# Why doesn't butter rot? **URL:** \ **Category:** Factual Questions\ **Created:** [August 24, 2010, 8:53am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268 "2010-08-24T08:53:57Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [August 24, 2010, 8:53am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/1 "2010-08-24T08:53:57Z")
Given that fat is a substance high in energy like carbohydrates, why don’t bacteria attack it the way they do starches and proteins, etc? Why doesn’t oil or fat go moldy before it goes rancid? In particular, why doesn’t butter rot? It has both water and fat mixed together! ---
**Author:** ![GiantRat](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@GiantRat](https://boards.straightdope.com/u/GiantRat)\ **Post date:** [August 24, 2010, 9:16am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/2 "2010-08-24T09:16:52Z")
Because it does? Butter goes bad, but it takes a while. I’ve witnessed this, and it was nasty. I’ll wait for a chemist to provide more info. As an anecdote to refute myself, though, my grandmother has always kept a stick of butter on the butter tray in the cupboard so there’s always one that’s easy to spread on her delicious bread/muffins/etc… I never feared it. Anything from grammie is safe and delicious. 🙂 ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [August 24, 2010, 9:33am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/3 "2010-08-24T09:33:16Z")
Most butter is salted, specifically to prevent spoilage. ---
**Author:** ![qpw3141](https://avatars.discourse-cdn.com/v4/letter/q/6bbea6/32.png) [@qpw3141](https://boards.straightdope.com/u/qpw3141)\ **Post date:** [August 24, 2010, 9:55am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/4 "2010-08-24T09:55:55Z")
Most bacteria cannot use the butter for food (because they are not evolved to ‘feed’ on fat) so nothing grows/feeds on it to turn it bad. If any bacteria that can use butter as a food takes hold the results are **nasty**. It’s much less common now because the bacteria that affect butter need higher temperatures to operate but in the days before refrigeration rancid butter was fairly commonplace. ---
**Author:** ![sandra\_nz](https://avatars.discourse-cdn.com/v4/letter/s/eada6e/32.png) [@sandra\_nz](https://boards.straightdope.com/u/sandra_nz)\ **Post date:** [August 24, 2010, 9:55am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/5 "2010-08-24T09:55:59Z")
Yeah, if you ever want to enjoy the magnificence of rancid butter, do like I do - be a household that rarely uses butter, and accidentally buy an unsalted block. The smell…oh god, the smell… ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [August 24, 2010, 10:10am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/6 "2010-08-24T10:10:16Z")
Hi All, Just to be clear (most are aware of this but from a couple of your replies it is not so clear to me): RANCID != ROTTEN Rancidification just means oxidation. I have seen plenty of rancid butter/fat/oil, but I have never seen or heard of mold growing on butter/pure fat/oil, and that includes unsalted/untreated butter/pure fat/oil. On the other hand I am familiar with the existence of oil-eating bacteria (has been discussed a lot recently relating to the BP oil spill), but its rarity, especially in the kitchen, is surprising to me. As I indicated in the OP, fat is an excellent source of energy, hell the body stores it for that very reason. There must be a good explanation as to why moldy butter etc isn’t more prevalent. ---
**Author:** ![qpw3141](https://avatars.discourse-cdn.com/v4/letter/q/6bbea6/32.png) [@qpw3141](https://boards.straightdope.com/u/qpw3141)\ **Post date:** [August 24, 2010, 10:28am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/7 "2010-08-24T10:28:57Z")
> [@iamnotbatman](#): > > Just to be clear (most are aware of this but from a couple of your replies it is not so clear to me): > RANCID != ROTTEN > > Rancidification just means oxidation. I’ve never heard that definition and neither Wictionary or OEDII agrees with it. > [@](#): > > I have seen plenty of rancid butter/fat/oil, but I have never seen or heard of mold growing on butter/pure fat/oil, and that includes unsalted/untreated butter/pure fat/oil. I’ve seen it but very rarely and it’s always been a very fine coating. It might have been growing on the milk solids in the butter. > [@](#): > > There must be a good explanation as to why moldy butter etc isn’t more prevalent. Possibly because any ‘free’ fat is consumed by larger organisms that act more quickly so no slow acting organism has ever evolved. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [August 24, 2010, 11:12am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/8 "2010-08-24T11:12:53Z")
I’ve occasionally seen fuzzy mold on butter. Give it time. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [August 24, 2010, 11:56am UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/9 "2010-08-24T11:56:59Z")
