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I have a sprayer that has a small glass bottle that holds oil for spraying pans etc. It has a very narrow neck.

I poured out most of the vegetable (100% canola) oil and put the glass part in the dishwasher, and after washing, a white-ish material appeared inside the bottle that was very, very close to glue- very sticky and very viscous, and a real hassle to clean out of the bottle.

  1. What went on here, chemically, if anyone knows?

  2. More relevantly to this SE, I've switched to olive oil now, could the same thing happen?

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2 Answers 2

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If it's sticky, the oil has polymerized. This can be speeded up with exposure to oxygen, sunlight, heat, and contamination with certain metals. The process may have started before you dumped the oil, and may have been accelerated in the dishwasher.

Canola oil is a semi-drying oil--as opposed to linseed oil or varnish, which will polymerize into a hard surface, or mineral oil which will never thicken due to oxidation. This is the process that creates sticky gunk on your range hood or other places near the stove that may not get cleaned promptly. It's also how cast iron is seasoned to develop a nonstick surface--in that case we do things to make it firmer and less sticky, such as polymerizing it in thin layers (and obviously exposing it to iron, which is a catalyst when it gets hot enough).

Also, it's not saponification as another answer claims. If you've made soap, you'll know the process is difficult. The lye (a much stronger base than what you'll find in detergents) needs intimate contact with the oil and lots of time. Little to no saponification would occur in a dishwasher.

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  • Thanks- are all cooking oils pretty much the same in this regard?
    – Spehro 'speff' Pefhany
    Commented Jul 27 at 14:12
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    @SpehroPefhany Technically they are not the same (in particular non-drying oils), but I think once you have enough heat and oxygen, they will all start to turn sticky and polymerize a little bit. With the possible exception of those that have antioxidant components, such as olive oil.
    – piojo
    Commented Jul 27 at 15:07
  • Sorry, this can't be right. Oil polymers aren't white. They also don't happen in a dishwasher - the conditions simply aren't right. I don't know for sure what that stuff is, but it's not oil suddenly polymerizing with itself during washing.
    – rumtscho ♦
    Commented Jul 29 at 11:57
  • @rumtscho They turn milky/chalky when they absorb water. And I don't think the OP has given enough details for you to determine that that bottle wouldn't have contained gunky oil.
    – piojo
    Commented Jul 29 at 12:25
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Something with a narrow neck can’t be effectively cleaned by a dishwasher. The jets of water simply can’t get to where they’re needed. Clean it by hand, with a bottle brush if needed.

Some of the detergent got in and partially saponified the oil, but the dishwasher was not able to rinse it out. This would have happened with any type of cooking oil.

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  • Or use a bottle washer, which might be in your closet if you've ever made beer or wine; or might not. They are a very handy specialized tool if you have one.
    – Ecnerwal
    Commented Jul 27 at 0:19
  • Thanks! "Saponify" is the term. Nothing like soap though. more like if you scraped all the adhesive off a stack of post-it notes.
    – Spehro 'speff' Pefhany
    Commented Jul 27 at 5:45

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