synergistic stressing action of high concentration of sugars and ethanol, especially after 24 h of SSF, upon the yeast cells [35]. This facts could also explain the higher concentrations of glycerol found in fermenting mashes in samples T0, Tp and T85. Glycerol is produced by yeast as a response to osmotic and ethanol stresses during fermentation [36] and its overproduction is one of the key factors which decreases ethanol yield. The presence of small amounts of lactic acid in mashes prepared from waste wheat–rye bread originated from the used raw material, rye flour is most often subjected to fermentation with its indigenous lactic acid bacterial microflora during which lactic acid is produced, which is then called the sourdough. However its initial concentration was negligible and did not increase during the fermentation what indicates that no further contamination occurred, hence it did not have inhibitory effect on the SSF processes [37].