As the questions says, I was wondering as I want to prepare a whey shake (with milk) and take it with me to drink later. Does it lose effectiveness and it is important to drink it right after mixing it or does it not matter?
For the most part this is speculation based on what I learned as from my college bio classes (which I took recently as a I just graduate as a bioengineer).
Obviously, there are circumstances in which mixing your protein into a shake may cause it to lose effectiveness. For instance, a shake with high bacterial content, such as if you put yogurt or something in it, might cause more rapid protein breakdown. Also, the presence of protein digesting enzymes in your shake would cause it to lose some potencty. How these enzymes would get into your shake, I'm not sure, but they could be present in something you else you mix into your shake. Also, keep in mind that protein powder is already in state where it is quick to degrade. So the degradation time of the protein in the shake is much lower than that of other protein sources.
Accourding to this, the half-life of whey is 3-4 hours after intake and casien is 6-8 hours. So depending on what type of protein you are using, half of it has degraded in 3-4 hours or 6-8 hours WHEN IT IS IN A DIGESTIVE ENVIRONMENT. I don't know what the half life of your shake would be, but I imagine it would be a lot more than this.
So the protein will definitely degrade, but to what extent, depends on what is in your shake. So avoid the preceding things that could cause loss of potency.
That being said, I think that you won't lose a lot of potency if you mix in the powder and keep it in the fridge for a few hours. Think of premade shakes... how long do those sit on grocery store shelves? And they are still "effective" are they not?
While there may be some slight degradation of the protein molecules I wouldn't expect enough to worry about.
The biggest risk is that the mix will go off. What's food for you is also food for bacteria and fungi. I would recommend keeping it in the fridge or somewhere cold before you drink.
Basically, a whey protein shake is just milk with extra bits of milk added to it so store it as you would milk.