Sterility of Nectar (1 Viewer)

How sterile is the nectar hummers get from various flowers versus what they get from our feeders? Has any university ever studied this?

Neither is sterile. I would guess that most flowers are a bit "cleaner" than an average feeder, since most plants that hummingbirds visit only produce nectar for a few days in each flower, whereas a feeder might get a haphazard cleaning every couple of weeks.

When nectar goes bad, hummingbirds won't drink it. That can take a little while: the sheer concentration of sugar makes it a difficult environment for many bacteria/fungi/etc. But sooner or later you'll see and/or smell some changes.

I'd guess flower nectar is a bit cleaner, simply because hummers will drain a flower of its juice and then it will produce new nectar. And, as Nartreb pointed out, flowers only exist for a short time.
With feeders, every bird that visits is injecting a little of its own mouth bacteria into the nectar: which is why it's so important to keep the feeders clean and refill frequently.
To avoid wasting nectar in my feeders, I monitor how much they are going through in a maximum 3-day period and only put in that amount of nectar. In summer and fall, I put in a lot more because there are so many more birds and they go through it much faster. Sometimes they will empty a full feeder in a single day in summer/fall.