Four sourdoughs (A to D) were produced under practical conditions by using a starter mixture of three commercially available sourdough starters and a baker's yeast constitutively containing various species of lactic acid bacteria (LAB). The sourdoughs were continuously propagated until the composition of the LAB flora remained stable. Two LAB-specific PCR-denaturing gradient gel electrophoresis (DGGE) systems were established and used to monitor the development of the microflora. Depending on the prevailing ecological conditions in the different sourdough fermentations, only a few Lactobacillus species were found to be competitive and became dominant. In sourdough A (traditional process with rye flour), Lactobacillus sanfranciscensis and a new species, L. mindensis, were detected. In rye flour sourdoughs B and C, which differed in the process temperature, exclusively L. crispatus and L. pontis became the predominant species in sourdough B and L. crispatus, L. panis, and L. frumenti became the predominant species in sourdough C. On the other hand, in sourdough D (corresponding to sourdough C but produced with rye bran), L. johnsonii and L. reuteri were found. The results of PCR-DGGE were consistent with those obtained by culturing, except for sourdough B, in which L. fermentum was also detected. Isolates of the species L. sanfranciscensis and L. fermentum were shown by randomly amplified polymorphic DNA-PCR analysis to originate from the commercial starters and the baker's yeast, respectively.The production of sourdough bread can be traced back to ancient times (24). The products are characterized by their unique flavor, enhanced shelf life and nutritional value, and favorable technological properties (16,25). Sourdough is an intermediate product and contains metabolically active microorganisms. The microbial ecology of the sourdough fermentation is determined by ecological factors described by Hammes and Gänzle (16). Endogenous factors are determined by the chemical and microbiological components of the dough, and exogenous factors are determined by temperature and atmosphere. In practice, strong effects are exerted by process parameters such as dough yield, amount and composition of the starter, number of propagation steps, and fermentation time. The impact of these parameters during continuous propagation of sourdough causes the selection of a characteristic microflora consisting of lactic acid bacteria (LAB) and usually yeasts. Microbiological studies have revealed that 43 species of LAB, mostly species of the genus Lactobacillus, and more than 23 species of yeasts, especially species of the genera Saccharomyces and Candida (6, 23), occur in this ecological niche. As shown for certain industrial sourdough processes (4), such microbial associations may endure for years, although the fermentation process runs under nonaseptic conditions.Based on common principles used in artisanal and industrial processes, Böcker et al. (5) defined three types of sourdough. Type I sourdoughs are produced with traditional techni...

Four sourdoughs (A to D) were produced under practical conditions, using a starter obtained from a mixture of three commercially available sourdough starters and baker's yeast. The doughs were continuously propagated until the composition of the microbiota remained stable. A fungi-specific PCR-denaturing gradient gel electrophoresis (DGGE) system was established to monitor the development of the yeast biota. The analysis of the starter mixture revealed the presence of Candida humilis, Debaryomyces hansenii, Saccharomyces cerevisiae, and Saccharomyces uvarum. In sourdough A (traditional process with rye flour), C. humilis dominated under the prevailing fermentation conditions. In rye flour sourdoughs B and C, fermented at 30 and 40°C, respectively, S. cerevisiae became predominant in sourdough B, whereas in sourdough C the yeast counts decreased within a few propagation steps below the detection limit. In sourdough D, which corresponded to sourdough C in temperature but was produced with rye bran, Candida krusei became dominant. Isolates identified as C. humilis and S. cerevisiae were shown by randomly amplified polymorphic DNA-PCR analysis to originate from the commercial starters and the baker's yeast, respectively. The yeast species isolated from the sourdoughs were also detected by PCR-DGGE. However, in the gel, additional bands were visible. Because sequencing of these PCR fragments from the gel failed, cloning experiments with 28S rRNA amplicons obtained from rye flour were performed, which revealed Cladosporium sp., Saccharomyces servazii, S. uvarum, an unculturable ascomycete, Dekkera bruxellensis, Epicoccum nigrum, and S. cerevisiae. The last four species were also detected in sourdoughs A, B, and C.Characterization of complex microbiota as they occur in food fermentation processes is facilitated by the development and application of sensitive and powerful molecular methods but is still a challenge. The microorganisms contributing to the characteristic properties of the food during the course of the fermentation process should be known in order to allow control of the process by selection of the appropriate technological condition and by using defined cultures. In a previous study, we reported the monitoring of lactic acid bacterium (LAB) population dynamics during the fermentation process in four continuously propagated sourdoughs by a LAB-specific PCR-denaturing gradient gel electrophoresis (DGGE) system (25). PCR-DGGE detects the 90 to 99% most numerous species of a community without discriminating living from dead cells or cells in a noncultivable state. The study revealed fluctuations within the LAB population, and under different ecological conditions, characteristic species prevailed. Because yeasts fulfill several important functions in bread making, the knowledge of their composition is also essential (15). They contribute to leavening (38) and produce metabolites such as alcohols, esters, and carbonyl compounds which contribute to the development of the characteristic bread flavor (7,9,16,20,21...

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