How do you know if an oil has gone bad/rancid?

by (20469) Updated July 13, 2012 at 4:09 AM Created October 05, 2010 at 3:17 AM

I see websites saying butter goes 'bad' after a few months in the fridge. How do I know if my butter has gone 'bad?' To me, it can sit in there for a year and still taste, smell, and look exactly the same as when I bought it. They say it goes rancid in a few days out of the fridge, but I take butter camping (in the winter so it's usually cool out) and it seems the same after 2 weeks of no fridge. What about coconut oil? Seems to me, these saturated fat oils are very stable.

Should I really worry about them going bad or is that just the industry being overly cautious and also hoping I will keep buying more if my stock gets old? Now I could see that for grain oils, that stuff is already compromised on arrival and is already bleached and perfumed, so any additional rancidity would probably be more hidden. But again, I just don't recall ever having seen any detectably rancidity even if it's been stored a long time. THe only time a grain oil has ever smelled rancid to me is after it has been heated and cooled many times, like when used as a fry oil at a restaurant. (Can you imagine the level of degradation and rancidity some of those restaurant fry oils must reach after hundreds of high temp heatings!!!)

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1658 · October 05, 2010 at 5:19 AM

Eva, If you are buying salted butter, you are not going to know it is rancid until you cook with it and your food tastes "off".

Unsalted, pastured butter will be really golden yellow (lots of VitD in it is the payoff), and will taste terrible once it has gone off. That said, properly refrigerated, it will last a while (less if stored in door, more if stored in back of fridge, even more if stored in deli drawer in the back). It is starting to go when the color pales, and when you can taste items from your refrigerator in it.

Coconut oil is very stable and should never be refrigerated. Unless you have HUGE swings in temperature in your house, it should last up to 2 years. But really, you are eating it daily so it will never last that long, unless you're buying it by the ton.

Olive oil will start to go rancid if not kept in a dark bottle in a dark place. If you're buying industrial sizes, refrigerator is best place.

Meat oils (lard, bacon fat, chicken fat, beef tallow) should be refrigerated except for the small quantities you'll use daily.

So aside from some other nut oils (walnut) or avocado oils, what other oils are you talking about?

7249 · July 13, 2012 at 4:09 AM

I go through butter and coconut oil to never have to worry about them going bad. I watch my olive oil, and keep no other oils in the condo. No need for them.

1636 · October 08, 2010 at 4:00 PM

Growing up we always left butter on the counter unless it was summer (since this was before air conditioning). I don't remember any time that it was ever "off" but, of course, we used it up pretty fast back then because no one knew about the evil saturated fat thing (note tongue planted firmly in cheek here)

Mom also always had a container of bacon fat on the counter next to the stove that was never refrigerated - just like her mother and grandmother before her - that was used for cooking almost every day and we all seemed to grow up OK. I never had weight or food issues until I left home and started eating college dorm food and fast food almost exclusively. So I guess maybe I just have a more relaxed attitude about this kind of stuff than a lot of people nowadays.

2143 · October 06, 2010 at 9:35 PM

I store butter in the freezer when I get it. I bought some expensive grass-fed butter from US Wellness and it's stored in the freezer but has a sort of off taste and I'm thinking maybe I should just throw it away. I've tasted REALLY rancid butter by mistake so I know the taste; this isn't like that but it doesn't taste like sweet creamy butter.

1410 · October 05, 2010 at 4:33 AM

You're right that saturated fats are far more stable. I think they should be called "stable" and "unstable" rather than saturated and unsaturated since it's more indicative of the way that they work when exposed to air.

I have a post on fats and oils- check it out- http://www.balancedbites.com/2010/07/fats-which-to-eat-and-which-to-ditch.html

There's a downloadable PDF guide to healthy ones and which to avoid.

239 · October 05, 2010 at 4:29 AM

I generally leave the current stick of butter out on the kitchen counter in a covered butter dish, except when the weather is hot enough to melt it. We use it up pretty quick so it hasn't ever gone bad for me. I have smelled and tasted butter that had gone off a couple of times in the past, but those times were due to a lunch bag being forgotten in a locker.

I haven't used vegetable oils (except for olive, peanut & sesame) in a couple of years, but I have at times in the past tasted oil from a brand new just opened bottle and detected rancidity. It's kind of metallic/musty/peppery sharpness, quite unpleasant. I've also tasted rancidity in cooking oils within days of the bottle being opened. Sometimes you can just smell it, a kind of puckery-metallic smell.

I read somewhere that vegetable oils are chemically deodorized so you can't detect rancidity in them even though they've already turned while still at the factory. And I was watching a TV show about food processing one day about the wonders of soybeans, and the host (can't remember his name, but he was fat, had curly dark hair, and kinda looked like Stephen Furst from Babylon 5) made some comment about how the oil would get rancid but that rancidity wasn't harmful. My ears sure bugged out at hearing that!

-2 · July 13, 2012 at 3:54 AM

Fresh oils usually have peroxide values well below 10 meq/kg.

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