Milk and dairy products are suitable for fast deterioration in terms of their quality. Naturally, it is not possible to obtain quality milk products from low quality milk. Milk means quality as raw material, obtained from disease-free and healthy animals in compliance with hygienic conditions, cooled quickly after being milked, preserved in cold until it reaches the enterprise, chemical composition conforming to standards, low number of bacteria, low enzyme activity and no foreign matter added milk is understood. Therefore, a number of quality control procedures are applied when the milk is accepted. Mainly sensory tests, alcohol test, oil test, dry matter analysis, bacteria count, acidity test and other necessary tests are performed. Tests are also carried out if water is suspected to be added to the milk, or if it is believed that carbonates, hydrogen peroxide or formaldehyde are added as preservatives. A test performed between quality control studies is the resazurin test.

The resazurine test performed between the dye reduction tests in milk is expressed by the color of the resazurine dye from dark blue to white when added to the milk. These colors are numbered for ease of application. These numbers are called the resazurin index.

The resazurine index is as follows:

• 6 Dark blue (milk not reduced)
• 5 Violet
• 4 Lilac
• 3 Red lilac
• 2 Lilac red
• 1 Pink
• 1 White (milk completely reduced)

Resazurin tests are based on two separate principles:

• The amount of time until a certain color opening occurs is determined
• After a certain period of time, which color occurs is determined.

These two principles are the same, only the reading is different.

In the laboratories, resazurin tests are performed within the scope of quality determination studies applied to raw milk among microbiological analyzes.