Microbiological and technological aspects of bread doughs leavened by Zymomonas mobilis

Zymomonas mobilis is a Gram negative bacterium with a fermentation metabolism similar to that of Saccharomyces cerevisiae. It has potential food applications in the development of leavened baked goods for people with adverse responses to the ingestion of baker’s yeast. The aim of this research was to assess the leavening performances (up to 24 h at 26°C) of Z. mobilis in bread doughs. Since it cannot ferment flour maltose, the association with Lactobacillus sanfranciscensis, possessing a constitutive maltose hydrolytic activity, or the direct addition of glucose to the dough (5 g/100 g flour) were investigated. The leavening performances of S. cerevisiae were chosen as benchmark. Preliminary baking trials were also conducted. Limited bacterium grown (from 7.5 to 8 log cfu/g), low dough development and CO2 production characterized samples leavened by Z. mobilis without glucose addition. Whit glucose, Zymomonas reached 9 log cfu/g and more than three times of ethanol was found. The addition of L. sanfranciscensis, slightly improved CO2 production but reduced the relative gas retention ability of the dough. The rheofermentographic analyses pointed out that glucose addition in samples leavened by Z. mobilis alone or coupled with L. sanfranciscensis caused a double raising of the doughs, contrarily to what happened with S. cerevisiae. As regards the preliminary baking trials, doughs leavened by Z. mobilis with glucose and by S. cerevisiae were compared: promising results were obtained in terms of bread development and alveolar crumb structure. In conclusion, results demonstrated that Z. mobilis in presence of glucose could efficiently leaven a bread dough, whereas the association with L. sanfranciscensis did not favour Zymomonas growth in a relevant manner: the pH dropped to values lower than 4, thus limiting Zymomonas leavening performances. The well-developed-breads obtained with Z. mobilis proved that this bacterium can be used to produce yeast-free leavened baked foods.