Frequency of 2,3,5-triphenyltetrazolium chloride (TTC) non-reducing bacteria in pasteurized milk

2,3,5-triphenyltetrazolium chloride (TTC) is a dye largely used for enumera tion of microbial colonies in solid culture media, being a key component of the dry rehydratable film system used for microbiological analysis of food . This dye is colorless in the oxidized form and red when reduced by microo rganisms, due to formation of formazan. In this study, TTC was added to Pla te Count Agar (PCA) for enumeration of microorganisms in thirty four pasteu rized milk samples, with the aim to verify the frequency of microorganisms that are unable to reduce TTC. Milk samples were decimally diluted in salin e and pour-plated in PCA plus 0.015% TTC. Colonies were counted after 24h a nd 48 h of incubation at 35 degrees C. From a total of 50,574 colonies, 19, 665 (38.88%) did not reduce TTC in 48h. It was observed that 571 (6.36%) co lonies that were colorless in 24h became red in 48h. From those that didn't reduce TTC in 48h, 233 were purified and Gram stained. 229 (98.71%) of the m were Gram positive cocci and bacilli. The results show that there is a hi gh percentage of microorganisms unable to reduce TTC in pasteurized milk, w hich cannot be detected by laboratory procedures based on the formation of red colonies.