeasts) and not gross lees (that including fruit debris). However, some winemakers conduct sur lie ageing on gross lees in barrels.
    Substances which are toxic to MLB also exist in lees, however. These include such substances as fatty acids C10 and C12, which can be absorbed into lees (fine lees and yeast hulls) during alcoholic fermentation.

    13 Preventing MLF

    Factors to help in preventing MLF occurring include the following:
  • Use of lysozyme (a protein that destroys the cell walls of the bacteria by catalyzing the hydrolysis of specific glucosidic links in the cell walls)
  • Keeping SO2 concentrations at MLF preventative levels - 0.8 mg/l molecular SO2 (or less accurately, a free SO2 concentration of 50 mg/l)
  • Keeping pH low - below 3.2
  • Sterile filtration
  • Keeping wine lees contact to a minimum and racking immediately upon completion of alcoholic fermentation
  • Use of a bayanus yeast strain for alcoholic fermentation (limits available nutrients for bacteria)
  • Keeping wine below 10°C (50°F) can provide limited assistance
  • Ensuring alcohol content is greater than 13%
  • Keeping maceration on skins to a minimum
  • Pasteurisation
    •

    14 MLF schedules: Partial MLF

    Partial MLF is used when some crisp acid/fruity character is desired, yet some softening and complexity from MLF is desired. There are two techniques used to achieve this:

    (1) The batch is split, one portion goes through full MLF and the other no MLF. The two portions are then blended back together. The problem to then overcome is a renewed MLF in the wine, since there is still malic acid present. The best method in providing stability is to sterile filter the wine. Alternatively, a thorough clearing and filtration of the wine coupled with a 0.8 mg/l molecular SO2 level can be successful in preventing renewed MLF.

    (2) The wine undergoes MLF and is closely monitored. When the winemaker is satisfied with the level of MLF (based on taste and analytical methods), the MLF is arrested. MLF is terminated by sterile filtration (<0.45 micron tight filter to remove yeast and lactic bacteria), a 1.5-2 mg/l molecular SO2 level, keeping wine lees contact to a minimum and racking when MLF termination is desired to reduce the MLB population, and possibly the use of lysosyme and cold temperatures/stabilisation.

    15 Wine styles suitable for MLF

    MLF is suitable for wines where a reduction in acidity is desired and the wine style is not so much focused towards varietal aromas and flavours. Where fruitiness and crispness/freshness (an impression given by acidity) is desired, MLF is generally avoided. Certain wines tend to lose their aroma through MLF because lactic aromas tend to dominate the fruity aromas.

    Some winemakers even use MLF in wines from aromatic grape varieties such as Gewürztraminer and Muscat (Riesling seems an exception where MLF is practically never practised) to improve body and mouth feel, without a significant reduction in fruitiness. Generally, however, it is avoided in these wine styles. That being said, there are exceptions. For example, Sauvignon Blanc, anothe aromatic variety, is sometimes put through (partial) MLF.

    Complex, full bodied whites often benefit from MLF. Chardonnay is a classic example. MLF gives a softer, more luscious wine with added complexity.

    Most reds undergo MLF. One of the main reasons for this is to ensure that the wine will not undergo MLF later in bottle; leading to lowered acidity, off aromas, and carbon dioxide.

    MLF has traditionally been avoided with regard to fruit, flower and vegetable wines. This is largely because an emphasis is place on primary ingredient ("fruit" or varietal) character. However, many non-grape wines may be suitable for MLF. Some winemakers routinely MLF apple wine, for instance, and reds such as blackberry are suitable candidates.

    Overall, MLF is a stylistic option that will suit some wines and not others. It is up to the winemaker to assess whether the individual wine in question will benefit or not from MLF. The above information hopefully provides clues as to how to discern this.

    16 References

    Asmundson, R.V. and Kelly, W.J. (1990). Temperature and ethanol effects on growth of Leuconostoc oenos. In: Yu, P.-L. (ed.) Fermentation technologies: Industrial applications, Elsevier, London, pp 128-131.
    Axelsson, L.T. (1993). Lactic acid bacteria: classification and physiology. In: Salminen S. and von Wright A. (eds.), Lactic acid bacteria, Marcel Dekker Inc., New York, pp 1-63.
    Bartowsky, E.J., Francis, I.L., Bellon, J.R., and Henschke, P.A. (2002). Is buttery aroma perception in wines predictable from the diacetyl concentration?, AGJWR, Volume 8, Number 3.
    Beelman, R.B., and Kunkee, R.E. (1985). Inducing simultaneous malolactic-alcoholic fermentation in red table wines. In "Malolactic Fermentation" (T.H. Lee, ed.), pp. 97-111. Aust. Wine Res. Inst., Urrbrae, South Australia.
    Bousbouras, G.E., and Kunkee, R.E. (1971). Effect of pH on malolactic fermentation in wine. Am. J. Enol. Vitic. 22, 121-126.
    Buckenhüskes, H.J. (1993). Selection criteria for lactic acid bacteria to be used as start cultures for various food comodities. FEMS Microbiol. Rev. 12, 253-271.
    Costello, P.J., Morrison, R.H., Lee, R.H., and Fleet, G.H. (1983). Numbers and species of lactic acid bacteria in wines during vinification. Food Technol. Aust. 35, 14-18.
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