Gather hot soapy water in the sink
Fill your sink with hot water and add some soap or dish cleaner to it. Shake so that the soap combines well with the water. Make sure the water is warm as it helps in removing the dirt and stains from the thermos quickly.
Wash the thermos in the sink
Place the thermos in the warm water and leave it for a few seconds so that the bacteria are removed. Thoroughly wash the thermos with the soapy water until it is clean. After that, rinse the thermos with cool running water and make sure it has no soapy water left. Set the thermos aside so that it can dry.
Add baking powder and white vinegar to the thermos
When the thermos is completely dry, add two tablespoons of baking soda in it. After adding the baking soda, pour a cup of white vinegar in the thermos. Baking soda and Vinegar are extremely effective is removing odours inside the thermos. They are natural cleaners and will help in removing any type of smell from the bottle.
Shake the thermos repeatedly
After adding both the baking soda and vinegar, close the lid of the thermos and shake it repeatedly for some time. You must continually shake the thermos so that the mixture of baking soda and vinegar reaches the inside from top to bottom.
Rinse with cold water
Remove the lid of the thermos and pour the mixture in the sink. After that, rinse the thermos again with cool water so that the vinegar ingredients are watered down. Once you have washed the thermos nicely, leave it for a few minutes to dry down. If you still find odours of milk inside the thermos, repeat the same process again.