I wouldn't put anything into your...
Stash, if it was just acquired. You've got to remember, that Detergent is probably at least 5-6 years old. And Powder Detergents have a relatively short shelf life.
I'd start using up that Detergent now. As... it's cleaning isn't going to get any better the longer you wait. You do have one good thing, going for you, as the detergent sounds like it hasn't caked inside the box. However with that said, overtime the STPP breaks down, and the Enzymes will kick in (inside the box) and die off.
If you are going to... store that Detergent, for use later. I'd recommend.. at the very least, wrap it up in 3/4 layers of Ziploc bags, to prevent any further moisture from breaking down the Detergent.
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That Cascade was probably manufactured in-between January 2008 and no later than May 2010.
With that said, It was the last version of powder Cascade to contain phosphates before the ban. It's a very strong.. formula containing Oxygen Bleach & Enzymes. Enough to handle most loads, and the most nastiest tea-stained mugs
I'd just start using it up though, if I were you. I'd hate to see it, go to waste :/