Physiology and Biochemistry of Sourdough Yeasts - Sourdough Biotechnology - page 168

Fig. 6.5 Schematic representation of metabolic routes of L-leucine in yeasts, leading to 4-methyl-2-
oxopentanoate, 3-methyl-1-butanol and 3-methylbutyrate (Adapted from [ 65 ] )

Especially, the levels of ethanol, methylpropanol, 2- and 3-methylbutanol as well as
of ethyl acetate and diacetyl were considerably increased in sourdoughs with the
addition of yeasts. Diacetyl is synthesised either by lactic acid bacteria or yeasts
[ 68 ]. Although it is difficult to distinguish between the contribution of the two
groups of microorganisms, processing parameters such as high water content and
temperature may favour the growth of yeasts [ 69- 71 ] and, consequently, their con-
tribution to the synthesis of volatile compounds during fermentation. There was a
clear difference between the volatile compound profiles of wheat dough fermented
with Dekkera bruxellensis compared to S. cerevisiae . Contrarily to S. cerevisiae ,
Dekkera sp. did not generate the same levels of 3-methylbutanol but formed high
concentrations of esters such as the odour-intense ethyl 2-methylbutanoate [ 72 ] .
Table 6.4 shows the metabolites synthesised by L. sanfranciscensis and S. cerevisiae
in doughs with different dough yields [ 51 ]. When the dough was inoculated with
L. sanfranciscensis , isobutanol, acetoin, 1-hexanol, ethyl octanoate and butyric acid
were found. When the dough was inoculated with S. cerevisiae at a dough yield
value of 146, the highest synthesis of volatile compounds was found, which included
alcohols and short- and medium-chain fatty acids.

6.4 Quorum Sensing on Sourdough Yeasts

Several reports have shown quorum sensing-like phenomena in fungal species. The
morphological transition, from the filamentous and mycelial form to the yeast form,
or vice versa, was always found. The regulation of the switch between the filamentous