My grandmother buys milk bags as they cost less, stores each milk bag in a pitcher as such, but can drink only 1/4 of the milk bag daily. So after opening each bag, she must close and fasten the milk bag, and to do so she has tried using a rubber band and a binder clip.
The problem:
Rubber bands and clips fail to be secure enough, since the milk always start to spoil on the 4th day.
So what else can be used to close the bag, to increase the endurance of the milk in the refrigerator?