Is it safe? maybe. Is it recommended? No.
If you're going to save bacon grease (or other fats), I'd recommend:
- straining it
- putting it into a different container to minimize the surface area exposed to air
- keeping the container in the fridge.
Although, the first one, I admit I don't do -- as I'm not cooking bacon every day, I've generally used up my supply by the time I've got more ... so I fill a glass jar, and let solids settle to the bottom ... then just use the stuff off the top as I need it, until I get so far down the jar that I hit the darker strata, when I then dispose of the whole thing. (I recycle jars for this, then pitch the whole thing ... you don't want to go washing bacon grease down the drain, it does nasty things to your pipes)
...
If you're going to do what you are, you'll want to make sure that you hold the pan at 250°F (121°C) for at least three minutes, preferably longer, to kill off botulism spores, unless you're fond of that whole paralysis thing.
Fat doesn't spoil like other foods. No bacteria can live in fat. Going rancid is a chemical reaction in which the fat molecules break down. So "leaving out" is not a problem.
The major factors in going rancid are light and air. The more light hits your fat, the sooner it goes rancid. Also, rancidity occurs when the fat is oxidized, meaning that if you prevent contact with air, your fat will last longer.
Both factors are minimized by transferring the fat to a tightly closed opaque container. With liquid oils, you use a dark colored bottle. As bottles are impractical for solid fat, just strain your grease into a jar, close it, and put it in a cupboard. The shelf life should be many months, probably more than a year.
Also, don't worry that you can get something nasty when the grease goes bad unnoticed. Rancid fat isn't a big safety risk, and ingesting it in small amounts is not problematic. If it stinks, throw it out. Before that, there is no problem eating it.
The above assumes pure, well-strained fat. Pieces of fried meat left over in the fat are a safety risk, even tiny ones.
Best Answer
Fat can be filtered through a coffee filter. It's slow, but it works. Make sure the fat stays well above the temperature at which it congeals.
Alternatively, you can "wash" the fat by mixing it well with some boiling water and then letting it cool and separate. The impurities (at least, the particulate matter) should go into the water and be more easily removable.