I have already stored used peanut oil in the refrigerator for one week. I had strained it through coffee filters, and it had looked very clean and clear. Now it looks cloudy and milky. Is it OK to use? I had used it to cook fish and hush puppies.
May depend on the temp. of your fridge, but when I have stored a brand new (i.e. unused) bottle of peanut oil in my fridge, it has gotten cloudy and thick, especially when the level in the bottle goes a little below full. When I wanted to use it and took the bottle out of the fridge, it "thawed out" as soon as it got warm, and looked & acted just like when I bought it.
Canola oil does not behave this way for me. So this might just be a property of peanut oil when it gets cold. Does your left over oil thaw out & look clear when you let it get to room temp.?
On the other hand if you have used it to cook fish, I imagine it will still have at least a slight fishy taste even if it is strained out perfectly. Maybe best not to use it for fine delicately flavored dessert items anyway.
Recommendations on peanut oil vary radically, claiming anywhere from 1 month to 3 years shelf storage after opening, most all claiming refrigeration not needed. If pure peanut oil it is likely towards the low side though, so I would tend towards refrigeration as well. The cloudiness should just be fats starting to solidify which is not a ratification or chemical reaction. It should be fine to use, but I would bring it back to room temperature and allow to clear before heating, or initially heat it slowly. If you begin to heat partially solidified oils quickly they may have a tendency to pop and splatter more than usual.
aswine is correct. The cloudiness is a function of the amount of saturated fats in the oil. The more saturated fat an oil contains, the higher the temperature it solidifies. As Joe commented, olive oil will become cloudy/solidify in the fridge as well.
This is the reason manufacturers 'hydrogenate' oils to produce margarines. The more hydrogen atoms they can attach to an oil the more 'saturated' it becomes and the more solid it is at room temperature. Lard, bacon grease, highly saturated, solid at room temperature. Some oils are less saturated so they stay liquid at room temperature and the fridge. Your peanut oil is in the middle range, so it just got cloudy in the fridge.
As far as microbes growing in the oil, they definitely can. Exposure to air (oxygen) and oil breakdown (exacerbated by heat) are bigger culprits. This breakdown has a few indicators, the primary one for me being a rancid smell. Keeping oils sealed from air and stored in the fridge/freezer will help slow down the breakdown.
Keep the oils tightly sealed, away from light and cool and refrigeration is generally not required. Oils I don't use often I buy in smaller quantities so I wind up using them up before they go rancid sitting on the counter.
Some oils, like walnut go rancid quickly and should be stored in the fridge. Experience will guide you here.
I personally don't worry too much about microbes in my oils. They usually start to smell bad before they are a health concern. I'm also not a doctor and don't cook for anyone with impaired immune systems, so you need to make your own calls here.
In the South, bacon grease is often left out at room temperature. This can be done due to its high salt content. Just make sure not to mix in other greases or oils.
Finally, (and this will likely start a fight) butter can be left out at room temperature (for a while) safely. This is great if you like spreading soft button on your toast. The length of time it can be left out is a function of the temperature of the room and the amount of salt in your butter. Some people use butter bells for this function.
Butter will not last as long on the counter as in the fridge. Also, your nose will tell you when you've left it out too long and it needs to be discarded. (We eat it too quickly at my house for that to happen though. :-)
Yes, it's safe.
Oil, even used oil, has a very small water activity, much too small to support the growth of microbes. Not only that, but you'll presumably heat up the oil to at least 300 °F for several minutes when you use it next, which will kill anything that would have grown.
In fact, storing the used oil in the refrigerator isn't necessary for safety reasons. I'd be more concerned about rancidity if you're planning to storing it for a while. Keeping it in an air-tight container in the dark is most important; glass is preferable. The cold temperature of the refrigerator will also help.
The reason the oil is cloudy is that you brought it to a temperature at which some of the fats in the oil solidified. That's the same thing that happens with bacon grease, only at a higher temperature. It's clear when warm, but at room temperature, it's opaque. Different fats have different melting/freezing temperatures, depending on the length of their fatty acids and whether they're saturated or unsaturated. Cloudiness is nothing to be concerned about.