pH – 6.9 It is used to determine the ability of an organism to produce mixed acids by fermentation of glucose and to overcome the buffering capacity of the medium.
The development of stable red color in the surface of the medium indicates sufficient acid production to lower the pH to 4.4 and constitute a positive test.
In the presence of atmospheric oxygen and 40% KOH, acetoin is converted to diacetyl and α-naphthol serves as catalyst to bring out red color complex.
A positive test is represented by the development of red color in 15 minutes or more after addition of the reagents, indicating the presence of diacetyl, the oxidation product of acetoin.
The test should be red, after standing for 1 hour because negative VP cultures may produce copper-like colour potentially resulting in a false positive interpretation, also because due to action of the reagents when mixed.