Evaluation of flow cytometry as a method for total bacterial count of raw milk / Validação da metodologia de citometria de fluxo para avaliação da contagem bacteriana do leite cru

AUTOR(ES)

Laerte Dagher Cassoli

DATA DE PUBLICAÇÃO

2005

RESUMO

The objective of this study was to evaluate the utilization of electronic flow cytometry to determine total bacterial count (TBC) of raw milk. In the first experiment, the effect of storage temperature, sample age and milk preservative type on TBC were evaluated. Additionally, the use of a single milk sample to performance regulatory milk analysis under the Normative Instruction 51 (NI-51) was tested. Effects were standardized as: three storage temperatures (0ºC – freezer, 7ºC – refrigerator and 24ºC – room temperature), four sample ages (1 (D1), 3 (D3), 5 (D5) and 7 (D7) days) and three milk preservatives (bronopol, azidiol and no preservative). Control treatment for TBC analysis was defined as refrigerated milk samples containing azidiol with 1 day of storage. For determination of milk components and somatic cell count (SCC), control treatment was defined as refrigerated milk samples containing bronopol with 1 day of storage. Results of the first experiment showed that two milk samples are necessary to performance regulatory milk analysis under the NI-51; one containing bronopol should be used for determination of milk components and SCC, and other containing azidiol for TBC. Milk samples used for TBC can be tested until 7 days after sampling when they are kept at 7ºC. Freezing or heating milk samples for TBC analysis should be avoided and addition of azidiol is always necessary. The second experiment was designed to determine a correlation between two methods of TBC, electronic flow cytometry and standard plate count. Milk samples collected from June to September (n = 155) were named as dry season samples and milk samples collected in November and December (n = 68) were named as rainy season samples. Each milk sample was used to run both methods of TBC. Results were expressed as individual bacterial count (IBC) and colony forming unit (CFU) for electronic flow cytometry (Bactocount) and standard plate count, respectively. The linear equations of correlation between IBC and CFU had similar patterns in both seasons, dry and rainy, indicating that a single equation can be used to transform IBC results in CFU along the year. The linear equation was defined as log(CFU) = 0.7224 x log(IBC) + 1.4617 with coefficient of correlation of 0.8125. The accuracy of Bactocount in estimating reference values, denoted by the standard error (s(y,x)), was 0.309 log CFU/mL. The results showed that Bactocount can be calibrated to express TBC readings in CFU and, consequently, be used to monitor milk quality.

ASSUNTO(S)

standard plate count citometria de fluxo leite routine method bactocount bacteriologia microbiologia de alimento flow cytometry milk quality bacterial count

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