Simplify the following text for a Grade 8 native English-speaking person.
Original: HTST pasteurized milk normally has a refrigerated shelf life of two or three weeks , but ultra pasteurized milk can last much longer when refrigerated , sometimes two or three months .
Simplified: It is the main reason for milk 's extended shelf life . High Temperature Short Time ( HTST ) pasteurised milk typically has a refrigerated shelf life of two to three weeks , whereas ultra pasteurised milk can last much longer , sometimes two to three months .