8/24/2016 ·Advantages Direct Microscopic Count Grid with 25 large squares ·fast,simple no incubation Cover glass little sample processing Slide small sample size Disadvantages ·fatigue live and dead bacteria counted no value for foods with low numbers Sample spread on a microsope slide" Cover glass Fix,defat,and stain the cells Calibrated microscope used Location of squares know the diameter of the field c Number of cells or clumps per field divided by the area of re kh the field-number per square millimeter Metabolism-Based Methods of Analysis Methylene Blue Reduction Test Measurements of metabolism or production of Color imparted to milk by metabolic products can be used to estimate microbial methylene blue disappears numbers as oxygen is removed due Most are minaturized and the basis of rapid test to the formation of methods reduced substances ·Redox potential electron transfers measured using potentiometer Bacterial metabolism dye reduction tests-use redox indicators or dyes causes the color to to estimate bacterial numbers disappear larger the number-faster the color changes Some foods contain high levels of reductive Quicker the color changes substances the larger the number of Commonly used for raw milk bacteria present >8/24/2016 7 Direct Microscopic Count • Advantages - fast, simple - no incubation - little sample processing - small sample size • Disadvantages - fatigue - live and dead bacteria counted - no value for foods with low numbers • Sample spread on a microsope slide • Fix, defat, and stain the cells • Calibrated microscope used - know the diameter of the field • Number of cells or clumps per field divided by the area of the field – number per square millimeter 25 26 Metabolism-Based Methods of Analysis • Measurements of metabolism or production of metabolic products can be used to estimate microbial numbers • Most are minaturized and the basis of rapid test methods • Redox potential - electron transfers measured using potentiometer - dye reduction tests – use redox indicators or dyes to estimate bacterial numbers  larger the number – faster the color changes • Some foods contain high levels of reductive substances • Commonly used for raw milk 27 Methylene Blue Reduction Test • Color imparted to milk by methylene blue disappears as oxygen is removed due to the formation of reduced substances • Bacterial metabolism causes the color to disappear • Quicker the color changes the larger the number of bacteria present 28