Salt, EDTA, and pH effects on rheological behavior of mold suspensions

The effects of surface properties on the rheological behavior of Aspergillus niger suspensions were studied at pH 2-7, with the addition of 0-0.10 m NaH2PO4, NaCl, CaCl2, AlCl3, and EDTA at shear rates of 0-21.6 s-1. The structural network concept was used to discuss the consequences of cell-cell interactions on the rheological behavior. Analysis of the data indicated that the suspensions containing CaCl2 nearly always had the smallest shear stress at all the pH values, indicating that, unlike with the yeast suspensions, Ca2+ does not contribute to the specific bonding of the mold cells. At low pH and salt concentrations, NaH2PO4 was better than AlCl3 for promoting bond formation between the cells. When either the salt concentration or the pH was increased, AlCl3 became a better bond formation agent than NaH2PO4. EDTA removes the charged groups from surfaces and encourages cell-cell contact via hydrophobic interactions. The shear stress and consistency index increased while the flow behavior index decreased with EDTA concentration

Mixed culture growth kinetics of Streptococcus thermophilus and Lactobacillus bulgaricus

A simple microbiological technique was used to differentially enumerate growth of Streptococcus thermophilus and Lactobacillus bulgaricus in a mixed culture. The growth of the microorganisms in the mixed culture was satisfactorily simulated with a set of modified logistic equations. This simple model was valid for various initial biomass concentrations and their ratios. It did not need substrate or product data for simulation of biomass growth, which may simplify the calculations in fermenter design. It was shown that our model may also be regarded as a special case of a common mixed culture model: Volterra's competition analysis

Growth kinetics of streptococcus thermophilus at subbacteriostatic penicillin G concentrations

Streptococcus thermophilus may be subjected to the effects of penicillin G in contaminated milk used for yogurt production. Sensitivity of this microorganism to penicillin G has been conventionally determined by the help of penicillin G-impregnated disks placed on solid media. It was observed that the bacteriostatic penicillin G concentration was much greater in liquid media than in solid media. The conventional disk method may not be appropriate for antibiotic sensitivity determinations if the microorganisms will be used in liquid culture. A simple mathematical model simulated the growth of S. thermophilus in liquid culture. Numerical values of this model's parameters were regarded as the measure of the antibiotic effect on the culture. In penicillin G containing fresh medium, small concentrations of antibiotic decreased the specific growth rate considerably. Increasing the antibiotic concentration caused only slight additional decline. Antibiotic shock, i.e., rapidly introducing penicillin G into an actively growing antibiotic-free culture, stopped growth of the penicillin G-resistant microorganisms, and no death was observed, but a fraction of the microorganisms were killed in the wild culture. Both the wild and the resistant cultures recovered from the shock in a few hours. Addition of penicillin G-resistant microorganisms together with the antibiotic dosage into the wild culture prevented death