Dye reduction test
- 2. INTRODUCTION Test to find bacterial contamination of food (especially milk). Based on utilization of oxygen by bacteria which reduces dye and decolorizes it. Faster the decolorization, more inferior is the bacteriological quality and vice-versa.
- 3. Methylene Blue Reductase Test(MBRT) • Take 10ml of milk sample in a sterile test tube. • Add 1ml of MBRT dye(dye concentration 0.005%). • Close the tube with sterilized rubber stopper. • Place in stand dipped in serological water bath maintained at 37±1⁰C. • Note time taken for decolorization.
- 4. FIGURE: Methylene Blue Dye Reduction
- 5. Grading Of Raw Milk Based On MBRT As per BIS 1479 (Part 3): 1977 criterion for grading of raw milk based on MBRT is as below: TIME QUALITY 5 hours and above Very good 3 to 4 hours Good 1 to 2 hours Fair Less than ½ hours Poor
- 6. Rate of reduction depends on the nature of the organism present in the milk. The rate of reduction by different microorganism is: E.coli (coliforms) > Streptococcus lactis > faecal Streptococci > micrococci> Thermoduric organisms > psychrotrophic organisms Light hastens reduction and therefore the tests should be kept covered. Creaming causes bacteria to surface, so the tubes should be inverted periodically. Any manipulation that increases oxygen affects the test like cold milk holds more oxygen, pouring milk back and forth and so on. Factors Affecting
- 7. RESAZURIN TEST • Same principle as MBRT • More sensitive resazurin dye used Mix 10 ml milk sample with 1ml of resazurin dye solution(0.05%). Place stopper and invert the tube to mix.Incubate at 37±1⁰C. Examine tubes at the end of an hour in the "one-hour test" or at the end of three successive hourly intervals in the "triplereading test."
- 9. Contd… Judgment of quality is based either on the color produced after a stated period of incubation or on the time required to reduce the dye to a given end-point. COLOR QUALITY Blue(no color change) Excellent Blue to deep mauve Good Deep mauve to deep pink Fair Deep pink to whitish pink Poor White Bad
- 10. OTHER FOODS TESTED(EXPERIMENTAL) • Maple sap • Red meet • Ground beef • Eggs • Seafood • Vegetables, etc.
- 11. ADVANTAGES • Simple, rapid, inexpensive • Only viable cells actively reduce the dye DISADVANTAGES • Not all organisms reduce the dye equally • Not applicable to food specimens that contain reducing enzymes