Effect of penicillin in concentration below the sensitivity level of biological methods on yoghurt cultures and yoghurt

The study was conducted to examine the effect of penicillin (0.003 IJ ml -1 of milk) in concentration below the sensitivity level of biological methods (Delvo SP and Resasurine test) on yoghurt cultures. According to the results obtained, the stated penicillin concentration prolonged the period of final incubation in the bulk starter (3 h and 20 min) and in yoghurt (3 h and 40 min). The total count of S. thermophilus colonies in the bulk starter and yoghurt is also reduced with a loss of one (log 7.32), that is, two logarithmic numbers (log 6.53) compared with the control samples (without penicillin). The total count of L. bulgaricus colonies was higher in the control samples compared with the experimental samples. Upon the final incubation period and storage for 18 h at 5 °C, there was a drop in the titratable acidity and an increase in pH values of the control samples. Microscopic examination of the control samples showed a gradual disappearance of streptococci with a complete prevalence of lactobacilli in yoghurt. Additionally, morphological changes occurred in streptococci (long chains) and lactobacilli (long and irregular rods)