Reinforced clostridial agar, best known for its use in the isolation of Clostridia species, has been shown to be selective for Lactobacillus bulgaricus found in yoghurt by suppressing the growth of Streptococcus thermophilus when the pH of the agar is 5·5. Str. thermophilus was isolated on plate count agar (PCA) incorporating 10 per cent milk by incubating at 37°C for a maximum of 24 h, since L. bulgaricus colony development was only sparse after this period of incubation.
Yeasts can be detected by the Bactometer 32 both in broth and in broth containing 10% yogurt culture. Several broths gave results which agreed well with the colony count. and Malt Extract broth incorporating 2% glucose was selected as satisfactory. With yogurt it was necessary to include antibiotic (20 ppm chloramphenico[) in the broth to suppress the bacterial signals and allow good detection of the yeasts. Results were available three days earlier than for a colony count, even with a yeast of slow multiplication rate.Impedance measurements have been used to determine bacterial numbers in a variety of foods during the last few years.
The Bactometers M120B and B32 were used to detect contaminant yeasts in fruit mix used for manufacture of yogurt. Both machines were able to detect yeasts, but the capacitance signal of the M120B gave results at least 2 hours faster than the impedance signal given by the B32. The correlation between yeast numbers and detection times in fruit mix was good for both systems, but the shorter detection time of the M120B is an advantage in a production laboratory.
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