Recently, while cleaning a neighbour's fridge (turned off for a few weeks), I came across a cup (closed with a lid). Inside the cup was, to my olfactory horror, congealed milk, with a steel (iron) teaspoon nicely wedged into it.
The milk had separated into a blue cheese-like solid, overlain with a semi-translucent liquid.
When I took the teaspoon out of the congealed mess, there was a layer of rust on the teaspoon where it was in the liquid. But, I observed almost no rust on the parts of the teaspoon in the blue-cheese-like solid and that exposed to the air.
The illustration below is a rough schematic of where the rust occurred (the 2 layers observed are labelled A and B, and the air in the top of the container is labelled C and the diagonal line represents the teaspoon:
A = congealed blue-cheese-like solid = virtually no rust on this part of the teaspoon
B = semi translucent liquid = heavily rusted part of the teaspoon
C = air = virtually no rust on this part of the teaspoon
How could the milk, or the liquid separation, cause rust on the teaspoon?
A hunch I have (and I could be way off the mark), is it possible that the corrosion could be due to, at least in some significant way, due to the fact that milk is slightly acidic $^{(1)}$?
$^{(1)}$ According to the International Livestock Research Institute, they state that
Fresh milk has a pH of 6.7 and is therefore slightly acidic
Related question and answer Silver and milk (or milk products)
there was a layer of rust on the teaspoon where it was in the liquid.
- the part of the teaspoon immersed in the separated liquid. $\endgroup$