Assessment of the efficiency of SimPlate™ total plate count color indicator (TPC CI) to quantify mesophilic aerobic microorganisms in pasteurized milk

The SimPlate™ TPC CI system is a rapid method to count mesophilic aerobic microorganisms (MAM) in foods, based on the use of resazurine to indicate bacterial growth. Its efficiency in pasteurized milk was evaluated using 142 pasteurized milk samples (38 type A, 43 type B and 61 type C) collected in Londrina, PR. The standard plating method, using Plate Count Agar (PCA) was used for comparison. The plates of both systems were incubated at 35ºC and read after 24h and 48h. The occurrence of false-positive and false-negative wells and the predominant microorganisms in them were also evaluated. The results were compared by simple correlation and mean variance analyses. The correlation (r) and mean variance values were 0.6811 and 0.7583 for the results obtained after 24h, respectively, and 0.9126 and 0.0842 for the results obtained after 48h, respectively. These results indicate that the performance of the system increases when the plates are incubated for 48h. When the three types of milk were evaluated separately, these values were 0.9285 and 0.0817 for type A milk, 0.9231 and 0.0466 for type B milk and 0.7209 and 0.1082 for type C milk. These results indicate that the better the quality of the milk the better the performance of SimPlate™ TPC CI. False-negative wells, found more frequently in samples with high MAM counts, were caused by Gram positive microorganisms, poorly detected by the SimPlate™ TPC CI system because they grew slowly and had low reduction capacity. The results indicated a higher efficiency of the SimPlate™ TPC CI system in the reading at 48h. O sistema SimPlate™ TPC CI é um método rápido para enumeração de microrganismos aeróbios mesófilos (MAM) em alimentos que utiliza a resazurina como substância indicadora de crescimento bacteriano. Para avaliar sua eficiência em leite pasteurizado, 142 amostras (38 de leite tipo A, 43 de leite tipo B e 61 de leite tipo C) foram colhidas em Londrina, PR, e analisadas pelo SimPlate™ TPC CI e pelo método de contagem em placas com ágar padrão de contagem (PCA). As placas de ambos os sistemas foram incubadas a 35ºC e as leituras realizadas em 24 e 48h. Também foi verificada a presença de resultados falso-positivos e falso-negativos bem como a microbiota láctea predominante nas cavidades do SimPlate™ TPC CI e a capacidade redutora dos diversos grupos de microrganismos. Os resultados foram comparados através de correlação simples e variância média. Considerando a leitura em 2