How long does it take for protein to go bad?

I can ONLY take Atkins- protein shakes pre-made in a can. (It is the one that I don't gag on & throw up). It only comes in 11oz. cans, problem is my pouch is 2 oz. (I am 2 week post-op) when I try to consume it quickly, (it makes me very ill and I feel like I am going to throw it up for at least an hour after drinking)... because I heard protein goes bad after it has been opened (or made) if it has been more than half an hour? even in the refrigerator & covered? I heard you loose nutritional value even if you freeze it? Has anyone heard this and is this true? I was advised by my Dr's office to break the shake up into halfs- drink the 5 1/2 oz. for breakfast... drink the rest at lunch, and do the same throughout the day. But how can I if I am gonna loose the protein? Any suggestions or what do you do?

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