Sara Salow, a 30-year-old mother, said she was humiliated as airline staff said that she had too many bags and must pay $150 for the small cooler filled with eight bags of frozen breast milk, reports Xinhua news agency.
"It was humiliating, they kept telling us it was because we were basic economy passengers. It really felt demeaning," said Salow, who was travelling with her husband and their 13-month-old son from Boston to Phoenix on December 7.
When she started crying, other passengers offered to take the cooler but the airlines employees denied all requests. She had to dispose of the cooler.
"The customer should have been allowed to fly with the breast milk and we apologise that a mistake was made in this case. We have clarified our policies with our team members," the airlines said in a statement on Thursday.
The service of major US airlines has come under intense fire many times this year, especially over issues of over-booking and luggage policies.
United Airlines has sparked global outrage in April for violently dragging a passenger off an over-booked flight, when the man refused to give up his seat.