To safely prepare potatoes for use in any kind of cooking:
Rinse and scrub well with a scrub brush designated only for scrubbing vegetables.
Cut away any eyes (new stem growths), cracked (shattered), damaged, or bruised areas.
This is what a bruise looks like:
A bruise is a darkened area close to the surface due to being bumped or a slight break in the skin allowing oxidation to occur, but the meat of the potato is still intact and firm with no necrosis.
If necrotic (soft, squishy, disintegrating) then it is rot.
If a potato has a bad odor, a fungal or fuzzy looking mold type growth, slime, sooty growth, liquid and/or bubbles, or other abnormally appearing conditions or discoloration then it should be discarded to avoid potential food poisoning or illness.
Rot can look several different ways depending upon the microorganisms causing it. Here are a couple of the most commonly seen:
Again, there are many others such as white rot, dry rot, soft rot, watery wound rot, brown rot, storage rot, charcoal rot, ring rot, nematode rot, etc. to name just a few.
Rot can be caused by many various molds, fungi, bacteria, parasitic worms (such as the often microscopic nematode invisible to the naked eye — although some of these non-segmented parasitic worms can be as large as 3 mm to 8 meters in length), as well as other infectious microorganisms and diseases.
Cut the potato in half slicing down the longest length and inspect inside for any signs of rot.
Discard and replace with another potato if any signs of rot exists. Do not consume any part or portion of a rotting potato; do not try to cut away the rotted appearing portion and eat the rest since the microorganism causing the rot has likely already spread to the rest of the potato.
TIP: Once you are certain there is no rot, peel potatoes if desired, rinse and place in bowl of cool tap water to prevent oxidation (browning) until you have finished peeling them all and are ready to use them. Oxidation does not cause illness or hurt in any way, it is just a chemical reaction due to contact with oxygen in the air after removing the skin or cutting up a raw potato. It just isn’t very appetizing in appearance.
Rot is caused by various unhealthy microorganisms which could potentially make you very ill. Rot is just another way of saying that it is spoiled.
It can occur on the surface, but it may also occur very deep down inside the center of the potato totally unseen until the potato has actually been opened up.
Even if baking, the potato should be cut open in this manner. Then, if safe, simply put the two halves back together and wrap in aluminum foil before placing in the oven to bake.
If microwaving, you may cut the potato open then carefully inspect either before or after cooking.
If a potato does have rot, be sure not to use the knife again — which was used to cut it open — before it has been first been rinsed then washed extremely well in hot soapy water!
Do not put the knife in the soapy water (or dishwasher) until you’ve rinsed off the infectious materials first or you could contaminate the dish washing water then cross contaminating any other dishes washed in it.
Disinfecting the knife with a bleach water solution is also recommended.
In fact, all dishes and utensils really should be rinsed prior to being washed in either the sink or the dishwasher for this reason.
Scrape off remaining food, rinse, wash, rinse, dry.
If there is known or suspected exposure to any spoiled or rotted foods, or dirty dishes with food particles on them are left sitting on the counter or in the sink for longer than 2 hours before washing, then treatment with a bleach water solution is highly recommended to avoid possible cross contamination resulting in potential illness. (See links below)
Some industrial and commercial conveyor belt dishwashers are already set up to do this unlike home dish washing machines. Otherwise, dishes have to be well rinsed first. A certain amount of bleach is also added along with soap to disinfect. The hot water temperature is also being closely monitored. Health inspections are done routinely to ensure these steps are being taken.
Day care centers, even (and especially) if using home style dishwashers, also must ensure these steps are being taken.
For more information on the difference between sanitizing and disinfecting as well as the recommended quantity of bleach to water for either a disinfecting solution or a sanitizing solution: