Fish Microbiology and Quality Assurance (2+2)

13. Determination of microbiological quality of food by dye reduction test

Dye reduction test is a simple and easy to perform enzymatic test useful for determining the microbiological quality of food samples such as milk. A milk sample containing large population of actively metabolizing microorganisms lower dissolved oxygen because of vigorous growth of microorganisms thereby lowering oxidation-reduction potential in the sample. The dye methylene blue, a redox indicator, becomes reduced in an anaerobic environment and loses its color. Thus, there is direct relation between the length of time required for reduction and the number of bacteria present. The speed at which methylene blue becomes colorless following addition to sample indicates quality of milk. The shorter the reduction time, lower the quality of milk. Based on dye reduction test four grades of milk quality is recognized.

Decolonization Time	Approx. no of microorganisms/ml	Quality grade
0- 20 minutes 20- 120 minutes 120- 330 minutes > 330 minutes	> 2 x 107 > 4 x 106 > 5 x 105 < 5 x 105	Very bad Bad Fair good

Materials required
1. Sterile test tubes with stopper
2. Milk sample
3. Metylene blue solution (1: 25,000)
4. Pipettes (10ml, 1ml)

Procedure

To sterile tests containing 10 ml of milk 1 ml metylene blue solution is added, mixed well and incubated in water bath at 35⁰C. The tubes are observed for color change at 30 minutes time intervals for up to six hours and time taken for turning the tubes colorless recorded.