In order to remove milk or oil stains, do use undiluted baby bottle detergent with the Hegen Bottle Brush and Teat Cleaner as this would aid in the breaking down of milk fats. The stains may sometimes be hard to remove if they have been left there for a period of time.
As such, ensure that cleaning takes place as soon as possible in order to avoid tough stains.
Note also that as breast milk has been stored in the fridge, the composition of the breast milk may get segregated after some time. As such, do swirl the content to ensure that the breast milk is well mixed after warming up as this will help to prevent fats in the breast milk from clinging onto the bottle and harder to be washed off effectively.