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I recently bought a wooden chopping board to replace my plastic one. In the course of my research I read that it's important to regularly rub mineral oil in (some people say once a week, some say once a month). It's said that it "protects" it and makes it last longer, but what is the oil actually doing?

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    I agree with @rumtscho on this one, oil is cosmetic. Plus, do you actually want to eat the stuff even if it's food grade? If you want your wooden cutting board to last the secret is to keep it dry. Don't soak it in water, basically.
    – GdD
    May 25 '21 at 9:56
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    I have several wooden chopping boards, some over 20 years old. They'e never been oiled
    – SiHa
    May 25 '21 at 17:58
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    @GdD I'm confused: what potential issue do you think consuming microscopic quantities of foods grade oil carries? May 25 '21 at 23:13
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    Mineral oils are petroleum distillates, and even though they are food safe that doesn't mean I'd be in a hurry to eat them @KonradRudolph. Plus, any oil will oxidize when exposed to oxygen and add off flavors, although with the small quantities involved it's unlikely to be noticed.
    – GdD
    May 26 '21 at 8:08
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    @GdD Sorry, you’ve essentially just restated your initial comment. Why would you not be “in a hurry to eat them”? They’re food safe. This means that, by definition, they’re safe to ingest. And, as you stated yourself, you wouldn’t even notice having done so. May 26 '21 at 8:34
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The oil is not actually doing anything to protect the wood - the idea that it does so is a very widespread myth. This is the conclusion of the guy who wrote the book on wood finishing, and I can confirm it from personal experience. You can continue using it for cosmetic reasons, I am personally partial to the look given by flax oil - although there are people who prefer the opposite, because cuts are much less noticeable on unoiled wood.

To get real protection on an wooden item, you have to apply a layer that actually seals the wood. The substances which do this well are shellac, nitro lacquers, acrylic finishes and alkydic finishes.

While you could use a safe-in-trace-amounts finish on a chopping board, the mechanical action of a knife will soon damage such a layer to a point where it becomes useless. Most people are just not interested in refinishing their chopping board a couple of times per week.

As for the source of the myth: Long before good finishes were discovered, people tried protecting wood. Since it was known that wood is most damaged by moisture, they used all kinds of water-repelling substances, such as different oils and waxes, sometimes also natural resins. The protection afforded by those is minimal, but it was all they had - and to the naked eye, it does look like a great protection, since oiled wood has an entirely different appearance, and water droplets pearl on it instead of visibly wetting it.

This is a concept that is known in science as "face validity" - you look at an approach and your intuition tells you that it must be working, so you are likely to conclude that it works. Scientists try to prove that methods work independently of their face validity, but cooks and woodworkers rarely do so :) And since there is a tradition, which convincingly does something, people continue applying it and promoting it.

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    Very interesting! So if water pearls on the surface does it eventually soak in or get through gaps? How come it appears to be working if it's not?
    – Tom
    May 25 '21 at 12:41
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    Here is a classic study that compared wood and plastic cutting boards to determine whether bacteria lived longer on wood boards than on plastic - the result was pretty clear. Bacteria die on a wood board even faster than they do on plastic. The only thing that seemed to help the bacteria hang on a bit longer was treating the wood with mineral oil, but not by enough to really be worth mentioning. Go figure.
    – J...
    May 25 '21 at 15:57
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    There are oil finishes for wood, and stuff sold as "oil" which is not oil at all. For example the widely available "lemon oil" sold for cleaning and protecting guitar fretboards is actually surgical spirit (rubbing alcohol) plus some lemon-scented perfume. Products based on genuine medical-grade non-mineral oils are something completely different, and have been used for at least 300 years, which is a long time for people to rely only on "intuition" that they work.
    – alephzero
    May 25 '21 at 16:48
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    @alephzero the age of a myth is not related to its veracity. People have been believing that rain dances bring rain, or that the sun rises because the Pharaoh calls it forth, for a lot longer than 300 years.
    – rumtscho
    May 25 '21 at 17:34
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    I'd always thought that "sealing" the wood (i.e., make it waterproof) was nice cosmetically but another purpose of oiling a board was to give the thirsty wood something safe to soak up. A board saturated with mineral oil wouldn't act like a sponge and soak up the juice from the raw chicken being cut.
    – spuck
    May 27 '21 at 20:43
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The oil protects the wood from drying out not from physical harm. Drying out can lead to the board warping, cracking, or even falling apart if you have a cutting board that is made of multiple pieces of wood.

I have solid single piece wood cutting boards that are decades old and still going strong with no oil but I've also had cutting boards split in half after only a few uses when someone ran it through the dishwasher, which removes the oil (detergent) and dries it out (heat drying cycle).

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    ran it through the dishwasher - there is an additional BIG problem - it isn't so much that the wood gets dried out, but in the wash cycle the wood absorbs a lot of water, causing it to swell, and then that same water gets dried out. Almost like (though not quite the same) a freeze/thaw cycle on concrete. If it didn't get so wet then I think the heat wouldn't really be such a problem. May 26 '21 at 2:30
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    Upshot - dishwashers are bad for wooden things generally.
    – Criggie
    May 26 '21 at 2:42
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    Sorry, this is incorrect. Wood drying out is a good thing, you want it to happen as much as possible, and wooden items made from fresh, undried weiß, are inferior. The warping from the dishwasher is an entirely different process.
    – rumtscho
    May 26 '21 at 6:30
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    Maybe a more accurate way to put it would have been to say quick changes in water content cause issues because it causes the wood to change shape/size. Oil can help prevent those quick changes. Also, green woodworking is totally a thing and there are advantages to using fresh wet wood in some circumstances like certain chair joints. Usually dry wood is what you want but not always. If you live in a very humid climate and you kiln dry wood to way below ambient that's going to cause problem too. And yes the dishwasher is bad for wood on many levels. May 26 '21 at 16:08
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    TL;DR = stop putting wooden things in the dishwasher!
    – Charleh
    May 27 '21 at 14:53

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