The viscoelastic properties of concentrated culture suspensions of
Escherichia coli in pH region from 4 to 9 were studied. It was confirmed from viable cell counts that microbial cells were viable in that pH region. Storage modulus for samples in pH region from 6 to 9,at a low strain of 0.02,shows small frequency dependence in lower frequency region, such as the characteristics of gel, because network structure formed by microbial cells behaves as a relaxation mechanism having long relaxation time. However, at a high strain of 1,storage modulus shows remarkable frequency dependence, since the network structure has been broken down. The values of G' and η',at the angular frequency of 0.1 rad/s are almost constant up to the strain amplitude of 0.01 for all samples. The values depend on pH, namely, the viscoelastic values at low pH 4, 5 samples are larger than those at high pH. With increasing strain amplitude, the values of storage modulus decrease. The strain region where elastic moduli are constant extends to higher strain for the samples at high pH 6, 7, 8. In the strain dependence of dynamic viscosity,a maximum is observed at the strain where the value of storage modulus becomes decreased. The differences in the viscoelastic properties depending on pH of media used to cultivate the microbes suggest that binding forces interacting among cells were affected by the pH. It was observed by electron microscope that soft matter like polysaccharides surrounding the cells was conspicuous in high pH samples.
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