Microbes need a fair amount of free water to grow. There just isn’t enough in butter - too much fat, not enough water. ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [August 24, 2010, 12:42pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/10 "2010-08-24T12:42:20Z")
> [@qpw3141](#): > > > [@iamnotbatman](#): > > > > Just to be clear (most are aware of this but from a couple of your replies it is not so clear to me): > > RANCID != ROTTEN > > > > Rancidification just means oxidation. > > I’ve never heard that definition and neither Wictionary or OEDII agrees with it. Why don’t you actually look it up before wasting my time having to provide the links for you: > **[rancidification - Wiktionary, the free dictionary](https://en.wiktionary.org/wiki/rancidification)** > **[Rancidification](https://en.wikipedia.org/wiki/Rancidification)** > > Rancidification is the process of complete or incomplete autoxidation or hydrolysis of fats and oils when exposed to air, light, moisture, or bacterial action, producing short-chain aldehydes, ketones and free fatty acids. > When these processes occur in food, undesirable odors and flavors can result. In processed meats these flavors are collectively known as warmed-over flavor. In certain cases the flavors can be desirable, as in aged cheeses. > Rancidification can detract from the nutritional val... What I said was basically correct. My point is that rancidification has nothing to do with bacteria. ---
**Author:** ![Hypno-Toad](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)\ **Post date:** [August 24, 2010, 12:46pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/11 "2010-08-24T12:46:53Z")
> [@iamnotbatman](#): > > What I said was basically correct. My point is that rancidification has nothing to do with bacteria. From your second link: “Microbial rancidity refers to a process in which micro-organisms such as \*\*bacteria \*\*use their enzymes, including lipases, to break down chemical structures in the fat.” ---
**Author:** ![qpw3141](https://avatars.discourse-cdn.com/v4/letter/q/6bbea6/32.png) [@qpw3141](https://boards.straightdope.com/u/qpw3141)\ **Post date:** [August 24, 2010, 12:54pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/12 "2010-08-24T12:54:39Z")
> [@iamnotbatman](#): > > > [@qpw3141](#): > > > > I’ve never heard that definition and neither Wictionary or OEDII agrees with it. > > Why don’t you actually look it up before wasting my time having to provide the links for you: I did. :rolleyes: At least, I looked up ‘rancid’: Wictionary: Being rank in taste or smell. OEDII: Of fat or oil, or foods containing these: having an unpleasant taste or smell as a result of decomposition; rank, stale, sour; (also) designating such a taste or smell. Also in extended use. Also: Merriam Webster: having a rank taste or smell. So what I posted was factually correct. And definitions are additive. Just because you find one that says something different does not invalidate all others. Maybe you should do a little more research before making dogmatic statements. 😉 ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [August 24, 2010, 12:58pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/13 "2010-08-24T12:58:54Z")
> [@Smeghead](#): > > Microbes need a fair amount of free water to grow. There just isn’t enough in butter - too much fat, not enough water. Good point, although butter does have a surprising amount of water in it (hence my use of it as the main example in my question). Butter is about 15% water. ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [August 24, 2010, 1:10pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/14 "2010-08-24T13:10:36Z")
> [@qpw3141](#): > > > [@iamnotbatman](#): > > > > Why don’t you actually look it up before wasting my time having to provide the links for you: > > I did. :rolleyes: > > At least, I looked up ‘rancid’: > > Wictionary: Being rank in taste or smell. > OEDII: Of fat or oil, or foods containing these: having an unpleasant taste or smell as a result of decomposition; rank, stale, sour; (also) designating such a taste or smell. Also in extended use. > > Also: > > Merriam Webster: having a rank taste or smell. > > So what I posted was factually correct. > > And definitions are additive. Just because you find one that says something different does not invalidate all others. > > Maybe you should do a little more research before making dogmatic statements. 😉 Wow. First of all, you looked up ‘rancid’, not ‘rancidification’ (the word I used), which would have quickly gotten you to the correct scientific definition, as opposed to the colloquial English usage definition that you found. Second of all, the onus is on you to find the correct definition before calling someone out for being wrong. When you say “Just because you find one that says something different does not invalidate all others” – that applies to YOUR post, not mine. YOU are the one who said I was wrong that rancidification means oxidation, and YOU are wrong – rancidification means oxidation. You obviously have to look up the scientific definition, and go out of your way to find it if you have to (although you don’t have to – it’s an extremely common definition), before telling someone they are wrong. Furthermore, you have a lot of balls to then not accept the fact that you were wrong and say that I should have done more research. I know the colloquial English usage of the term Rancid. I should hope to God most people do. I was obviously referring to the more rigorous definition. omg. wipe sweat from brow. ---
