Are there any way to preserve the juice of lemon for, say, one week without a change in its taste?
Even deep-freezing it makes it little bit bitter!
I do not think that there is a better way they not to squeeze the lemon until you need them :D
Regardless, if you want to do testing and are concerned about the bitter notes that develop , I recommend doing the research with grapefruit instead as it turns bitter is a matter of minutes and has a much stronger bitter flavor then the lemons do. That way you at least have a shorter turn around time.
You need to freeze them and You can find all tips here http://drinklemonwater.com/ways-to-preserve-lemon-juice/