Page 18 - MALOLACTIC FERMENTATION – IMPORTANCE OF WINE LACTIC ACID BACTERIA IN WINEMAKING

CITRATE
100-700 ppm
GLUCOSE FRUCTOSE 300-1000 ppm
MALATE
1000-4000 ppm
MALOLACTIC FERMENTATION IN WINE
PRODUCTION OF DIACETYL – THE METABOLISM OF CITRIC ACID
LAB are well known for their ability to produce diacetyl (2,3-butanedione), an intensely aro- matic diketone that is characterized by a buttery, nutty aroma. Although small quantities of diacetyl (0.2 to 0.3 mg/L) can be produced by the alcoholic fermentation of yeast, subsequent increases in diacetyl content are typically associated with the growth of LAB and MLF (Laurent et al. 1994, Martineau et al. 1995, Davis et al. 1985, and Bartowsky and Henschke 2004). The aroma threshold of diacetyl in wine is low (0.2 to 2.3 mg/L) and is dependent on wine type (Martineau et al. 1995). Depending on the style and type of wine, the production of low amounts of diacetyl (1 to 4 mg/L) contribute a buttery sensory character and are considered desirable. However, the formation of concentrations in excess of 5 to 7 mg/L can be detri- mental to wine quality and may cause spoilage (Rankine et al. 1969, and Davis et al. 1985).
Diacetyl is produced by O. oeni as an intermediate in the metabolism of citric acid. In this pathway, the intermediate, pyruvic acid, is reductively decarboxylated to diacetyl via a-ace- tolactate. Since diacetyl is chemically unstable, it can be further reduced by active cells of O. oeni or by yeast to less flavour-active end products, the keto-alcohol acetoin and the diol, 2,3 butanediol (Ramos et al. 1995, Bartowsky and Henschke 2004, and Bauer and Dicks 2004). In addition to citric acid, the metabolism of diacetyl by LAB is closely associated with the me- tabolism of sugars and malic acid as shown in figure 3... (Complete text available in printed version of the book.)
Acetaldehyde-TPP
Acetyl-P
ATP
ATP ATP Pyruvate
Acetyl-CoA
Fatty acids LIPIDS
L-LACTATE
670-2680 ppm
D-LACTATE
100-200 ppm
DIACETYL
2-8 ppm
acetoin
ACETATE
100-200 ppm
Figure 3. Metabolism of heterofermentative lactic acid bacteria (Source: Krieger 2005)
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pH, temp