**Author:** ![qpw3141](https://avatars.discourse-cdn.com/v4/letter/q/6bbea6/32.png) [@qpw3141](https://boards.straightdope.com/u/qpw3141)\ **Post date:** [August 24, 2010, 1:22pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/15 "2010-08-24T13:22:18Z")
> [@iamnotbatman](#): > > Wow. First of all, you looked up ‘rancid’, not ‘rancidification’ (the word I used), which would have quickly gotten you to the correct scientific definition, as opposed to the colloquial English usage definition that you found. But sandra-nz used ‘rancid’. You then took another definition in order to make some spurious, pedantic, point that was irrelevant to the original question. > [@](#): > > Second of all, the onus is on you to find the correct definition before calling someone out for being wrong. If you actuall **read** what I said, I didn’t say you were wrong. I just said that two dictionaries did not agree with you. > [@](#): > > When you say “Just because you find one that says something different does not invalidate all others” – that applies to YOUR post, not mine. Nope. Someone used the term ‘rancid’ in a manner completely approropriate to its dictionary definition. You then started nit picking on the basis of a completely different word. All I was doing was defending someone’s use of a word. > [@](#): > > YOU are the one who said I was wrong that rancidification means oxidation, and YOU are wrong – rancidification means oxidation. You obviously have to look up the scientific definition, and go out of your way to find it if you have to (although you don’t have to – it’s an extremely common definition), before telling someone they are wrong. If you’d like to just calm down a bit and stop shouting, you might like to go back and look at your first post in this thread. You know, the one where you say: “RANCID != ROTTEN”. > [@](#): > > Furthermore, you have a lot of balls to then not accept the fact that you were wrong and say that I should have done more research. I know the colloquial English usage of the term Rancid. I should hope to God most people do. And yet, even knowing that, you make a post that effectively contradicts the accepted definition. > [@](#): > > I was obviously referring to the more rigorous definition. omg. wipe sweat from brow. Yes, I can actually imagine that you have worked up a sweat. You seem to have become **extremely** excited over the normal usage of a commonly accepted definition. 😃 ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [August 24, 2010, 1:28pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/16 "2010-08-24T13:28:37Z")
> [@iamnotbatman](#): > > Good point, although butter does have a surprising amount of water in it (hence my use of it as the main example in my question). Butter is about 15% water. Microbial growth isn’t just related to the absolute amount of water in some food. Some of that water can be bound up tightly in starches or proteins and is effectively unavailable for any microbes trying to grow nearby. The ratio of “free” water is defined as [water activity](http://en.wikipedia.org/wiki/Water_activity). Most common food spoilage bacteria cannot grow on foods with a water activity below 0.9, and most molds can’t grow with a water activity below 0.8. There are some very hardy bacteria that can tolerate down to 0.5, but these are very slow growing and harmless to humans. In comparison, butter typically has a [water activity around 0.9](http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T6R-49NPW08-5K&_user=489286&_coverDate=12%2F31%2F1992&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1440223285&_rerunOrigin=google&_acct=C000022678&_version=1&_urlVersion=0&_userid=489286&md5=8efa76e1325079a6eb84515047175fe9), right on the threshold of allowing bacterial growth. Water activity is also reduced by lowering the temperature or adding salt, so chilling or salting the butter can prevent spoiling by bacterial growth. This is, of course, totally independent of oxidation with the air. ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [August 24, 2010, 1:43pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/17 "2010-08-24T13:43:46Z")
