The phrase "oldest milk" refers to milk that has been kept for a long time and is no longer fresh.
Sentences with «oldest milk»
When it's time for a feeding, serve oldest milk first. (parents.com)
The team even offers free presentations to pupils about recycling to explain the production process involved in turning old milk bottle tops into their new play trail. (qaeducation.co.uk)
My mom had recently given me a big box of old milk glass vases that belonged to my grandmother. (blesserhouse.com)