BACKGROUND: Viscosity-time plots for plasmid-bearing E. coli cells undergoing alkaline lysis are reported in this study. The plots demonstrate generic features that reflect the progress of fermentation and allow an assessment of the genomic DNA denaturation following cellular release into the alkaline solution. This rheological analysis could offer useful insights to the state of fermentation or the selection of operational specifications and predictions of the performance of subsequent downstream operations. RESULTS: Studies showed a distinct change in the rheological profile throughout the batch fermentation, with different viscosity versus time profiles for lag, exponential and stationary microbial growth phases. The DNA denaturation time was found to increase with fermentation time from about 120 s after 3 h of fermentation to about 180 s after 7 h of fermentation. CONCLUSION: The increase of denaturation time was mainly caused by a rise in the genomic content of cells during the exponential growth phase. The viscosity-time profiles were found to provide a good indication of the cellular contents, reflecting the physiological changes occurring during a batch fermentation process. (C) 2009 Society of Chemical Industry