Why is the Sudan test for skim milk negative?

The Sudan test for skim milk is negative because skim milk contains little to no fat, which is the primary component that the Sudan dye detects. The Sudan dye specifically binds to lipids, and since skim milk has had most of its fat removed, there are insufficient lipids present for the test to yield a positive result. Consequently, the absence of fat in skim milk leads to a negative outcome in the Sudan test.