# "Refrigerate after opening"
**URL:** https://boards.straightdope.com/t/refrigerate-after-opening/187089
**Category:** Factual Questions
**Created:** [July 8, 2003, 9:15pm UTC](https://boards.straightdope.com/t/refrigerate-after-opening/187089 "2003-07-08T21:15:38Z")
**Posts on this page:** 5
**Page:** 2
### Author:  [@Chefguy](https://boards.straightdope.com/u/Chefguy)
#### Post date: [July 9, 2003, 6:52pm UTC](https://boards.straightdope.com/t/refrigerate-after-opening/187089/21 "2003-07-09T18:52:34Z")
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> \*Originally posted by Elemenopy \*
> \*\*May I squeeze in a semi-related question? How long can you keep a half-jar of, say, marinara sauce in the fridge without it going bad? I try to use it within a week, but I rarely get the chance to. I use it more as a condiment than as actual pasta sauce. \*\*
Tomatoes are pretty high in acid and the stuff will keep for quite awhile. Make sure you’re not licking the spoon and putting it back in the jar, keep fingers away from the rim, etc. Some items like this keep better if stored upside down, for some reason.
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### Author:  [@Cervaise](https://boards.straightdope.com/u/Cervaise)
#### Post date: [July 9, 2003, 8:11pm UTC](https://boards.straightdope.com/t/refrigerate-after-opening/187089/22 "2003-07-09T20:11:14Z")
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> \*Originally posted by Fear Itself \*
> **Neither my honey nor my pancake syrup has ever developed mold or bacteria colonies**
My honey had a yeast infection once.
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> \*Originally posted by Esprix \*
> **Specifically, a bottle of barbecue sauce - do you really have to keep it refigerated after opening? Maybe just keep it in a cool place?**
So let me get this straight: You want to keep something _in_ the closet? 😉
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### Author:  [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)
#### Post date: [July 10, 2003, 1:07am UTC](https://boards.straightdope.com/t/refrigerate-after-opening/187089/23 "2003-07-10T01:07:25Z")
> [@](#):
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> \*Originally posted by Cervaise \*
> **My honey had a yeast infection once.**
I bet that made your biscuit taste funny.
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### Author:  [@Smeghead](https://boards.straightdope.com/u/Smeghead)
#### Post date: [July 10, 2003, 9:39pm UTC](https://boards.straightdope.com/t/refrigerate-after-opening/187089/24 "2003-07-10T21:39:47Z")
It’s all about water activity, people, which is defined as the water vapor pressure over your condiment divided by the water vapor pressure over pure water. All of these substances have tons of nutrients that microbes would love to devor. Water is the limiting factor. And not just water, but free water, meaning water that’s available for them to use.
In mayo, for instance, there’s tons of water, but it’s all tied up in the emulsion, so bacteria are unable to grow. Even if the eggs that were used to make it were infected with _Salmonella_, it won’t be able to grow to dangerous levels. However, if you stick a knife in there with, say, a chunk of bread stuck on it, and the bread sticks in the mayo jar, there will be free water in the bread and lots of nutrients in the mayo, so the bacteria will go bonkers. Unless you refrigerate it, of course.
Honey is the same deal. The water is all tied up with the sugars. There are a few molds and yeasts that can grow in it if conditions are favorable, but it’s not common.
I don’t know the water activity of BBQ sauce off the top of my head (I know, I know. I’m sorry), but I’d hazard a guess that it’s along the same lines.
pH effects definitely help limit microbial growth, but water activity plays a vital role as well. One of these days, I’ll dig around and find my food microbiology textbook and provide some hard numbers.
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### Author:  [@Padeye](https://boards.straightdope.com/u/Padeye)
#### Post date: [July 11, 2003, 12:01am UTC](https://boards.straightdope.com/t/refrigerate-after-opening/187089/25 "2003-07-11T00:01:54Z")
Who the hell has a bottle of Hershey’s syrup long enough for it to go bad? What’s wrong with you people?
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