The idea is that for long term space flight such as the trip to Mars cosmonauts will be able to transform milk getting to expiration date to kefir so as to extend food shelf life. They will use freeze dried milk in the experiment, which seems to be a standard product in the food supplies of the ISS.
Well, properly stored means in the dark and away from heat and radiation. The first two parts are easy, but not the third part. We really do not know what is the effect of cosmic radiation to food in general but if it is like radiation to food on earth, it is not good. In any case people get bored of drinking milk every day, they may just want to try something else