Look, I said: > [@](#): > > Rancidification just means oxidation. Your direct reply to that statement was: > [@](#): > > I’ve never heard that definition and neither Wictionary or OEDII agrees with it You can be pedantic if you want, but that sounds to me like you are saying I am wrong. When you say: > [@](#): > > Someone used the term ‘rancid’ in a manner completely approropriate to its dictionary definition. You then started nit picking on the basis of a completely different word. First of all, ‘rancidification’ is not a completely different word. And no I’m not nit-picking, the entire point of my OP is ‘why does butter not rot’? Obviously it is important that if people use the word ‘rancid’ we all understand that it typically does not equal rotten. It means smelly, bad, oxidized, but it typically never means rotten. You say: > [@](#): > > And yet, even knowing that, you make a post that effectively contradicts the accepted definition. Nope. Again, this is either a lie or you are really just trying to be incredibly smug for the sake of pissing me off. Nothing I said contradicts the accepted definition, whether that me yours or the more scientific one. Either way, the definition for ‘rancid’ includes, among more colloquial uses, oxidation. And again, my whole point, which is the only important issue to me here, was to point out that if you use the term ‘rancid’, don’t use it to mean ‘rotten’. ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [August 24, 2010, 1:46pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/18 "2010-08-24T13:46:07Z")
> [@lazybratsche](#): > > > [@iamnotbatman](#): > > > > Good point, although butter does have a surprising amount of water in it (hence my use of it as the main example in my question). Butter is about 15% water. > > Microbial growth isn’t just related to the absolute amount of water in some food. Some of that water can be bound up tightly in starches or proteins and is effectively unavailable for any microbes trying to grow nearby. The ratio of “free” water is defined as [water activity](http://en.wikipedia.org/wiki/Water_activity). Most common food spoilage bacteria cannot grow on foods with a water activity below 0.9, and most molds can’t grow with a water activity below 0.8. There are some very hardy bacteria that can tolerate down to 0.5, but these are very slow growing and harmless to humans. > > In comparison, butter typically has a [water activity around 0.9](http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T6R-49NPW08-5K&_user=489286&_coverDate=12%2F31%2F1992&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1440223285&_rerunOrigin=google&_acct=C000022678&_version=1&_urlVersion=0&_userid=489286&md5=8efa76e1325079a6eb84515047175fe9), right on the threshold of allowing bacterial growth. Water activity is also reduced by lowering the temperature or adding salt, so chilling or salting the butter can prevent spoiling by bacterial growth. > > This is, of course, totally independent of oxidation with the air. Awesome! Thanks for the intelligent reply! ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [August 24, 2010, 2:31pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/19 "2010-08-24T14:31:42Z")
> [@](#): > > Yes, I can actually imagine that you have worked up a sweat. You seem to have become **extremely** excited over the normal usage of a commonly accepted definition. 😃 OK, but you have to understand, the term ‘rancid’ shouldn’t be used recklessly to describe any and everything that has “a rank smell or taste” [definition via google]. The simplest dictionary definitions are often misleading, as they leave out important details that can be found by looking just a little deeper, or going to a less terse dictionary source. In the case of ‘rancid’, it should be understood that the _origin_ of the “rank smell or taste” is generally understood to be an oil or fat that has undergone a form of chemical decomposition, typically oxidation. The definition via google above, in its complete form, is: > [@](#): > > (used of decomposing oils or fats) having a rank smell or taste usually due to a chemical change or decomposition Any definition much simpler than the above is simply leaving out crucial information. So when you say I’m getting excited about a “normal usage of a commonly accepted definition” you come off to me as smug and ignorant, and I hope you can forgive me for getting agitated. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 24, 2010, 3:35pm UTC](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268/20 "2010-08-24T15:35:05Z")
Rancid butter forms butyric acid. Nasty smelling stuff (what the guys on Whale Wars throw at the whalers…in one episode they were a mile behind the ship they hit with the butyric acid and remarking at the awful smell from even that far behind). Butyric acid is also in vomit to give you an idea. Seriously gross stuff (albeit otherwise harmless). [Next page](https://boards.straightdope.com/t/why-doesnt-butter-rot/551268.md?page=2)