Diacetyl Citric Acid Utilization

Citric acid is present in wine at levels 0.5 g/L (Zoecklein et al., 1995) and may be of secondary importance (compared with malate conversion) in energy-yielding bacterial metabolism (Kandler, 1983). More important to the winemaker, citrate may serve as a substrate in the formation of senso-rially important metabolites. 

Both yeast and bacteria are capable of utilizing the acid in production of diacetyl, acetoin, and acetic acid (see Fig. 1-11 A). Diacetyl resulting from yeast activity is typically below threshold, ranging from 0.2 to 0.3 mg/L. Its familiar buttery properties are perceived at higher concentrations and result from post-fermentation growth of LAB. Depending on the intrinsic 

OM3 NADPH+H+ NADP 9 3 NADPH+H+ CHOH NADP C H3 CJHOH 

Diacetyl can be produced by either homolactic or heterolactic pathways of sugar metabolism (via free pyruvate) or by utilization of citric acid (see Figs. 1-1 lA and 1-1 IB). In this case, citric acid is first converted to oxaloacetic and acetic acids. The former is then decarboxylated to pyruvate which undergoes a second decarboxylation and condensation with thiamine pyrophosphate (TPP) to yield active acetaldhyde, which reacts with another pyruvate to yield a-acetolactate which undergoes oxidative decarboxylation to yield diacetyl and its equilibrium products see Fig. 1-11 A. In the case of other LAB, the precursor, a-acetolactate is not produced. Here active acetaldehyde, produced as described above, reacts with acetyl CoA to yield diacetyl see Fig. 1-1 IB. 

Prahl and Nielsen (1995) report the reversible reaction of diacetyl and SO2, resulting in rapid decreases of 30-60%. Unfortunately, the reaction is transitory, and objectionable levels of diacetyl may return after several weeks of storage. 

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During growth, malic and citric acid utilization by lactic acid bacteria may occur concomitantly, although utilization of citric acid proceeds at a much slower rate (Pimentel et al., 1994). Thus, complete conversion of citric acid does not necessarily coincide with completion of MLF, and levels of citric acid remaining in the wine post-MLF may be sufficient to stimulate bacterial formation of diacetyl and acetic acid. 

To minimize the potential for secondary growth, winemakers should consider racking, SO2 addition, and acidulation (where the pH has increased above 3.5) as soon after completion as possible. In this case, tartaric acid should be utilized rather than malic or citric acids, which may provide a continuing source of carbon for LAB. Despite its relative low cost, citric acid should not be used for wines except immediately prior to sterile botding. Its use as an acidulant in wines destined for continuing cellar aging may result in the formation of excess diacetyl, (see Sec 1.4.5). 

Fig. 1-11. (a) Utilization of citric acid in formation of diacetyl from a-acetolactate. (b)... 

Diacetyl may be synthesized by either homolactic or heterolactic pathways of sugar metabolism as well as by utilization of citric acid (Fig. 2.9). Citric acid is hrst converted to acetic acid and oxaloacetate the latter is then decarboxylated to pyruvate. Although earlier reports indicated that diacetyl synthesis by lactic acid bacteria does not proceed via a-acetolactate (Gottschalk, 1986), more recent evidence suggests that this pathway is active in lactic acid bacteria (Ramos et al., 1995). Here, pyruvate undergoes a second decarboxylation and condensation with thiamine pyrophosphate (TPP) to yield active acetaldehyde. This compound then reacts with another molecule of pyruvate to yield a-acetolactate, which, in... 

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