Willow 2.0 Milk Bags - our unique spill-proof bags, with a slightly updated shape. Willow 2.0 Milk Bags are BPA free* and food safe* for baby, and spill proof for sanity. Each 4 oz. disposable Milk Bag features a one-way valve so milk goes in and doesn’t come out. Tuck a fresh bag inside Willow®, pump, and then pour it or store it. They’re also fridge and freezer safe to help your supply stay fully stocked.