Equipment: Test tubes, stoppers, test tube racks, measuring cylinders, spatulas, warming cabinet, beakers (50 ml), filter paper Chemicals: Methylene blue solution (1%, according to Löffler), UHT milk (whole milk), various types of yoghurt and yoghurt ferment. Methylene blue is a widely used vital dye in microscopy for staining living material. Löffler's methylene blue solution is a solution in water that is used to stain bacteria, fungi and blood parasites.

Fill four test tubes with 15 ml of milk each. Add 20 drops of methylene blue solution to each test tube, close with a stopper and shake vigorously. Add two spatulas of different types of yoghurt or the yoghurt ferment to three glasses, close the glasses and shake vigorously again. Label the test tubes. Incubate all four glasses overnight at 30-40 °C in an incubator.

In this way, the jars are warmed and are perfectly prepared for the lactic acid bacteria in the yoghurt. If I were to pour the warm milk-yoghurt mixture into cold jars, the bacteria would not feel as comfortable due to the coldness of the jars and would become less active or die completely. The result is examined the next day.

a) Look at the test tubes. Briefly describe the changes you can observe in the four approaches ________________________________________________________________________________________________________________________________________________________________________________________________________________________ b) Make a sketch of your experimental results ________________________________________________________________________________________________________________________________________________________________________________________________________________________________

a) The formation of yoghurt from milk takes place with the help of a specific process. Name this and explain it briefly. Also talk about the bacteria involved _______________________________________________________________________________________________________________________________________________________________________ ___________________________________ b) How can the different taste of milk and white natural yoghurt be explained? _______________________________________________________________________________________________________________________________