The situation. I bought two packs of powdered skim milk. I think I used a part of it for making pulvoron and pastillas. I bought the two at different times. I thought the first was already consumed so I bought another one. I ended up having two unconsumed packs. I let them in cellophane but placed inside a microwavable box for added protection. I bet on the fact that skimmed milk was a dried product and should be shelf stable. No refrigeration needed.
I took it out of the glass shelf after a couple of months. The skimmed milk was not white. It became yellowish. I dumped it right in garbage can without a second thought.
Is the milk that turned yellow already spoiled? Maybe or maybe not. Remember that the top of freshly harvested milk is yellow and some popular commercial milk formulas have yellow tint. The native soft cheese or kesong puti is white but most commercially manufactured cheeses are yellowish.
We experienced opening a yellow evaporated milk. Color was not convincing for him so he dared taking a sip and found out that it was really bad.
Here are other thoughts about milk turning yellow.
When powdered milk goes bad it turns rancid, has a distinctive odor, and also may turn yellow in color. If you were to mix it up to drink, you’d be able to taste the “off”-ness. (Kathieo )
Discard any powdered milk that develops an off odor or yellow coloration. These are signs that the milk has finally gone bad. (frugalliving)
This one is not closely related but could be useful. The milk fat itself is yellow, and emulsions often don’t survive the freezing process. That’s why frozen milk appears yellow – the emulsion has broken and the colour of it’s dispersed phase can be seen. answers.yahoo)