变质 (biàn zhì): to go bad, to deteriorate

The milk in the fridge had been there for a week, had gone bad, and gave off a sour smell; it couldn't be drunk anymore.

In summer the weather is hot, food easily goes bad, so it's best to eat it the same day.

They used to be close friends who shared everything, but later a misunderstanding made their friendship deteriorate.

These fruits look fine, but inside they've already gone bad; you'll know with one bite.

Due to improper storage, this batch of medicine has deteriorated and can no longer be used for patients.