I've been told that casein paints are great to use but that they will spoil over time, unlike acrylics and oils.
How long will casein paints last in the tube? Is there any way to extend their shelf life?
I do not think you have to worry about commercial casein paints in tubes unless you are leaving them alone for months. Doubt they would spoil, one site said you can refrigerate them if you wish, but do not freeze.
Maybe you are thinking about egg casein which is mixed fresh from egg whites? Or other methods that start from powders? Those would go bad but the tubes seem to be stable.
FAQ 1 - good overview of casein tubes. source of quote.
FAQ 2 - more focused on freshly mixed caseins
FAQ 3 - about underpainting with casein, talks about true casein spoiling though
I have about 15 tubes of Shiva casein paint tubes that were left in a box in my garage for 8 years, I am mainly an oil painter, never try to paint with acrylics or casein. So I tried them, they were a bit thick but I added few drops of water and I was flabbergasted, they all were good, they dry way too fast for my painting style, but I am going to learn how to use them, I think they will work great for an under painting.