Even if the culture contains citrate-forming strains, diacetyl formation does not always occur in amounts necessary to impart the desired flavor. Several reasons may account for reduced citrate fermentation and diacetyl synthesis. First, there may simply not be enough citrate in the milk. Although milk contains, on average, about 0.15% citrate, this amount varies, depending largely on the diet of the cow. Therefore, sodium citrate is frequently added to the milk to provide a consistent source of substrate. If the temperature during incubation is too high (>24°C), growth of the ho mo lactic lactococci will be favored, too much acid will be produced, and the citrate-fermentors may be inhibited. Not only will the product have a "lacks flavor" defect, but it will instead have a harsh acid flavor.
In contrast, if the incubation temperature is too low (< 20°C), not enough acid is produced and the flavor will be flat or "lacks acid." Acid production is also necessary for diacetyl formation, since the citrate transport system is not acti vated unless the pH is below 5.5. In fact, maximum diacetyl synthesis occurs between pH 5.0 and 5.5.Finally, once diacetyl is made, low pH inhibits the reduction reactions that convert di-acetyl to acetoin and 2,3-butanediol.One other factor that is critical for synthesis of diacetyl is oxygen, which can stimulate diacetyl formation by as much as thirty-fold. Several mechanisms are responsible for the oxygen effect. As noted above, pyruvate, the metabolic precursor of di-acetyl, can serve as the substrate for several alternative enzyme reactions, including lactate dehy-drogenase. In the presence of high atmospheric oxygen, lactate dehydrogenase activity is reduced, and the oxidative decarboxylation reaction responsible for diacetyl synthesis is enhanced. In addition, by oxidizing NADH, oxygen also slows the rate at which diacetyl is reduced to acetoin or 2,3-butanediol. For these reasons, it may be useful to stir air into the final product during the agitation step. In fact, maximum di-acetyl production occurs about seventy-two hours after manufacture, so fresh product may not have as good a flavor as one that is slightly "aged".The flavor and texture defects that occur in cultured buttermilk are, in general, similar to those in yogurt.The most common flavor defect is "lacks flavor," caused by insufficient diacetyl formation due to reasons outlined above. Excess acidity is also objectionable to consumers. Texture defects include wheying off, too thin, or too viscous.The latter may be caused by body-forming bacteria that over-produce EPS. Milkborne defects, such as yeasty flavor, unclean, and rancidity, are caused by poor quality milk, especially milk that had been contaminated with psy-chrotrophic microorganisms.
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