Does fat spoil?

Why doesn’t oil go bad?

It does. Oil becomes rancid when left exposed to air. IIRC, it’s due to toxins excreted by bacteria.

Well then why don’t people need to put in the the fridge?

I IIRC’ed incorrectly. It’s not bacteria, but the air itself as well as heat and light that causes oil to become rancid. So to preserve oil, keep tightly sealed in a cool, dark place. I WAG that refrigerating it would extend its shelf life if you plan to keep it for an extended period of time without using it. Many references available by googling “rancid oil”. Wiki link.

Why doesn’t bacteria eat oil?

For one thing it’s too dry. Bacteria need moisture to replicate. For another it’s pretty darn anaerobic, and most bacteria are aerobic. I used to store garlic cloves in olive oil until I learned that the environment was conducive to clostridium botulinum bacterium growth. There is enough moisture in fresh garlic to be problematic.

I assume by ‘dry’ you mean ‘lacking in water content’? Cuz oil isn’t dry in the ordinary sense of the term.

Yeah, that’s what I meant. Sorry. In the field we talk about “dry” or anhydrous liquids a lot, when what we mean is lacking in water. Hey, like a dry martini. That works too.

To echo this statement, you should NEVER try to make your own garlic oil, unless you do it by simmering/sauteing the cloves in the garlic and then using it right away. Garlic can have botulism spores, and on the cloves, thisd doesn’t pose a problem because they won’t replicate and make botulism toxin, which is what gives you botulism poisoning. When it’s in the oil, the anaeroibic conditions let the spores breed like made and mkae the toxin.

Oils don’t normally “go bad” the way meats and other foods do, because that refers to bacterial spoilage. Oils don’t have water, and water is absolutely necessary for bacterial growth. Garlic supplies the moisture necessary for the above-mentioned spoilage of garlic-infused olive oil.

Oils can go rancid; this is a simple oxidation process, it’s normally not dangerous (never in my experience, actually), and I find that a tiny bit of rancidity adds to the aroma. In industrial-sized amounts, oils can be preserved by storage under nitrogen. Refrigeration won’t work quite as well, and may cause other problems.

Well then why doesn’t water go bad?

Generally nothing for an organism to feed upon in the water. Put a sprinkle of sugar in a glass of water and you may be unimpressed with its drinkablility in a week or so.

Bacteria need water and nutrients. Water has the water (obviously) but not the nutrients. Oil has the nutrients but not the water. Nothing can survive on just water. That’s right, nothing! Plants need sun, too. And water doesn’t go rancid because it’s already oxidized. It’s stable.

To the OP:

Hell yes. Have you ever found a two-year-old steak or pork loin in the freezer? I have, and it’s damn near inedible.

So if I pour water into a jar of oil and leave it out, it will go bad?

Water won’t go bad, but it can harbor bad stuff.

Rancid oil is absolutely bad for your health. Think of trans-fats. Okay, not instant death, but not something you want regular exposure to.

Two year old meat in the freezer may be perfectly okay if it was stored well, or in a non-self-defosting freezer. The only thing that can really hurt it is freezer burn, and freezer burn only affects poorly packed meat, especially in self-defrosting freezers.

Also, different oils / fats go bad at different rates. Lipids (oils & fats) can go bad by sulfur kicking out some of the hydrogen and forming something nasty. This happens quicker with un-saturated fats. (Unsaturated fats have places where the Hydrogen is naturally missing, making it easier for foreign elements to insert themselves.) So, the healthier stuff tends to go bad quicker. Hydrogenated oils (which usually turn into fats), like Crisco have Hydrogen added to them to make them less susceptible to going rancid. (I.e. to give them a longer shelf-life. Think along the lines of Twinkies: I don’t think those things can ever go bad.)

Another way that lipids go bad is through reuse. Many times, cooks (at home and in restaurants) will strain oils from deep fryers and reuse the same oil a number of times. This is perfectly fine for the first few strainings, but, eventually, the oil starts to break down and turn dark brown. (Those folks you may have seen on TV collecting used oil and driving their car on it use the discarded stuff. I’ve been driving behind one; their exhaust really does smell like french fries!)

As for water going bad, well it can, sorta. If the container of water somehow gets bacteria in it, it can certainly make the water dangerous / nasty / bad. Granted, you could clean up the water in ways that simply wouldn’t work for the lipids, but it would still be bad by most people’s definitions. I remember from my Boy Scout days our Scoutmaster (who was in the Army) telling us how to check emergency water rations (which were canned) by slapping the cans and listening for the right kind of slosh.

No, because the water, being much denser than oil (and also immiscible in it) sinks straight to the bottom.

That is almost a prefect combination for spoilage. The air can’t really reach the interface between the air and water, but over time, it will get a little funky. This is a problem in many situations – for example, petroleum, in the ground is safe from bacterial attack, but in a storage tank, water, bacterial spores and air do get in, and can make a mess. They don’t eat all the oil rapidly (else ocean oil spills would be less of a problem,) but they live somewhat at the interface, consuming some, and excreting waste products that contaminate the petroleum. Point is, at an oil/water interface bacteria can thrive.

Having eaten perfectly good pemmican (ground jerky and dried berries mixed with copious amounts of saturated fat) that had stood in room temperature for 15 months or so, I can attest to the practical keeping qualities of rendered fat.