Physiology and Biochemistry of Sourdough Yeasts - Sourdough Biotechnology - page 166

the release of medium-chain fatty acids and of 2(5H)-furanones presumably
originating from peroxidation of membrane-associated UFAs, followed by a
sequence of b -oxidation reactions [ 58, 59 ]. The biosynthesis or integration of UFAs
in the yeast membrane is reported as a mechanism to detoxify H2O2 and protect
cells from oxidative stress [ 59 ] and this has already been observed in L. helveticus
[ 60 ]. ROS with consequent peroxidation of membrane fatty acids has been observed
in S. cerevisiae after a cycle of freezing/thawing [ 61 ] .
Sterols are membrane-associated lipids that play an important role in the tolerance
to physico-chemical stresses including ethanol exposure. The ability of sterols to
increase stress tolerance is well documented [ 62 ]. Oxygen is required for the cycliza-
tion of squalene to sterols. Thus, the presence of oxygen during the early phase of
yeast fermentation is considered a limiting factor for growth and fermenting due to
the membrane-stabilising effect of sterols. Also the synthesis of UFAs is prevented by
anaerobic conditions. When sterol and UFA biosynthesis is prevented by anaerobic
conditions, yeasts integrate sterols and UFAs from the fermentation media in their
membranes. Cereals contain a large variety of nutrients including phytosterols, which
serve as a source of membrane sterols for yeasts also in anaerobic conditions.

6.3 Minor Yeast Metabolites in Sourdough

In addition to ethanol and CIO 2 , yeasts generate a large spectrum of metabolites
during sourdough fermentation. Yeasts are responsible for characterising the aroma
profile, or so-called “fermented taste” in the production of bread and alcoholic
beverages. This profile consists of a complex mixture of flavour compounds but the
characteristic impact on flavour is determined mainly by fusel alcohols and their
derivatives. Commonly known fusel alcohols are 3-methyl-1-butanol, 2-methyl-1-
butanol and 2-methyl-1-propanol. When ammonia is used as the nitrogen source,
fusel alcohols can be synthetized via the isoleucine, valine and leucine (ILV) pathway
[ 63 ]. However, their concentration drastically increases when branched chain amino
acids (BCAAs) are present in the media. Yeasts convert free amino acids mainly by
the Ehrlich pathway [ 64 ] (Table 6.3 ). The Ehrlich pathway assumes the conversion
of the BCAAs to fusel alcohols by three enzymatic steps. The first step is a transam-
inase, in which the amino group of BCAAs is transferred to 2-oxoglutarate, resulting
in branched chain oxoacids and glutamate. The second step is the decarboxylation
step that converts the branched chain oxoacid to the branched chain aldehydes. The
last step is the reduction step in which branched chain aldehydes are reduced to
branched chain alcohols or so-called fusel alcohols ([ 65 ] Fig. 6.5 ). For instance,
S. cerevisiae and other sourdough yeasts convert leucine and phenylalanine to
3-methylbutanol and 2-phenylethanol, respectively [ 64 ] . The Strecker reaction during
baking also generates a-dicarbonyl compounds such as methylglyoxal
(2-oxopropanal), which leads to the corresponding aldehydes and acids [ 66 ] .
The presence of yeasts ( Saccharomyces and Hansenula genera) during sourdough
fermentation favoured an increased synthesis of alcohols, esters and some carbonyl
compounds compared to sourdough fermentation without the addition of yeasts [ 67 ] .