People rub it on babies' butts, it has been used as a laxative, new-age healers turn it into essential oils, and mechanics grease gears with it and pour it into engines to keep things running smoothly. Mineral oil—a broad class of odorless, colorless oils—is used in the cosmetics industry, science labs, factories, industrial settings, and...pretty much everywhere else. But you'll rarely find it used in the one place where it's needed most: the kitchen.
Rifle through the cabinets of most home kitchens and you'll likely find bottles of corn oil, canola oil, safflower oil, peanut oil, olive oil, and sesame oil. You may even find the odd bottle of hazelnut or walnut oil, if the home cook is fancy like that. It'd be a rare day, though, that'd you'd stumble across a jug of food-grade mineral oil.
That's a mistake, because mineral oil belongs in every kitchen. It's not a cooking oil like the others, but it has a limited yet important set of applications.
What is Mineral Oil Good For?
There are two important things mineral oil is good for in the kitchen. The first is wood, and the second is metal. In both applications, be sure to use mineral oil that's marked as "food-grade"—you don't want to eat nasty additives that can be found in other types of mineral oil.
Conditioning Wood With Mineral Oil
First, and probably most importantly, wood: Any unvarnished wood in the kitchen, including wood cutting boards, butcher block countertops, and kitchen islands, needs to be conditioned with oil to keep it in prime condition. Cutting boards and butcher blocks that are allowed to dry out will eventually warp and crack, especially at the seams where the pieces of wood join together.
Mineral oil soaks into the wood, helping to keep it plump and protected. It doesn't just keep wood in good condition, though—it also acts as a water-repellant barrier. It will reduce the chances that the wood will stain when wine is splashed on it; it will also reduce the chances the wood will rot from prolonged contact with water.
How often should you oil a cutting board or butcher block? Pretty much whenever it looks like it's dry, which you can tell by sight or by sprinkling a little water on top; if the water absorbs into the wood, it's time for an oiling (read more about how to do it here).
If you have butcher block countertops, it's often a good idea to do several frequent applications of mineral oil early on, to be sure that the wood is fully saturated. As time goes on, you can decrease the frequency to once a month or once every other month, or whatever your schedule allows.
Most wood cutting boards come well oiled, but over time, and especially with frequent washing, they'll need to be reconditioned with oil. First, make sure the boards are fully dry, in particular if you've washed them recently.
Whether you're treating a butcher block or cutting boards, oiling them is as simple as liberally pouring the oil all over them, spreading it around to cover all surfaces, and then giving them time to drink it up. I often leave them overnight, returning in the morning to wipe up any excess oil that remains unabsorbed.
If you have butcher block countertops and wood cutting boards, you can do them all at once, flooding the countertops with oil, and then soaking your cutting boards on top.
Protecting Metal With Mineral Oil
Any home cooks who own carbon-steel knives will also need mineral oil. There's a lot to like about carbon steel knives, but unlike ones made from rust-resistant stainless steel, carbon steel is vulnerable to moisture. It's not as simple as drying them well after washing, either—stored for enough time in a humid environment, the knives will eventually rust.
The solution is to rub them lightly with food-grade mineral oil after washing and drying them. The oil forms a protective barrier on the blades, preventing moisture in the air from corroding the metal.
Even kitchens with stainless steel surfaces can benefit from a mineral-oil treatment. While the stainless steel doesn't need a coating of mineral oil, the oil can still be used to clean and polish the metal, giving it a gleaming finish.
Why Use Mineral Oil?
What is it about mineral oil that makes it so well suited to these kitchen applications? In a nutshell: resistance to oxidation.
Have you ever noticed surfaces in your kitchen that have become slightly sticky? This is usually due to cooking oil that has oxidized. Oil, as you can imagine, gets on every surface in a kitchen: it spits and spatters, drips, smears, and vaporizes. Eventually it forms a thin layer on cabinets, on the hood or vent above your stove, on fans and ceilings...everywhere.
Over time, those cooking oils oxidize, a complex process that eventually leads to molecules linking together to form larger molecular chains, called polymers. As these large polymers form, the oil becomes more viscous. Mix in the inevitable dust and dirt that settles on the thickening oil, and you get that awful sticky coating that can be so hard to remove.
Perhaps some of you are seeing the word "polymer" here and making the connection to the process of seasoning cast iron. It's basically the same process: With enough heat and time, those polymer chains in the oil move past the thick and sticky stage to become a totally hard substance—the seasoning on your cast iron skillet. (If you've ever seasoned a cast iron pan and found it sticky to the touch after, that's a sure sign that you haven't polymerized the oil well enough; it needs more heat and time, and maybe a less heavy coating of oil, to complete the transformation).*
*Just to further fill in the picture, this is the same phenomenon of "drying" oils (such as linseed oil and other oils sometimes used to make varnish), which are designed to polymerize quickly, forming a hard, durable, and protective barrier on woods, paint, etc.
Mineral oil is less susceptible to oxidation than those cooking fats. This means it can remain fluid even after exposure to heat and the passage of time. It's not perfect in this regard—it, too, can eventually oxidize. A lot of mineral oil has additives to help ward against this, antioxidants and other ingredients that help keep the base oil as close to its original state for as long as possible. Food-grade mineral oil has antioxidant additives like Vitamin E that can play this function while remaining safe to ingest.
This is why mineral oil is an absolutely terrible seasoning oil for cast iron. It simply won't form the durable, nonstick layer of polymers you need. But what makes it bad for seasoning a pan is what makes it great for protecting cutting boards and knives. It won't grow thick, sticky, and rancid with time.
What Kind of Mineral Oil Should You Buy?
There are a number of products out there marketed as being specifically for oiling cutting boards and butcher blocks and knives. Some contain additives, such as beeswax, that may have beneficial effects on wood, but I've found the most important thing I look for in the mineral oil I use on my cutting boards and knives, aside from being safe to ingest, is quantity: I want a lot on hand, so I can really flood my cutting boards and countertops.
While ordering mineral oil online is convenient, you can also find inexpensive food-grade mineral oil at hardware stores and at pharmacies, so there's really no excuse for you to not have some mineral oil around your kitchen. Your wood and metal surfaces will thank you.