TL;DR
Clean a grinder dry -- empty it, brush the burrs and hopper with a dry brush, and grind a spoonful of uncooked rice to pull out oily residue. Water is the one thing a grinder mechanism cannot take: steel burrs rust, and salt plus moisture corrodes metal fast, which is why salt grinders should use ceramic burrs rather than steel.
Why grinders are a dry cleaning job
A grinder is a mechanism, not a container. Inside there is a burr set -- two interlocking toothed rings -- plus a shaft, a spring, and an adjustment nut.
Water gets into all of those places easily and comes out of none of them. A grinder washed at the sink will look fine for a week and then start to squeak, stiffen, and eventually seize.
On steel burrs the damage is faster still. Water plus steel equals rust, and rust in a burr set both discolors your pepper and destroys the cutting edges that make grinding work.
This is the same reasoning that keeps water away from other mechanism tools, a theme running through the guide to cleaning kitchen tools without damaging them.
How do you clean a pepper grinder?
Empty the hopper, then take the grinder apart as far as the design allows and brush the burrs, the shaft, and the inside of the hopper with a dry pastry brush or a clean toothbrush. Grind a tablespoon of uncooked rice through the mechanism to absorb oils, discard it, and wipe the exterior with a barely damp cloth.
The dry cleaning routine
1. Empty the hopper. Pour the remaining peppercorns or salt into a bowl or a bin. Old peppercorns lose their aroma anyway, so this is a good moment to replace them.
2. Take it apart as far as it goes. Most grinders unscrew at the top knob or the base. Lay the parts out in the order they came off, and photograph the arrangement if it looks complicated.
3. Dry brush everything. A dry pastry brush, a clean toothbrush, or a small paintbrush reaches into the burr teeth. Work over a sink or a bin because a surprising amount of dust comes out.
4. Grind rice through it. A tablespoon of plain uncooked white rice run through the burrs is mildly abrasive and absorbs oils. It clears the sticky film that dry brushing leaves behind. Discard the rice powder.
5. Wipe the outside. A barely damp cloth for the body and the knob, then dry immediately. Wooden and painted bodies especially should never get properly wet.
6. Reassemble and refill. Check the grind adjustment still works through its full range before you fill it.
Why salt is harder on a grinder than pepper
Salt is corrosive to metal, and it does its worst work in the presence of even trace humidity.
A steel burr set in a salt grinder pits and rusts, sometimes within months. That is why grinders intended for salt should have ceramic burrs, which do not corrode at all.
Salt also cakes. Humidity in the air is enough to make it clump inside the hopper and jam the mechanism, which is the usual reason a salt grinder stops turning. Dry brushing the caked salt out solves it, and a few grains of dry rice stored in the hopper help keep it flowing.
If your grinder is labeled for pepper only, do not fill it with salt. That instruction exists because the burrs are steel.
The oily film problem in pepper grinders
Peppercorns contain volatile oils, which is the point of them. Those oils coat the burrs and the inside of the hopper over time and go slightly stale, which is why an old grinder produces pepper that smells flat.
Rice grinding is the standard fix, and it works because the rice is both abrasive and absorbent. Grind it, look at the powder, and repeat if it comes out grey or brown.
Coarse dry salt does the same job in a pepper grinder. Use it once and then run a few peppercorns through to clear the salt out.
Absorbent scrubbing with rice or salt is exactly how unsealed stone tools are cleaned as well, as covered in how to clean a mortar and pestle.
Can you wash a grinder with water?
Not the mechanism. Water rusts steel burrs, corrodes the shaft and spring, and swells wooden bodies. If a part has genuinely been contaminated and must be washed, wash only the removable hopper if it is glass or acrylic, dry it completely, and keep water off the burr assembly.
Acrylic hoppers should also be kept away from the dishwasher, where heat and detergent cloud and craze the plastic.
Electric and battery grinders
An electric grinder adds a motor housing that must never be submerged or rinsed.
Remove the batteries or unplug it first. Take off the hopper, dry brush the burrs, and wipe the housing with a barely damp cloth followed immediately by a dry one.
Do not run rice through an electric grinder unless the manufacturer says it is fine. Some motors are not built for it and rice is harder than peppercorns.
Storage and the mistakes to avoid
Do not store grinders directly above the stove. Steam and cooking grease drift upward, and that is how the exterior becomes tacky and the pepper inside goes stale early.
Do not soak any part of a grinder. Even a glass hopper attached to a metal collar traps water in the joint.
Do not use bleach on a grinder. It offers nothing here, chloride bleach pits stainless steel, and bleach must never be combined with ammonia, with vinegar or any other acid, or with rubbing alcohol, because those mixtures release toxic gases.
Do not force a jammed mechanism. Empty it, brush it, and check for a caked lump before applying more turning force, which is how adjustment threads strip.
Refill with fresh spice. Whole peppercorns keep their aroma for roughly a year in a sealed container away from heat and light. The same storage logic applies to oils, covered in how to clean an oil dispenser bottle, and to bulbs, covered in how to clean a garlic keeper.
Cleaning a grinder improves flavor and keeps the mechanism working. It does not treat or prevent anything.
Source: USDA Food Safety and Inspection Service guidance on cleaning food contact utensils
FAQ
Can you wash a pepper grinder with water?
No, not the mechanism. Water rusts steel burrs, corrodes the shaft and spring, and swells wooden bodies, and none of it drains back out. Clean grinders dry with a brush and a rice grind. Wash only a detachable glass or acrylic hopper, and dry it completely first.
How do you clean the burrs in a salt or pepper grinder?
Empty the grinder, take it apart as far as the design allows, and brush the burr teeth with a dry pastry brush or a clean toothbrush. Then grind a tablespoon of uncooked rice through the mechanism to absorb oils and clear the film, and discard the powder.
Why has my salt grinder stopped working?
Almost always caked salt jamming the mechanism, or corrosion on steel burrs. Empty it, dry brush the burrs and hopper, and check the adjustment nut turns freely. Grinders used for salt should have ceramic burrs, because salt and moisture corrode steel quickly.
Can you put salt in a pepper grinder?
Only if the burrs are ceramic. Steel burrs, which most pepper grinders use, corrode and pit when exposed to salt, and once the cutting edges are damaged the grinder never works properly again. Check the manufacturer label before switching.
Why does my pepper taste flat even though the grinder works?
Stale volatile oils coating the burrs and the hopper, plus old peppercorns. Grind rice through the mechanism to strip the oily film, wipe the hopper, and refill with fresh whole peppercorns. Whole pepper holds its aroma roughly a year when stored away from heat and light.
How often should you clean a grinder?
Every few months for a grinder in regular use, and any time the grind feels stiff or the pepper smells dull. The routine is short -- empty, dry brush, grind rice, wipe the outside -- and it prevents the caking and oily buildup that eventually jam the mechanism.
Small tools take minutes once you know the method. The kitchen around them takes rather longer. If you would rather hand that off, get a quote or read more about house cleaning services.