عنوان مقاله [English]
The methylene blue reduction method measures bacterial density in milk in term of the time interval required, after starting incubation, for a dye - milk mixture with a characteristic blue color to become white. The principle of the test is based on a shift of the redox potential of the milk due to metabolic action of bacteria present. As the potential falls, the methylene blue is decolorized. This method is used to grade raw milk, especially for manufacturing purposes. The small amount of equipment and materials and the simplicity of the method are the most important advantages of this test. In this study methylene blue reduction test and standard plate count were compared using 112 samples of raw milk obtained from bulk milk, at the time of receipt, during fall and winter, 1998. Samples were also examined for their acidity, leukocyte count, temperature at the time of receipt and specific gravity, all of which used for evaluating the microbiological quality of raw milk. Correlation cofficient between Log10 standard plate count with methylene blue reduction time and Leukocyte count were low(r = 0.41 and 0.28 respectively) No significant correlation was observed between Log10 SPC with the tests of specific gravity (p = 0.072) temperature at the time of receipt (p = 0.235) and e acidity (p = 0.745). This study shows that none of these tests are valuable for assessing bacterial quality of raw milk, and they cannot result in improved coefficient of determination (R2) in multifactorial regression equation. It is concluded that the methylene blue reduction test is an unsuitable test for assesment of bacterial quality of raw cow milk and it should be replaced with other methods which enumerate microorganisms in milk, such as spiral plating system, plate loop method or flow cytometry.