I cooked was milk together with rice, sugar, and cinnamon for more than 50 minutes yesterday. It was stored in the fridge for a night. Today I found small and sparse pink spots (~3mm radius, one or two max for each single-dose small bowl).
Any idea of what these can be? Can these signal any safety problem with the storage or boiling of the milk? Is it safe to ignore them?
There is no way to be sure from the information you have given us. If in doubt, throw it out!
If you could take a photo of the spots, and edit into the question, that might help. Also a description of your process would be helpful to determine if there is a problem with something you are doing. Particularly how you stored it (as one mass? if so how big is that? if not, how much per aliquot? etc.).
Milk, sugar and rice make an excellent medium for growing bacteria and yeasts. If your food was warm enough for long enough (a few hours between about 60 C and 4 C), then it could easily grow bacteria or yeasts to a visible colony size.
Pink spots could be colonies of coloured bacteria such as Serratia marcescens (often found as pink stains in around drains in sinks, showers, baths, or other damp spots like the back of the fridge) or yeasts such as Rhodotorula genus, or they could just be pink stain from the cinnamon. There are other potential bacterial species that you might not see as colonies, as these are often pale yellow or beige/tan and would blend in with the cooked colour of your dish.
Boiling doesn't sterilize (you need higher pressure to get a higher boiling point of water for that). It will kill off many species of bacteria, but not necessarily, and you can contaminate between boiling and storing in the fridge. For the fridge storage, you would need to separate the mass into small lots and cool rapidly. It may be that your fridge is too full or too empty to cool properly. Another possibility is that you put the food in there too hot and the fridge couldn't cool effectively, so acted as an incubator, keeping the food warm.
