Malolactic fermentation phase

The malolactic fermentation phase begins during the growth phase, as soon as the total population exceeds lO UFC/ml. It continues and is completed during the stationary phase, or sometimes at the beginning of the death phase. In very favorable conditions with a limited concentration of malic acid, malolactic fermentations are often completed even before the end of the growth phase. The optimum population in these cases exceeds 10 UFC/ml. As soon as a sufficient biomass is formed, malic acid is degraded. The malolactic acid bacterial activity is always present but depends on various conditions, especially the temperature. The transformation of 2 g of malic acid per liter can take more time than 4 g/1 if the population level attained is lower. 

At the end of alcoholic and malolactic fermentation, the wine undergoes clarification and stabilization prior to bottling the phase inappropriately called "aging." This is the final step in winemaking, which involved those operations required to permit the expression of features considered necessary to the wine s overall quality. Nuances that distinguish wine styles also develop. 

Malolactic fermentation (MLF) in wine is by definition the enzymatic conversion of L-malic acid to L-lactic acid, a secondary process which usually follows primary (alcoholic) fermentation of wine but may also occur concurrently. This reduction of malic acid to lactic acid is not a true fermentation, but rather an enzymatic reaction performed by lactic acid bacteria (LAB) after their exponential growth phase. MLF is mainly performed by Oenococcus oeni, a species that can withstand the low pFi (<3.5), high ethanol (>10 vol.%) and high SO2 levels (50 mg/L) found in wine. More resistant strains of Lactobacillus, Leuconostoc and Pediococcus can also grow in wine and contribute to MLF especially if the wine pH exceeds 3.5 (Davis et al. 1986 Wibowo et al. 1985). The most important benefits of MLF are the deacidification of high acid wines mainly produced in cool climates, LAB contribute to wine flavour and aroma complexify and improve microbial sfabilify (Lonvaud-Funel 1999 Moreno-Arribas and Polo 2005). 

Figure 1.19 Mass spectra of acetaldehyde PFB-oxime (a), diacetyl mono PFB-oxime (b), acetoin PFB-oxime derivative (c), and o-chlorobenzaldehyde PFB-oxime (d) recorded in the GC/MS analysis of standard solution performed in positive ion chemical ionization mode using methane as reagent gas (reagent gas flow 1 mL/min ion source temperature 200 °C). Flamini et al., (2005) Monitoring of the principal carbonyl compounds involved in malolactic fermentation of wine by synthesis of 0-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine derivatives and solid-phase-microextraction positive-ion-chemical-ionization mass spectrometry analysis, Journal of Mass Spectrometry, 40, p. 1561. Copyright John Wiley Sons, Ltd. Reproduced with permission...

Paper chromatography is extensively used to follow the progress of the malolactic fermentation and the simplicity of this method is such that practically all wineries can carry it out. It is based on the difference in R values between malic and lactic acids using a mobile phase consisting of acetic acid and butanol. Bromothymol blue is incorporated into the mobile phase as indicator and thus the acids appear as yellow spots on a blue background. The absence of a spot for malic or lactic acid indicates that the MLF has or has not, respectively, taken place. 

Estimating microbiological population density and diversity plays important and often pivotal roles at several junctures in the winemaking process. For instance, it is frequently necessary to determine changes in microbial populations during the preparation of starter cultures, growth and decline phases of malolactic fermentation, or monitoring potential Brettanomyces infections. 

In the production of wines requiring malolactic fermentation, the bacterial microflora passes through several phases (Figure 6.1). During the first days of fermentation, as soon as the tanks are filled, they are present in very variable quantities—most often from 10 to 10" UFC/ml. The extent of the population depends on climatic conditions during the... 

If wine is not sulfited after malolactic fermentation, bacteria continue to survive for months. Carre (1982) observed a small decrease from 10 UFC/ml to 10 UFC/ml after 6 months of conservation in a wine stored at 19°C with a pH of 3.9 and an ethanol volume of 11.25%. Sulfiting immediately after the end of malolactic fermentation is intended to accelerate this death phase. No significant viable population should be left in the wine. Even if they can no longer multiply very actively. 

Experience has shown that, even in sugar-containing media, lactic acid bacteria during their growth phase do not produce acetic acid and decompose only malic acid (Section 3.8.3). The complete depletion of malic acid, however, greatly increases the risk of serious alterations when the wine still contains sugar. For example, malolactic fermentation sometimes occurs before the wine has... 

During the second fermentation phase, the complete transformation of sugar into alcohol is generally very quick. It is carried out at 18-20°C to preserve aroma components. Afterwards, the favorable conditions permit the easy initiation of malolactic fermentation. Despite the existence of two distinct phases, carbonic maceration requires less time than traditional winemaking. This method is therefore well adapted for wines that are quickly put on the market. 

As soon as fermentation is nearly complete, the barrels are topped off with juice from the same lot. Sluggish fermentations can often be reactivated by topping off the barrel with a wine lot that has recently completed a successful fermentation. This technique is equivalent to using a starter (10%) composed of a population in the stationary phase—resistant to inhibition factors. At the end of alcoholic fermentation, the barrels are stirred daily until sulfiting (Section 13.7.6). Wines undergoing malolactic fermentation are not sulfited until its completion. 

The word vinification has been used in this work and is part of the technical language of the French tradition of winemaking. Vinification describes the first phase of winemaking. It comprises all technical aspects from grape maturity and harvest to the end of alcoholic and sometimes malolactic fermentation. The second phase of winemaking winematuration, stabilization and... 

---