The most dominant flavor of yogurt is sourness, due to lactic acid produced by the starter culture. Most yogurts contain between 0.8% and 1.0% lactic acid and have a pH below 4.6.
In the absence of sweetener or added flavors, most consumers can detect sourness when the pH is below 5.0. Other organic acids, including formic and acetic, may also be produced by the culture, but at much lower concentrations, and they generally make only modest contributions to yogurt flavor.
There are, however, other metabolic products produced by the culture that accumulate in yogurt and contribute significantly to flavor development. The most important of these is acetaldehyde, a two carbon aldehyde.Although normally present at less than 25 ppm concentration, this is still sufficient to give yogurt its characteristic tart or green apple flavor. Both S. thermophilus and L. delbrueckii subsp. bulgaricus can produce acetaldehyde; however, the rate and amount produced depends on the strain and on the growth conditions.
There appear to be at least two metabolic routes by which acetaldehyde is produced (Figure 4-3). In one pathway, an aldolase enzyme hydrolyzes the amino acid, threonine, and acetaldehyde and glycine are formed directly. It is also possible to form acetaldehyde from pyruvate (generated from glucose metabolism). The pyruvate can either be decarboxy-lated, forming acetaldehyde directly, or converted first to acetyl CoA, and then oxidized to acetaldehyde. By understanding how these pathways function, it may soon be possible to apply molecular metabolic engineering approaches to enhance or control acetaldehyde levels in yogurt. Finally, diacetyl and acetoin may also be produced by the yogurt culture; however, they are usually present at concentrations below a typical taste threshold.
In the United States, most yogurt is flavored with fruit and other flavorings. Fruits are usually added in the form of a thick puree, with or without real pieces of fruit.These ingredients obviously dilute or mask the lactic acid and ac-etaldehyde flavors. In fact, the trend in the United States is to produce mild-flavored yogurts with less characteristic yogurt flavor. Thus, strains that produce little acetaldehyde are often used in yogurt cultures.
The texture and rheological properties of yogurt are, perhaps, just as important to
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