13 Apr

Reductase Test

The reductase test is a test used to determine the quality of milk.  This test measures the oxidation reduction potential due to the exhaustion of dissolved oxygen by microorganisms.  Methylene blue loses its color as it becomes reduced.  In the reductase test 1mL of methylene blue is placed in 10mL of milk.  The tube is sealed with a rubber stopper and slowly inverted three times to mix.  It is placed in a water bath at 35 C. and examined at intervals up to 6 hours.  The time is takes for methylene blue to become colorless is the methylene blue reduction time for (MBRT).  The shorter the MBRT, the lower the quality of milk.  An MBRT of 6 hours is very good however an MBRT of 30 minutes indicates a very poor milk quality.  The test is based on the assumption that all bacteria in milk lower the oxidation reduction potential at 35 C.  Nonpathogenic bacteria  which do not grow at this temperature would not produce a positive result.  Streptococcus lacis and Escherichia coli are strong reducers and are present in raw milk.